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Differential distribution of GABAA receptor subunits on bulbospinal serotonergic and nonserotonergic neurons of the ventromedial medulla of the rat

Spinally projecting neurons of the ventromedial medulla (VMM) compose an important efferent pathway for the modulation of nociception. These neurons receive a substantial γ‐aminobutyric acid (GABA)‐ergic input, but the GABA receptor that mediates this input is unknown. This study examined the distribution of GABAA receptor α1 and α3 subunits in serotonergic and nonserotonergic neurons of the VMM that project to the dorsal horn in the rat. A pledget of GelfoamTM soaked in Fluoro‐Gold was placed at the thoracolumbar junction of the spinal cord to label spinally projecting neurons. Alternate sections of the medulla were then incubated with a mixture of antisera to either serotonin and the α1 subunit, or to serotonin and the α3 subunit of the GABAA receptor. Nearly 30% of spinally projecting neurons in the VMM were immunoreactive for the α1 subunit. A similar percentage of spinally projecting neurons in the VMM were immunoreactive for the α3 subunit, although diffuse cellular labeling combined with intense staining of processes in the neuropil precluded a rigorous semi‐quantitative estimation of this population. No α1‐subunit‐immunoreactive neurons colocalized serotonin. In contrast, serotonergic neurons were immunoreactive for the α3 subunit. However, these double‐labeled neurons were a modest percentage of the serotonergic population. A small percentage of spinally projecting serotonergic neurons was immunoreactive for the α3 subunit. These results suggest that significant numbers of spinally projecting serotonergic and nonserotonergic neurons of the VMM possess GABAA receptors that differ in their respective subunit compositions and that both classes of neurons may mediate the antinociception produced by the microinjection of GABAA receptor antagonists in the VMM. J. Comp. Neurol. 384:337–348, 1997. © 1997 Wiley‐Liss, Inc.