Brain Stem GABA Pathways and the Regulation of Baroreflex Activity

Since the inhibitory neurotransmitter γ-aminobutyric acid (GABA) has been implicated in virtually all neuronal systems, it would be surprising if it were not also involved in the pathways responsible for maintaining and controlling blood pressure and heart rate. However, the exact role that it plays in cardiovascular control is still under intensive investigation and GABAergic inhibitory connections between cell groups are still being mapped anatomically and functionally. In addition, in recent years it has become apparent that GABA can act on at least two types of receptor (GABAA and GABAB), which are both capable of mediating pre- and postsynaptic inhibition but function via different ionic mechanisms. Agonists acting at the GABAA-receptor subtype cause inhibition by opening of Cl− channels whereas agonists at the GABAB-receptor subtype cause postsynaptic inhibition via a K+-conductance (Newberry and Nicoll, 1985) and presynaptic inhibition of transmitter release probably through a reduction in Ca2+-conduc-tance (see Huston et al., 1990).