Induction and maintenance of free postsynaptic membrane thickenings in the adult superior cervical ganglion.

The superior cervical ganglion (SCG) of adult rats was exposed to GABA, either by long lasting microapplication (implantation of glass bulbs for 1-24 days) or in short term experiments (external application up to 6 h). Autoradiography showed that [3H] GABA accumulated selectively in satellite cells. The GABA produced the following effects: (1) Specialized membrane thickenings--similar in fine structural appearance to those seen as postsynaptic membrane thickenings at Gray type I synapses--were formed at the extrasynaptic dendritic surface of principal ganglion cells. (2) Morphometry revealed that the surface to volume ratio of dendrites increased significantly corresponding to an enlargement of their extrasynaptic surface as a result of the formation of spine-like projections. (3) Electrophysiology confirmed that, at least after short term application, the action potentials induced by preganglionic stimulation were heavily suppressed. These results suggest that, in the course of depressed ganglionic activity, so-called free postsynaptic membrane thickenings are generated and maintained in the SCG of adult rats even in the absence of significant axonal degeneration. The discussion focuses on two points: (1) possible similarities between the conditions of neurons after denervation and under the influence of GABA; (2) a possible role of GABA and other substances with inhibitory action in synaptogenesis.