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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.