The timed infusion paradigm for melatonin delivery: What has it taught us about the melatonin signal, its reception, and the photoperiodic control of seasonal responses? J. Pineal Res. 1993: 15: 161-190. Abstract: This review summarizes the evidence showing that the duration of the nocturnal secretory profile of pineal melatonin (MEL) is critical for eliciting seasonally appropriate reproductive physiological and behavioral responses in mammals. We review experiments using the timed infusion paradigm (TIP) to deliver MEL either systemically or centrally to pinealectomized hamsters and sheep. In this paradigm, MEL is infused, usually once daily, for a specific number of hours and at a predetermined time of day. This experimental strategy tests most directly those features of the MEL signal that are necessary to trigger photoperiodic responses. The data suggest that the duration of the MEL stimulation is the critical feature of the MEL signal for both inhibitory and stimulatory effects of the hormone on the photoperiodic control of reproductive development in juvenile Siberian hamsters, and for the photoperiodic control of reproductive and metabolic responses in adult Siberian and Syrian hamsters and sheep. The use of the TIP reveals the importance of the frequency of the signal presentation of MEL and suggests the importance of a period of low-to-absent circulating concentrations of the hormone. The TIP also reveals that the characteristics of the MEL signal that regulate male sexual behavior are similar to those that are critical for reproductive and metabolic responses in Syrian hamsters. We summarize the locations of possible functional MEL target sites identified by combining the TIP with traditional brain lesion techniques. Evidence from such studies suggests that the integrity of the suprachiasmatic nucleus (SCN) region in Siberian hamsters and the anterior hypothalamus in Syrian hamsters is necessary for the response to short-day MEL signals. The TIP has been used to deliver MEL to putative target sites for the hormone in the brain of juvenile and adult Siberian hamsters. The results of these preliminary experiments suggest that the regions of specific MEL binding in this species, especially the SCN, are effective sites where MEL may stimulate short-day-type responses. In contrast, results from intracranial application of MEL in sheep suggest the medial basal hypothalamus as a critical site of action. Finally, we also discuss potential applications of the TIP for identification of brain MEL target sites, understanding of other photoperiodic phenomena and responses, and resolution of the cellular/molecular basis underlying the reception and interpretation of MEL signals. It is our collective view that the T P has played, and will continue to play, a pivotal role in elucidation of the function of MEL in the photoperiodic control of seasonal mammalian responses and that the duration of the MEL signal is the critical parameter of the nocturnal secretion profile of the hormone for the photoperiodic control o...

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This study was conducted to determine whether the pineal indoleamine melatonin mediates the effects of photoperiod on the capacity of estradiol to inhibit LH secretion in the ewe. Patterns of serum melatonin were characterized in pineal-intact ovariectomized ewes treated with sc Silastic estradiol implants and exposed to 90-day alternations between long and short photoperiods. High fluctuating levels of serum melatonin were found during the night, with the duration of elevated serum levels corresponding to the length of the dark period. Transfer from long to short photoperiods caused a rapid change in the duration of nightly melatonin secretion and reduced the negative feedback potency of estradiol upon LH secretion during the natural anestrous season. In pinealectomized ewes, the night-time rise of melatonin was absent, and transfer from long to short days failed to reverse the capacity of estradiol to inhibit LH secretion during anestrus. Nightly infusions of melatonin restored patterns of this indoleamine similar to those observed in pineal-intact ewes exposed to the 90-day alternation between long and short days. The melatonin infusions also restored the reproductive response to the inductive photoperiod: in every ewe, the negative feedback effects of estradiol upon LH secretion were diminished after transfer from long to short days. The amplitude and latency of this escape matched those of pineal-intact animals. We conclude that the pineal mediates the reproductive response of the ewe to inductive photoperiods through its daily rhythm of melatonin secretion.

Although dependent on the integrity of a central pacemaker in the suprachiasmatic nucleus of the hypothalamus (SCN), endogenous daily (circadian) rhythms are expressed in a wide variety of peripheral organs. The pathways by which the pacemaker controls the periphery are unclear. Here, we used parabiosis between intact and SCN-lesioned mice to show that nonneural (behavioral or bloodborne) signals are adequate to maintain circadian rhythms of clock gene expression in liver and kidney, but not in heart, spleen, or skeletal muscle. These results indicate that the SCN regulates expression of circadian oscillations in different peripheral organs by diverse pathways.A wide variety of physiological events and behaviors exhibit pronounced endogenous daily (circadian) rhythms. Experimental destruction of the suprachiasmatic nucleus of the hypothalamus (SCN) results in arrhythmicity that is reversed by neurotransplantation of this structure. Persistence of rhythmicity depends on the operation of transcriptional-translational feedback loops among the products of critical genes, including Per1, Per2, and Bmal1 within the SCN. Circadian oscillations of Per1, Per2, and Bmal1 also occur in a variety of peripheral organs. Evidence has accumulated for both neural and humoral control of peripheral rhythms. For example, serum shock initiates oscillations of mPer1 expression in cultured hepatocytes and HeLa cells (1), and implants of fibroblasts that receive no innervation adopt the circadian phase of the host (2). Endocrine signals, including glucocorticoids, angiotensin II, and retinoic acid can shift the phase of peripheral clock gene expression (3-5). In addition, metabolites such as glucose may directly entrain peripheral oscillators or act indirectly to induce endocrine signals that regulate circadian rhythms of gene expression (6). On the other hand, the autonomic innervation of peripheral organs provides a potential pathway for entrainment (7-9). For example, SCN lesions that compromise catecholamine rhythms eliminate oscillations of clock gene expression in mouse liver (10).Neural and endocrine pathways for peripheral entrainment are not mutually exclusive. Furthermore, different organs may vary in their dependence on one or another entraining signal. Such variation is not without precedent. Whereas behavioral rhythms appear to depend on humoral outputs of the SCN (11), endocrine rhythms may rely on axonal projections (12).The technique of parabiosis offers unique advantages for investigation of the importance of blood-borne cues in the control of a variety of physiologic systems. This approach has been exploited in the study of cockroach circadian rhythms (13). Despite its useful application to study of metabolic signals in mice (14, 15), the effect on peripheral circadian rhythms of establishing vascular exchange without neural communication has not previously been investigated in vertebrates. We now report that parabiotic linkage of SCN-lesioned mice to intact partners reinstates circadian rhythmicity in some, but not o...

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