Effects of Microinjections of Triazolam into Medial Preoptic Area on Sleep and Brain Temperature in Rats

A variety of hypnotic compounds including triazolam (1), pentobarbital (2) and ethanol (3) have been shown to induce sleep when microinjected into the medial preoptic area (MPA) of the anterior hypnothalamus. The mechanism by which they produce this effect, however, is not clear. Lesions of a basal forebrain area including the preoptic area decrease sleep (4), while electrical stimulation of this area increases it (5). As an integrative structure, the preoptic area is also involved in a variety of other physiological processes. It contains cells sensitive to osmotic, and cardiovascular signals (6) as well as glucose and steroids (7). It is a thermoregulatory site in mammals, and contains a high concentration of strongly warmsensitive and cold-sensitive neurons that are involved in autonomic and behavioral thermoeffector activities (8). This raises the possibility that the effect of microinjection of hypnotic compounds on sleep is secondary to druginduced changes in temperature. In the past we have explore this possibility using rectal (1) and intraperitoneal temperature (9). After microinjection of both vehicle and triazolam into the medial preoptic area, there was an increase in core temperature, which did not differ between the two treatments. Thus, although the act of microinjecting materials into the MPA appears to produce a relatively consistent transient rise in temperature, there was no evidence to suggest that triazolam’s hypnogenic property is secondary to drug-induced alterations in core temperature. It is possible, however, that its effects on temperature are more subtle, involving more localized changes. Under some circumstances brain temperature may vary from trunk temperature (10), and although in general they move in parallel, changes in preoptic temperature may precede alterations in abdominal temperature in association with sleep or eating in the rat (11). Indeed, rat brain and rectal temperature may even transiently move in opposite directions in response to some behaviors (12). Thus, we believe that an examination of the possibility that druginduced changes in sleep are secondary to effects on temperature requires direct measures of brain temperature Effects of Microinjections of Triazolam into Medial Preoptic Area on Sleep and Brain Temperature in Rats