The potential role of LH in the initiation of the follicular phase in dogs was investigated by treating anoestrous bitches with highly purified pig LH (n = 16) or saline (n = 8) three times a day for 7 days, beginning in either early anoestrus (days 94-116 of the cycle) or mid-anoestrus (days 124-145). Treatment with LH induced pro-oestrus within 7 days (n = 16), and oestrus (n = 12) and fertile ovulations (n = 7) at 16 + 3 days, while pro-oestrus in bitches treated with saline did not occur until 46-166 days after the start of treatment. Six of the seven ovulating bitches whelped normal litters. The bitches in which pro-oestrus but not oestrus occurred were all treated in early anoestrus. During treatment, plasma oestradiol in bitches treated with LH increased from 8 + 2 pg ml-1 to 20 + 5 pg ml-1 within 1 day, and reached higher peak values (45 + 7 pg ml-1) (P < 0.05) than those observed in saline-treated controls (9 + 3 ng ml-1). These results demonstrate that LH treatment alone can terminate anoestrus by inducing a normal follicular phase in dogs. The results also suggest that, in normal cyclic bitches, anoestrus is the result of insufficient LH secretion, and that spontaneous pro-oestrus could be the result of increased LH secretion in the presence of already adequate concentrations of FSH.

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In this study, we evaluated the effects of applying Ciosin® (containing 0.25 mg/ml cloprostenol, an analogue of prostaglandin F2α (PGF2α)) at the time, five or seven hours after hypophysation resolving dose (experiment 1). In the experiment 2, at the time of hypophysation resolving dose, females were induced with 1.0 to 7.0 mL Ciosin /kg. Reproductive parameters and plasma levels of 17α-20β-dihydroxy-4-pregnen-3-one (DHP) and PGF2α were evaluated at the time of spawning for both experiments. Neither the timing of Ciosin application closer to the spawning nor its higher concentrations resulted in increased reproductive parameters or elevated PGF2α and DHP plasma levels. The absence of PGF2α peak at the time of ovulation in both experiments can be attributed to the rapid increase and subsequent decrease in exogenous PGF2α levels shortly after application. Interestingly, multivariate analysis in experiment 2 revealed two main clusters: one characterized by spawning failure, poor-quality spawns and low DHP levels; and the other by showing successful spawning, high fecundity and elevated DHP levels. Since it is known that the DHP binding to the nuclear progestin receptor induces prostaglandin receptor expression, these facts may explain why better reproductive performances were associated with higher endogenous DHP levels, but not with changes in exogenous PGF2α application. Although positive correlations were observed between DHP levels and good reproductive performance, the reasons behind the substantial variation in DHP levels among females subjected to the same treatments remain unknow. An emerging focus is to elucidate the pattern of expression of PGF2α receptors and its relationship with DHP in Piaractus mesopoatamicus.