Pharmacological characterization of postsynaptic potentials evoked in the bimodal pacemaker neuron of Helix pomatia L

Stimulation of various peripheral nerve trunks evokes very similar compound postsynaptic potentials (PSP) composed of one or more excitatory postsynaptic potentials (EPSP) followed by fast and slow inhibitory postsynaptic potentials (IPSP) on the identified RPal neuron of Helix pomatia L. Evoked EPSPs were reduced or blocked by nicotine, atropine and d-tubocurarine. The two components of IPSP were different in their pharmacological sensitivity. Slow IPSP was partly or totally eliminated by ergometrine and chlorpromazine and was reduced by atropine, nicotine as well as by propranolol. Fast IPSP was reduced only in the presence of ergometrine and could not be blocked by either of the applied drugs. Participation of cholinergic transmission seems to be essential in the evoked EPSP but its partial involvement in the slow IPSP can also be supposed. A dopaminergic mechanism may take part in the generation of both components of IPSP but the receptors responsible for the slow IPSP were sensitive to other catecholamine antagonists as well, referring to a more complex origin, or to the involvement of an unknown transmitter. Comparison of PSPs evoked by stimulation of different nerves shows that presynaptic areas belonging to various peripheral sources are overlapped on the RPal neuron, and they probably act by similar transmitter substances.