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Are Cannabinoids Neurotoxic?
Introduction
Cannabis is the worlds most commonly used illicit drug (United States
Department of Health and Human Services, 2000). While the debate continues as to
whether cannabis laws should change, so does the debate on the extent of suggested
negative effects of cannabis use (Hall, 1997). The possible negative effect of cannabis
and its major psychoactive component Δ9
-tetrahydrocannabinol (THC), on the brain and
specifically neurotoxicity, is one of the many topics brought up in such debates. While
there is a large number of research articles investigating cannabinoid neurotoxicity, there
has only been one review dedicated to this field (Scallet, 1991). Since this review was
written, over a decade ago, the landscape of cannabinoid research has changed
significantly. The existence and nature of the CB1 and CB2 receptors has been widely
studied (Felder & Glass, 1998), on top of that there has been the discovery of anandamide
(Devane et al., 1992) (AEA), sn-2-arachidonylglycerol (Mechoulam et al., 1995) as well
as other endo-cannabimimetics (Lambert & Di Marzo, 1999). Therefore, it seems that a
new review may add to the field.
When discussion neurotoxicity, it is important to consider what is actually meant
by neurotoxicity. The Interagency Committee on Neurotoxicology (ICON) adopted the
definition that neurotoxicity is “any adverse effect on the structure or function of the
central and/or peripheral nervous system by a biological, chemical or physical agent and
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may result from direct or indirect actions or reflect permanent or reversible changes in the
nervous system” (see Scallet, 1991).
While the ICON definition includes reversible changes, it is important to consider
that while a drug is still in an animal, it will still have an effect and even if this effect is
adverse, it is hardly reasonable to consider a drug neurotoxic if the adverse effect
disappears as soon as the drug is cleared from the system. Hence, before any parameters
are measured, it is important to have a “washout phase” where the animal is given no
drug. Indeed, this is especially important when examining THC, as its highly fat soluble
nature and active metabolites allows it to persist in the plasma for weeks (Grotenhermen,
2003). In humans, a long washout phase is important not only to exclude residual drug
actions, but also to allow any possible withdrawal effects to subside (Smith, 2002).
Many studies have claimed to show evidence for cannabinoid-induced
neurotoxicity on the basis that chronic administration of cannabinoids leads to altered
morphological or neurochemical parameters. In light of the definition of neurotoxicity
mentioned above, this is not sufficient, as changes need to be negative to be considered
neurotoxic. Although some researchers might suggest that any changes in the brain are
negative, this is obviously not the case. Consider the example of large-scale differences
seen between the brains of animals raised in standard and enriched environments (van
Praag et al., 2000). Here we have neurochemical and morphological changes that
correlate with positive changes in behaviour, so all neuronal changes are not bad. One
could then argue that there would be some effects that would be obviously good, like
neuronal proliferation, and some that would be obviously bad, such as cell death, but in
some instances the same effect can be seen in both toxic and beneficial situations. For
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example, the proliferation of astrocytes is seen associated with Alzheimer’s disease
(Coria et al., 1993; Luth et al., 2003) as well as in the brains of animals raised in enriched
environments (Kempermann et al., 1997; Walsh et al., 1969). Therefore, with the
exception of cell death, unless there is data which shows negative behavioural changes
that are directly correlated with a physical (morphological or neurochemical) change,
then the physical change can not be considered as evidence for neurotoxicity.
A similar problem is raised when considering behavioural results. Just because a
behaviour is changed, does not mean that there has been a negative impact on an animal.
Indeed, the same change of behaviour can be seen as positive by one group of
experimenters and negative by another. For instance, increased exploratory behaviour in
an open field test is seen as a negative by someone investigating the hippocampal toxicity
of kainic or domoic acid (Strain & Tasker, 1991), but is seen as a positive by someone
examining the anxiolytic action of postnatal handling (Padoin et al., 2001). Fortunately,
in cannabinoids research, much of the data gathered is in regards to learning and memory,
and in most cases it can be agreed that a reduction in learning or memory is a negative
impact. But where other behaviour is studied the question of whether or not any given
effect is ‘negative’ needs to be considered.
The example used above shows us another caveat. If a lesion or neurotoxin brings
about a behavioural effect e.g. kainic acid induced increase in exploratory behaviour in
an open field test, it does not mean that an intervention which produces the same
behavioural effect produces the same neurotoxicity, such as postnatal handling.
Unfortunately, a researcher can not always look for neurological changes induced
by a drug and then check to see if there is a behavioural correlate. Therefore, we must
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closely compare neurochemical/neurohistological experiments, which show a physical
change, with behavioural experiments, which show a behavioural change, in order to
make a conclusion about proposed neurotoxicity.
Neuronal Morphology
The classic first step when investigating whether or not a given chemical is
neurotoxic, is to give it to an animal and then investigate, using histological methods,
whether there has been any cell death or at least alteration of cellular morphology.
The first study into the effect that cannabinoids have on neuronal morphology was
Harper et al ., (1977). This study reported that rhesus monkeys exposed to the smoke of 1
to 3 2.6% THC cannabis cigarettes a day or 0.7mg/kg THC a day for 6 months showed
widening of the synaptic cleft, material in the synaptic cleft and nuclear inclusion bodies
after an 8-month washout phase. Unfortunately there were many methodological
problems with this study. There were no statistical analysis, the sample sizes were
extremely small, consisting of one active cannabis smoke treated animal, one IV treated
animal and the controls were two completely untreated monkeys and one monkey who
smoked alcohol extracted cannabis leaf which was presumed to be THC free. Indeed, the
apparatus in which the animals were placed in order to administer the cannabis smoke, a
procedure that most of the control animals were free of, seemed to be so constrictive and
unnatural, that it could be a serious source of bias.
Scallet et al., (1990) repeated this experiment, with larger sample numbers and
showed that after a wash out phase of 7 months, rhesus monkeys who were exposed to
the smoke of a single cannabis cigarette containing 2.6% THC, every day for 12 months,
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showed no statistically significant changes in synaptic characteristics, neuronal size, the
number of apical or basilar dendrites or the number or length of dendritic branches, in
comparison to monkeys who were either non-exposed or smoked ethanol-extracted
cannabis. Andrews et al ., (1989, as cited by Scallet et al ., 1991) also found no
neurohistological changes after dosing moneys IV with either 0.1 or 1mg/kg of THC for
90 days.
