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In order to localize the cells expressing SHT,, receptors in the rat brain, we used in situ hybridization histochemistry to visualize the distribution of the mRNA coding for SHT,, receptors. Oligonucleotides derived from different parts of the coding region of the rat 5-HT,, receptor gene were used as hybridization probes. 5-HT,, binding sites were visualized on consecutive sections by receptor autoradiography using 3H-8-hydroxy-2-(di-rpropylamino)tetralin as ligand. The highest levels of hybridization were observed in the dorsal raphe nucleus, septum, hippocampus, entorhinal cortex, and interpeduncular nucleus. Positive hybridization signals were also present in other areas, such as the olfactory bulb; cerebral cortex; some thalamic and hypothalamic nuclei; several nuclei of the brainstem, including all the remaining raphe nuclei, nucleus of the solitary tract, and nucleus of the spinal tract of the trigeminus; and the dorsal horn of the spinal cord.The distribution and abundance of SHT,, receptor mRNA in different rat brain areas generally correlate with those of the binding sites, suggesting that 5-HT,, receptors are predominantly somatodendritic receptors.Several different types of S-HT receptors have been pharmacologically defined (Peroutka, 1990). The recent molecular cloning ofthe genes or the cDNAs for four ofthese receptors, 5-HT,,, 5-HT,,, 5-HT,,, and 5-HT, (see Hartig, 1989, for a review; Branchek et al., 199 l), has suggested that they all belong to the monomeric G-protein-coupled receptor family (Dohlman et al., 1987). The distribution of 5-HT,, receptors has been examined in the rat brain by means ofreceptor autoradiographic technique (Marcinkiewicz et al., 1984;Pazos and Palacios, 1985) using a selective tritium-labeled ligand, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (Arvidsson et al., 198 1;Gozlan et al., 1983). Recently, 5-HT,, receptors have been also visualized by immunoautoradiography (El Mestikawy et al., 1990), using an antibody against a synthetic peptide derived from the predicted sequence of the rat 5-HT,, receptor. The light microscopic autoradiographic techniques used until now did not allow the unequivocal establishment of the cellular localization of receptors, which could only be inferred indirectly (Palacios and Dietl, 1988). A presynaptic localization of 5-HT,, receptors on the cell body and dendrites of serotoninergic cells, which would be responsible for the autoinhibition of 5-HT cells in the dorsal raphe (Wang and Aghajanian, 1977) is supported by biochemical (Hjorth et al., 1982), lesion (Weissman-Nanopoulos et al., 1985;Verge et al., 1986) and electrophysiological (Aghajanian et al., 1988) experiments. The cellular localization of 5-HT,, receptors in the projection areas of the raphe nuclei is less clear (Palacios and Dietl, 1988). However, in the hippocampus, lesion experiments suggested a localization of the receptors to neurons intrinsic to this region (Hall et al., 1985) and electrophysiological experiments pointed in particular to the hippocampal pyramidal c...

Several transcription factors are expressed at higher levels in the waking than in the sleeping brain. In experiments with rats, the locus coeruleus, a noradrenergic nucleus with diffuse projections, was found to regulate such expression. In brain regions depleted of noradrenergic innervation, amounts of c-Fos and nerve growth factor-induced A after waking were as low as after sleep. Phosphorylation of cyclic adenosine monophosphate response element-binding protein was also reduced. In contrast, electroencephalographic activity was unchanged. The reduced activity of locus coeruleus neurons may explain why the induction of certain transcription factors, with potential effects on plasticity and learning, does not occur during sleep.

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We have recently shown that the expression of two immediate-early genes, c-fos and NGFI-A, is strongly affected by sleep deprivation, In this work, we investigated c-fos and NGFI-A expression after periods of spontaneous wakefulness or sleep. We used in situ hybridization and immunocytochemistry to detect the corresponding mRNA and protein levels, respectively. A first group of rats (S-L) was sacrificed during the light hours at the end of a long period of sleep. A second group (W-L) was sacrificed under similar conditions, except that during the last half hour the animals had been spontaneously awake. A third group (W-D) was sacrificed during the dark hours after a long period of continuous wakefulness. We found that c-fos and NGFI-A expression in several brain areas was increased in W-L and W-D rats with respect to S-L rats. Some of these areas, including the cerebral cortex, basal ganglia, and colliculi, may have been activated by the increased sensory and motor activity associated with waking. The activation of other areas, such as the medial preoptic area of the hypothalamus and some brainstem nuclei, may be more directly related to sleep regulation. These results indicate that many regions showing an increased expression of immediate early genes after wakefulness induced by sleep deprivation are also activated by periods of spontaneous wakefulness.