Circadian change of dopaminergic neuron activity: effects of constant light and melatonin

Twenty-four hour profiles of tuberoinfundibular (TI), nigrostriatal and mesolimbic dopaminergic (DA) neuronal activities were assessed in estrogen-primed ovariectomized rats using DOPAC and DOPA levels in terminal regions of DA neurons. Significant decreases in DOPAC and DOPA levels in the median eminence were observed at 17.00 and 21.00 h, which corresponded with higher serum prolactin levels. DOPAC or DOPA levels in the striatum and nucleus accumbens were, however, significantly higher during the dark (21.00-05.00 h) phase. In rats kept under conditions of continuous light, no late afternoon decline in median eminence DOPA was observed; this decline could be reinstated by repeated injections of melatonin between 18.00 and 01.30 h for 3 days. In summary, circadian rhythms of central DA neurons were shown and melatonin may play an entraining role.