The effects of 3-mercaptopropionic acid on the cardiovascular system and the content of GABA in specific areas of the brain: further evidence for GABAergic involvement in central cardiovascular control

Abstract

A role has been proposed for the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) in central cardiovascular control. This proposa 1 was based on the cardiovascular effects of agents which block or mimic the action of GABA on the post-synaptic membrane. This dissertation reported the cardiovascular effects of 3-mercaptopropionic acid (3-MP), an agent which inhibits GABA biosynthesis in the pre-synaptic nerve terminal in anesthetized guinea pigs. 3-MP interrupts GABAergic transmission by decreasing the amount of GABA in the brain. The effect of 3-MP on mean arterial pressure, heart rate and barorflex-induced bradycardia was determined as well as the mechanism(s) involved in observed changes. The content of GABA in four regions of the brain (hypothalamus, medulla, cerebellum and occipital cortex) was determined at the end of each experiment. In anesthetized guinea pigs, 3-MP (195 mg/kg, i.p.) elicited a biphasic response (Type I) in the majority of animals. This response consisted of sympathetically-mediated hypertension and tachycardia superseded by vagally-mediated bradycardia. The other response (Type II) consisted of only the sympathetically-mediated effects. The Type II response was associated with animals in which the vagus nerves were functionally impaired. In all brain regions measured, the GABA levels of both Types I and II were significantly lower than those of control animals. The sympathetically-mediated effects of 3-MP were reversed by chlordiazepoxide, a GABA-facilitory agent. Therefore, the 3-MP-induced cardiovascular effects appeared to reflect GABAergic activity in the bra in which resulted from a reduction of GABA content. The two phases of the Type I response may have resulted from a reduction of GABA content in specific brain regions. A reduction in hypothalamic GABA levels appeared to be related to the sympathetic activation and a reduction in medullary GABA levels appeared to be related to the vagal activation. In unparalyzed animals, 3-MP elicited convulsive movements as well as the described effects on the cardiovascular system. The centrally acting anticonvulsant phenytoin stopped 3-MP-induced motor manifestations of seizure activity without altering either blood pressure or heart rate. Therefore, the cardiovascular effects of 3-MP appeared to occur independent of the convulsive effect of this agent. These results support the hypothesis of GABAergic involvement in the central control of autonomic outflow to the cardiovascular system. The inability of phenytoin to reverse the cardiovascular effects of 3-MP suggests that these effects were independent of 3-MP-induced seizures.