Immunohistochemical distribution of 10 GABAA receptor subunits in the forebrain of the rhesus monkey Macaca mulatta

GABA(A) receptors are composed of five subunits arranged around a central chloride channel. Their subunits originate from different genes or gene families. The majority of GABA(A) receptors in the mammalian brain consist of two alpha-, two beta- and one gamma- or delta-subunit. This subunit organization crucially determines the physiological and pharmacological properties of the GABA(A) receptors. Using immunohistochemistry, we investigated the distribution of 10 GABA(A) receptor subunits (alpha 1, alpha 2, alpha 3, alpha 4, alpha 5, beta 1, beta 2, beta 3, gamma 2, and delta) in the fore brain of three female rhesus monkeys (Macaca mulatta). Within the cerebral cortex, subunits alpha 1, alpha 5, beta 2, beta 3, and gamma 2 were found in all layers, alpha 2, alpha 3, and beta 1 were more concentrated in the inner and outer layers. The caudate/putamen was rich in alpha 1, alpha 2, alpha 5, all three beta-subunits, gamma 2, and delta. Subunits alpha 3 and alpha 5 were more concentrated in the caudate than in the putamen. In contrast, alpha 1, alpha 2, beta 1, beta 2, gamma 2, and delta were highest in the pallidum. Most dorsal thalamic nuclei contained subunits alpha 1, alpha 2, alpha 4, beta 2, beta 3, and gamma 2, whereas alpha 1, alpha 3, beta 1, and gamma 2 were most abundant in the reticular nucleus. Within the amygdala, subunits alpha 1, alpha 2, alpha 5, beta 1, beta 3, gamma 2, and delta were concentrated in the cortical nucleus, whereas in the lateral and basolateral amygdala alpha 1, alpha 2, alpha 5, beta 1, beta 3, and delta, and in the central amygdala alpha 1, alpha 2, beta 3, and gamma 2 were most abundant. Interestingly, subunit alpha 3-IR outlined the intercalated nuclei of the amygdala. In the hippocampus, subunits alpha 1, alpha 2, alpha 5, beta 2, beta 3, gamma 2, and delta were highly expressed in the dentate molecular layer, whereas alpha 1, alpha 2, alpha 3, alpha 5, beta 1, beta 2, beta 3, and gamma 2 were concentrated in sector CA1 and the subiculum. The distribution of GABA(A) receptor subunits in the rhesus monkey was highly heterogeneous indicating a high number of differently assembled receptors. In most areas investigated, notably in the striatum/pallidum, amygdaloid nuclei and in the hippocampus it was more diverse than in the rat and mouse indicating a more heterogeneous and less defined receptor assembly in the monkey than in rodent brain.