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Melatonin chimeras alter reproductive development and photorefractoriness in Siberian hamsters
B J Prendergast1, I Zucker, S M Yellon
1Department of Psychology, University of California, Berkeley 94720, USA.
Abstract:
Nightly melatonin (MEL) durations > 8 h provoke gonadal regression and decreases in body mass, whereas signals < 7 h stimulate gonadal and somatic growth in male Siberian hamsters. The authors sought to determine the minimum frequency of short MEL signals sufficient to induce the long-day phenotype in several photoperiodic traits. D,L-propranolol (hereafter propranolol) injections shortened MEL signals on the night of treatment without altering MEL on the subsequent night; this permitted interpolation of short MEL signals at variable frequencies against a background of long MEL signals (chimeras). Hamsters kept in short days (10 h light/day, 10L) were injected with propranolol 6 h after dark onset for 28 consecutive weeks beginning at 30 days of age (Week 0) either every other day or once every 3, 6, or 9 days. Control animals were injected with saline or with propranolol during the light phase or were transferred to long days (16L) at Week 0. Hamsters in 16L underwent rapid gonadal development and increases in body mass and displayed summer pelage color, as did hamsters treated with propranolol every other day. Animals treated with propranolol less frequently than every other day uniformly maintained undeveloped gonads and winter-like body weights, but pelage color became proportionately darker with increased frequency of propranolol treatments. The onset of spontaneous testicular development in 10L was unaffected by propranolol injections. After termination of injections at Week 28, testicular regression was not observed in most 10L animals that previously had undergone spontaneous testicular development; however, 40% of hamsters that had been injected with propranolol every 3rd night did manifest the winter phenotype after Week 28. In an alternating sequence, short MEL signals completely override long signals and induce the summer phenotype. Threshold frequencies differ for MEL stimulation of long-day pelage and gonadal phenotypes. The timing and development of refractoriness to MEL does not depend in any simple manner on the number of long MEL signals or on the accumulation of a reaction product produced by long, and depleted by short, MEL signals.
Insights
Short nightly melatonin signals, when frequent enough, can induce long-day adaptations in Siberian hamsters. The frequency required differs for pelage and gonadal changes, indicating distinct thresholds for these photoperiodic traits.
Area of Science:
- Endocrinology
- Chronobiology
- Animal Physiology
Background:
- Melatonin (MEL) duration influences seasonal reproduction and growth in male Siberian hamsters.
- Longer MEL durations (>8h) induce gonadal regression and body mass decrease, while shorter durations (<7h) stimulate growth.
- Understanding the minimum frequency of short MEL signals to induce long-day phenotypes is crucial for photoperiodism research.
Purpose of the Study:
- To determine the minimum frequency of short melatonin signals required to induce the long-day phenotype in Siberian hamsters.
- To investigate the differential effects of MEL signal frequency on various photoperiodic traits, including gonadal development, body mass, and pelage color.
Main Methods:
- D,L-propranolol injections were used to artificially shorten nightly MEL signals at varying frequencies (every other day, every 3, 6, or 9 days) in hamsters housed in short days (10L).
- Control groups included saline injections, light-phase propranolol injections, and transfer to long days (16L).
- Photoperiodic traits such as gonadal size, body mass, and pelage color were assessed over 28 weeks.
Main Results:
- Frequent short MEL signals (every other day) induced rapid gonadal development, body mass increase, and summer pelage, mimicking long-day conditions (16L).
- Less frequent short MEL signals maintained winter-like body mass and undeveloped gonads, with pelage color darkening proportionally to treatment frequency.
- Distinct threshold frequencies were observed for the induction of long-day pelage and gonadal phenotypes.
Conclusions:
- Short melatonin signals can override long melatonin signals to induce the summer phenotype, with threshold frequencies varying for different traits.
- The frequency of short MEL signals, not just the number of long signals, is critical for regulating photoperiodic responses.
- Refractoriness to melatonin does not appear to depend on the simple accumulation of a reaction product.