Superfusion of the hypothalamus with gamma-aminobutyric acid

Summary

The third ventricle and the aqueduct of anaesthetized cats were cannulated and the posterior area of the hypothalamus was stimulated with a monopolar electrode. Electrical stimulation of the posterior area evoked a rise of the arterial blood pressure which was inhibited by the injection of 0.2 ml of gammaaminobutyric acid (GABA, 2M) into the third ventricle. The impairment of the pressor response to electrical stimulation was accompanied by a fall of the “resting” blood pressure and depression of the respiration, presumably by action of GABA on brain areas in the vicinity of the fourth ventricle. In another series of experiments the posterior area was labelled with (±)-³H-noradrenaline and 2 h later superfused with a push-pull cannula and stimulated with the tip of the cannula. Superfusion with GABA (0.1 or 1 M) evoked a dose-dependent increase of release of catecholamines and enhanced the pressor response to electrical stimulation. 1×10⁻³ M of GABA enhanced the pressor response without increasing the spontaneous release of catecholamines but potentiated the output of radioactivity during electrical stimulation. Superfusion with sucrose (1 M) did not influence either pressor response or release of radioactive compounds. Superfusion with GABA increased slightly but significantly the relative concentration of catechols in the effluents and reduced that of normetanephrine. Pretreatment of animals with pargyline and tropolone evoked a pronounced increase of the relative concentrations of catechols in the effluent, while those of 3-methoxy-4-hydroxyphenylglycol and 3-methoxy-4-hydroxymandelic acid were strongly reduced. It is concluded that superfusion with high concentrations of GABA enhances the pressor response by increasing the release of catecholamines, while the effect of low GABA concentrations is due to facilitation of release of catecholamines from the adrenergic nerve endings during electrical stimulation.