Developmental and functional aspects of grafting of the suprachiasmatic nucleus in the Brattleboro and the arrhythmic rat.

The development of suprachiasmatic nuclei (SCN) dissected from fetal rats and grafted in adult rat brains has provided additional insights in the normal ontogeny of the SCN. The SCN survives rather easily and develops to its typical adult cytoarchitectonical arrangement of contiguous clusters of vasopressin (VP)-, vasoactive intestinal polypeptide (VIP)- and somatostatin (SOM)- immunoreactive cells. Neither site of implantation, nor the establishment of efferent or afferent connections of the grafted SCN seems to be essential to allow it to develop normally into this distinguishing cytology. This independent maturation does certainly not contradict with its known endogenous and independent potency of circadian pacemaker function in the brain. If the fetal SCN is grafted in such a way that it could merge with the parenchyma of the brain of a VP-deficient Brattleboro rat, the VP neurons of the SCN often establish efferent connections with the genuine target areas of this nucleus as could be shown immunocytochemically. When the fetal SCN is grafted homotopically in the brain of SCN-lesioned rat (or hamster), the surviving SCN neurons are able to reverse the arrhythmicity of these rats. Free-running circadian rhythm of drinking or motor behaviour in constant darkness are induced within weeks after grafting. A correlation between this restorative effect and the immunocytochemical staining pattern of the SCN in the transplant and/or the afferent and efferent connections between graft and host brain, could, however, not be shown conclusively. Transplants with surviving SCN are also seen when arrhythmicity was still present, which made us conclude that there has to be a neural connection between graft and host rather than a neurohumoral control in order to explain the restorative effect of the SCN graft in SCN-lesioned animals.