CHOLINERGIC TRANSMISSION AT MECHANOSENSORY AFFERENTS IN THE CRAYFISH TERMINAL ABDOMINAL-GANGLION

Electrical stimulation of mechanosensory afferents innervating hairs o n the surface of the exopodite in crayfish Procambarus clarkii (Girard ) elicited reciprocal activation of the antagonistic set of uropod mot or neurones. The closer motor neurones were excited while the opener m otor neurones were inhibited. This reciprocal pattern of activity in t he uropod motor neurones was also produced by bath application of acet ylcholine (ACh) and the cholinergic agonist, carbamylcholine (carbacho l). The closing pattern of activity in the uropod motor neurones produ ced by sensory stimulation was completely eliminated by bath applicati on of the ACh blocker, d-tubocurarine, though the spontaneous activity of the motor neurones was not affected significantly. Bath applicatio n of the acetylcholinesterase inhibitor, neostigmine, increased the am plitude and extended the time course of excitatory postsynaptic potent ials (EPSPs) of ascending interneurones elicited by sensory stimulatio n. These results strongly suggest that synaptic transmission from mech anosensory afferents innervating hairs on the surface of the tailfan i s cholinergic. Bath application of the cholinergic antagonists, d-tubo curarine (vertebrate nicotinic antagonist) and atropine (muscarinic an tagonist) reversibly reduced the amplitude of EPSPs in many identified ascending and spiking local interneurones during sensory stimulation. Bath application of the cholinergic agonists, nicotine (nicotinic ago nist) and oxotremorine (muscarinic agonist) also reduced EPSP amplitud e. Nicotine caused a rapid depolarization of membrane potential with, in some cases, spikes in the interneurones. In the presence of nicotin e, interneurones showed almost no response to the sensory stimulation, probably owing to desensitization of postsynaptic receptors. On the o ther hand, no remarkable changes in membrane potential of interneurone s were observed after oxotremorine application. These results suggest that ACh released from the mechanosensory afferents depolarizes intern eurones by acting on receptors similar to vertebrate nicotinic recepto rs.