Our objective was to test the hypothesis that prolactin (PRL) 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 PRL concentration in control rams peaked at 103.4G22.1 ng/ml in late regression and then steadily declined (P!0.01) to 19.5G4.3 ng/ml, whereas PRL in treated rams was always %4.0 ng/ml. Suppression of PRL 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 PRL were unmasked when rams were given highly stimulative i.v. injections of GNRH (single 3 mg dose) and NIH-oLH-S24 (three 5 mg doses given 20 min apart) respectively. PRL insufficiency also appeared to slow down 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. We conclude that PRL is an important part of the intricate regulation of the pituitary-gonadal system in moderately seasonal DLS rams.

When the LH signal in the ram is changed from one of large and infrequent pulses to one of small and frequent pulses, the testes quickly become more responsive to LH and testosterone secretion is elevated, perhaps because the number and (or) binding affinity of testicular LH receptors have increased. An experiment was undertaken in the nonbreeding season (July) with 10 adult Dorset \m=x\Leicester \m=x\Suffolk rams that were about 3.5 years of age and 69 \ m=+-\2 kg in body weight. Rams were given injections into the jugular vein of either 5 \g=m\g NIH\x=req-\ LH-S24 (in 1 ml saline) or vehicle every 80 min for 6 days. LH treament produced a series of LH pulses that occurred three times more frequently and were 70% less in amplitude than pulses in the control rams, without causing mean LH concentration to increase. Endogenously produced LH pulses were not evident in the treated rams after LH injection began. The modified LH-pulse pattern elevated mean testosterone concentration by 150% (assessed on days 2 and 5), and caused the cumulative testosterone response to LH pulses, estimated by multiplying testosterone-pulse amplitude by frequency per 6 h, to increase progressively by 180% (days \m=-\2 through 5). Enhanced testicular steroidogenic activity, presumably due to greater enzymatic activity and cholesterol availability within Leydig cells, was not associated with increases in either the concentration or affinity of LH\x=req-\ binding sites in the testis (assessed on days 3 and 6).The testes of adult rams secrete four to five times more testosterone in the breeding season than in the nonbreeding season (1, 2). Increased testos¬ terone secretion in the breeding season is the result of high-frequency, low-amplitude LH releases stimulating highly responsive testes (3). Increases in the number of Leydig cells (4), and (or) in the number of LH receptors per Leydig cell (5) could explain why the small LH pulses of the breeding season are able to trigger very large elevations in testosterone secretion. The LH-pulse pattern itself seems to be involved in maintaining testicular re¬ sponsiveness in that the elevation in testosterone secretion in response to an LH release is propor¬ tionately greater when LH releases are small and frequent (6).We hypothesized that establishing a high-fre¬ quency, low-amplitude pattern of LH pulses in rams would enhance the responsiveness of the testes to LH by increasing the number and (or) binding affinity of LH receptors. In a short-term study in the nonbreeding season, adult rams were given a series of LH injections to produce an LHpulse pattern resembling that of the breeding season. Changes in the pattern of testosterone se¬ cretion were assessed in relation to possible changes in gonadotropin receptors within the testis, as well as in FSH and PRL secretion which in rodent spe¬ cies also affect testosterone secretion (7,8).

Two previously uncharacterized radioactive estrogen conjugates, 17beta-estradiol-17-beta-D-glucuronide (3-hydroxyestra-1,3,5(10)-trien-17beta-D-glucopyranosiduronate) and 17alpha-estradiol-17beta-D-glucuronide (3-hydroxyestra-1,3,5(10)-trien-17alpha-yl-beta-D-glucopyranosiduronate), have been identified in small but significant amounts in avian urine and in a ratio of approximately 2:1 after intramuscular injection of [4-14C]estrone.

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