GONADOTROPIN STORAGE PATTERNS IN THE EWE DURING THE ESTROUS-CYCLE OR AFTER LONG-TERM TREATMENT WITH A GNRH AGONIST

The storage pattern of gonadotrophins in the ewe pituitary was investi gated during the oestrous cycle and after desensitization to GnRH usin g long-term treatment with a GnRH agonist, buserelin. Oestrous cycles in ewes were synchronized with progestagen sponges. Animals were alloc ated to two experiments. In the first, ewes were killed 36 h (before t he preovulatory surge, n=4), 48 h (end of rile preovulatory surge, n=5 ), 72 h (post-ovulation, n=4) and 240 h (luteal phase, n=3) after spon ge removal. In the second experiment, another progestagen sponge was i nserted in ewes 84 h after removal of the first sponge. Four ewes were infused continuously with buserelin (50 mu g/day) for 15 days before killing. A further four ewes received no buserelin (controls). Pituita ries were collected and processed for immunocytochemistry to detect mo no-hormonal (LH or FSH) and multihormonal (LH/FSH) cells. The percenta ges of LH or FSH immunoreactive cells in the pituitary were lower at t he end of the preovulatory surge (7.4 +/- 0.3% and 1.2 +/- 0.3% respec tively) compared with the other stages (11.4 +/- 0.5% and 5.4 +/- 0.7% respectively). Analysis of dual immunostaining showed the existence o f monohormonal cells for LH and multihormonal cells (LH/FSH). No monoh ormonal cell for FSH was detected except at the end of the preovulator y surge when a few monohormonal FSH cells appeared (0.1 +/- 0.01% of p ituitary cells). The percentage of monohormonal LH cells in the pituit ary gland was similar in all studied stages of the oestrous cycle, whe reas the percentage of multihormonal cells was lower at the end of til e surge. In agonist-treated ewes,:he percentages of LH or FSH immunore active cells (5.3 +/- 0.5% and 1.5 +/- 0.8% respectively) were decreas ed compared with controls (9.4 +/- 1% and 7.5 +/- 1.1% respectively). Analysis of the double immunostaining revealed a few monohormonal FSH cells (0.2 +/- 0.01% of pituitary cells) in agonist-treated ewes but n ot in controls. The percentage of monohormonal LH cells in the pituita ry gland increased from 1.9 +/- 0.2% in controls to 3.8 +/- 0.3% in ag onist-treated ewes, whereas multihormonal cells dropped from 7.5 +/- 1 .1% to 1.3 +/- 0.7%. Our data suggest, therefore, that multihormonal c ells contribute to gonadotrophin secretion, either during the preovula tory surge of the oestrous cycle or during the 'flare-up' effect initi ally induced by a FSH agonist. Moreover, the appearance of monohormona l FSH cells in some conditions reflects a differential regulation of L H and FSH.