Effects of intra-raphe injection of amino acids on cardiovascular function in rats

The effects of microinjection of an excitatory amino acid (glutamate, 10 micrograms) or several inhibitory amino acids (taurine 10 micrograms, GABA 10 ug or glycine 10 ug) into the dorsal raphe region on cardiovascular function were assessed in rats under pentobarbital sodium. Intra-raphe administration of glutamate, but not saline, caused an increase in the mean arterial pressure. By contrast, intra-raphe administration of taurine, GABA or glycine, but not saline, caused a decrease in both the mean arterial pressure and the heart rate. The glutamate-induced hypertension or both the hypotension and the bradycardia induced by taurine, GABA or glycine was antagonized by pretreatment with intra-raphe injection of a serotonergic receptor antagonist (1 ug cyproheptadine). In addition, the vasopressor and bradycardia responses to an intravenous dose of epinephrine (2.5 ug/kg) were assessed in saline-treated rats and amino acid-treated rats. Intra-raphe injection of glutamate produced a significant decrease in reflex bradycardia compared to the controls. On the other hand, administration of taurine, GABA or glycine into the dorsal raphe region led to an enhancement of epinephrine-induced bradycardia. Again, the reduction or the facilitation of the epinephrine-induced bradycardia following administration of these amino acids was antagonized by pretreatment with cyproheptadine. The results suggest that the serotonergic receptor mechanisms in the dorsal raphe region play a role in the elaboration or modulation of the cardiovascular responses to amino acids (including glutamate, taurine, GABA and glycine).