Melatonin injections affect circadian behavior and SCN neurophysiology in Djungarian hamsters

• Published in: The American journal of physiology Vol. 264; no. 3 Pt 2; p. R615
• Main Authors: Margraf, R R, Lynch, G R
• Format: Journal Article
• Language: English
• Published: United States 01.03.1993
• Subjects: Animals; Circadian Rhythm - drug effects; Cricetinae; Electrophysiology; Melatonin - pharmacology; Motor Activity - drug effects; Neurons - drug effects; Neurons - physiology; Phodopus; Photoperiod; Suprachiasmatic Nucleus - drug effects; Suprachiasmatic Nucleus - physiology
• ISSN: 0002-9513
• DOI: 10.1152/ajpregu.1993.264.3.R615

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.