How do Fetal Grafts of the Suprachiasmatic Nucleus Communicate with the Host Brain?

Fetal grafts containing the hypothalamic suprachiasmatic nucleus (SCN), the site of an endogenous circadian pacemaker, can reinstate behavioral rhythms in lesioned recipients but the precise routes of communication between the graft and the host brain remain unknown. Grafts containing the SCN may convey temporal information to the host brain via neural efferents, diffusible factors, or a combination of both. We examined graft-host connections in anterior hypothalamic homografts (hamster-to hamster) and heterografts (rat-to hamster) implanted in the third ventricle by: (a) applying the carbocyanine dye, dil, directly onto homo- and heterografts in fixed tissue sections; and (b) using a donor-specific neurofilament (NF) antibody to immuno-cytochemically visualize heterograft efferents. Dil applied onto either homografts or heterografts labeled relatively few graft efferents which could be followed only short distances into the host brain. In contrast, NF-labeled heterograft efferents were both more numerous and extended for longer distances into the host brain than anticipated on the basis of dil tract tracing. The results suggest that anterior hypothalamic grafts implanted in the third ventricle provide substantial input to the adjacent host hypothalamus although it is not known whether these projections arise from SCN cells or from other extra-SCN hypothalamic tissue within these grafts. Nor is it known whether these projections are functional. To determine if neural efferents are required for the restoration of rhythmicity after grafting, we have encapsulated fetal anterior hypothalamus in a permselective polymer which prevents neurite outgrowth but allows diffusible signals to reach the host brain. Polymer-encapsulated grafts of fetal anterior hypothalamus from wild-type hamster fetuses have been implanted into the third ventricle of heterozygote tau mutant, SCN-lesioned hamsters. Because the free-running period of tau mutant hamsters is significantly shorter than that of wild-type hamsters, restored rhythms when they occur can be unambiguously attributed to the presence of donor tissue. Encapsulated grafts that survive contain neuropeptide cell markers characteristic of the intact SCN, but the survival rate of encapsulated neural tissue is low. Nevertheless, if we find that even a few encapsulated grafts restore donor-specific rhythms, this would suggest that diffusible signals emitted from SCN grafts may be sufficient to support circadian function. It may be that the SCN in the intact animal communicates with the rest of the brain by redundant signals, either efferent fibers or diffusible signals. Alternatively, different circadian rhythms may be mediated by distinct output signals from the SCN.