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Overall, the data support a simple "dual site" hypothesis which proposes that melatonin acts independently in mediobasal hypothalamus to affect gonadotrophin secretion but acts in the pituitary gland to affect PRL secretion (Figure 8). Since mammalian species are remarkably similar in the photoperiodic control of PRL secretion it is likely that the melatonin-PRL control mechanism in the pituitary gland has been conserved during the evolution. This may be because all photoperiodic species share common requirement for an endocrine signal of summer. PRL is a pleiotrophic hormone inducing multiple effects on growth and metabolism and may thus consitute such a signal. PRL induces the development a summer pelage, for example, which is a common feature in mammals, and is clearly adaptive. The presence of high concentrations of melatonin receptors in the pars tuberalis in all photoperiodic mammals so far studied (54), is consistent with a generalised role of the pars tuberalis in the regulation of PRL secretion. In addition, the pars tuberalis is a conspicuous feature in the anatomy of the pituitary gland in reptiles and amphibians (17), and undergoes marked seasonal activation in spring in some species (e.g. frog: 65), thus the melatonin regulation of this tissue may have developed at an early phase of vertebrate evolution and constitutes an ancestral mechanism.

In contrast, the marked variation between species in the photoperiodic regulation of GnRH/gonadotrophin secretion indicates that there has been divergence in the neural mechanisms mediating the effect of melatonin on the gonadal cycle. The selective pressure to produce different responses to the melatonin signal must relate to the requirement to time conception and birth to optimise reproductive success, and this varies between species related to differences in body size, gestation length and ecology

Figure 7. Timing of the seasonal peak in the blood concentrations of PRL and FSH in wild (mouflon), feral (Soay) and domesticated breeds of sheep selected for a prolongation of the lambing seaon (31,45).

Figure 8. The "dual site hypothesis which proposes that the melatonin signal that transduces the effects of photoperiod acts: 1) in the mediobasal hypothalamus (MBH) via neural network to affect GnRH/gonadotrophin secretion and the gonadal axis, and (2) in the pituitary gland via the pars tuberalis (PT) to affect PRL secretion and the associated pleiotrophic responses.

Figure 8. The "dual site hypothesis which proposes that the melatonin signal that transduces the effects of photoperiod acts: 1) in the mediobasal hypothalamus (MBH) via neural network to affect GnRH/gonadotrophin secretion and the gonadal axis, and (2) in the pituitary gland via the pars tuberalis (PT) to affect PRL secretion and the associated pleiotrophic responses.

(8). Currently the phenotype of the melatonin responsive cells in the MBH is unknown, but the prediction is that different inhibitory/stimulatory neural pathways will be targeted by melatonin in the different species to mediate the species-specific reproductive response.

Healthy Sleep

Healthy Sleep

A Guide to Natural Sleep Remedies. Many of us experience the occasional night of sleeplessness without any consequences. It is when the occasional night here and there becomes a pattern of several nights in arow that you are faced with a sleeping problem. Repeated loss of sleep affects all areas of your life The physical, the mental, and theemotional. Sleep deprivation can affect your overall daily performance and may even havean effecton your personality.

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