GABAAReceptors Mediate Trophic Effects of GABA on Embryonic Brainstem Monoamine NeuronsIn Vitro

The inhibitory neurotransmitter GABA may act as a trophic signal for developing monoamine neurons in embryonic rat brain, because GABA neurons and their receptors appear in brainstem during generation of monoamine neurons. To test this hypothesis, we used dissociated cell cultures from embryonic day 14 rat brainstem, which contains developing serotonin (5-HT), noradrenaline (tyrosine hydroxylase; TH), and GABA neurons. Immunocytochemistry and reverse transcription-PCR (RT-PCR) revealed the presence of multiple α, β, γ, and δ subunits in these cultures. Competitive RT-PCR demonstrated high levels of β3 subunit transcripts. Expression of functional GABAAreceptors was demonstrated using36Cl−flux assays. To investigate GABAergic regulation of neuronal survival and growth, cultures were treated for 1–3 din vitrowith 10 μmGABA and/or GABAAantagonist (bicuculline or the pesticide dieldrin). The effects of treatments were quantified by analysis of immunoreactive 5-HT, TH, and GABA neurons. GABAAreceptor ligands differentially regulated neuronal survival and growth depending on neurotransmitter phenotype. GABA exerted positive effects on monoamine neurons, which were countered by bicuculline (and dieldrin, 5-HT neurons only). By itself, bicuculline produced inhibitory effects on both 5-HT and TH neurons, whereas dieldrin potently inhibited 5-HT neurons only. GABA neurons responded positively to both antagonists, but more strongly to bicuculline. Taken together, these results demonstrate that the activation/inhibition of GABAAreceptors produces opposite effects on the development of embryonic monoamine and GABA neurons. This suggests that these neurotransmitter phenotypes may express GABAAreceptors that differ in fundamental ways, and these differences determine the developmental responses of these cells to GABAergic stimuli.