Pharmacological Characteristics and Ionic Bases of a Two Component Postsynaptic Inhibition

SEVERAL types of postsynaptic inhibition have been observed in molluscan neurones; in Aplysia, the most common is a rapidly decaying inhibitory postsynaptic potential (IPSP) with pharmacological characteristics which have been describad by Taue and Gerschenfeld1. In their study of the role of acetylcholine (ACh) in the nervous system of Aplysia, they noted that cells receiving this inhibitory input are hyperpolarized by ionophoretic injection of ACh, and that both the ACh potential and the IPSP are blocked by d-tubocurarine. They also demonstrated that the polarization at which the ACh potential reverses is similar to that of the equilibrium potential for the IPSP, implying that the two potentials have the same underlying ionic mechanism. They concluded, on the basis of these and other data, that these IPSPs are cholinergic. Further work on similar cholinergic IPSPs in the land snail indicated that these potentials are the result of changes in the permeability of the postsynaptic membrane to chloride2 with some possible contribution from potassium ions3.