ARTIFICIAL SWEETENERS, NaCl, AND KCl STIMULATE PERIPHERAL TRIGEMINAL NERVE CHEMORECEPTORS

Vajini N. Atukorale, Matthew W. Greene, and Wayne L. SilverDepartment of Biology, Wake Forest University, Winston-Salem, NC 27109

atukvn5@wfu.edu

MethodsRats were anesthetized with urethane (ethyl carbamate: 1 g/kg injected i.p.). Two cannulae were inserted into the trachea of each rat. One cannula allowed the rat to breathe room air. The second cannula, inserted into the nasopharynx, was connected via a pump to a reservoir containing Ringer’s solution. Stimuli (1.0 ml) were injected into the flow of Ringer’s (10 ml/min), which was allowed to drip from the rat’s nose. Rats were restrained in a head holder and the ethmoid nerve was exposed and placed on a pair of platinum-iridium wire hook electrodes.

Multiunit neural activity from the ethmoid nerve was summated using an averaging circuit. The data were analyzed by taking the maximum height of the integrated response after stimulus delivery. Responses are reported as a percentage of the standard response to 10µM capsaicin. Concentrations are reported for the injected solutions.

Introduction The trigeminal nerve provides sensory information from the eyes, nose, and mouth. It is a multisensory nerve, responding to a variety of irritants in the environment. Several receptor proteins associated with the trigeminal nerve mediate these responses. Artificial sweeteners, which are of obvious economic importance, are reported to be irritating at high concentrations. As irritants they should stimulate the trigeminal nerve.

The present study examined peripheral trigeminal nerve responses to increasing concentrations of three artificial sweeteners (Sodium Saccharin, Sodium Cyclamate, and Acesulfame K), NaCl, and KCl.

Literature Cited

Finger TE, Böttger B, Hansen A, Anderson KT, Alimohammadi H, and Silver WL. (2003) Solitary chemoreceptor cells in the nasal cavity serve as sentinels of respiration. PNAS. 100:8981-8986.

Dinh, Q.T., Groneberg, D.A., Mingomataj, E., Peiser, C., Heppt, W., Dinh, S., Arck,  P.C., Klapp, B.F., and Fischer, A. (2003) Expression of substance P and vanilloid receptor (VR1) in trigeminal sensory neurons projecting to the mouse nasal mucosa. Neuropeptides 37, 245-250.

Kuhn C, Bufe B, Winnig M, Hofmann T, Frank O, Behrens M, Lewtschenko T, Slack JP, Ward CD, Meyerhof W. (2004). Bitter taste receptors for Saccharin and Acesulfame K. J Neurosci. 24: 10260-10265.

Riera C, Damak S, Coutre JL. (2006). Food flavors and the sweetener Saccharin activate the transient receptor potential vanilloid subtype 1 (TRPV1) channel. Chem Senses. 31: A49-A50.

ConclusionsThe artificial sweeteners tested, Sodium Saccharin, Sodium Cyclamate, and Acesulfame K, elicit graded responses (with respect to increasing concentrations) from the ethmoid branch of the trigeminal nerve. Artificial sweeteners stimulate TRPV1 receptors (capsaicin, irritant receptors) (Riera et al., 2006) and T2R receptors (bitter receptors) (Kuhn et al., 2004) in vitro. As both of these receptor proteins are associated with trigeminal nerves (Dinh et al., 2003; Finger et al., 2003), either or both may contribute to the trigeminal nerve responses to the sweeteners tested.

NaCl and KCl also elicited responses from the trigeminal nerve. Therefore, Na+ and K+ may have played a role in the response to the sweeteners tested. However, the thresholds for NaCl and KCl were higher than those of the sweeteners, suggesting that the sweeteners must be stimulatory by themselves.

Future experiments will examine trigeminal nerve responses to the sweeteners in the presence of TRPV1 and T2R receptor blockers.

Figure 1. Electrophysiological setup. Stimuli (1.0 ml) were delivered via a syringe into Ringer’s solution flowing through the rat’s nose. Responses were digitized and recorded on an IBM computer with Acknowledge software (Biopac).

Figure 2. Stimuli tested.

Figure 1. Experimental Setup

Amplifier

Electrodes

Audio Monitor

Integrator Data Acquisition

System

Pump

← ←

↓

Nasopharyngeal tube

Stimulus injection port

Rat Ringer’s

NaCl KCl

Figure 2. Stimuli Tested

Capsaicin

10 µM Capsaicin

Acesulfame K

10 sec

Sodium Cyclamate

Sodium Saccharin

10mM 100mM 250mM 1000mM750mM500mM

NaCl

KCl

Figure 3. Integrated Responses

0

20

40

60

80

100

120

10 100 250 500 750 1000

Concentration (mM)

% C

apsa

icin

Res

po

nse

NaCl

Sodium Saccharin

Sodium Cyclamate

Acesulfame K

KCl

Figure 4. Concentration-Responses Curves

Figure 3. Multiunit, integrated responses to the stimuli tested at increasing concentrations.

Figure 4. Concentration-response curves for the stimuli tested. Responses are reported as a percent of the response to 10 µM capsaicin. Error bars have been omitted for clarity. N=4

Acesulfame-K (potassium salt)

Sodium Saccharin Sodium Cyclamate

NH

O

HO

O

C

N- K+

S

O O

O

O

S

N- Na+

C

O

OO

NH

S

O

O

O- Na+