Intrapreoptically microdialyzed and microinjected norepinephrine evokes different thermal responses.

Abstract

Norepinephrine (NE) microdialyzed into the medial preoptic area (MPO) evokes a core temperature (Tco) fall in contrast to the rise when it is microinjected. Because prostaglandin E2 (PGE2) is a contaminant of the microinjection procedure per se, we determined whether it might account for these differential thermal responses. NE (1 microgram/microliter) was bilaterally microinjected into the MPO of conscious guinea pigs treated 20 min prior with a PG synthetase inhibitor, indomethacin (Indo, 10 mg/kg, im). Under these conditions, the latency of the NE-induced Tco rise was prolonged (138 +/- 18 min). When Indo was administered both 20 min before and 20 min after NE microinjection, NE was hypothermizing. NE (10 micrograms/microliter at 2 microliters/min for 3 h) microdialyzed into the MPO, lateral septum, or anterior hypothalamus caused Tco falls, whereas it induced no Tco change when dialyzed into the lateral preoptic area, indicating site specificity. PGE2 (1 microgram/microliter) caused a Tco rise when it was dialyzed intra-MPO. Microdialysis of PGE2 and NE together neutralized each other's effects. Indo given at the end of intra-MPO NE dialysis blocked the usual recovery of Tco from its lowered value. These results indicate that NE and PGE2 in the MPO may reciprocally influence the Tco of guinea pigs. The data further suggest that PGE may account for the different responses to microinjected and microdialyzed NE.