Effects of pinealectomy and subsequent melatonin implants on activity rhythms in the house finch (Carpodacus mexicanus)

Summary

Activity patterns were analyzed in house finches which were pinealectomized and subsequently implanted with melatonin capsules. Most pinealectomized house finches showed loss of circadian perch-hopping rhythms in constant darkness (DD). Total amount of activity in DD was about 175% higher in pinealectomized than in intact finches. In a light-dark cycle (LD) with 8 h L and 16 h D, activity in pinealectomized birds was redistributed such that an increase in activity during phase lead counter-balanced a decrease during L. Mean duration of phase lead increased from 1/2 h in intact finches to about 3 1/2 h in pinealectomized birds.

Melatonin capsules implanted in pinealectomized finches shortened phase lead in LD and lengthened phase trail, suggesting a role for melatonin levels in setting the phase of the activity rhythm apart from phasic changes in melatonin release. Melatonin did not restore circadian rhythmicity in pinealectomized finches and instead was associated with a decline in signs of residual, damped rhythmicity. Amount of 24-h activity was positively correlated with percentage of total activity occurring during daytime; both of these were inversely correlated with melatonin dosage. In pinealectomized finches, the amount of DD activity correlated positively with phase lead activity in LD, but not with amount of activity during L. The data are interpreted in terms of the hypothesis that phase lead represents a component of activity rhythms governed by a damped oscillator of non-pineal location but influenced by melatonin.