Scallet et al ., (1987) examined the hippocampal morphology of rats that had been
dosed orally with THC 5 days a week for 90 days with 20mg/kg Monday to Thursday and
60mg/kg on Friday. After a 7 month washout phase, the THC treated animals were found
to have a significant decrease in the cross sectional area of neurons and their nuclei, as
well as a decrease in the synaptic density in the CA3 region of the hippocampus, in
comparison to vehicle treated controls. Cerebellar Purkinje cells were unaffected. A
second group of animals was dosed with either 10mg/kg or 20mg/kg THC, 5 days a week
for 90 days. After an 8-week washout phase Golgi impregnation showed a reduction in
the length of the outermost branchlets of CA3 neurons. Interestingly, in the second group
of rats, there was no reduction in the size of neurons or synaptic density.
Landfield et al ., (1988) treated rats 5 days a week for 8 months, with either
4mg/kg or 8mg/kg of THC orally. The 8mg/kg group originally received 10mg/kg but
this dose was lowered during the first month because the animals found it aversive.
Directly after the last dose of THC the CA1 region of the hippocampi from the high dose
animals were shown to have a decreased number of pyramidal cell per area of section.
Whether this reduction in pyramidal cell density was due to decreased cell volume or
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actual loss of cells was not explored. It was also shown that there was an increase in dark
membranous inclusions in hippocampal astrocytes in THC treated animals.
Chan et al ., (1996) dosed rats orally with 0-500mg/kg THC 5 times a day, for
various time periods. Some of the rats suffered convulsions from such high doses of THC
and so at 9 and 15 months, the neurohistology of only the convulsive rats was
investigated. It was reported that “no treatment-related neuropathological changes were
observed in any tissue evaluated by step section”. Unfortunately, there is no report on the
number of rats in this sample, or on the specific parameters measured.
Lawston et al ., (2000) showed that rats that were subcutaneously injected twice
daily with 2mg/kg of the synthetic CB1 receptor agonist WIN 55,212-2 for 21 days,
exhibited cellular alterations in the hippocampus. Using MAP-2 antibodies, which act as
a dendritic marker it was shown that there was increased staining in the subiculum and
CA3 region of the hippocampus, as well as increased staining in the lower blade of the
dentate gyrus in comparison to vehicle treated controls. It was also shown that there was
a decrease in MAP-2 staining in the CA1 region of the hippocampus and an increase in
cresyl violet staining in the lower blade of the dentate gyrus. The authors also stated that
the dendrites of CA1 neurons in treated animals appeared as disjointed segments, which
seemed twisted or broken with a beaded aspect, rather than continuos smooth, structures,
as was seen in their controls animals. Although this is an intriguing claim, there are no
statistics associated with it, and so the definitiveness of this claim can not be verified.
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Neurochemistry
On top of neurohistological methods, another technique used to investigate the
potential neurotoxicity of a chemical, is to look for a neurochemical change associated
with it. If a chemical causes an alteration in a neurochemical system, and this is
associated with a negative behavioural change, then this fits into the ICON definition of
neurotoxicity. In neurochemical studies one must be very careful to allow for an adequate
washout phase, to make sure that any potential change is not a transient one that will
dissipate as the cannabinoids are cleared from the body.
Walters and Carr (1986) were the first to investigate the neurochemical alterations
induced by chronic cannabinoid administration after a significant washout phase. In this
experiment, pregnant rats were dosed with a cannabis extract daily so that they received
20mg/kg THC, for 20 days prior to impregnation until 20 days after giving birth, when
the offspring were weaned, which means approximately 60 days of dosage. It was
reported that at both 10 and 20 days after birth, the offspring had a significant decrease in
the Bmax of D2 receptors in striatal tissue samples in comparison to vehicle treated
controls. It was also reported that at day 20 after birth, the offspring had a significant
decrease in the K d of α1 receptors. At days 40 and 60 after birth, the offspring showed no
significant changes in Bmax or K d of either receptor studied. Tyrosine hydroxylase activity
was also studied, and in comparison to the controls it was shown to be significantly
decreased in the striatum at days 20 and 40 after birth, and in the cortex at day 40 after
birth. It is interesting to note that the cannabinoid treated animal gained significantly less
weight during pregnancy and that their offspring significantly weighed significantly less
and had lighter brains than the controls. The time points when there was a significant
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alteration of neurochemical parameters were nearly exclusively during times when the
animals were still receiving cannabinoids (apart from one tyrosine hydroxylase activity
recording), as they were still suckling at mothers who were being treated with
cannabinoid extract. All recorded neurochemical parameters had returned to normal after
being free of cannabinoids for 20 days. This could point to the fact that any alteration in
receptor binding characteristics were due to the direct effect of cannabinoids, and did not
reflect any neurotoxicity, though, the lowered brain weight induced by cannabinoid
treatment could refute that.
The experiment of Walters and Carr (1986) was repeated, though this time instead
of pregnant rats receiving cannabis extract, they were exposed to either THC (10mg/kg),
Δ8-tetrahydrocannabinol (Δ8-THC) (1mg/kg) or cannabidiol (CBD) (10mg/kg) (Walters
& Carr, 1988). The same neurochemical parameters where recorded and the rats were
dosed in the same fashion. It was reported that CBD was the only cannabinoid to alter D 2
receptor K d at any time point and no cannabinoid altered D2 receptor Bmax. CBD was
reported to significantly increase D2 K d in offspring at 10 and 20 days after birth in
comparison to control. It was reported that THC and Δ8-THC significantly increased α1
receptor Bmax at day 20 after birth, while CBD significantly increased α 1 receptor K d at
day 10 in comparison to control. It was reported that tyrosine hydroxylase activity was
significantly decreased at day 60 after birth by Δ8-THC and CBD.
When comparing these two experiments, it is interesting to note that although
they used the exact same methods in regards to everything apart from the fact that
Walters and Carr (1986) used a cannabis extract, while Walters and Carr (1988) used
separate cannabinoids, their results completely disagree. This indicates that either a
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cannabinoid apart from the ones used by Walters and Carr (1988) are responsible for the
results of Walters and Carr (1986), that cannabinoids administer together have nearly the
opposite effect from when they are administered separately, or that there was a
methodological error in one of these experiments.
