PLASMA-CONCENTRATIONS OF PROLACTIN IN OVERTLY PSEUDOPREGNANT AFGHAN HOUNDS AND THE EFFECT OF METERGOLINE

The effect of metergoline, a 5-hydroxytryptamine (serotonin) antagonis t, on the plasma, concentrations of prolactin in overtly pseudopregnan t 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 f or 2-3 weeks and in three Afghan hounds that were not pseudopregnant a t the time of blood sampling. In the overtly pseudopregnant bitches th e plasma concentrations of prolactin before treatment (35.5 +/- 8.5 mu g l(-1)) were significantly higher than the plasma concentrations of prolactin of the three bitches that were not pseudopregnant (6.3 +/- 0 .5 mu g l(-1)); the latter values were similar to those of non-pseudop regnant beagle bitches during the total luteal phase. The six pseudopr egnant Afghan hounds were treated for 10 days with the antiserotoniner gic drug metergoline. At 2 h after the onset of treatment with metergo line, the mean plasma concentration of prolactin had decreased to 10.8 +/- 2.9 mu g l(-1). The plasma concentrations of prolactin continued to decline to 5.4 +/- 1.0 mu g l(-1) at 4 h and to 1.0 +/- 0.1 mu g l( -1) during treatment days 3-10. Signs of pseudopregnancy, such as swel ling of the mammary glands and digging, decreased during the treatment period. The treatment was associated with mild behavioural side effec ts such as whimpering and aggressiveness. These side effects are proba bly not related to suppression of prolactin but are due to a direct ef fect on serotoninergic pathways in the brain. It is concluded that hig h plasma concentrations of prolactin are associated with the developme nt and maintenance of pseudopregnancy. The serotonin antagonist meterg oline strongly suppresses plasma concentrations of prolactin in pseudo pregnant dogs and decreases the clinical signs of pseudopregnancy.