VASOACTIVE-INTESTINAL-PEPTIDE EFFERENT PROJECTIONS OF THE SUPRACHIASMATIC NUCLEUS IN ANTERIOR HYPOTHALAMIC TRANSPLANTS - CORRELATION WITH FUNCTIONAL RESTORATION OF CIRCADIAN BEHAVIOR

Circadian rhythmicity can be restored by transplantation of fetal ante rior hypothalamic (AH) tissue containing the suprachiasmatic nucleus ( SCN) into hosts rendered arrhythmic by SCN ablation. However, the natu re of the SCN effector pathways mediating functional recovery has rema ined elusive. To examine implant-derived SCN innervation of the host, AH homografts (hamster-to-hamster) and heterografts (mouse- or rat-to- hamster) were employed and the distribution of vasoactive intestinal p eptide (VIP) within the SCN terminal fields was evaluated. A compariso n was made between cases where circadian locomotor activity was restor ed and cases where circadian rhythmicity remained disrupted following AH transplantation. A dense aggregation of VIP neurons and processes w as identified in each transplant that restored behavioral rhythmicity in the host. In these cases, SCN-derived VIP fibers were integrated wi th the host brain and could be identified in host terminal fields typi cally innervated by SCN-VIP fibers. A correlation was noted between VI P innervation of the host paraventricular thalamic nucleus (PVT) and r estoration of circadian rhythmicity. Neither qualitative nor quantitat ive differences in transplant VIP projections were noted between AH ho mografts and heterografts. These results demonstrate that SCN VIP neur ons in AH transplants send an appropriately restricted set of efferent projections to the host brain and suggest that SCN efferent projectio ns to the PVT may participate in mediating the functional recovery of circadian locomotor activity. (C) 1995 Academic Press, Inc.