Cellular and subcellular localization of the GABA(B) receptor 1a/b subunit in the rat periaqueductal gray matter.

The inhibitory effects of gamma-aminobutyric acid (GABA)ergic neurotransmission in the periaqueductal gray matter (PAG) are mediated, at least partly, by metabotropic GABA(B) receptor subtypes whose cellular and subcellular localization is still unknown. We performed immunohistochemical experiments with an antibody against GABA(B) receptor subtype 1a/b (GABA(B)R(1a/b)) by using light and electron microscopy. On light microscopy, GABA(B)R(1a/b) immunoreactivity (IR) was in all columns, defined by cytochrome oxidase histochemistry. Neuropil labeling was strongest in the lateral portion of dorsolateral PAG. Labeled neurons, albeit not numerous, were in ventrolateral, dorsal, and medial subdivisions and were sparser in dorsolateral PAG. Labeling was mostly on the soma of PAG neurons. Sometimes GABA(B)R(1a/b) IR spread along proximal dendrites; in these cases bipolar neurons were the most common type. On electron microscopy, GABA(B)R(1a/b) IR was mainly on dendrites (54.92% of labeled elements) and axon terminals (21.90%) making synapses with labeled and unlabeled postsynaptic elements. Presynaptic labeling was also on unmyelinated and myelinated axons (overall 8% of all labeled elements). Postsynaptically, GABA(B)R(1a/b) IR was at extrasynaptic sites on dendritic shafts; spines were always unlabeled. On axon terminals, GABA(B)R(1a/b) IR was on extrasynaptic membranes and sometimes on presynaptic membrane specializations. Of the labeled elements, 13.03% elements were distal astrocytic processes (dAsPs) surrounding both symmetric and asymmetric synapses whose pre- and postsynaptic elements were often labeled. Immunoreactive dAsPs were around the soma and dendrites of both labeled and unlabeled neurons. These findings provide insights into the intrinsic PAG organization and suggest that presynaptic, postsynaptic, and glial GABA(B) receptors may play crucial roles in controlling PAG neuronal activity.