Pharmacological Specificities of Neurones in an Elementary Central Nervous System

Abstract

RECENT pharmacological investigations by us¹ on the central nervous system of the mollusc Aplysia depilans revealed acetylcholine as a possible synaptic transmitter of inhibition. The evidence was obtained by electrophysiological methods; different cells in the abdominal ganglion of this animal were impaled by glass microelectrodes according to previously described techniques² and their reactions observed in the presence of different drugs. It was thus shown that externally applied acetylcholine in concentrations up to 10⁻¹² hyperpolarizes the membrane and mimics the inhibitory postsynaptic potentials in cells having inhibitory input (conventionally designated as H-cells), whereas other cells having only excitatory input (D-cells) are depolarized by acetylcholine (Fig. 1A). In both cases the impedance of the membrane drops to a low level. The action of acetylcholine on H-cells is reversed when the level of polarization of the membrane is increased by artificially applied current. The reversal-level for acetylcholine potentials and inhibitory postsynaptic potentials have equal values, suggesting an identical selective permeability variations of the membrane. In D-cells the equilibrium-level is close to the zero membrane potential, indicating a different and non-selective permeability change. Local electrophoretical injections of acetylcholine disclosed a type of response which is identical in synaptic regions as well as in proximal extrasynaptic membranes; but the axonal synaptic fields are more sensitive to acetylcholine than the somatic membrane where, as usual in molluscs, there are no synaptic terminals³. D-Tubocurarine and atropine in external solutions reduce the inhibitory postsynaptic potentials as well as the action of acetylcholine, whereas eserine first lengthens the inhibitory effect and then reduces the size of inhibitory postsynaptic potentials by inactivating the postsynaptic membrane; excitatory postsynaptic potentials in H-cells are not affected. Consequently, we concluded that the inhibitory synapse in the Aplysia central nervous system is cholinergic.