Module 17: Circannual rhythms and seasonal depression

how do meadow voles differ in their rest-activity patterns throughout the year?

winter they are diurnal, while in the summer they become nocturnal

• because of testosterone changes in the seasons

what are the two distinct mechanisms mammals use to control their annual rhythms?

• endogenous circannual clock

• endogenous interval timer

what is the entrainment cue for circannual rhythms?

the day length

what made the ground squirrel study shown in Figure 4 ““heroic”?

it lasted 4 years under constant lab conditions, tracking hibernation in 5 groups of squirrels without environmental cues

what did researchers observe about hibernation cycles in constant conditions?

hibernation bouts recurred annually even without external seasonal cues, indicating an internal annual rhythm

why is the ground squirrel’s annual rhythm described as ‘circannual’?

because the rhythm is close to - but not exactly - a full year, showing it’s internally generated like a circadian rhythm, but on a yearly timescale

how do SCN lesions affect circannual rhythms in mammals?

circannual rhythms still occur after SCN lesions, but they can no longer be entrained to the annual light cycle (day length).

what role does the SCN play in photoperiod-based annual rhythms?

The SCN measures day length using light input from the retina; without it, animals can’t track seasons or exhibit photoperiodic rhythms

how does the brain measure day length, and what hormone reflects this?

the SCN uses light pulses marking sunrise/sunset to measure day length. This info controls melatonin duration, which reflects night length

what effect does melatonin have on seasonal reproduction, and does it differ between species?

Yes. In hamsters, melatonin is antigonadal (inhibits reproduction); in sheep, it is progonadal (stimulates reproduction

what pathway does melatonin use to suppress reproductive activity?

melatonin acts on receptors in the hypothalamus and pituitary to inhibit GnRH, reducing FSH and LH, and thus lowering testosterone and estrogen production

what is meant by the chronobiotic effect of melatonin?

refers to melatonin’s ability to shift circadian rhythms - advancing the clock when taken in the evening and delaying it when taken in the morning

can melatonin influence the circadian clock in the SCN?

yes, in some species melatonin receptors in the SCN allow it to shift or entrain the circadian phase, especially with repeated administration

how does melatonin help synchronize fetal rhythms?

melatonin crosses the placenta and helps entrain the fetal SCN to the mother’s circadian rhythm, preventing postnatal jet lag