Sex difference in the endocrine response of adult domestic chickens to gonadotrophin-releasing hormone-I
The responsiveness and sensitivity to an injection of chicken [Gln 8]-gonadotrophinreleasing hormone (GnRH-I) is lower, and the duration of increased plasma luteinizing hormone (LH) is more prolonged, in laying hens than in adult cockerels (Callus domesticus). This thesis sought to further characterise and establish an oestrogendependency of these sex differences in vivo and in vitro, and to determine the mechanism for sexual differentiation of GnRH-I-induced LH secretion. The magnitude and profile of the GnRH-I-stimulated LH response, and the sensitivity to GnRH-I were shown to be functions of the anterior pituitary gland. There was less LH in gonadotroph cells from laying hens than from adult cockerels, but no sex difference between juveniles. The low responsiveness to GnRH-I of laying hens was reproduced by oestrogen treatment of adult cockerels in vivo and pituitary cells from juvenile chickens in vitro, through depression of the total and readily releaseable pool (RRP) of pituitary LH. The secretion of LH from GnRH-I-stimulated isolated pituitary glands was biphasic, consisting of a spike and a plateau. Sexual differentiation of the magnitude of the spike of LH release was determined by the smaller RRP of LH, and the absence of an extracellular Ca2 -independent and an L-type-Ca 2 channel-dependent component of LH secretion from pituitary glands from laying hens, compared with that from adult cockerels. A decrease in spike-to-plateau phase ratio of GnRH-I-induced LH secretion was observed in vitro in pituitary glands from hens at onset of sexual matuFity. This decrease in ratio was simulated in pituitary glands from adult cockerels in vitro after incubation with 176-oestradiol. The membrane fluidity of pituitary cells from laying hens was lower than that from adult cockerels and shown to be induced in pituitary cells from juvenile chickens by treatment with 17B-oestradiol. Attempts to establish an oestrogen-dependency of the sexually differentiated sensitivity of adult chickens to GnRH-I, or to relate it to a sex difference in pituitary GnRH-receptors were unsuccessfuL The sex difference in GnRH-I-induced duration of increased plasma LH could not be reproduced in vitro and therefore depends on an extrapituitary mechanism. These sex differences became fully established after sexual maturation, with similar GnRH-I-induced LH responses being found in adult cockerels and juveniles of both sex. It is therefore concluded that sexual differentiation of these LH responses results from the maturational increase in plasma 178-oestradiol in hens.
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