HOST RESETS PHASE OF GRAFTED SUPRACHIASMATIC NUCLEUS - A 2-DG STUDY OF TIME-COURSE OF ENTRAINMENT

The object of the present experiment was to examine whether in an inta ct animal implanted with a hypothalamic graft, the phase of the host a nd grafted suprachiasmatic nucleus (SCN) would become synchronized. To this end, we first established the time at which daily fluctuations i n local cerebral glucose utilization were maximal in the SCN in our po pulation of adult hamsters. Next, we verified that rhythms of (C-14)2- deoxyglucose uptake could be measured on the day after birth in pups t hat were to provide donor tissue. Host and donor animals were housed i n opposite light:dark cycles. We then transplanted fetal SCN tissue in to the third ventricle of intact hamsters, placed the grafted animals in constant darkness with access to running wheels and examined the ph ase of metabolic activity in host and donor SCN. For several days afte r grafting, there was no circadian fluctuation in the metabolic activi ty of either the host SCN or of the grafted SCN. During this time, the circadian locomotor rhythms were not disrupted, suggesting that pacem aker activity was not interrupted. By day 14 after transplantation, me tabolic activity in the host SCN was elevated during subjective day an d host and donor SCN were in synchrony, invariably with the phase of t he host animal. We conclude that a signal from the host SCN resets the grafted SCN and not vice versa and that pacemaker cells communicate w ith each other rather than exerting independent effects on target site s.