Ali et al ., (1989) also investigated what neurochemical effects chronic THC
administration produced in the rat, after a significant washout phase. Rats were dosed
orally for 90 days, 5 days a week, with 0, 10 or 20mg/kg THC then sacrificed either
directly after the last dose or after a 2-week washout phase. It was reported that when
rats were sacrificed directly after the last dosing, neither the high or low dose groups had
any significant alteration in the Bmax of GABA, muscarinic acetylcholine, dopaminergic,
or mu, kappa and delta opioid receptors in the hippocampus in comparison to the controls
or in dihydroxyphenylacetic acid (DOPAC), 5-HT or 5-hydroxyindoleacetic acid (5-
HIAA) concentration in the hypothalamus, septum or caudate nucleus. In rats sacrificed
after a 2-week washout phase there was found to be a significant decrease in the Bmax of
GABA receptors in the high dose group, though no other changes were found. In order to
repeat these results, and to see whether chronic THC treatment had any effect on the
properties of sigma opioid receptors or various allosteric GABAA receptor sites, a second
experiment was designed. In this experiment, rats were treated in the same fashion as the
first experiment with the addition of a fourth dosage group that received 20mg/kg
Mondays-Thursday and 60mg/kg on Friday. In this experiment, binding was only
investigated after a 2-week washout phase. It was reported that there was no significant
alteration in the Bmax of sigma receptors, GABA receptors, or any of the GABAA
allosteric sites at any dosage level, in disagreement with their earlier results. In the end,
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these results are inconclusive, and no other study has looked into the effect of chronic
cannabinoid treatment of the GABA receptor since.
Ali et al ., (1991) examined the effects of chronic THC or cannabinoid treatment
in rats and the rhesus monkey respectively. Rats were dosed with orally with 10 or
20mg/kg of THC 5 days a week, for 90 days and sacrificed 2 hours or 2 months after the
last dose. It was reported that at either 2 hours or 2 months, neither of the doses produced
a significant effect on the Bmax of muscarinic acetylcholine receptors in comparison to
vehicle treated controls. Rhesus monkeys were treated for either 2 or 7 days a week for 1
year with the smoke of 1 2.6% cannabis cigarette, the smoked of an ethanol extracted
cannabis cigarette or no smoke and then sacrificed 7 months after the last treatment. It
was reported that there were no statistically significant dose-related changes in dopamine,
DOPAC, serotonin or 5-HIAA concentration, in either the caudate nucleus or
hypothalamus.
Westlake et al ., (1991) investigated whether chronic treatment with THC altered
the Bmax or K i of the cannabinoid receptor in either rats or rhesus monkey. Rats were
dosed with 0, 10 or 20mg/kg 5 days a week or 20mg/kg Monday to Thursday and
60mg/kg Friday for 90 days and sacrificed 60 days after the last dose. It was reported that
there were no significant changes induced by any dose in either the Bmax or the K i of the
cannabinoid receptor in the cortex, striatum, cerebellum, hippocampus or brainstem in
comparison to the control. Rhesus monkeys were exposed to the smoke of either ethanol
extract cannabis or 2.6% THC cannabis, 7 days a week for a year and then sacrificed 7
months after the last exposure. It was reported that there were no significant changes in
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the Bmax or the K i of the cannabinoid receptor in the cortex, caudate nucleus or the
cerebellum in comparison to the control.
Animal Behavioural
As mentioned earlier if a chemical is to be considered neurotoxic it must produce
not only some measurable change in neuroanatomy or neurochemistry, but it must
produce a negative behavioural change. This behavioural change must also continue after
the drug has left the body.
The first experiment that investigated what behavioural effects chronic
cannabinoid treatment may have, after a significant wash out phase was Fehr et al .,
(1976). The rats were dosed orally with an ethanolic cannabis extract containing 10mg/kg
THC daily for 30, 60 or 90 days or 20mg/kg THC for 180 days, and then after a 1-month
washout phase. After the washout phase the animals ability in a moving belt task and the
Hebb-Williams maze were investigated. The moving belt task involved the rats walking
on a moving belt, where the surrounding area is electrified so that if a paw is placed off
the belt in any direction the animal will receive a shock. It was reported that tasks after
the 1-month washout phase, none of the 10mg/kg dosing regimes produced any
significant effect in any of the in comparison to the control but the 20mg/kg group
performed significantly slower at the Hebb-Williams maze and spent significantly more
time off the moving belt than the controls.
Stiglick and Kalant (1982) examining the effects of chronic cannabinoid
administration on the performance in an 8 or 12-arm radial maze. Rats were dosed orally
with an ethanolic cannabis extract so that they received 20mg/kg daily for 3 or 6 months.
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After a 1-month washout phase, the animals that were dosed for 3-month were tested
with an 8 or 12-arm radial maze, and the animals that were dosed for 6-months were
tested with an 8-arm radial maze. It was reported that after the 1-month washout phase,
all cannabinoid treated animals learned the radial arm maze significantly slower than the
vehicle treated controls. It was reported that the control and cannabinoid treated animals
did not eat significantly different amounts and hence this result is unlikely to be because
of a lowered motivation to eat food caused by cannabinoid treatment.
Stiglick and Kalant (1983) conducted a similar experiment, although this time
pure THC was used and the behaviour of rats chronically treated with THC in a two-way
shuttle box avoidance test and a 12-arm radial maze was also investigated. Rats were
treated orally with 20mg/kg THC daily for 3 months. Testing in the 12-arm radial maze
began after a 34-day washout phase. Rats were then tested in an open field test after a 77-
day washout phase, then a differential reinforcement of low rate responding-20 (DRL-20)
test after a 92-day washout and finally the shuttle box avoidance test after a 132-day
washout. It was reported that treated with THC had no effect on open field test
exploratory behaviour in comparison to control. It was shown that THC treatment
effected the animals’ ability to learn the 12-arm radial maze in comparison to the vehicle
treated control, but that this effect was restricted to the first 11 days of performance. It
was reported that THC treated animals received significantly less food rewards during the
DRL-20 test in comparison to controls, but that this effect only last for the first 13 days of
testing. It was shown that THC treatment had no significant effect on the performance in
the shuttle box avoidance task.
