Morphological and functional development of the suprachiasmatic nucleus in transplanted fetal hypothalamus

The development of the suprachiasmatic nucleus (SCN) in fetal rat hypothalamus transplanted to the adult brain was studied using morphological and functional methods. Anterior hypothalamic tissue was transplanted into the third ventricle, lateral ventricle or subarachnoid space of intact, adult hosts from E17 fetuses. These transplants developed the cytoarchitectonic and immunohistochemical staining characteristics of SCN, clusters of parvocellular neurons expressing vasopressin- and vasoactive intestinal polypeptide-like immunoreactivity in adjacent cellular populations, irrespective of the exact location of the transplanted tissue in the host brain. The functional status of the transplants placed in the rostral third ventricle and the foramen of Monroe was analyzed and compared to host SCN using in vitro recording of neuronal firing rate and measurement of metabolism using the 2-deoxyglucose (2-DG) technique. During subjective day, neuronal firing rates and 2-DG uptake were high in discrete cell groups within the transplants which were subsequently demonstrated to exhibit the cytoarchitectonic and immunohistochemical characteristics of SCN. The firing rates and 2-DG uptake in these areas were lower during the subjective night. This pattern of activity closely resembles that of the intact SCN. In contrast, neither transplanted anterior hypothalamic area, lacking an identifiable SCN-like structure, nor posterior hypothalamic area showed day-night differences in firing rate or 2-DG uptake. These observations indicate that SCN transplanted into intact adult hosts exhibits morphological and functional differentiation nearly identical to the host and that the transplanted SCN maintains circadian function which is probably entrained to the host SCN.