Discharge modulation of rat dorsal raphe neurons during sleep and waking: effects of preoptic/basal forebrain warming

In cats, putative serotonergic neurons (PSNs) recorded from the dorsal raph e nucleus (DRN) across the sleep-wake cycle exhibit the so-called rapid eye movement sleep-off (REM-off) discharge pattern. Since, the sleep-wake disc harge patterns of DRN neurons in behaving rats is poorly known, the present study examined this neuronal populations. The PSNs recorded in this study exhibited: (1) progressive decrease in discharge rate from waking to NREM t o REM sleep; (2) long action potential duration, and (3) reduction of disch arge rate after systemic administration of a selective 5-HT1A agonist, (+/- )-8-hydroxy-2-(di-n-propylamino) tetralin hydrobromide (8-OH-DPAT). Evidenc e supports the hypothesis that NREM sleep is modulated by thermoregulatory mechanisms localized in the preoptic area and adjacent basal forebrain (POA /BF). We previously reported that POA/BF warming suppresses the discharge o f wake-promoting neurons in the posterior hypothalamus and the basal forebr ain. Since the DRN is one component of the brainstem arousal system and rec eives projections from POA/BF, we examined the effects of local POA/BF warm ing by 1.5-2.0 degrees C during waking on the discharge of DRN neurons. POA /BF warming reduced the discharge in 14 of 19 PSNs and in 12 of 17 other wa ke-related neurons in the DRN. DRN neuronal discharge reduction occurred wi thout accompanying EEG frequency or behavioral changes. These results sugge st that PSNs recorded in DRN in unrestrained and unanesthetized rats exhibi t a "wake-active REM-off" discharge pattern and further support the hypothe sis that the POA/BF warm-sensitive hypnogenic system induces sleep by a coo rdinated inhibition of multiple arousal systems including that modulated by the DRN. (C) 2000 Elsevier Science B.V. All rights reserved.