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Nakamura et al ., (1991) investigated the effects that chronic THC administration
had on performance in a variation of the 8-arm radial maze in the rat. Rats were dosed
with 5mg/kg THC I.P. 6 days a week, for 90 days. During administration the rats were
trained in an 8-arm radial maze where, after they had entered 4 arms, the rats were
removed from the maze for 5 seconds or 1 hour, and the placed back in the center. During
treatment, THC treated animals made significantly more errors in both the 5-second and
1-hour delay than the vehicle treated controls. 15 days after discontinuation of THC
treatment the THC treated animals still made significantly more errors than the controls,
but after 30 days the difference was no longer significant. Because the behavioural deficit
corrected after such a short time, it could be possible that it was just due to residual THC.
Unfortunately, plasma THC was not recorded, so this possibility can not be addressed.
Paule et al ., (1992) investigated the effect of cannabinoids on two operant tasks in rhesus
monkeys. Rhesus monkeys were exposed to the smoke of 1 2.6% cannabis cigarette 2 or
7 days a week, the smoke of 1 ethanol extracted cannabis cigarette 7 days a week or no
smoke for 1 year. Before exposure, animals were then trained approximately 30 times on
two tasks, a progressive ration (PR) operant test, where the number of presses to receive
the reward increased after each reward, but reverted back to 1 press at the end of each
day. The other task was a Conditioned Position Responding (CPR) test, involving
pressing a button in response to a light to receive a food reward where the correct button
changed depending on the color of the light. During dosing, the animals either continued
training (the active group) or were not trained (the residual group). In the active group, by
the end of their 12 months of treatment, the cannabinoid-exposed animals received
significantly fewer rewards or performed significantly slower that the cannabinoid free
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animals in the PR, and CPR experiments. After the cannabinoid exposure the animals
were studied for 7 months where they received no drugs, during this time, the residual
group began training again. The data for the active group was not fully reported, but the
residual group still showed significantly impaired in performance in the CPR task after a
7 month abstinence period.
Human Neurological Imaging
Campbell et al., (1971) investigated the possibility of cannabis-induced cerebral
atrophy using pneumoencephalograms, a technique where the cerebral ventricles are
inflated with air in order to display their size. They found that subjects who had used
cannabis ‘consistently’ showed markedly bigger ventricles than the controls, and hence
cerebral atrophy. This study was seriously flawed as the cannabis using subjects were
polydrug abusers, which the controls were not, they were all under psychiatric treatment,
which the controls were not and worst of all, the first 4 cannabis using subjects were
know to have abnormal scans at the start of the trial. With the invention of the CAT scan
in the late 1970s, two studies used this far more sensitive technique to investigate the
claims of Campbell et al ., (1971).
Co et al ., (1977) and Kuehnle et al ., (1977) used CAT scans to look for any gross
cerebral atrophy in 12 and 19 chronic cannabis users respectively. Despite a lack of
control for any factor apart from sex, there were found to be no significant changes in
cerebral volume in comparison to controls.
Block et al ., (2000a) were the first to investigate the possibility of brain atrophy
induced by chronic cannabis use using magnetic resonance imaging. The cannabis using
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subjects had used cannabis for at least 2 years, on average every day. It was found that
the cannabis users had no significant differences in gross brain, frontal lobe, temporal
lobe, parietal lobe, occipital lobe, cerebellar and hippocampal volume in comparison to
the controls.
Block et al ., (2000b) investigated the cerebral blood flow of abstinent chronic
cannabis users, using positron emission tomography (PET). The same cohort as Block et
al ., (2000a) received supervised abstinence from cannabis, alcohol and caffeine use for
an average of 44 hours, and a minimum of 33 hours before the PET scan. It was shown
that cannabis users had significantly lower regional blood flow in the posterior
cerebellum than the controls. Although this could be attributed to residual drug levels,
this seems unlikely as an experiment by the same laboratory reported that the cerebellar
blood flow increased during acute THC treatment (O'Leary et al., 2002). The possibility
that this decrease in blood flow is an effect of withdrawal can not be excluded until the
experiment is repeated with a longer period of abstinence.
Human Neuropsychological
There is a large history of neuropsychological testing in chronic cannabis users,
and a review the entire body of research is outside the scope of this article (for a review
see Gonzalez et al., 2002) so the most recent and illustrative examples are cited here.
Pope and Yurgelun-Todd (1996) compared the cognitive function in heavy and
light cannabis using college students. The rational behind the study was that if heavy
users were compared to light users then the difficulty of matching controls would be
avoided, as it was presumed that light and heavy cannabis users would come from the
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same socio-economic background. Light users were defined as subjects who had smoked
cannabis a maximum of 9 times in the last month, and had no urinary traces of
cannabinoids. Heavy users were defined as subjects who had smoked cannabis a
minimum of 22 times in the last month and had traces of cannabinoids in their urine.
Subjects had 19 hours of supervised abstinence before they received a battery of
cognitive tests. It was reported that the heavy users did not performed significantly worse
than the light users in any test apart from the Wisconsin card sorting test and the
California verbal learning test. Interestingly, these results did not reach significance if the
genders were compared individually. Because of the short abstinence period, this
difference could be attributed to both residual cannabinoid action (because if there were
cannabinoids in the urine, there could be cannabinoids in the blood) or withdrawal
effects.
Fletcher et al ., (1996) investigated the effects chronic cannabis use has on
cognitive function in a young and an older group of Costa Rican men. The older group
had a mean age of 45 years old and had smoked for an average of 34 years. The young
group had a mean age of 28 and had smoked for an average of 8 years. The controls were
appropriately matched in regards to age, sex and socio-economic background. The
subjects were required to abstain from cannabis use for 72 hours, and any subjects who
had cannabinoids in their urine were excluded. Short-term memory, working memory,
and attentional skills were measured in each subject. It was reported that the older group
showed a significant deficit in short-term memory in comparison to the control group, but
no difference of any kind was seen in the young group. It seems unlikely that this result is
because of residual cannabinoids, as the urine tests were negative for cannabinoids. It
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also seems unlikely that this effect was caused by cannabinoid withdrawal as withdrawal
symptoms last for 7 days after the end of cannabinoid administration (Smith, 2002) but
cannabinoids persist in urine for longer than this especially in heavy users (Ellis et al.,
1985). Therefore, if subjects had negative urine samples, they must have stopped
cannabis use a long enough time ago, that withdrawal symptoms would have passed.
