Implant and Lesion Experiments

The simplest interpretation of the experimental studies using the cerebral microimplants and the HPD model is that the melatonin signal that transduces the effects of photoperiod acts at different sites to regulate gonadotrophin and PRL secretion. The caudal region of the MBH appears to be the principle site for the control of gonadotrophin secretion, while the pars tuberalis/pituitary gland appears to be the principle site for the control of PRL secretion. The initial observation that microimplants of melatonin placed in the MBH simultaneously affects both FSH and PRL

Figure 6. Long-term changes in blood plasma concentrations of FSH in HPD Soay rams treated with a pulsatile infusion of GnRH for 10 weeks (or no treatment—c) while exposed to: a) short days (8L:16D, SD) and b) long days (16L:8D, LD). Note, a similar response irrespective of the ambient photoperiod (38).

Figure 6. Long-term changes in blood plasma concentrations of FSH in HPD Soay rams treated with a pulsatile infusion of GnRH for 10 weeks (or no treatment—c) while exposed to: a) short days (8L:16D, SD) and b) long days (16L:8D, LD). Note, a similar response irrespective of the ambient photoperiod (38).

secretion (Figure 3), is therefore explained by the diffusion of melatonin from the implant to the two closely adjacent control sites. In the MBH, the increased exposure to melatonin activates gonadotrophin secretion, while in the pituitary gland the same change supresses PRL secretion. This produces the inverse relationship between the two endocrine systems which is such a notable feature in sheep seasonality (e.g. Figure 1).

In the Syrian hamster, selective electrolytic lesions of the dorso-medial region of the MBH which shows enhanced binding of 2-(125I)iodomelatonin, blocks the inhibitory effect of short days, or appropriately programmed daily infusions of melatonin on gonadotrophin secretion and testicular activity (51). This surgical intervention, however, spares the inhibitory PRL response. Since the GnRH neurosecretory cells, that regulate gonadotrophin secretion are not located close to the putative target site in the MBH, it is presumed that melatonin acts via the neural networks which modulate pulsatile GnRH secretion. Dopaminergic, serotonergic and opioidergic pathways are particularly implicated (52,66). For the control of PRL in the pituitary gland, mela-tonin is thought to act via the pars tuberalis, thus in both the hypothalamus and pituitary gland the target cells expressing the melatonin receptor are one component of a cellular relay.

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