Use of push-pull perfusion techniques in studies of gonadotrophin-releasing hormone secretion in mares

Push-pull perfusion was used to study GnRH secretory ability of the hypothalamus in anoestrous, transitional, dioestrous and oestrous Pony mares. The technique involved placement of a concentric (tube within a tube) cannula into the area of the medial basal hypothalamus and perfusing a carrier medium (artificial cerebrospinal fluid) through the inner tube whilst aspirating from the outer tube so that the flow rate within the hypothalamic tissue was essentially constant. The perfusion rate was 0.5 ml/10 min and samples were collected at 10-min intervals for 10-15 h. The carrier medium, which contained GnRH, was acidified and frozen until measurement by radioimmunoassay. Blood samples were taken from the jugular vein simultaneously for measurement of LH. Nineteen mares were perfused, representing anoestrus (N = 3), transition (N = 6), dioestrus (N = 7), and oestrus (N = 3). GnRH secretion was minimal during anoestrus, with the majority of samples being below the level of detection of the assay (1.9 pg/tube). During the transition phase, the overall mean secretory rate was increased, and there appeared to be more secretory episodes. During the breeding season, GnRH secretion was markedly increased over anoestrus and transition, with oestrous mares secreting significantly greater amounts of GnRH than dioestrous mares. There were no significant differences among groups in number of secretory peaks, or the interval between peaks. There was, however, a significant (P less than 0.0001) difference among groups in overall mean secretory rate. These results suggest that GnRH secretion is increased in early transition, thus accounting for the increased FSH secretion that takes place then, but LH is not increased.(ABSTRACT TRUNCATED AT 250 WORDS)