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We investigated the effects of daily melatonin (MEL) injection on phase angle of entrainment, duration of wheel-running activity (alpha), and frequency of suprachiasmatic nuclei (SCN) neuronal discharge in the photo-nonresponsive phenotype of the Djungarian hamster, Phodopus sungorus. Photo-nonresponsiveness is characterized by an absence of physiological adjustments to short days (SD). With respect to wheel-running activity, photo-nonresponsive hamsters have a large negative phase angle of entrainment and a compressed alpha under SD. These hamsters also have a delayed nocturnal MEL pulse. These circadian differences are correlated with the daily profile of SCN neuronal activity. In the present experiments, daily MEL injections to photo-nonresponsive hamsters resulted in molt, gonadal regression, and expansion in alpha until entrainment to lights off. Vehicle-injected controls did not exhibit any of these responses. SCN neuronal activity patterns recorded from MEL-injected photo-nonresponders, but not vehicle-injected controls, resembled electrical activity profiles of photoresponsive hamsters. These results demonstrate that MEL induces "photoresponsiveness" in previously photo-nonresponsive hamsters, that MEL modifies circadian behavior to resemble that of photoresponders, and that MEL injections affect the circadian rhythm of SCN neuronal firing.
Margraf et al. (Mon,) studied this question.