State-dependent pattern of Fos protein expression in regionally-specific sites within the preoptic area of the cat

Clinical and experimental data have shown that the preoptic area of the hypothalamus (POA) is involved in the generation and maintenance of NREM sleep. However, the activity of specific populations of POA neurons during REM sleep, NREM sleep and different waking conditions is still not firmly established. Consequently, we performed a quantitative, regionally-specific analysis of the Fos immunoreactivity of neurons in the POA of the cat during NREM sleep and REM sleep induced by microinjections of carbachol into the nucleus pontis oralis (REMc), as well as during quiet and alert wakefulness. We observed that while the total number of Fos immunoreactive neurons in the POA did not change as a function of these behavioral states, state-specific differences in neuronal activity were detected in restricted regions of the POA. An increase in the number of Fos+ neurons was observed in the rostral tip of the suprachiasmatic nucleus (SCN) during NREM (83.4 ± 25.6) compared to quiet wakefulness (5.1 ± 1.3, p < 0.05) but not with the other behavioral states. In the median preoptic nucleus (MnPN), the number of Fos immunoreactive neurons was greater during NREM sleep (39.5 ± 6.1) compared with quiet wakefulness (13.5 ± 1.4, p < 0.05) and REMc (16.2 ± 2.0, p < 0.05). State-specific Fos immunoreactive neurons were not observed in the ventro-lateral preoptic nucleus (VLPO). Finally, there was no significant increase in the number of Fos+ neurons during REMc in any of the subregions of the POA. In conclusion, within the POA, a selective neuronal activation during NREM sleep was found only in the MnPN. In addition, our data suggest a potential role of the SCN in NREM sleep. Finally, based on the distribution of Fos+ neurons in the entire POA, we conclude that the neuronal network involved in the regulation of NREM sleep is dispersed and intermingled with waking-related neurons.