6.6 Hormones and hibernation

6.6.1 Melatonin

Syrian hamsters, which display pronounced circadian temperature fluctuations before hibernation, lose these circadian cycles on entry to hibernation, and start to regain them shortly before arousal. Cycles are distorted during the early recovery period, suggesting that the SON oscillator has either been switched off or de-synchronized in hibernation. Another monoamine, melatonin, is involved in making these adjustments. A hormone rather than a neurotransmitter, melatonin is secreted by the pineal gland on the dorsal side of the brain. Removal of this gland can cause disturbances in both the ‘clock’ and the ‘calendar’ settings of mammals. Melatonin travels both to the pre-optic area of the hypothalamus, where it can reset the set-point for Tb, and to the SON where it can adjust circadian and seasonal rhythms. Continuous infusion of melatonin inhibitor into the circulation from subcutaneous implants can lead to a decrease in the duration of bouts of torpor in hibernating ground squirrels (Figure 42) (Pitrosky et al., 2003). A second important function of melatonin is to inhibit sexual activity and the production of gonadal steroid hormones through the mediation of the pituitary gland, which is itself under the control of neurons within the hypothalamus. Both events appear to be under the principal control of the SON, since its destruction by electrolytic lesions leads to an impairment of circadian rhythms and the ability to hibernate. More recently it was shown that an inhibitor of melatonin, on the other hand, can decrease the duration of hibernation by reducing both the number of hypothermic bouts and the amount of lipid present in BAT.

Pitrosky, B. et al. (2003) Research report – S22153, a melatonin antagonist dissociates…, Behavioural Brain Research, 138. Elsevier Science

Figure 42 Hypothermic bouts, induced by short day laboratory regimes (10 h light and 14 h dark) with T _a of 5° C. The trace is T _b recorded by telemetry every 5 min for 18 weeks in a Syrian hamster. Trace (b) is from an animal with a slow-release implant containing the highly selective melatonin inhibitor S22153. Trace (a) is a control animal. Each row is temperature data for 48 h. Where there is a dark band, T _b is 38° C. The clear areas show periods where T _b is around 32° C. Note the prolonged periods of hypothermia (torpor) in the control animal, compared with trace (b) in which torpor bouts from an animal exposed to melatonin inhibitor for the duration of the experiment are drastically curtailed. These data provide evidence that torpor bouts are induced by melatonin