GABA receptors mediate cerebral vasodilation in the unanesthetized goat.

The effects of gamma-aminobutyric acid (GABA) and muscimol upon cerebral blood flow were evaluated in the unanesthetized goat. Cerebral blood flow was continuously measured by means of an electromagnetic flow probe chronically implanted on the internal maxillary artery after occlusion and thrombosis of the distal extracerebral vessels. Administration of GABA (1-100 micrograms) directly into the cerebral circulation produced dose-dependent increases in cerebral blood flow, without accompanying systemic effects. Muscimol mimicked the effects of GABA at doses 10 times lower. Administration of picrotoxin (1-3 mg) into the internal maxillary artery did not significantly change cerebral blood flow, but inhibited in a dose-dependent manner the vasodilation induced by GABA. Selective blockade of beta-adrenergic or muscarinic cholinergic receptors by propranolol or atropine, respectively, did not modify the cerebrovascular response to the GABAergic agonists. These results indicate that GABA increases total cerebral blood flow, acting on specific receptor sites in the cerebral blood vessels. The absence of influence of picrotoxin on resting cerebral blood flow suggests that the GABAergic receptors are not tonically activated under physiological conditions.