Evidence for involvement of separate mechanisms in the production of analgesia by electrical stimulation of the nucleus reticularis paragigantocellularis and nucleus raphe magnus in the rat

The effect of ambient temperature (T_A) on the rectal temperature (T_RE) response to intraventricular injection of bombesin has been evaluated in conscious adult male rats. AtT_A= 4 °C, bombesin (50 ng-1 μg) caused a marked hypothermia which was dose-dependent both in terms of the magnitude and of the duration of the response. The bombesin-induced hypothermia was reduced atT_A= 24 °C, whereas atT_A= 31 or 33 °C, the peptide (1 μg) failed to affect T_RE. AtT_A= 36 °C, bombesin 1–10 μg induced anelevation in T_RE. The hyperthermia observed at high T_A could be reversed to hypothermia by transferring rats to cold. The analogs [D-Trp⁸]bombesin or [D-Leu¹³]-bombesin, tested under the same conditions, failed to produce significant changes in T_RE. These findings demonstrate that bombesin appears to act in the brain as a poikilothermic agent by disrupting thermoregulation at temperatures below or above thermoneutrality.