Contrasting effects of ibotenate lesions of the paraventricular nucleus and subparaventricular zone on sleep-wake cycle and temperature regulation

The suprachiasmatic nucleus (SCN), the circadian pacemaker for the brain, p rovides a massive projection to the subparaventricular zone (SPZ), but the role of the SPZ in circadian processes has received little attention. We ex amined the effects on circadian rhythms of sleep, body temperature, and act ivity in rats of restricted ibotenic acid lesions of the ventral or dorsal SPZ that spared the immediately adjacent paraventricular hypothalamic nucle us (PVH) and the SCN. Ventral SPZ lesions caused profound reduction of meas ures of circadian index of sleep (by 90%) and locomotor activity (75% reduc tion) but had less effect on body temperature (50% reduction); dorsal SPZ l esions caused greater reduction of circadian index of body temperature (by 70%) but had less effect on circadian index of locomotor activity (45% redu ction) or sleep (<5% reduction). The loss of circadian regulation of body t emperature or sleep was replaced by a strong ultradian rhythm (period <simi lar to>3 hr). Lesions of the PVH, immediately dorsal to the SPZ, had no sig nificant effect on any circadian rhythms that we measured, nor did the lesi ons affect the baseline body temperature. However, the fever response after intravenous injection of lipopolysaccharide (5 mug/kg) was markedly decrea sed in the rats with PVH lesions (66.6%) but not dorsal SPZ lesions. These results indicate that circadian rhythms of sleep and body temperatures are regulated by separate neuronal populations in the SPZ, and different aspect s of thermoregulation (circadian rhythm and fever response) are controlled by distinct anatomical substrates.