Host resets phase of grafted suprachiasmatic nucleus : a 2-DG study of time course of entrainment

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