Effects of Stereotactic Introduction of Baclofen in the Medullary Cardiovascular Nuclei of Rats

In rats anesthetized with urethane (1.7 g/kg, i. p.), we investigated the effects of stereotaxic microinjections of baclofen into the medullary nuclei involved in the neural control of cardiovascular activity (nuclei paramedianus, ambiguus, and reticularis lateralis). Changes in the hemodynamic parameters (systolic and diastolic blood pressure and heart rate) were measured. Injections of the above GABA_B receptor agonist (10^–7, 10^–6, or 10^–5 M, 0.1 μl) into the medullary cardiovascular nuclei was accompanied by changes in the blood pressure, the magnitude and direction of which depended not only on the baclofen concentration but also on the site of injection (into one nucleus or another). Injections of the agent into the nucl. ambiguus at a 10^–7 M concentration resulted in an increase in the blood pressure, but a 10^–5 M concentration provided significant reduction of the systolic and diastolic blood pressure. If baclofen was injected into the nucl. reticularis lat., the blood pressure also increased or decreased but the concentration dependence was opposite to the above described. Injections of the agent into the nucl. paramedianus were always accompanied by significant increases in the blood pressure. Changes in the heart rate following baclofen injections into the nuclei under study were insignificant. The specificities of the baclofen-induced effects are probably related to peculiarities of the functioning of GABA_B receptors, the activation of which may mediate the effects of multiple neuronal mechanisms.

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