Investigation Into the Regulation of the Circadian System by Dopamine and Melatonin in the Adult Siberian Hamster (Phodopus sungorus)

Dopamine and melatonin have both been implicated in mediating maternal influences on the developing circadian system of altricial rodents. The aim of these studies was to investigate their role in the entrainment of the circadian system of the adult Siberian hamster (Phodopus sungorus). In-situ hybridization revealed that D₁-dopamine receptor (D₁-R) mRNA was expressed in the adult suprachiasmatic nucleus (SCN) at levels comparable to neonates. As dopamine has been postulated to mimic photic stimulation during early development, experiment 1 compared the effects of a D₁-R agonist and a light pulse on free-running wheel running rhythms in hamsters maintained in constant dim red light. A phase response curve to light was generated, revealing clear phase delays early in the subjective night, and large phase advances in the late subjective night. However, the D₁-R agonist (SKF 81297, 2 mg/kg, s.c.) did not produce consistent phase shifts at any circadian phase. Experiment 2 tested the ability of this dopaminergic agonist to modulate photic responses of the circadian system. Free-running animals were pre-treated with SKF 81297 (2 mg/kg, s.c.) 30 min before a 15 min light pulse given early or late in the subjective night. This agonist had no effect on the magnitude of phase shifts at either circadian time. In experiment 3, light pulses at CT13–15 induced expression of the immediate early gene c-fos in the SCN, as assessed by immunocytochemistry for the protein product. In contrast, SKF 81297 (2 mg/kg, s.c.) at the same phase did not induce c-fos in the SCN, despite marked c-fos induction in the caudate-putamen, nor did it affect photic induction of c-fos in the SCN. To investigate whether dopamine might be involved in nonphotic regulation of the circadian system in adult hamsters, experiment 4 compared the response of free-running hamsters to a series of injections of SKF 81297 (2 mg/kg, s.c.) or melatonin (1 mg/kg, s.c.), since melatonin receptor expression in the SCN also persists into adulthood. Animals were treated every 23.5 h for 6 days. The serial injections of melatonin produced cumulative phase advances of up to 3 h when delivered in late subjective day, but not when presented in late subjective night. Hamsters did not respond to SKF 81297 or vehicle treatment at either circadian phase. Moreover, pre-treatment with the dopaminergic agonist did not affect the phase-advancing effects of melatonin when both were given in the serial injection protocol. These results demonstrate clear phase-dependent effects of light pulses and melatonin on circadian rhythms in Siberian hamsters, but suggest that D₁-Rs in the SCN no longer modulate photic or melatonin-dependent entrainment pathways in the adult.