Development of circadian rhythms in rats with lesions of serotonergic system

In order to assess the role of the serotonergic system in the development of overt circadian rhythms in the rat, serotonin neurons in the brain were destroyed either by thermocoagulation of the median raphe (MRL) or by an intracerebroventricular injection of the neurotoxin, 5,7-dihydroxytryptamine (DHT). The reductions in serotonin content induced by two manipulations with MRL and DHT were 41% and 100% in the striatum, 40% and 66% in the hypothalamus, and 62% and 88% in the hippocampus, respectively. Neither manipulation eliminated the expression of circadian rhythms in corticosterone (CS) secretion, locomotor activity and drinking behavior, and changed the phase relationship in the overt CS rhythm. Also, 5,7-DHT treatments did not significantly affect the free-running period in locomotor activity. However, the emergence of CS circadian rhythm was delayed for one week in both MRL and DHT groups compared to the intact control ones. These results suggested that a serotonergic system would not be essential for the generation of the endogenous rhythm and the photoentrainment of overt circadian rhythms, but seems to participate in the only development of CS rhythms during the early stage of life.