GABAB receptor subunits, R1 and R2, in brainstem catecholamine and serotonin neurons.

Abstract GABA B receptors have been implicated in the GABAergic modulation of catecholaminergic and serotonergic pathways in the central nervous system. The GABA B receptor may require two subunits, GABA B R1 and GABA B R2, for functional activity. Using dual immunofluorescent labelling on adjacent cryostat sections, we investigated the presence of immunoreactivity for the GABA B R1 and GABA B R2 subunits in brainstem catecholamine (tyrosine hydroxylase-immunoreactive) and serotonin (tryptophan hydroxylase-immunoreactive) neurons. All neurons (>98%) examined in catecholamine groups A1, A2, A5, A6, C1, and serotonin groups B1–3 and B6–8 were immunoreactive for the GABA B R1 subunit. All A5 and A6 neurons (>97%) and at least 86% of A1, A2, C1, B2, B3, B7 and B8 neurons examined were GABA B R2-immunoreactive. The proportion of neurons with immunoreactivity for the GABA B R2 subunit varied between 0% and 99% for B1 neurons, and between 35% and 93% for B6 neurons. Statistical analysis showed that similar proportions of sampled neurons were immunoreactive for GABA B R1 and GABA B R2 in the A1, A5, A6, C1, B2 and B7 cell groups, whereas a smaller proportion of A2, B1, B3, B6 and B8 neurons were GABA B R2-immunoreactive than GABA B R1-immunoreactive. In general, our results suggest that GABA B R1 and GABA B R2 co-exist in the great majority of brainstem catecholamine and serotonin neurons. In the neurons that lack GABA B R2, the GABA B R1 subunit may act alone or with another protein.