Abstract
The avian circadian system is regulated by four circadian components: the pineal gland, the suprachiasmatic nuclei (SCN) of the hypothalamus, the retinae and deep brain extraocular photoreceptors (EOP). The interactions among these oscillators are hypothesized to be crucial for the generation of avian circadian rhythms in the neuroendocrine loop model (Cassone and Menaker, 1984). This thesis focuses on testing the neuroendocrine loop model by using multiple approaches including behavioral, neuroendocrine, electrophysiological, and mathematical techniques. The main results are: (1) Mathematical model simulation indicates that stable oscillation can be reached by co-inhibitory oscillators that are 180° out of phase. This mathematical model defines the damping coefficients of each component. (2) By measuring behavioral activity and brain metabolism, the sparrow vSCN have been found to be damped oscillators, i.e., in the uncoupled condition (pinealectomy), the vSCN oscillation damp out in constant darkness (DD) (Chapter IV). vSCN damping can be reversed by rhythmic administration of exogenous melatonin (Chapter V) via inhibition. (3) Melatonin binding sites are widely distributed in avian visual structures with the binding in those structures exhibiting circadian rhythms with the antiphase of the endogenous melatonin rhythm. The melatonin binding rhythm appears to be regulated by a circadian clock (Chapter IV, Appendix I). (4) Melatonin synchronization occurs through entrainment rather than masking, and this entrainment depends upon the SCN (Chapter VI). (5) Melatonin also influences circadian rhythmicity of visual sensitivity at the retinal level (electroretinogram, ERG, Chapter VII). (6) The study of guanethidine on locomotor rhythm suggests that the SCN affect the output pathways of pineal melatonin synthesis (Appendix II). These studies support the neuroendocrine loop model of avian circadian organization. The neuroendocrine loop model may extend to the mammalian circadian system with significant modification of the damping coefficients. In mammals, pineal damping would be so high that the mammalian pineal almost completely relies on SCN output. The SCN of mammals, on the other hand, would be dominant pacemakers with very low damping.
Lu, Jun (1994). The multi-oscillatory circadian system in the house sparrow, Passer domesticus. Texas A&M University. Texas A&M University. Libraries. Available electronically from
https : / /hdl .handle .net /1969 .1 /DISSERTATIONS -1554708.