Abstract
The responsiveness and sensitivity to an injection of chicken [G ln8]-gonadotrophinreleasing hormone (GnRH-I) is lower, and the duration of increased plasm a luteinizing
horm one (LH) is more prolonged, in laying hens than in adult cockerels (G a llu s
dom esticus). 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 G nRH -I-stim ulated 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-Ca2+ 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 G nRH-I-induced LH secretion was observed in vitro in
pituitary glands from hens at onset of sexual maturity. This decrease in ratio was simulated
in pituitary glands from adult cockerels in vitro after incubation with 17B-oestradiol. The
membrane fluidity of pituitary cells from laying hens was low er than that from adult
cockerels and shown to be induced in pituitary cells from juvenile chickens by treatment
with 176-oestradiol. A ttem pts 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 G nRH -receptors were unsuccessful. The sex difference in G nRH -I-induced
duration of increased plasma LH could not be reproduced in vitro and therefore depends
on an extrapituitary mechanism.
These sex differences becam e fully established after sexual m aturation, with sim ilar
GnRH-I-induced LH responses being found in adult cockerels and juveniles o f both sex.
It is therefore concluded that sexual differentiation of these LH responses results from the
maturational increase in plasma 1713-oestradiol in hens.
