ABSTRACT: GABAA receptors are composed of five subunits arranged around a central chloride channel. Their subunits originate from different genes or gene families. The majority of GABAA receptors in the mammalian brain consist of two ?-, two ?- and one ?- or ?-subunit. This subunit organization crucially determines the physiological and pharmacological properties of the GABAA receptors. Using immunohistochemistry, we investigated the distribution of 10 GABAA receptor subunits (?1, ?2, ?3, ?4, ?5, ?1, ?2, ?3, ?2, and ?) in the fore brain of three female rhesus monkeys (Macaca mulatta). Within the cerebral cortex, subunits ?1, ?5, ?2, ?3, and ?2 were found in all layers, ?2, ?3, and ?1 were more concentrated in the inner and outer layers. The caudate/putamen was rich in ?1, ?2, ?5, all three ?-subunits, ?2, and ?. Subunits ?3 and ?5 were more concentrated in the caudate than in the putamen. In contrast, ?1, ?2, ?1, ?2, ?2, and ? were highest in the pallidum. Most dorsal thalamic nuclei contained subunits ?1, ?2, ?4, ?2, ?3, and ?2, whereas ?1, ?3, ?1, and ?2 were most abundant in the reticular nucleus. Within the amygdala, subunits ?1, ?2, ?5, ?1, ?3, ?2, and ? were concentrated in the cortical nucleus, whereas in the lateral and basolateral amygdala ?1, ?2, ?5, ?1, ?3, and ?, and in the central amygdala ?1, ?2, ?3, and ?2 were most abundant. Interestingly, subunit ?3-IR outlined the intercalated nuclei of the amygdala. In the hippocampus, subunits ?1, ?2, ?5, ?2, ?3, ?2, and ? were highly expressed in the dentate molecular layer, whereas ?1, ?2, ?3, ?5, ?1, ?2, ?3, and ?2 were concentrated in sector CA1 and the subiculum. The distribution of GABAA 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.
