Sleep-related c-Fos protein expression in the preoptic hypothalamus: effects of ambient warming

Preoptic area (POA) neuronal activity promotes sleep, but the localization of critical sleep-active neurons is not completely known. Thermal stimulati on of the POA also facilitates sleep. This study used the c-Fos protein imm unostaining method to localize POA sleep-active neurons at control (22 degr ees C) and mildly elevated (31.5 degrees C) ambient temperatures. At 22 deg rees C, after sleep, but not after waking, we found increased numbers of c- Fos immunoreactive neurons (IRNs) in both rostral and caudal parts of the m edian preoptic nucleus (MnPN) and in the ventrolateral preoptic area (VLPO) . In animals sleeping at 31.5 degrees C, significantly more Fos IRNs were f ound in the rostral MnPN compared with animals sleeping at 22 degrees C. In VLPO, Fos IRN counts were no longer increased over waking levels after sle ep at the elevated ambient temperature. Sleep-associated Fos IRNs were also found diffusely in the POA, but counts were lower than those made after wa king. This study supports a hypothesis that the MnPN, as well as the VLPO, is part of the POA sleep-facilitating system and that the rostral MnPN may facilitate sleep, particularly at elevated ambient temperatures.