THE CONTROL OF CIRCADIAN GLYCOGEN RHYTHMS

The photocontrol of biological in higher animals reaches all extensions of the organism. The mechanism is so deeply integrated within the living system that even the metabolism is governed by the external periodic input to our body. We know little about the communication channels which link the optic sensors to the metabolic effectors. We may guess that they pass through opticalhypothalamic connections, whether direct or indirect, to the endocrine and autonomous nervous pathways which govern the enzymatic servomechanisms in our cells. Rhythms appear on all leve1ss1-34 of this communication system, which brings up the question of how the metabolic rhythms may be coupled to other periodic functions and inputs of the body. The chemical activity is also morphologically bound both to intracellular mitochondriae and to different cell systems ; we may thus wish to compare the rhythms in different organs. Since some parts of the communication system and metabolic mechanisms develop rather late in the phylogeny and ontogeny, one would expect a feeble rhythm control in the embryo, though slowly maturing before and after birth. Several of the mentioned aspects have been investigated on the circadian liver glycogen rhythm. The 24-hour liver glycogen rhythm was first observed by F ~ r s g r e n ~ ~ ~ ' in 1927. The discovery was instrumental in forcing the physiologists to realize the fundamentality of the biological rhythms. Similar variations in e.g. temperature and activity were well accepted, owing to the ease of observation, but they were more regarded as reflex responses to light and feeding, confined to the milieu externe of the organism. Now, rhythms were suddenly found right in the cells, in the milieu interne, and they even stayed during starvation. Since Forsgren's first description, numerous studies of the liver glycogen rhythm have been made, under various conditions and in different animals.