cDNA cloning and expression of a gamma-aminobutyric acid(A) receptor epsilon-subunit in rat brain

A cDNA encoding a GABA(A) receptor subunit was isolated from rat brain. The predicted protein is 70% identical to the human epsilon -subunit. It was r ecently reported [Sinkkonen et al. (2000), J. Neurosci., 20, 3588-3595] tha t the rodent epsilon -subunit mRNA encoded an additional sequence (approxim ate to 400 residues). We provide evidence that human and rat epsilon -subun it are similar in size. The distribution of cells expressing the GABA(A) ep silon -subunit was examined in the rat brain. in situ hybridization histoch emistry revealed that epsilon -subunit mRNA is expressed by neurons located in septal and preoptic areas, as well as in various hypothalamic nuclei, i ncluding paraventricular, arcuate, dorsomedial and medial tuberal nuclei. T he mRNA was also detected in major neuronal groups with broad-range influen ce, such as the cholinergic (basal nucleus), dopaminergic (substantia nigra compacta), serotonergic (raphe nuclei), and noradrenergic (locus coeruleus ) systems. Immunohistochemistry using an affinity-purified antiserum direct ed towards the N-terminal sequence unique to the rat epsilon -subunit revea led the presence of epsilon -subunit immunoreactivity over the somatodendri tic domain of neurons with a distribution closely matching that of mRNA-exp ressing cells. Moreover, using in situ hybridization, alpha3, theta and eps ilon GABA(A) subunit mRNAs were all detected with an overlapping distributi on in neurons of the dorsal raphe and the locus coeruleus. Our results sugg est that novel GABA(A) receptors may regulate, neuroendocrine and modulator y systems in the brain.