Serotonin antagonist-induced lowering of prolactin secretion does not affect the pattern of pulsatile secretion of follicle-stimulating hormone and luteinizing hormone in the bitch.

Dopamine agonists decrease plasma prolactin concentration and shorten the duration of anoestrus in the bitch. In order to determine whether this shortening results from decreased prolactin release or is due to another dopamine agonistic effect on the pulsatile release of FSH and LH, eight anoestrous beagle bitches were treated with a low dose of the serotonin antagonist metergoline (0.1 mg per kg body weight, twice daily) starting 100 days after ovulation. Six-hour plasma profiles of LH and FSH were obtained 7 days before, immediately before, 1 week after, and then at 2-week intervals after the start of the treatment with the serotonin antagonist until signs of pro-oestrus appeared. Plasma prolactin concentration was measured three times weekly from 75 to 142 days after ovulation and thereafter once weekly until the next ovulation, and was observed to decrease significantly after the start of treatment. The length of the interoestrous interval in the treated dogs was, however, not different from that in the preceding pretreatment cycle or from that in a group of untreated bitches. During the first weeks of treatment no changes were observed in the pulsatile plasma profiles of FSH and LH. Four weeks after the start of the treatment with the serotonin antagonist there was an increase in the mean basal plasma FSH concentration and the mean area under the curve for FSH, without a concurrent increase in LH secretion. The increase in FSH secretion continued until late anoestrus. In conclusion, the serotonin antagonist-induced lowering of plasma prolactin concentration was not associated with shortening of the interoestrous interval. The plasma profiles of LH and FSH were similar to those observed during physiological anoestrus, but different from those observed during anoestrus shortened by treatment with a dopamine agonist. Hence the prematurely induced oestrus observed during administration of dopamine agonists cannot be explained by a decreased plasma prolactin concentration but must be due to some other dopamine agonistic effect, probably increased FSH secretion. The observations in this study further strengthen the hypothesis that an increase in circulating FSH is essential for ovarian folliculogenesis and consequently the termination of anoestrus in the bitch.