Pope et al ., (2001) investigated the cognitive function of chronic cannabis users
over a 28-day abstinence period. There were 3 groups: subjects who had used cannabis at
least 5000 times and were smoking daily at the start of the study, subjects who had used
cannabis at least 5000 times but had used cannabis 12 times or less in the last three
months and the control group, who had used cannabis no more than 50 times. The
subjects were tested on a battery of tests on day 0, 1, 7 and 28 of abstinence. The former
users showed no significant difference in any of the tests. The current users showed
significant differences in many of the tests on day 0, 1 and 7, but by day 28 there was
only one test where they performed significantly worse, but when the test was controlled
for verbal IQ, the difference was no longer significant. The experiment shows that
washout phases of 19-72 hours, as used in other experiments probably are not long
enough to remove the residual effects of cannabinoids. Interestingly, the mean time of
use for the cannabis using group was 13 years. When compared to the results of Fletcher
et al ., (1996) this time length is far closer to the young group (9 years) who showed no
negative cognitive impact from cannabis use. Perhaps this indicates that cannabis use
needs to continue for more than 13 years to be damaging.
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Summary
The results from the various experiments investigating chronic cannabinoid
treatment on neuronal morphology are summed up in table 1. In the rat experiments
reported here, all but one reported that cannabinoids alters neuronal morphology,
particularly in the hippocampus, and the one study that did not, used an unknown sample
size and techniques. In the rhesus monkey, the results have been inconclusive, but seem
to show that 1 2.6% THC cannabis cigarette a day is not enough to induced
neurohistological changes.
Studies investigating the neurochemical effect of chronic cannabinoid treatment
have had mixed results (Table 2). It has been reported that administration of cannabinoids
to pregnant rats produced neurochemical alterations in their offspring up to 40 days after
weaning (Walters & Carr, 1986), but when this experiment was repeated the alterations
found were completely different (Walters & Carr, 1988). Other experiments have found
no significant alterations in the neurochemical systems investigated in either the rat or the
rhesus monkey. It is possible that there is a critical period in which cannabinoids can alter
the neurochemistry of the rat.
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T a b l e 1 . S t u d i e s i n v e s t i g a t i n g t h e e f f e c t s o f c a n n a b i n o i d s o n n e u r o
n a l m o r p h o l o g y
C h a n g e ? +
- - + + - +
T a b l e 2 . S t u d i e s i n v e s t i g a t i n g t h e e f f e c t s o f c a n n a b i n o i d s o n
n e u r o c h e m i s t r y
C h a n g e ? + + - - -
W a s h o u t P h a s e
8 m o n t h s
7 m o n t h s
N o n e
7 m o n t h s
N o n e
N o n e
n o n e
W a s h o u t P h a s e
0 - 4 0
d a y s
0 - 4 0
d a y s
0 - 2 w
e e k s
2 h o u r s - 2 m o n t h s
7 m o n t h s
6 0
d a y s
7 m o n t h s
D o s i n g L e n g t h
3 m o n t h s
1 y e a r s
9 0 d a y s
9 0 d a y s
8 m o n t h s
1 3 o r 2 4 m o n t h s
2 1 d a y s
D o s i n g L e n g t h
~ 6 0 d a y s
~ 6 0 d a y s
9 0 d a y s
9 0 d a y s
1 y e a r
9 0 d a y s
1 y e a r
D o s i n g r e g i m e
1 - 3 2 . 6 % T H C c a n n a b i s c i g a
r e t t e s o r 0 . 7 m g / k g T H C I . V . d a i l y
1 2 . 6 % T H C c a n n a b i s c i g a r e t t e d a i l y
0 . 1 - 1 m g / k g T H C I . V . d a i l y
2 0 - 6 0 m g / k g T H C o r a l l y 5 x w e e k
4 - 8 m g / k g T H C o r a l l y 5 x w e e k
0 - 5 0 0 m g / k g T H C o r a l l y 5 x w e e k
2 m g / k g W I N 5 5 , 2 1 2 - 2 S . C . 2 t i m e s d a i l y
D o s i n g r e g i m e
2 0 m g / k g T H C o r a l l y d a i l y t o d a m s
1 0 m g / k g T H C a n d C B D , 1 m
g / k g Δ 8 - T H C o r a l l y d a i l y t o d a m s
1 0 - 2 0 m g / k g T H C o r a l l y 5 x w e e k
1 0 - 2 0 m g / k g T H C o r a l l y 5 x w e e k
1 2 . 6 % T H C c a n n a b i s c i g a r e t t e 2 - 7 x w e e k
1 0 - 6 0 m g / k g T H C o r a l l y 5 x w e e k
1 2 . 6 % T H C c a n n a b i s c i g a r e t t e d a i l y
A n i m a l
R h e s u s M o n k e y
R h e s u s M o n k e y
R h e s u s M o n k e y R a t R a t R a t R a t
A n i m a l R a t R a t
R a t R a t
R h e s u s M o n k e y
R a t
R h e s u s M o n k e y
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A u t h o r s
H a r p e r e t a l . , 1 9 7 7
S c a l l e t e t a l . , 1 9 9 0
A
n d r e w s e t a l . , 1 9 8 9
S c a l l e t e t a l . , 1 9 8 7
L a n d f i e l d e t a l . , 1 9 8 8
C h a n e t a l . , 1 9 9 6
L
a w s t o n e t a l . , 2 0 0 0
A u t h o r s
W a l t e r s a n d C a r r , 1 9 8 6
W a l t e r s a n d C a r r , 1 9 8 8
A l i e t a l . , 1 9 8 9
A l i e t a l . , 1 9 9 1
W
e s t l a k e e t a l . , 1 9 9 1
T a b l e 3 . S t u d i e s i n v e s t i g a t i n g
t h e e f f e c t s o f c a n n a b i n o i d s o n a n i m a l b e h a v i o u r
C h a n g e ? + + + ? +
T a b l e 4 . S t u d i e s i n v e s t i g a t i n g t h e e f f e c t s o f c a n n a b i n o i d s o n n e u r o l o g i c a l i m
a g i n g i n h u m a n s
C h a n g e ? + - - - +
T a b l e 5 . S t u d i e s i n v e s t i g a t i n g t h e e f f e c t s o f c a n n a
b i n o i d s o n n e u r o p s y c h o l o g i c a l t e s t i n g i n h u m a n s
C h a n g e ?
