NICOTINIC ACETYLCHOLINE RECEPTORS SUBSERVING NOCICEPTION IN THE DOG HINDLIMB

Abstract

Properties of nociceptors excited by acetylcholine in the hindlimb were investigated on conscious dogs utilizing vocalization as a measure of nociception. All compounds were administered into the femoral artery through a chronically indwelling catheter. Acetylcholine and two nicotinic compounds, nicotine and DMPP, produced vocalization, withdrawal or shaking of the hindlimb, biting and struggling. Three muscarinic compounds, bethanechol, methacholine and McN-A-343, failed to cause any sign indicative of nociception in the hindlimb even with large doses producing severe signs of generalized parasympathetic excitation. The nociceptive responses to acetylcholine were blocked by prior (1 to 2 min) administration of hexamethonium, tetraethylammonium, or DMPP in doses equal to those of acetylcholine, but not by comparable doses of l-hyoscyamine. The nociceptive responses to equi-effective doses of histamine or bradykinin were not affected either by any of these blockers. These results indicate that the afferent neuroreceptors which give rise to the nociceptive responses to acetylcholine administered into the femoral artery are nicotinic in nature.