Cellular localization of GABA receptor alpha subunit immunoreactivity in the rat hypothalamus: relationship with neurones containing orexigenic or anorexigenic peptides.

gamma-Aminobutyric acid (GABA), the major inhibitory neurotransmitter in the brain, acts via two different type of GABA receptors. GABA(A) receptors are composed of five subunits that belong to eight different classes. Depending on their subunit composition, distinct pharmacological and electrophysiological properties are obtained. GABA is produced in certain hypothalamic neurones known to be involved in control of feeding behaviour. We report the detailed immunohistochemical localization of four GABA(A)R alpha subunits in hypothalamic regions associated with the regulation of feeding behaviour. Immunoreactive structures for all studied GABA(A)R alpha subunits were observed in the hypothalamus, but with subunit-specific staining patterns. GABA(A)R alpha(1) immunoreactivity was most prominent in the dorsomedial hypothalamic nucleus and in the lateral hypothalamic area (LHA), whereas GABA(A)R alpha(2), alpha(3) and alpha(5) subunits exhibited particularly strong immunoreactivity in the ventromedial hypothalamic nucleus. In comparison, GABA(A)R alpha subunit immunoreactivities were generally weak in the arcuate nucleus. In the ventromedial part of the arcuate nucleus, neuropeptide Y- and agouti-related peptide-containing cell bodies, which also are known to be GABAergic, were immunoreactive for only the GABA(A)R alpha(3) subunit, whereas pro-opiomelanocortin- and cocaine- and amphetamine-regulated transcript- containing cell bodies located in the ventrolateral subdivision of the arcuate nucleus, showed GABA(A)R alpha(1), alpha(2) and alpha(3) subunit immunoreactivity. In the LHA, GABA(A)R alpha(3) subunit immunoreactivity was demonstrated in both melanin-concentrating hormone (MCH) and orexin-containing neurones. In addition, MCH neurones contained GABA(A)R alpha(2) immunoreactivity. In neurones of the tuberomammillary nucleus, GABA(A)R alpha(2) and alpha(5) subunits were colocalized with histidine decarboxylase, a marker for histamine-containing neurones.