- - ?
W a s h o
u t P h a s e
1 m o n t h
1 m o n t h
7 7 - 1 3 2 d a y s
1 5 - 3 0 d a y s
7
m o n t h s
W a s h o
u t P h a s e
N o n e
N o n e
N o n e
N o n e
4 4 h o u r s
W a s h o
u t P h a s e
1 9 h o u r s
7 2 h o u r s
0 - 2 8 d a y s
D o s i n g L e n g t h
3 0 - 1 8 0 d a y s
3 - 6 m o n t h s
3 m o n t h s
9 0 d a y s
1 y e a r
I m g a i n e T e c h n e q u e
p n e u m o e n c e p h a l o g r a m
C A T
C A T
M R I
P E T
H i s t o r y o f c a n n a b i s u s e
L i g h t u s e r s : U s e d < 9 t i m e s i n l a s t m o n t h . H e a v y u s e r s : u s e d > 2 2 t i m e s l a s t m o n t h
O l d u s e r s : U s e d f o r 3 4 y e a r s . Y o u
n g u s e r s : U s e d f o r 9 y e a r s
E x - u s e r s : U s e d > 5 0 0 0 t i m e s , b u t n o t i n l a s t 3 m o n t h s . C u r r e n t u s e r s : U s e d > 5 0 0 0 t i m e s
D o s i n g r e g i m e
1 0 - 2 0 m g / k g T H C d a i l y o r a
l l y
2 0 m g / k g T H C d a i l y o r a
l l y
2 0 m g / k g T H C d a i l y o r a
l l y
5 m g / k g T H C 6 x w e e k I . P .
1 2 . 6 % T H C c i g a r e t t e 2 - 7 x w e e k
H i s t o r y o f c a n n a b i s u s e
“ C o n s i s t e n t ” u s e
“ C h r o n i c ” u s e
“ C h r o n i c ” u s e
D a i l y u s e f o r 2 y e a r s
D a i l y u s e f o r 2 y e a r s
A n i m a l
R a t
R a t R a t R a t
R h e s u s M o n k e y s
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A u t h o r s
F e h r e t a l . , 1 9 7 6
S t i g l i c k a n d K a l a n t , 1 9 8 2
S t i g l i c k a n d K a l a n t , 1 9 8 3
N a
k a m u r a e t a l . , 1 9 9 1
P a u l e e t a l . , 1 9 9 2
A u t h o r s
C a m p b e l l e t a l . , 1 9 7 1
C o e t a l . , 1 9 7 7
K
u e h u l e e t a l . , 1 9 7 7
B l o c k e t a l . , 2 0 0 0 a
B l o c k e t a l . , 2 0 0 0 b
A u t h o r s
P o p e a n d Y
u r g e l u n e t a l . , 1 9 9 6
F l e t c h e r e t a l . , 1 9 9 6
P o p e e t a l . , 2 0 0 1
Experiments investigating the behaviour of rats and rhesus monkeys have
reported that it is altered, even after a significant washout phase, after chronic
cannabinoid treatment (Table 3). The one experiment which did not show this
conclusively was Nakamura et al ., (1991), which showed that the behavioural effects
were reversed after a 30 day washout, but not a 15 day washout phase. It is unknown
whether this is due to the persistence of cannabinoids in the plasma. Largely though, it
has been shown that cannabinoids in doses between 5 and 20mg/kg produce a residual
change in behaviour in the rat, and that this change is negative in nature, as it involves a
decrease in spatial processing as shown by reduced performance in 8 and 12-arm radial
mazes. In order to be conclusive about the action of chronic cannabinoids in monkeys,
more experiments need to be run.
In humans it has been shown that cannabinoids probably do not produce any
change in brain volume (Table 4). Block et al ., (2000b) reported that during abstinence,
cannabinoids produced a change in regional brain flow but the abstinent period was
probably not long enough to conclude that this was not an effect of residual cannabinoids
in the blood, or because of cannabinoid withdrawal.
Investigations into the cognitive state of cannabis users have shown very
interesting results (Table 5). The results of Pope and Yurgelun-Todd (1996) which
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indicated that chronic cannabinoid usage resulted in cognitive impairment was probably
due to an insufficient washout phase. Fletcher et al ., (1996) reported that even after a
washout phase that was probably long enough for residual cannabinoids to be cleared and
withdrawal symptoms to subside, subjects who had used cannabinoids for a mean length
of 34 years performed significantly worse than their age matched controls in short term
memory tasks. On the other hand, it was reported that subjects who had used for a mean
of 9 years did not. This experiment indicates that perhaps significant cognitive
impairment only happens after extended cannabinoid treatment. Pope et al (2001) showed
that subjects who had used cannabis more than 5000 times but had been generally
abstinent for the last 3 months performed no worse than the controls. They also reported
that while subjects who had used cannabis more than 5000 times and were still using
daily performed significantly worse than the controls, they performed no worse than the
controls after 28 days abstinence.
As mentioned earlier, in order for a chemical to satisfy to ICON definition for
neurotoxicity, it must produced a physical change (neurochemical/neurohistological/
neurological) and this change must be associated with a negative behavioural effect.
Cannabinoids have satisfied this criterion in doses between 8 and 20mg/kg in the rat, but
not in the rhesus monkey. In humans there is no clear evidence that chronic cannabinoid
physical alters the brain or cognition, so there is no evidence that cannabinoids are
neurotoxic.
Even though cannabinoids have been shown to be protective against many
induced neurotoxic events (for review, see Fowler, 2003) in a model proposed by
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Guzman (2003), it could be possible for cannabinoids to be protective against large
neurotoxic insults, yet cause neurotoxicity slowly over time.
In order to gain more knowledge in area, studies investigating rhesus monkeys
that are dosed in a more controlled fashion than smoke, and at a higher dose than 1
cannabis cigarette could prove useful. Instead of getting further away from human users,
this could in fact bring the experimental paradigm closer to human users, who often
smoke more than one cannabis cigarette a day (Foltin et al., 1989). Also studies
investigating the cognitive function of humans who have used cannabis for 30 years or
longer could show whether permanent cognitive impairments can be induced by extended
cannabis use.
In conclusion, there is evidence that chronic cannabinoid treatment is neurotoxic
to the rat, but not to the rhesus monkey or to humans.
