Medullary gamma-aminobutyric acid (GABA) receptors and the regulation of blood pressure in the rat.

The cardiovascular effects of a GABAergic agonist (muscimol) and antagonist (bicuculline methiodide) were determined after microinjection (100 nl) into ventrolateral vasodepressor and vasopressor neuron pools. In urethane-anesthetized rats, these neuron pools were identified bilaterally by observing pressor or depressor responses after the microinjection of L-glutamate (333 ng). The bilateral microinjection of muscimol (0.1-10 ng/site) into vasodepressor sites caused a dose-related (0.1-10.0 ng/site) increase in the blood pressure, heart rate and pulse pressure. In ventrolateral vasopressor sites, muscimol (0.1-10.0 ng/site) caused a marked fall in blood pressure, heart rate and pulse pressure. The depressor response elicited by aortic nerve stimulation (2 V, 25 HZ, 3 msec) was blocked and reversed by muscimol microinjections into vasodepressor sites. In these sites, muscimol also caused a similar reversal of the depressor response induced by stimulation of pulmonary C-fibers (J-receptors) after the right atrial administration of phenyldiguanide (30 micrograms/kg). All the effects of muscimol were blocked or reversed by the bilateral microinjection of bicuculline methiodide into the vasodepressor and vasopressor sites. These results demonstrate the importance of ventrolateral vasodepressor and vasopressor neuron pools in the maintenance and reflex regulation of blood pressure and suggest that intrinsic GABAergic systems may serve to modulate activity in these neuron pools.