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NEUROHUMORAL CONTROL OF CIRCULATION AND HYPERTENSION

Stimulation and blockade of GABA_A receptors in the raphe pallidus: effects on body temperature, heart rate, and blood pressure in conscious rats

Department of Pharmacology and Toxicology, Indiana University School of Medicine, Indianapolis, Indiana 46202

Submitted 10 January 2003 ; accepted in final form 24 February 2003

Studies in anesthetized rats have implicated GABA_A receptors in the region of the medullary raphe pallidus (RP) at the level of the facial nucleus in sympathetic nervous regulation of both heart rate and thermoregulatory mechanisms. Therefore, we examined the effect of microinjection of muscimol, a GABA_A receptor agonist, and of bicuculline methiodide (BMI), a GABA_A receptor antagonist, into the same region of the RP on heart rate, blood pressure, and core body temperature in conscious rats. Microinjection of BMI (40 pmol) into the RP evoked tachycardia that appeared within 1 min and was maximal within 10 min but had little or no effect on blood pressure or body temperature. Microinjection of muscimol (10–80 pmol) at the same sites in the RP evoked marked dose-related decreases in body temperature that developed more slowly (i.e., maximum decreases appearing at 60–75 min after 80 pmol) but had no effect on heart rate or blood pressure. Injection of either agent at sites outside the region had lesser or no effect on the measured parameters. These findings suggest that activity of neurons in the region of the RP plays an important role in the maintenance of body temperature but not heart rate under baseline conditions in conscious rats. Specifically, thermoregulatory neurons in this region appear to be tonically active and contribute to maintenance of body temperature under baseline conditions, while cardiac sympathetic premotor neurons in the RP are not active under these circumstances and thus do not support basal heart rate in conscious rats.

raphe nuclei; conscious rat; heart rate; blood pressure; body temperature

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