Melatonin Chimeras Alter Reproductive Development and Photorefractoriness in Siberian Hamsters

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.

Testicular development in Siberian hamsters depends on frequency and pattern of melatonin signals

“…As in previous experiments, melatonin signal integration over the course of several weeks was trait specific (7,30). In contrast to the categorical effects of the various propranolol treatments on the gonadal axis (i.e., either fully stimulatory or inhibitory), pelage coloration exhibited a graded dependence on short melatonin signal frequency: pelage color of PR3-treated hamsters was intermediate between that of PR2-and Sal-treated hamsters.…”

“…We concluded previously that in hamsters with regressed or undeveloped gonads short melatonin signals stimulate gonadal development only if spaced no more than 48 h apart, i.e., that the number of successive long-duration melatonin signals determines reproductive responses to intervening short melatonin signals (30). The present data contradict this conjecture.…”

“…Paradoxically, inductive effects on reproductive development are evident when short melatonin signals are presented every other day against a background of long melatonin signals; i.e., melatonin-responsive neuroendocrine targets bridge a gap of 48 h between short melatonin signals when long-duration melatonin signals are interspersed (30). These data suggested that the presence of any melatonin (even long-duration signals) at approximately daily intervals may serve to maintain reproductive responsiveness to short-melatonin signals; this experiment did not, however, assess whether the pattern of melatonin signals contributes to melatonin signal processing.…”

Interval Timer Control of Puberty in Photoinhibited Siberian Hamsters

“…1A). Melatonin production in PROP-treated hamsters remains suppressed for the remainder of the night and recovers its normal pattern the next night (Prendergast et al, 1998). Timed PROP administration given 6 h after dark onset transforms a longduration melatonin signal (~10 h) to a short melatonin signal (~5 h) that resembles that produced in long days (Fig.…”

“…Refractoriness to melatonin does not depend in any simple manner on the number of prior longduration melatonin signals (Prendergast et al, 1998). Once the interval timer is triggered, subsequent disruption of long-duration melatonin signals has variable effects on the course of gonadal growth, depending on the stage of development in which the signals are altered (Anchordoquy and Lynch, 2000).…”