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Am J Physiol Regul Integr Comp Physiol (February 27, 2003). doi:10.1152/ajpregu.00016.2003
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Submitted on January 10, 2003
Accepted on February 24, 2003

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

Dmitry V Zaretsky1, Maria V Zaretskaia1, and Joseph A DiMicco1*

1 Department of Pharmacology and Toxicology, Indiana University School of Medicine, Indianapolis, Indiana, USA

* To whom correspondence should be addressed. E-mail: jdimicco{at}iupui.edu.

Studies in anesthetized rats have implicated GABAA 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 GABAA receptor agonist, and of bicuculline methiodide (BMI), a GABAA 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 one minute and was maximal within 10 minutes, 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 minutes 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.




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