References
ALI, S.F., NEWPORT, G.D., SCALLET, A.C., GEE, K.W., PAULE, M.G., BROWN, R.M. & SLIKKER , W., JR . (1989).
Effects of chronic delta-9-tetrahydrocannabinol (THC) administration on neurotransmitter
concentrations and receptor binding in the rat brain. Neurotoxicology, 10, 491-500.
ALI, S.F., NEWPORT, G.D., SCALLET, A.C., PAULE, M.G., BAILEY, J.R. & SLIKKER , W., JR . (1991). Chronic
marijuana smoke exposure in the rhesus monkey. IV: Neurochemical effects and comparison toacute and chronic exposure to delta-9-tetrahydrocannabinol (THC) in rats. Pharmacol Biochem
Behav, 40, 677-82.
A NDREWS, A.M., SCALLET, A.C., UEMURA, E., ALI, S.F., BAILEY, J.R., PAULE, M.G., BROWN, R.M. & SLIKKER JR ,
W. (1989). Ultrastructural evaluation of hippocampus in rhesus monkeys after intravenous delta-9-
tetrahydrocannabinol (THC): 90 Day study. EMSA Proc, 47, 956-7.
BLOCK , R.I., O'LEARY, D.S., EHRHARDT, J.C., AUGUSTINACK , J.C., GHONEIM, M.M., ARNDT, S. & HALL, J.A.
(2000a). Effects of frequent marijuana use on brain tissue volume and composition. Neuroreport ,11, 491-6.BLOCK , R.I., O'LEARY, D.S., HICHWA, R.D., AUGUSTINACK , J.C., PONTO, L.L., GHONEIM, M.M., ARNDT, S.,
EHRHARDT, J.C., HURTIG, R.R., WATKINS, G.L., HALL, J.A., NATHAN, P.E. & A NDREASEN, N.C. (2000b).
Cerebellar hypoactivity in frequent marijuana users. Neuroreport , 11, 749-53.
CAMPBELL, A.M., EVANS, M., THOMSON, J.L. & WILLIAMS, M.J. (1971). Cerebral atrophy in young cannabis
smokers. Lancet , 2, 1219-24.
CHAN, P.C., SILLS, R.C., BRAUN, A.G., HASEMAN, J.K. & BUCHER , J.R. (1996). Toxicity and carcinogenicity of
delta 9-tetrahydrocannabinol in Fischer rats and B6C3F1 mice. Fundam Appl Toxicol , 30, 109-17.
8/6/2019 Are Cannabinoids Neurotoxic-12
http://slidepdf.com/reader/full/are-cannabinoids-neurotoxic-12 24/25
CO, B.T., GOODWIN, D.W., GADO, M., MIKHAEL, M. & HILL, S.Y. (1977). Absence of cerebral atrophy in
chronic cannabis users. Evaluation by computerized transaxial tomography. JAMA, 237, 1229-30.
CORIA, F., MORENO, A., R UBIO, I., GARCIA, M.A., MORATO, E. & MAYOR , F., JR . (1993). The cellular pathology
associated with Alzheimer beta-amyloid deposits in non-demented aged individuals. Neuropathol
Appl Neurobiol , 19, 261-8.DEVANE, W.A., HANUS, L., BREUER , A., PERTWEE, R.G., STEVENSON, L.A., GRIFFIN, G., GIBSON, D., MANDELBAUM,
A., ETINGER
, A. & MECHOULAM
, R. (1992). Isolation and structure of a brain constituent that binds tothe cannabinoid receptor. Science, 258, 1946-9.
ELLIS, G.M., JR ., MANN, M.A., JUDSON, B.A., SCHRAMM, N.T. & TASHCHIAN, A. (1985). Excretion patterns of
cannabinoid metabolites after last use in a group of chronic users. Clin Pharmacol Ther , 38, 572-
8.
FEHR , K.A., K ALANT, H. & LEBLANC, A.E. (1976). Residual learning deficit after heavy exposure to cannabis
or alcohol in rats. Science, 192, 1249-51.FELDER , C.C. & GLASS, M. (1998). Cannabinoid receptors and their endogenous agonists. Annu Rev
Pharmacol Toxicol , 38, 179-200.
FLETCHER , J.M., PAGE, J.B., FRANCIS, D.J., COPELAND, K., NAUS, M.J., DAVIS, C.M., MORRIS, R., K RAUSKOPF, D.
& SATZ, P. (1996). Cognitive correlates of long-term cannabis use in Costa Rican men. Arch Gen Psychiatry, 53, 1051-7.
FOLTIN, R.W., FISCHMAN, M.W., BRADY, J.V., CAPRIOTTI, R.M. & EMURIAN, C.S. (1989). The regularity of
smoked marijuana self-administration. Pharmacol Biochem Behav, 32, 483-6.
FOWLER , C.J. (2003). Plant-derived, synthetic and endogenous cannabinoids as neuroprotective agents. Non- psychoactive cannabinoids, 'entourage' compounds and inhibitors of N-acyl ethanolamine
breakdown as therapeutic strategies to avoid pyschotropic effects. Brain Res Brain Res Rev, 41,
26-43.
GONZALEZ, R., CAREY, C. & GRANT, I. (2002). Nonacute (residual) neuropsychological effects of cannabis
use: a qualitative analysis and systematic review. J Clin Pharmacol , 42, 48S-57S.
GROTENHERMEN, F. (2003). Pharmacokinetics and pharmacodynamics of cannabinoids. Clin Pharmacokinet ,42, 327-60.
GUZMAN, M. (2003). Neurons on cannabinoids: dead or alive? Br J Pharmacol , 140, 439-440.
HALL, W. (1997). The recent Australian debate about the prohibition on cannabis use. Addiction, 92, 1109-
15.HARPER , J.W., HEATH, R.G. & MYERS, W.A. (1977). Effects of Cannabis sativa on ultrastructure of the
synapse in monkey brain. J Neurosci Res, 3, 87-93.
K EMPERMANN, G., K UHN, H.G. & GAGE, F.H. (1997). More hippocampal neurons in adult mice living in anenriched environment. Nature, 386, 493-5.
K UEHNLE, J., MENDELSON, J.H., DAVIS, K.R. & NEW, P.F. (1977). Computed tomographic examination of
heavy marijuana smokers. JAMA, 237, 1231-2.
