Role of the Preoptic Area in the Inter- relationship Between Thermoregulatory and Sleep Regulatory Mechanisms

S everal brain regions are implicated in the regulation of sleep. Similarly a number of brain regions are also involved in the regulation of body temperature. In the hierarchy of neural structures regulating the body temperature, the preoptic area (POA) plays an important role (1-4). But we cannot talk about such a hierarchy for sleep regulation. At the same time, there is growing evidence in favor of the theory that the hypothalamus is the major center regulating slow wave sleep (SWS), sleep-wake cycle, and even wakefulness, though the role of the brain stem in the genesis of rapid eye movement (REM) sleep, and the thalamus in the genesis of the EEG spindles, cannot be underestimated. Chemical stimulation and neurotoxic lesion studies showed that the medial preoptic area (mPOA) of the hypothalamus, play an important role in the regulation of sleep (5-7). There is drastic reduction, but no abolition of sleep, after the destruction of the neurons in the mPOA. The major deficit which is observed in the rats is the inability to maintain sleep, rather than sleep initiation (7,8). Sleep, sleep-wake cycle and even circadian cycle of sleep were still present in those lesioned rats. Earlier lesion studies had shown a major thermoregulatory deficit in the POA lesioned animals (9,10). This had given rise to the feeling that the POA plays a major role in thermoregulation, and a less important, rather subservient, role in sleep regulation. But recent neurotoxic lesion studies, in which the neurons are destroyed, sparing the fibers of passage, had shown that the mPOA lesion does not abolish the thermoregulatory ability of the animal. But it only resets the body temperature at a higher level (11). The range of body temperature variation is also increased, resulting in exaggerated ultradian and circadian fluctuations of body temperature in the lesioned rats. Several terms were used for describing the area in the anterior hypothalamus which regulates the body temperature and sleep. Though the term POA is most commonly used, other terms like anterior hypothalamus, preoptic-anterior hypothalamic area (POAH), the basal forebrain are very frequently used (12-20). In addition to these, the terms like mPOA, and lateral preoptic area, which would restrict anatomically the area