Photo-induction of hypothalamic gonadotrophin releasing hormone-I mRNA in the domestic chicken: A role for oestrogen?

Photoinduced changes in GnRH neuronal function were investigated in prepubertal and in mid-pubertal cockerels and somatically mature hens. Photostimulation of short day mid-pubertal cockerels and somatically mature out-of-lay hens for 7 days significantly increased (P < 0.05) total hypothalamic gonadotrophin releasing hormone-I (GnRH-I) mRNA. The increase in GnRH-I mRNA was associated with increased (P < 0.05) plasma LH in the hens but not mid-pubertal cockerels. Photostimulation of short day prepubertal cockerels for 7 days also stimulated LH release (P < 0.05) but in contrast did not increase total hypothalamic GnRH-I mRNA. Plasma LH and hypothalamic GnRH-I mRNA were depressed in (P < 0.001) short day prepubertal cockerels chronically treated with oestradiol benzoate (0.5 mg/kg, on alternate days). However, photostimulation of oestrogenized prepubertal cockerels for 7 days stimulated LH release (P < 0.001) and increased hypothalamic GnRH-I mRNA (P < 0.001). It is concluded that photostimulatory inputs to GnRH neurones have the potential to increase GnRH-I mRNA transcription or stability and to increase GnRH-I release. The extent to which increased levels of GnRH-I mRNA or increased GnRH release from GnRH neurones are observed after photostimulation may depend on the interaction between the drive on GnRH-I neurone function, which increases at the onset of puberty, and the inhibitory action of oestrogen produced locally in the hypothalamus.