Sleep-waking discharge patterns of median preoptic nucleus neurons in rats

Several lines of evidence show that the preoptic area (POA) of the hypothalamus is critically implicated in the regulation of sleep. Functionally heterogeneous cell groups with sleep-related discharge patterns are located both in the medial and lateral POA. Recently a cluster of neurons showing sleep-related c-Fos immunoreactivity was found in the median preoptic nucleus (MnPN). To determine the specificity of the state-related behaviour of MnPN neurons we have undertaken the first study of their discharge patterns across the sleep-waking cycle. Nearly 76 % of recorded cells exhibited elevated discharge rates during sleep. Sleep-related units showed several distinct types of activity changes across sleep stages. Two populations included cells displaying selective activation during either non-rapid eye movement (NREM) sleep (10 %) or REM sleep (8 %). Neurons belonging to the predominant population (58 %) exhibited activation during both phases of sleep compared to wakefulness. Most of these cells showed a gradual increase in their firing rates prior to sleep onset, elevated discharge during NREM sleep and a further increase during REM sleep. This specific sleep-waking discharge profile is opposite to that demonstrated by wake-promoting monoaminergic cell groups and was previously found in cells localized in the ventrolateral preoptic area (vlPOA). We hypothesize that these vlPOA and MnPN neuronal populations act as parts of a GABAergic/galaninergic sleep-promoting ('anti-waking') network which exercises inhibitory control over waking-promoting systems. MnPN neurons that progressively increase activity during sustained waking and decrease activity during sustained sleep states may be involved in homeostatic regulation of sleep.