Newly identified GABAergic neurons in regions of the ventrolateral medulla which regulate blood pressure.

Glutamic acid decarboxylase (GAD), the enzyme which synthesizes the inhibitory transmitter gamma-aminobutyric acid (GABA), was localized immunocytochemically within cells and processes distributed throughout the ventrolateral medulla. In caudal regions, GAD-stained cells were adjacent to the 'precerebellar' lateral reticular nucleus and partially overlapped the A1 area of norepinephrine synthesizing neurons. The largest number of labeled neurons filled the rostral ventrolateral medulla (RVL), coinciding with and extending beyond the C1 adrenergic area. GAD-positive cells also occupied the nucleus reticularis parvocellularis, raphe magnus (RM) and lateral wings of RM in the region of the pararaphe. Intrinsic GAD-containing cells in the ventrolateral medulla may tonically inhibit sympathoinhibitory neurons in the caudal ventrolateral medulla (CVL) and sympathoexcitatory neurons in the RVL.