Two articles by Lawrence Morin, PhD Provide a Comprehensive and Integrative View of the Circadian Rhythm System and its Functions

Two recent articles by Lawrence Morin, PhD illustrate his proclivity for comprehension and integration. In “Neuroanatomy of the extended circadian rhythm system” published in Experimental Neurology, he surveyed what is known and, perhaps more instructively, what is not known about the anatomy of brain structures related to circadian rhythms. In “Nocturnal Light and Nocturnal Rodents: Similar regulation of disparate functions?” accepted for publication in the Journal of Biological Rhythms, he suggested a grand synthesis within chronobiology that integrates disparate areas of study which are typically treated in isolation.

In the article on neuroanatomy Dr. Morin presented the internal anatomy of the suprachiasmatic nucleus (SCN), the site of the body’s primary internal clock, by reviewing the distribution of cell types in the SCN and internal connections among its regions. Next he reviewed the current state of knowledge about the neuronal tracts leading to the SCN, focusing on the retinohypothalamic tract which brings information about light from the eyes to the SCN, and discussing five other tracts leading to the SCN from various regions of the brain. He reviewed pathways from the SCN to other regions, commenting on the methodological issues associated with tracing connections between one part of the brain and another. Then he provided a similar overview of the internal anatomy and external connections of the intergeniculate leaflet, which plays an important role in synchronizing internal rhythms with environmental conditions. Throughout the article he provided leads to areas for future research, among them the interactions between the sleep regulatory system and the circadian system, the extent to which individual cells within the SCN project to multiple targets, and the reciprocal relationships between the SCN and the intergeniculate leaflet.

In the article on the effects of nocturnal light on nocturnal rodents, Dr. Morin provided an extended argument for the thesis that various effects typically treated in isolation share a single light-activated, time-limited mechanism mediated through the circadian visual system. His central argument is that circadian clock phase shifts and suppression of locomotion in nocturnal rodents share several essential features. They are induced simultaneously by exposing nocturnal rodents to brief pulses of light. Both are mediated through photoreceptors connected to the SCN. Both effects last for a fixed interval even after the stimulus is removed. Both can be maintained by extending the exposure beyond the length of the interval and both are associated with higher levels of irradiance. In addition, he provided evidence that suppression of nocturnal pineal function, facilitation of an adrenal corticosterone surge and a drop in core body temperature can also be viewed as components of a whole body response to nocturnal light that involves multiple systems.