TRACING SCN GRAFT EFFERENTS WITH DIL

The suprachiasmatic nuclei (SCN) regulate circadian rhythmicity in many biological and behavioral responses. Hamsters are made permanently arrhythmic by bilateral destruction of the SCN. Circadian locomotor rhythmicity is restored by fetal tissue transplants placed in the 3rd ventricle (3V). If intact animals are implanted with fetal SCN grafts, they maintain locomotor activity rhythms when the host SCN are subsequently destroyed. The mechanism(s) whereby the SCN (either grafted or in situ) regulate locomotor rhythmicity is not known. Evidence from other graft models point to the possibility of efferents to appropriate targets in the host. In the present study, efferent connections of transplanted fetal SCN were examined using the carbocyanine dye, Dil. Intact or SCN-lesioned animals were sacrificed 7 or 40 days after receiving fetal SCN grafts into 3V. Dil crystals were placed on the grafts in fixed brains which were then incubated for 3-6 weeks before sectioning. Sections bearing Dil-labelled efferents from the graft were photographed and then stained for immunoreactive VIP and NP cells to locate donor SCN. Although labelled efferents were observed in a majority of the grafts, most were confined to the limits of the graft. The few labelled efferents that entered the host tissue when the graft seemed to merge with the host did not extend very far regardless of whether the graft contained immunohistochemical evidence for donor SCN or not. The observation of limited graft-host connectivity suggests either that a limited number of efferents is sufficient to support circadian locomotor rhythmicity, or that the mechanism mediating restoration of function entails a diffusible substance.