LOCALIZATION OF SUBSTANCE-P RECEPTORS IN CENTRAL NEURAL STRUCTURES CONTROLLING DAILY RHYTHMS IN NOCTURNAL RODENTS

Brain areas involved in the genesis and control of daily rhythms recei ve a prominent neural input that contains the neurotransmitter substan ce P (SP), a peptide putatively involved in the synchronization of cir cadian rhythms by environmental light. We investigated the localizatio n of receptors to SP in the suprachiasmatic nucleus of the hypothalamu s (SCN) and in the intergeniculate leaflet of the thalamus (IGL) of th e rat and hamster using in situ hybridization histochemistry and immun o-histochemistry. Consistently with that previously described in the r at a neuronal population distributed along the lateral margin and at t he dorso-latero-caudal aspect of the hamster SCN expresses moderately the mRNA encoding the SP receptor In both rat and hamster, immunohisto chemical data confirm the previous finding and reveal an almost comple te absence of SP receptors in the ventral part of the SCN, which recei ves a direct projection from the retina. In the IGL of both species, n umerous neurons prominently express the mRNA encoding the SP receptor. The immunostaining shows a high density of SP receptors on perikarya and dendrites throughout the nucleus. A dense staining is also observe d on individual cells and processes bordering the lumen of blood vesse ls in the SCN and IGL. These observations in nocturnal rodents reveal the following features: (1) a novel neuronal subpopulation located at the latero-dorsal edge of the SCN that likely receives a non-retinal S P-containing input; (2) a large number of neurons in the IGL that migh t receive a SP-containing input arising from the raphe (3) the absence of SP receptors in the retinorecipient part of the SCN, raising the q uestion of the biological significance of SP-containing fibers distrib uting within the ventral part of this nucleus; (4) the presence of per ivascular cells bearing a high density of SP receptors, which might co ntribute to the cyclic regulation of local blood flow.