Circannual rhythm

Melatonin is an indole-derived anterior pituitary hormone that causes downstream inhibition of a-MSH-induced melanogenesis. Melatonin is antiamnesic, synchronizes circadian and circannual rhythms and is metabolized to 5-methoxytryptamine. Melatonin acts via GPCRs MT1 and MT2 (which both couple through Gai and cAMP decrease). MT1 may also couple via Gao and Gaq to activate PLC (and hence increase cytosolic Ca2+) and via G Gy activation of PLA2. Melatonin can further interact with nuclear receptor superfamily orphan retinoid receptors RZR/ROR. Melatonin fluctuates with a circadian rhythm and is elevated in blood during the night. Melatonin is accordingly of social importance in relation to shift work and jet-lag. Melatonin and 5-methoxytryptamine occur in some plants (Table 5.8). 

MT-R) [Downstream inhibition of a-MSH-induced melanogenesis (at lOnM) antiamnesic, synchronizes circadian circannual rhythms]... 

The relationships between immune functions of the host and metastasis as a function of stress have been examined in mice bearing Lewis lung carcinoma subjected to rotational stress. Since some immune parameters were shown to display rhythmic variations, the occurrence of seasonal factors has been also taken into consideration. Indeed, the increase in metastasis number and volume caused by rotational stress varied in magnitude with a highly significant circannual rhythm, the acrophase coinciding with summer solstice. 

Whatever the reason is for the observed reduction in avoidance behavior in P. cinereus during winter, we hope our experience alerts other investigators to the possibility of seasonal or captivity and repeat-testing effects on the natural response patterns of amphibians, and that one way to detect possible effects is to record the same behavioral response through time with and without previous exposure to test stimuli. Further research is necessary concerning the influence of circannual rhythms on amphibian behavior in winter. 

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