LAMBERT, D.M. & DI MARZO, V. (1999). The palmitoylethanolamide and oleamide enigmas : are these two
fatty acid amides cannabimimetic? Curr Med Chem, 6, 757-73.LANDFIELD, P.W., CADWALLADER , L.B. & VINSANT, S. (1988). Quantitative changes in hippocampal structure
following long-term exposure to delta 9-tetrahydrocannabinol: possible mediation by
glucocorticoid systems. Brain Res, 443, 47-62.
LAWSTON, J., BORELLA, A., R OBINSON, J.K. & WHITAKER -AZMITIA, P.M. (2000). Changes in hippocampal
morphology following chronic treatment with the synthetic cannabinoid WIN 55,212-2. Brain
Res, 877, 407-10.
LUTH, H.J., APELT, J., IHUNWO, A.O., ARENDT, T. & SCHLIEBS, R. (2003). Degeneration of beta-amyloid-
associated cholinergic structures in transgenic APP SW mice. Brain Res, 977, 16-22.MECHOULAM, R., BEN-SHABAT, S., HANUS, L., LIGUMSKY, M., K AMINSKI, N.E., SCHATZ, A.R., GOPHER , A., ALMOG,
S., MARTIN, B.R., COMPTON, D.R. & ET AL. (1995). Identification of an endogenous 2-monoglyceride, present in canine gut, that binds to cannabinoid receptors. Biochem Pharmacol ,
50, 83-90.
NAKAMURA, E.M., DA SILVA, E.A., CONCILIO, G.V., WILKINSON, D.A. & MASUR , J. (1991). Reversible effects of
acute and long-term administration of delta-9-tetrahydrocannabinol (THC) on memory in the rat.
Drug Alcohol Depend , 28, 167-75.
8/6/2019 Are Cannabinoids Neurotoxic-12
http://slidepdf.com/reader/full/are-cannabinoids-neurotoxic-12 25/25
O'LEARY, D.S., BLOCK , R.I., K OEPPEL, J.A., FLAUM, M., SCHULTZ, S.K., A NDREASEN, N.C., PONTO, L.B.,
WATKINS, G.L., HURTIG, R.R. & HICHWA, R.D. (2002). Effects of smoking marijuana on brain
perfusion and cognition. Neuropsychopharmacology, 26, 802-16.
PADOIN, M.J., CADORE, L.P., GOMES, C.M., BARROS, H.M. & LUCION, A.B. (2001). Long-lasting effects of
neonatal stimulation on the behavior of rats. Behav Neurosci, 115, 1332-40.PAULE, M.G., ALLEN, R.R., BAILEY, J.R., SCALLET, A.C., ALI, S.F., BROWN, R.M. & SLIKKER , W., JR . (1992).
Chronic marijuana smoke exposure in the rhesus monkey. II: Effects on progressive ratio andconditioned position responding. J Pharmacol Exp Ther , 260, 210-22.
POPE, H.G., JR ., GRUBER , A.J., HUDSON, J.I., HUESTIS, M.A. & YURGELUN-TODD, D. (2001). Neuropsychological
performance in long-term cannabis users. Arch Gen Psychiatry, 58, 909-15.
POPE, H.G., JR . & YURGELUN-TODD, D. (1996). The residual cognitive effects of heavy marijuana use in
college students. JAMA, 275, 521-7.
SCALLET, A.C. (1991). Neurotoxicology of cannabis and THC: a review of chronic exposure studies inanimals. Pharmacol Biochem Behav, 40, 671-6.
SCALLET, A.C., UEMURA, E., A NDREWS, A., ALI, S.F., MCMILLAN, D.E., PAULE, M.G., BROWN, R.M. & SLIKKER ,
W., JR . (1987). Morphometric studies of the rat hippocampus following chronic delta-9-
tetrahydrocannabinol (THC). Brain Res, 436, 193-8.
SCALLET, A.C., UEMURA, E., A NDREWS, A.M., CRAVEN, J., R OUNTREE, R., WILSON, S., ALI, S.F., BAILEY, J.R.,
PAULE, M.G. & SLIKKER , W., JR . (1990). Morphometric neurohistological studies of rhesus
monkeys after chronic marijuana smoke (MS) exposure. Soc Neurosci Abstr , 16, 1116.
SMITH, N.T. (2002). A review of the published literature into cannabis withdrawal symptoms in humanusers. Addiction, 97, 621-32.
STIGLICK , A. & K ALANT, H. (1983). Behavioral effects of prolonged administration of delta 9-tetrahydrocannabinol in the rat. Psychopharmacology (Berl) , 80, 325-30.
STIGLICK , A. & K ALANT, H. (1982). Learning impairment in the radial-arm maze following prolonged
cannabis treatment in rats. Psychopharmacology (Berl) , 77, 117-23.
STRAIN, S.M. & TASKER , R.A. (1991). Hippocampal damage produced by systemic injections of domoic acid
in mice. Neuroscience, 44, 343-52.
U NITED STATES DEPARTMENT OF HEALTH AND HUMAN SERVICES, O.O.A.S. (2000). National household survey of
drug abuse: main findings 1998. Rockville (MD): Substance Abuse and Mental Health Services
Administration, Office of Applied Studies.VAN PRAAG, H., K EMPERMANN, G. & GAGE, F.H. (2000). Neural consequences of environmental enrichment.
Nat Rev Neurosci, 1, 191-8.
WALSH, R.N., BUDTZ-OLSEN, O.E., PENNY, J.E. & CUMMINS, R.A. (1969). The effects of environmentalcomplexity on the histology of the rat hippocampus. J Comp Neurol , 137, 361-6.
WALTERS, D.E. & CARR , L.A. (1986). Changes in brain catecholamine mechanisms following perinatal
exposure to marihuana. Pharmacol Biochem Behav, 25, 763-8.
WALTERS, D.E. & CARR , L.A. (1988). Perinatal exposure to cannabinoids alters neurochemical development
in rat brain. Pharmacol Biochem Behav, 29, 213-6.WESTLAKE, T.M., HOWLETT, A.C., ALI, S.F., PAULE, M.G., SCALLET, A.C. & SLIKKER , W., JR . (1991). Chronic
exposure to delta 9-tetrahydrocannabinol fails to irreversibly alter brain cannabinoid receptors. Brain Res, 544, 145-9.