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10 uM Capsaicin 10 uM Capsaicin 10 uM Capsaicin 10 uM Capsaicin 10 uM Capsaicin 1uM Capsazepine 3 uM Capsazepine 10 uM Capsazepine. 1 min.

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10 uM Capsaicin 10 uM Capsaicin 10 uM Capsaicin 10 uM Capsaicin 10 uM Capsaicin

1uM Capsazepine 3 uM Capsazepine 10 uM Capsazepine

1 min

The Effect of VR1 Blockers on Peripheral Trigeminal Nerve Responses to IrritantsSallie Allgood and Wayne L. SilverDepartment of Biology, Wake Forest University, Winston-Salem, NC 27109

Introduction:

The trigeminal nerve innervates the head and face and terminates in free nerve endings in the nasal mucosa, oral cavity and the eyes. These nerve endings detect noxious chemical stimuli and act as a warning system to help prevent damage to these areas.

On the trigeminal nerve fibers, there are many receptors that respond to different types of stimuli. One of these receptors is the Vanilloid Receptor 1 (VR1). VR1 is known to respond to heat, acids as well as capsaicin (the active ingredient in chili peppers), but exactly how it contributes to trigeminal stimulation by compounds other than capsaicin is not well known.

These preliminary experiments were preformed to determine how VR1 may contribute to the trigeminal response to irritating compounds. Whole-nerve recordings were obtained from Sprague-Dawley rats and responses to irritating compounds were recorded both in the presence and absence of the VR1 blockers capsazepine (a selective VR1 antagonist) and ruthenium red (a nonselective inhibitor that blocks Ca2+ transport through membrane channels).

Discussion:

These preliminary experiments show how the ethmoid nerve responds to various compounds in solution, and how VR1 blockers affect this response. The response to capsaicin decreases in the presence of the VR1 blockers. Thus, capsaicin exerts its effect on the trigeminal nerve through VR1s. Additionally, because capsaicin’s effect was almost eliminated with high concentrations of capsazepine, capsaicin appears to exert its effect solely through VR1s.

The response to high concentrations of propionic acid is also decreased in the presence of the VR1 blockers suggesting that this compound stimulates trigeminal nerve fibers partly through VR1s. Other acid receptors (ASIC and P2X) may also play a role.

The VR1 blockers have no affect on the ethmoid nerve’s response to nicotine and cyclohexanone. This suggests that these compounds do not stimulate trigeminal nerve through VR1s.

Future experiments will use this experimental protocol to examine the role of VR1s in response to additional irritating trigeminal stimuli. Blockers for other known receptors on the trigeminal nerve will also be tested.

Methods:

Adult Sprague-Dawley rats were anesthetized with Urethane (ethyl carbamate: 1.0 g/kg injected i.p.). When they were fully anesthetized, two cannulae were placed in the trachea. One cannula was inserted toward the lungs, to allow for breathing, and the second was inserted up into the nasopharynx. Then the rat’s head was restrained and the ethmoid nerve was isolated and placed on two platinum-iridium electrodes.

Rat Ringer’s (5.4mM KCl, 5mM HEPES, 135mM NaCl, 1.8mM CaCl2 ) was flowed through the nasal cavity continuously (10 mL/min) using the nasophayngeal cannula. Irritants (capsaicin, nicotine, cyclohexanone and propionic acid) were injected into the Ringers in 0.5 cc aliquots. These stimuli were delivered alone or along with either capsazepine or ruthenium red in order to block the response from VR1.

The whole nerve response was integrated and stored and analyzed using a data acquisition system. The height of the integrated response was used to determine the strength of the response.

Figure 2Concentration-response curves for the four compounds tested. Concentrations which elicited similar responses were used in the subsequent experiments.

Figure 3 An example of a neural recording. The top trace is the raw neural response and the bottom trace is the integrated response. This example shows the response of the ethmoid nerve to 10μM capsaicin decreasing in the presence of capsazepine.

Figures 4-11These figures show effect of ruthenium red and capsazepine on ethmoid nerve responses to capsaicin, nicotine, cyclohexanone and propionic acid. Each stimulus was presented alone or with one of three different concentrations of either capsazepine or ruthenium red. The response is calculated as the percent height of the integrated response as compared to the response of either 10 uM capsaicin, 0.308 mM nicotine, 114.1 mM cyclohexanone or 36.4 mM propionic acid when presented alone. Error bars (when present) represent one standard error of the mean (n=1 or 2).

Figure 1: Experimental Set-Up. Stimuli were injected into the Ringer’s flow with a syringe in 0.5cc aliquots. The solutions flowed through the nasal cavity and dripped out of the nose. Neural responses were recorded from the ethmoid nerve and the integrated response was used for data analysis.