AJP - Regulatory, Integrative and Comparative Physiology, Vol 264, Issue 3 615-R621, Copyright © 1993 by American Physiological Society
Melatonin injections affect circadian behavior and SCN neurophysiology in Djungarian hamsters
R. R. Margraf and G. R. Lynch
Department of Biology, Wesleyan University, Middletown, Connecticut 06459.
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.
