Hemodynamic defense response to thyrotropin-releasing hormone injected into medial preoptic nucleus in rats

The role of thyrotropin-releasing hormone (TRH) and glutamate in central cardiovascular control was studied by microinjections (50 nl) of these agents into the medial or median preoptic nuclei of conscious rats (n = 49) with continuous recording of mean arterial pressure, heart rate, blood flow, and vascular resistance in hindquarter, renal, and mesenteric blood vessels. In addition, the effect of TRH on renal sympathetic nerve activity was studied in anesthetized rats. TRH (2.4-240 pmol) elicited the typical hemodynamic pattern of the "defense response" consisting of increased blood pressure, tachycardia, hindquarter vasodilation, and constriction of renal and mesenteric blood vessels. Maximum changes in cardiovascular variables after the 24-pmol dose were +12 +/- 2 mmHg (mean arterial pressure), +73 +/- 15 beats/min (heart rate), -21 +/- 6% (hindquarter resistance), +15 +/- 6% (renal resistance), and +31 +/- 6% (mesenteric resistance), P less than 0.05 compared with saline. In anesthetized rats, TRH at the 2.4-pmol dose increased renal sympathetic nerve activity (greater than 200%, n = 5, P less than 0.05 compared with control) with no effect on blood pressure or renal flow. Glutamate (10 or 100 nmol) produced a similar pattern of hemodynamic changes as TRH. Peak effects after the 100-nmol dose of glutamate were +16 +/- 2 mmHg (mean arterial pressure), +57 +/- 11 beats/min (heart rate), -31 +/- 3% (hindquarter resistance), +29 +/- 9% (renal resistance), and +87 +/- 22% (mesenteric resistance), P less than 0.05 compared with saline. The glutamate N-methyl-D-aspartate (NMDA) receptor blocker MK-801 (300 micrograms/kg iv) attenuated the pressor-tachycardic responses to TRH and the pressor-mesenteric constrictor responses to glutamate.(ABSTRACT TRUNCATED AT 250 WORDS)