&p.1:Abstract The effects of nimodipine, an L-type calciumchannel antagonist, on nicotine-induced locomotor activ-ity were investigated in drug-naive rats. Nicotine(0.4 mg/kg IP) produced significant increases in locomo-tion following acute administration. However, when ratswere given injections of nimodipine (5, 10, or 20 mg/kgIP) 1 h prior to the test drug, nicotine-induced locomotoractivity was altered. Nimodipine 5 mg did not signifi-cantly block locomotor activity produced by nicotine. Incontrast, pretreatment with 10 and 20 mg nimodipinesignificantly blocked nicotine-induced locomotor activi-ty. These findings clearly indicate that nicotine-inducedlocomotion is altered by nimodipine in a dose-dependentfashion. Results further suggest that the effect of nicotineon locomotion is calcium-dependent.

&kwd:Key words Nicotine · Locomotor activity · Calcium antagonists · Nimodipine · Dopamine&bdy:

Introduction

Several groups have reported that dopamine release with-in the nucleus accumbens is preferentially increased,compared to other CNS regions, following administra-tion of drugs commonly abused by humans, includingnicotine (Rowell et al. 1987; Di Chiara and Imperato1988). Although these drugs share in common an abilityto increase nucleus accumbens dopamine turnover, theirmechanisms of action may differ. For example, cocaineblocks the reuptake of released dopamine (Ritz et al.1987) and can cause a calcium-dependent dopamine re-lease as well (Hurd and Ungerstedt 1989). Nicotine, onthe other hand, is thought to increase nucleus accumbenssynaptic dopamine levels by stimulating the soma ofneurons in the ventral tegmental area which project tothe nucleus accumbens (Nisell et al. 1994). However,

like cocaine, nicotine’s dopamine releasing actions seemto be calcium-dependent (Westfall et al. 1989).

Following depolarization of the cell membrane by anaction potential, calcium enters the cytoplasmic spacethrough voltage-sensitive calcium channels. Once in thecytoplasm calcium plays a critical role in assisting fusionof the synaptic vesicles with the plasma membranewhere neurotransmitters are released through appropriatechannels. There exist at least three types of voltage-sen-sitive calcium channels: the L-type, the N-type, and theT-type. The L- and N-type are thought to play a domi-nant role in calcium dependent neurotransmitter release,whereas the T-type is only thought to participate indi-rectly in neurotransmitter release (Hirning et al. 1988;Kato et al. 1992). There is evidence that suggests boththe L- and N-type calcium channels play a role in the re-lease of dopamine induced by drugs of abuse (Pani et al.1990; Harsing et al. 1992). Pani et al. (1990) found thatcocaine-induced dopamine release is blocked by L-typecalcium channel blockers, whereas Harsing et al. (1992)reported that dopamine release by nicotine is attenuatedby omega-conotoxin, an N-type calcium channel antago-nist.

In addition to increasing synaptic dopamine, drugs ofabuse are also known to induce locomotor activity fol-lowing administration at appropriate doses. Since dopa-mine antagonists and selective dopamine lesions havebeen used to attenuate the increased locomotion causedby these drugs, there is a general consensus that drug-in-duced locomotion is dopamine-dependent (Swerdlow etal. 1986). Locomotion induced by drugs such as cocaineor nicotine, which increase synaptic dopamine levels viacalcium-dependent mechanisms, should be prevented inthe presence of a calcium channel blocker. In fact, Paniet al. (1990) recently reported that cocaine-induced do-pamine release and locomotor activity were blocked bythe L-type calcium antagonists.

As aforementioned, nicotine induces both dopaminerelease and increases locomotion, and dopamine releasedepends on upon extracellular calcium (Westfall et al.1989). Since nicotine-induced locomotor activity is

C. Hart · N.A. Kisro · S.L. Robinson · C. Ksir (✉)Department of Psychology and Neuroscience Program,Box 3415 University Station, University of Wyoming,Laramie, WY 82071, USA&/fn-block:

Psychopharmacology (1996) 128:359–361 © Springer-Verlag 1996

O R I G I N A L I N V E S T I G AT I O N

&roles:Carl Hart · Noelle A. Kisro · Stacy L. RobinsonCharles Ksir

Effects of the calcium channel blocker nimodipineon nicotine-induced locomotion in rats

&misc:Received: 7 February 1996 / Final version: 16 August 1996

thought to be dependent upon nicotine’s ability to in-crease dopamine release, we reasoned that nicotine-in-duced locomotor activity should be attenuated by an L-type calcium antagonist, as has been previously demon-strated with cocaine. Thus, the present study investigatedthe effects of nimodipine, an L-type calcium channelblocker, on nicotine-induced locomotor activity in rats.

Materials and methods

Animals

Twenty-four drug-naive adult male Sprague-Dawley rats (Univer-sity of Wyoming Animal Colony, Laramie, Wyoming, USA),weighing 275–375 g, were used in this experiment. Animals werehoused four per cage and allowed free access to water and food.Lights were maintained on a 12-h light-dark cycle.

Locomotor apparatus

Locomotor activity was observed in four acrylic 41×41×30 cm testchambers inside Omnitech Digiscan activity monitors. Each moni-tor was equipped with two arrays of infrared photocell beams (8×8photocells, model RXYZCM-8) for detection of animals’ move-ments. Total distance moved (measured in cm), number of verticalmovements, and number of stereotypy (repeated movement epi-sodes) were analyzed. Data were printed at 5-min intervals duringhabituation, pretreatment, and testing.

