The pharmacology of recurrent inhibition in the supraoptic neurosecretory system

Abstract

A pharmacological study of recurrent inhibition in the supraoptic neurosecretory system of the cat has been conducted in an attempt to identify the inhibitory transmitter in this pathway. Acetylcholine (muscarinic component), glycine and GABA were considered as possible transmitters mediating this inhibitory pathway. Antagonists to the depressant actions of these substances on neurosecretory cells were tested on both unit responses and the inhibitory pathway. Although atropine, strychnine, picrotoxin and bicuculline were able to block the pharmacologic depressions, they were all ineffective in antagonizing the recurrent inhibition when applied either microelectrophoretically or intravenously. It is thus unlikely that either acetylcholine, glycine or GABA participate in this inhibitory pathway. Norepinephrine was also considered as a candidate for inhibitory transmitter. However, since recurrent inhibition was present in cats whose norepinephrine-containing terminals were destroyed by pretreatment with 6-hydroxydopamine, we do not think norepinephrine is the transmitter. The possibility that the neurosecretory product ADH might also serve a synaptic function and mediate the recurrent inhibition was investigated by microelectrophoretic administration of ADH to neurosecretory cells. The fact that a good correlation was found between those cells exhibiting recurrent inhibition and those depressed by ADH supports the idea that ADH may serve as a transmitter in this recurrent inhibitory pathway.