The HPD Soay ram model has also been used to assess whether melatonin acts in the pituitary gland to affect gonadotrophin secretion (38). This has involved treating HPD rams with a standard pulsatile infusion of GnRH to reactivate the gonadotrophin/testicular axis in animals exposed to long and short days (different melatonin signal, Figure 6). The replacement of GnRH is necessary since the HPD surgery destroys the terminals of the GnRH neurones and blocks the endogenous GnRH drive to the gonadotrophs which become quiescent (12,43). The chronic treatment with pulsatile GnRH for 10 weeks induced an increase in LH and FSH secretion, and full maturation of the testes in HPD rams, but there was no effect of the prevailing photoperiod on these reproductive responses. This was despite differences in prolactin secretion between the groups. These results support the conclusion that mela-tonin does not act in the pituitary gland to affect GnRH-induced gonadotrophin secretion, and the changes in PRL secretion which result from the local action of melatonin in the pituitary gland do not affect gonadotrophin secretion in the adult ram.
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