RELATIONSHIP OF SECRETION OF GNRH IN-VITRO TO CHANGES IN PITUITARY CONCENTRATIONS OF LH AND FSH AND SERUM CONCENTRATIONS OF LH DURING LACTATION IN SOWS

The objectives of this study were (1) to determine whether release of GnRH in vitro was related to concentrations of LH and FSH in serum and-pituitaries and to oestradiol in serum of sows at mid- or late lactation, and (2) to determine whether weaning at mid-lactation changes concentrations of these hormones from values expected at mid-lactation to values expected at late lactation. Multiparous crossbred sows were killed on day-14 (n = 5) or day 28 (n = 5) of lactation or on day 14 post partum after litters had been weaned on day 10 (n = 5). Blood samples were taken every 6 h for 4 days before sows were killed, and the preoptic suprachiasmatic area, medial basal hypothalamus, stalk median eminence, anterior pituitary and ovaries were collected at slaughter. Sows killed on day 14 after having their litters weaned on day 10 had more (P < 0.01) preovulatory follicles (> 6 mm in diameter) than lactating sows killed on day 14 or 28 (7.0 +/- 1.2 versus 0.2 +/- 0.1 and 1.5 +/- 0.8, respectively). Concentrations of LH, FSH and oestradiol in serum during 90 h before slaughter were greater (P < 0.05) in weaned sows and lactating sows killed on day 28 than in lactating sows killed on day 14 (LH: 0.72 +/- 0.3 and 0.68 +/- 0.3 versus 0.45 +/- 0.2 ng ml-1; FSH: 39.3 +/- 2.7 and 5 7.3 +/- 4.0 versus 28.8 +/- 1.6 ng ml-1; oestradiol: 10.0 +/- 1.6 and 5.6 +/- 0.7 versus 2.7 +/- 0.2 pg ml-1, respectively). Weights of anterior pituitaries did not differ among groups, but concentrations of LH in anterior pituitaries increased (P < 0.05) and concentrations of FSH in pituitaries tended to increase although the increase was not significant (P < 0.10) from day 14 to day 28 of lactation, whereas pituitary concentrations of gonadotrophins in weaned sows were intermediate (LH: 0.45 +/- 0.2, 0.94 +/- 0.2 and 0.65 +/- 0.1 mg g-1; FSH: 8.7 +/- 1.2, 13.8 +/- 2.6 and 10.9 +/- 1.2 mg g-11 for day 14 lactating, day 28 lactating and weaned groups, respectively). Content of GnRH in the preoptic area, and medial basal hypothalamus did not differ among groups. Content of GnRH in stalk median eminence was lower (P < 0.05) in weaned sows, but was similar among sows killed on day 14 or day 28; however, the proportion of residual GnRH released by the stalk median eminence in response to a challenge with K+ in vitro was greater (P < 0.05) for sows at day 14 than for weaned sows and sows at day 28 (21.5 +/- 4.5 versus 13.7 +/- 2.8 and 7.0 +/- 1.6%, respectively), indicating an increase in rate of release of GnRH in vivo and a consequent stimulation of synthesis and release of LH and FSH. These results are consistent with the hypothesis that suckling/lactation alters endogenous rate of GnRH secretion, thereby influencing secretion of gonadotrophins and follicular development.