Serum melatonin profiles and endocrine responses of ewes exposed to a pulse of light late in the dark phase.

To determine whether effects of light pulses on the photoperiodic time measuring system involve changes in pineal gland function, melatonin profiles were determined in groups of ewes maintained under 10-h light, 14-h dark (10L:14D) or 10L:10D:1L:3D. Ewes exposed to 10L:14D had a significantly (P less than 0.01) longer duration of melatonin secretion (15.0 +/- 0.4 h, mean +/- SE) than ewes under 10L:10D:1L:3D (9.0 +/- 0.4 h). The 1-h pulse of light therefore acted as a dawn signal in the latter group. During a period of extended darkness imposed to study endogenous control of melatonin release, there was no change in the duration of elevated melatonin in control ewes (16.1 +/- 0.5 h), but a significant (P less than 0.05) lengthening occurred in pulsed ewes (13.2 +/- 1.4 h). PRL responses to a bolus iv injection of TRH (50 ng/kg BW) were significantly (P less than 0.01) smaller in control ewes (478 +/- 134 ng/ml) compared with pulsed ewes (1578 +/- 175 ng/ml), with responses in the latter group resembling those observed in ewes on long days. A 1-h pulse of light late in the dark phase, therefore, resulted in a melatonin pattern normally observed under long days in ewes, and this was associated with other endocrine functions also characteristic of sheep on long days. It is concluded that pulses of light modify activity of the pineal gland which in turn interacts with the photoperiodic time-measuring system via melatonin. The increase in duration of melatonin secretion observed in pulsed ewes under extended darkness suggests that the melatonin rhythm is under the control of two oscillators coupled to dusk and dawn, and that these oscillators interact more strongly when compressed by an interrupted dark phase.