Drugs

Nicotine hydrogen tartrate (Sigma, St Louis, Mo., USA) was dis-solved in saline and neutralized to pH 7.2–7.4. The nicotine hy-drogen tartrate dose was calculated in terms of nicotine baseweight. Nimodipine (Research Biochemicals, Natick, Mass.,USA) was dissolved in warm propylene glycol. Propylene glycolalso served as the vehicle. All drugs were administered IP in a vol-ume of 1 ml/kg.

Procedure

Animals were randomly assigned to one of four groups: 5 mg/kgnimodipine/nicotine (n=6), 10 mg/kg nimodipine/nicotine (n=6),20 mg/kg nimodipine/nicotine (n=6), or vehicle/nicotine (n=6).Rats were weighed and then placed in the locomotor apparatus fora 1-h habituation period. Following habituation, each animal wasthen given a pretreatment injection of nimodipine (5, 10, or20 mg/kg) or vehicle, and locomotor activity was monitored forthe next hour. Immediately thereafter, rats were administered aninjection of nicotine (0.4 mg/kg). Behavioral activity was thenmonitored for 1 h.

All animal procedures were in strict accordance with the NIHGuide for the Care and Use of Laboratory Animalsand were ap-proved by the institutional Animal Care Committee.

Results

Figure 1 shows that nicotine produced a clear increase inlocomotor activity from 5–10 min post-injection whenrats were pretreated with the vehicle. However, whennimodipine was given as the pretreatment, this effectwas altered in a dose-dependent manner. A two-factorANOVA with repeated measures revealed a significant

difference on total distance moved between the nimodi-pine/nicotine groups and the vehicle/nicotine group(F=6.18, df=3, 20, P<0.01). In addition, there was a sig-nificant effect of post-nicotine injection time (F=31.99,df=11, P<0.01) and a significant group by time interac-tion (F=3.07, df=33, 220, P<0.01). Further analysis ofthe 5-min post-injection time point (peak nicotine effect)showed that the 10 mg/kg nimodipine/nicotine and20 mg/kg nimodipine/nicotine groups were significantlydifferent from the vehicle/nicotine group and the5 mg/kg nimodipine/nicotine group such that nicotine-in-duced locomotion was blocked in the former, while es-sentially being unaltered in the later (P<0.01) (Fig. 2).

When an ANOVA was used to analyze baseline andpretreatment differences between the four groups no sig-nificant effects were observed. In addition, no significanteffects were noted when the number of vertical move-ments and number of stereotypy were analyzed.

Discussion

There exists considerable evidence that CNS calciumchannels are involved in at least some of the effects ofdrugs commonly abused by humans. For example, self-administration of both cocaine and morphine is de-

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Fig. 1 Mean±SEM (bars) total distance moved. The arrow indi-cates the time of nicotine injections&/fig.c:

Fig. 2 Mean±SEM (bars) for total distance moved 5 min afternicotine injections. The asteriskindicates the groups that were sig-nificantly different from the vehicle group&/fig.c:

creased if animals are pretreated with an L-type calciumchannel blocker (Kuzmin et al. 1992; Martellotta et al.1994). It has been further demonstrated that cocaine-in-duced dopamine release and locomotion are blocked byL-type calcium channel blocker (Pani et al. 1990). Therole of neuronal calcium channels in nicotine-inducedbehaviors has also been investigated. Recently it hasbeen reported that nicotine-induced behavioral effects,such as antinociception and self-administration, are at-tenuated by L-type calcium channel blockers (Damaj etal. 1993; Martellotta et al. 1995). Additionally, it hasbeen reported that isradipine, an L-type calcium channelantagonist, blocks nicotine’s discriminative stimulus cuein rats (Schecthter and Meehan 1992).

When the L-type calcium antagonist nimodipine wasadministered prior to nicotine injections, nicotine-in-duced hyperactivity was clearly antagonized in a dose-dependent fashion. Presumably, by blocking L-type cal-cium channels, calcium’s ability to enter the intracellularpool was restricted, thereby decreasing the number ofcalcium ions that participated in the fusion of synapticvesicles with cell membrane. Consequently, there was adecrease in the amount of neurotransmitter released.

These results are consistent with the argument thatnicotine-induced locomotion is dependent upon nico-tine’s ability to cause calcium-dependent neurotransmit-ter release. Because there exists a general consensus thatmesolimbic dopaminergic transmission underlies nico-tine-induced locomotion, dopamine is the likely neuro-transmitter candidate (Clarke et al. 1988). However, ourfindings appear to be in some conflict with those recent-ly reported by Harsing et al. (1992). They concluded thatnicotine-induced dopamine release in the striatum is me-diated by the N-type calcium channel. These conclusionswere based on the observation that omega-conotoxin, anL- and N-type calcium channel blocker, inhibited nico-tine-induce dopamine release, whereas diltiazem, an L-type calcium antagonist, did not. There exist at least twopossible explanations for the apparent discrepancy in thetwo studies. First, the Harsing et al. experiment was con-ducted in vitro. In contrast, the present experiment wascarried out in intact behaving animals. Secondly, theHarsing et al. study reported inhibition of nicotine-in-duced dopamine release in the striatum. However, it isthe mesolimbic dopamine pathway which is thought tounderlie drug-induced, specifically nicotine-induced, lo-comotor activity.

The present results lend further support to the hypo-thesis that nicotine-induced locomotion is mediatedthrough calcium-dependent dopaminergic mechanisms.

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