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The effects of metergoline, a 5-hydroxytryptamine (serotinin) antagonist, on the plasma concentrations of prolactin in overtly pseudopregnant Afghan hounds and on the clinical symptoms of overt pseudopregnancy were studied. Plasma concentrations of prolactin and progesterone were determined in six Afghan hounds with signs of overt pseudopregnancy for 2-3 weeks and in three Afghan hounds that were not pseudopregnant at the time of blood sampling. In the overtly pseudopregnant bitches the plasma concentrations of prolactin before treatment (35.5 +/- 8.5 micrograms l-1) were significantly higher than the plasma concentrations of prolactin of the three bitches that were not pseudopregnant (6.3 +/- 0.5 micrograms l-1); the latter values were similar to those of non-psueodopregnant beagle bitches during the total luteal phase. The six pseudopregnant Afghan hounds were treated for 10 days with the antiserotoninergic drug metergoline. At 2 h after the onset of treatment with metergoline, the mean plasma concentration of prolactin had decreased to 10.8 +/- 2.9 micrograms l-1. The plasma concentrations of prolactin continued to decline to 5.4 +/- 1.0 micrograms l-1 at 4 h and to 1.0 +/- 0.1 microgram l-1 during treatment days 3-10. Signs of pseudopregnancy, such as swelling of the mammary glands and digging, decreased during the treatment period. The treatment was associated with mild behavioural side effects such as whimpering and aggressiveness. These side effects are probably not related to suppression of prolactin but are due to a direct effect on serotoninergic pathways in the brain. It is concluded that high plasma concentrations of prolactin are associated with the development and maintenance of pseudopregnancy. The serotonin antagonist metergoline strongly suppresses plasma concentration of prolactine in pseudopregnant dogs and decreases the clinical signs of pseudopregnancy.

Suppression of the prolactin level in the peripheral blood by chronic treatment with bromocryptin throughout the oestrous cycle of the cow, had no effect on: 1) LH concentrations during the cycle, 2) progesterone secretion of the corpus luteum, 3) duration of the early luteal phase, the luteal phase and the period of luteal regression, 4) oestradiol secretion during the follicular phase, 5) number and quality of follicles greater than 2 mm, 6) number of binding sites on granulosa cells and thecal tissue of the preovulatory follicle and 7) the concentration of oestradiol, androstenedione and progesterone in the follicular fluid of the preovulatory follicle. Follicles from bromocryptin treated and control heifers did not possess specific binding sites for prolactin. It is concluded that prolactin is not involved in the regulation of the oestrous cycle in cattle.

Both prolactin and the suckling stimulus are involved in the suppression of LH levels during lactation in the sow. Reduction of the prolactin level by treatment with bromocriptine will result in a significant increase of the LH level, followed by a further significant increase after weaning. The suppression of LH release by the two factors is caused by a blockade at the hypothalamic level. The LH-response to an injection of LHRH is not affected by the presence or absence of prolactin and the suckling stimulus and therefore rules out the pituitary as the primary target organ.

The development of pre-ovulatory bovine follicles from the onset of oestrus until ovulation may be divided into four phases on the basis of the patterns of the steroid levels in the follicular fluid. Changes in the morphology of the follicular wall and the oocyte coincide with these patterns. Prior to the pre-ovulatory LH surge steroid synthesis is directed towards the production of oestradiol-17 beta, and androstenedione is the major substrate of aromatase. After the LH peak, steroid synthesis switches to the production of progesterone shortly prior to ovulation. The membrana granulosa cell-layer shows morphological luteinization at the time. The elevated levels of LH and FSH observed in the follicular fluid suggest that changes in the micro-environment of the maturing bovine oocyte are regulated by these gonadotrophins.