[The suprachiasmatic nucleus: cellular approach to clock functioning].

The hypothalamic suprachiasmatic nucleus (SCN), the circadian clock in mammals, generates and maintains a variety of daily rhythms. The present review is an attempt to synthesis experimental data on the anatomical organisation and cellular activities within SCN. The clock exhibits an endogenous rhythmic activity and can also be entrained by environmental synchronisers such as the light/dark cycle. It can be also influenced by internal signals such as the rhythmic secretion of melatonin which is under control of SCN activity. This tiny structure contains a variety of peptides organised in a specific distribution. It receives three main inputs from the retina (glutamate), the intergeniculate leaflet (NPY) and the dorsal raphe (serotonin). VIP containing cells located in the ventral part of SCN receive all these afferences and innervate the whole structure. VIP, PHI and GRP are likely implicated in the entrainment of the clock. The vasopressin (VP) cells exhibiting an endogenous rhythmic synthesis are considered as an output of the clock. The specific induction of immediate early genes (c-fos, jun B) within SCN by light pulses during the subjective night suggests the participation of these genes in the process of cellular entrainment by the photic input. The demonstration of a rhythmic astrocytic activity within SCN suggests an active involvement of this cellular population in the functioning of the clock facilitating or not neuronal communication. Cellular disturbances such as a decrease in VIP or VP cell population, reduction in the amplitude of functional cellular rhythms, astrocytic proliferation could explain some pathologies observed with ageing.