The interaction of GABA and glutamate on the cardiovascular responses of horizon tal limb of diagonal band of Broca (hDB)

Introduction: We previously shown that microinjection of glutamate into the hDB of rat elicited cardiovascular depressive responses. Microinjection of AP5 (an NMDA receptor antagonist, 2.5 mM, 50 nl) and CNQX (an AMPA receptor antagonist, 1 mM, 50 nl) caused no significant changes in the blood pressure and heart rate. Microinjection of bicuculline (BMI: a GABAA receptor antagonist, 1 mM, 50 nl) resulted in the increased blood pressure and heart rate. In this study we investigated the possible interaction of GABAergic and glutaminergic systems by coinjection of the antagonists of both systems. Methods: Experiments were performed on 27 urethane anesthetized rats. Drugs were microinjected into the hDB using micropipettes. The arterial pressure and heart rate were continuously recorded and repeated measures ANOVA was used for data analysis. Results: Our results showed that coinjection of 50 nl of BMI (1mM ) and AP5 (2.5 mM) significantly (P < 0. 01) decreased the presser effects of BMI. Also, coinjection of 50 nl of BMI (1 mM) and CNQX (1 mM) significantly (P < 0.01) decreased the presser effects of BMI. Microinjection of the doses of BMI and two glutamate receptor antagonists produced the same results. Conclusion: These findings suggest that the cardiovascular effects of the blockade of GABAergic system depend on the activation of local NMDA and AMPA receptors. A possible explanation for the results is that, GABAergic system tonically inhibits the glutaminergic neurons.