CHOLINERGIC TRANSMISSION FROM MECHANOSENSORY AFFERENTS TO AN IDENTIFIED NONSPIKING INTERNEURON IN THE CRAYFISH PROCAMBARUS-CLARKII GIRARD

Pharmacological properties of excitatory synaptic transmission from me chanosensory afferents to an identifiable nonspiking interneuron of cr ayfish were studied by drug perfusion experiments using acetylcholine (ACh) agonists and antagonists. Application of carbachol, a general ag onist of ACh, caused sustained depolarization of the interneuron and a decrease in the peak amplitude of its excitatory synaptic response to sensory stimulation on the soma side. Similar depolarization was obse rved during application of carbachol under the low-Ca2+, high-Mg2+ con dition. The peak amplitude was also reduced by application of nicotine and tetramethylammonium, both of which also caused sustained depolari zation of the interneuron. By contrast, perfusion of muscarinic agonis ts, muscarine, oxotremorine and pilocarpine, reduced the peak amplitud e without affecting the membrane potential of the interneuron. Perfusi on of nicotonic antagonists of ACh, d-tubocurarine and hexamethonium, caused reduction of the peak amplitude Without any change ill the memb rane potential. A muscarinic antagonist atropine was also effective in blocking the synaptic transmission but at higher concentration than d -tubocurarine. The results suggest that the ACh receptors on the nonsp iking interneuron belong to a previously characterized class of crusta cean cholinergic receptors resembling the nicotinic subtype of vertebr ates.