A comparison of the cholinergic properties of the leopard frog vestibular organs

Previous data from this group indicated that the main cholinergic effect on the afferent resting firing rate of the posterior canal is an atropine-sensitive, strychnine-resistant facilitation, while the main effect on the saccule is a strychnine-sensitive inhibition. In the present research we compared the effect of acetylcholine (ACh) on the afferent whole-nerve resting discharge of all vestibular organs of the frog. All three semicircular canals, utricle and lagena responded to ACh and carbachol (CCh) perfusion with an increase in resting discharge, while the saccular afferent discharge was mainly inhibited. The perfusion with 10 μ M physostigmine potentiated both facilitatory and inhibitory responses to ACh in all organs tested. On the other hand, CCh was much more potent than ACh in producing the facilitatory response but not the inhibitory response. As already observed in the posterior canal, the facilitation observed in the other vestibular organs was reversibly blocked by 1 or 10 μ M atropine but not by 1 μ M strychnine. In addition, the irreversible non-selective muscarinic antagonist propylbenzilylcholine mustard irreversibly abolished the facilitatory response but not the inhibitory one, while 50 μ M tetraethylammonium blocked the inhibitory response without affecting the facilitatory response or producing inhibition by itself. These results suggest that the predominant cholinergic response on the resting discharge is muscarinic in all vestibular organs of the frog except the saccule.