8-OH-DPAT-sensitive neurons in the nucleus raphe magnus modulate thermoregulatory output in rats

The nucleus raphe magnus (NRM) is purported to be a relay through which peripheral thermoafferent information is transmitted to thermointegrative centers located in the preoptic/anterior hypothalamus (POAH). Therefore, suppression of neural activity in the NRM should reduce thermoregulatory responses to peripheral thermal challenges, but not affect responses elicited by manipulation of POAH temperature. At low ambient temperatures lidocaine injections into the NRM of nonanesthetized rats resulted in decreases in POAH temperature, oxygen consumption, and electromyographic activity. At a warm ambient temperature, lidocaine injections into the NRM decreased the elevations in oxygen consumption and electromyographic activity elicited by cooling the POAH. The effects of lidocaine injections were duplicated by injection of a 5-HT(1A) agonist 8-hydroxy-dipropylaminotetralin (8-OH-DPAT) into the NRM. The effect of 8-OH-DPAT was eliminated by pre-treatment with a selective autoreceptor antagonist. These results suggest that NRM 5-HT neurons are modulating the relationship between output of thermointegrative centers and thermoregulatory effector responses rather than processing thermoafferent information.