POSTSYNAPTIC GEPHYRIN IMMUNOREACTIVITY EXHIBITS A NEARLY ONE-TO-ONE CORRESPONDENCE WITH GAMMA-AMINOBUTYRIC ACID-LIKE IMMUNOGOLD-LABELED SYNAPTIC INPUTS TO SYMPATHETIC PREGANGLIONIC NEURONS

Peripheral regulation of cardiovascular function is fundamentally infl uenced by central excitation and inhibition of sympathetic preganglion ic neurons in thoracic spinal cord. This electron microscopy study inv estigated whether the gamma-aminobutyric acid (GABA)-ergic and glycine rgic inhibitory innervation of sympathetic preganglionic neurons arise s from mutually exclusive afferent populations. Sympathetic preganglio nic neurons were retrogradely labeled with cholera beta subunit. GABAe rgic terminals were identified using strict quantitative statistical a nalyses as those boutons containing significantly elevated levels of G ABA-like immunogold labeling (GABA(+)). Glycinergic terminals were cla ssified as those boutons opposite postsynaptic gephyrin immunostaining containing background levels of GABA-like immunogold labeling (gephyr in(+)/GABA(-) association). Approximately 43% of the synaptic terminal s that contacted sympathetic preganglionic somata and proximal dendrit es and that were opposite gephyrin were GABA(-); the remaining 57% wer e GABA(+). Only two GABA(+) boutons (4%) that synapsed on identified s ympathetic preganglionic neuron (SPN) processes were not opposite geph yrin immunostaining(GABA(+)/gephyrin(-) association). GABA(-)/gephyrin (+) associations were anticipated given prior anatomical, physiologica l, and pharmacological data. The observed nearly one-to-one correspond ence between postsynaptic gephyrin immunoreactivity and GABA(+) bouton s was unexpected. Prior physiological and pharmacological experiments suggest that the postsynaptic effects of GABAergic inputs to sympathet ic preganglionic neurons are mediated by activation of GABA(A) recepto rs. Those data, the present results, and other molecular, biochemical, and anatomical studies of gephyrin in the central nervous system (CNS ) are consistent with two hypotheses: 1) Postsynaptic gephyrin is asso ciated with GABA(A) receptors in the membranes of sympathetic pregangl ionic neurons, and 2) GABA(+)/gephyrin(+) associations do not necessar ily predict colocalization of GABA and glycine within single boutons s ynapsing on sympathetic preganglionic somata and dendrites. (C) 1995 W iley-Liss, Inc.