Sanford L, Baker S
Prolactin regulation of testosterone secretion and testes growth in DLS rams at the onset of seasonal testicular recrudescence. [JOURNAL ARTICLE]
Reproduction 2009 Sep 15.
In this study, we tested the hypothesis that prolactin acts at both the pituitary and testis levels to regulate testosterone secretion in the adult ram. The focus was on the mid-regression to mid-redevelopment stages of a photoperiod-condensed "seasonal" testicular cycle. DLS rams (six per group) were given daily s.c. injections of bromocriptine (4 mg) or vehicle during the entire period. Serum prolactin concentration in control rams peaked at 103.4 +/- 22.1 ng ml-1 in late regression and then steadily declined (P<0.01) to 19.5 +/- 4.3 ng ml-1, whereas prolactin in treated rams was always </=4.0 ng ml-1. Suppression of prolactin tended (P<0.10) to increase the amplitude of natural LH pulses (transition stages) or reduce the number of LH receptors in the testis (regressed stage), although neither change disturbed testosterone levels in peripheral blood. These subtle changes were accompanied by significant (P<0.05) alterations in the capability of the pituitary to release LH (85% more) and of the testes to secrete testosterone (20% less). These effects of prolactin were unmasked when rams were given highly stimulative i.v. injections of GnRH (single 3 microg dose) and NIH-oLH-S24 (three 5 microg doses given 20 min apart), respectively. Prolactin insufficiency also appeared to slow the "seasonal" rise in FSH secretion and slightly delayed (2 weeks) the times when the testes began to grow and were first significantly (P<0.05) enlarged from the regressed state. In conclusion, prolactin is an important part of the intricate regulation of the pituitary-gonadal system in moderately seasonal DLS rams.
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