Inhibition of vasopressin release to carotid occlusion by gamma-aminobutyric acid and glycine.

1 In cats anaesthetized with pentobarbitone sodium or chloralose, the amino acids, gamma-aminobutyric acid (GABA) and glycine, were applied to the ventral surface of the brain through paired Perspex rings placed across the medulla. 2 Applied to a region situated at the transition between medulla and cord, both amino acids greatly attenuated and even abolished the vasopressin release in response to carotid occlusion. Glycine was about 100 times more potent than GABA and effective in a concentration of 0.1 mg/ml. The pressor response to carotid occlusion was not affected. 3 Applied to a region situated 5 to 6 mm more rostrally, the amino acids did not affect vasopressin release but in strong concentrations, greatly attenuated the pressor response to carotid occlusion. 4 The two responses to carotid occlusion, vasopressin release and the pressor response, can thus be influenced independently. 5 It is concluded that the pathways carrying afferent impulses from the baroreceptors in the carotid sinus reach the ventral surface of the brain stem at two regions. At both, synaptic transmission can be blocked by the application of an inhibitory amino acid and thus prevent either the release of vasopressin at the caudal site, or the increase of vasomotor tone at the rostral site.