Morphofunctional interactions between galanin and GnRH-containing neurones in the diencephalon of the ewe. The effect of oestradiol.

In rodents, the neuropeptide galanin (Gal) is involved in controlling the release of gonadotrophin-releasing hormone (GnRH). In the female, this peptide is colocalized in a subpopulation of GnRH neurones and its expression is stimulated by oestradiol. In the ewe, the morphofunctional relationship between these two neuronal peptides is poorly understood. The morphological interaction between Gal and GnRH was studied in ewes treated with oestradiol or with colchicine and in control animals. Five ewes were treated for 6h with oestradiol implants, a treatment known to induce a preovulatory surge of GnRH, and compared with five control animals. In addition, four animals received an intracerebroventricular injection of colchicine known to increase the intracellular level of galanin immunoreactivity. The morphological relationship between the two peptides was investigated by immunofluorescence using specific antibodies on the same sections, and the results were analysed using confocal microscopy. In colchicine-treated ewes, numerous Gal-immunoreactive neurones were found in the preoptic area in the vicinity of GnRH-immunoreactive neurones, but the two peptides were never observed in the same neurone. In all animals, Gal-ir fibres were observed to be in apposition to GnRH-containing perikarya in the preoptic area and these appositions were more numerous in oestradiol-treated ewes than in control animals. In contrast with rodents, galanin was not colocalized with GnRH in the neurones of the preoptic area of ewes, but this peptide could control GnRH neuronal secretion through axosomatic interactions. However, the presence of synaptic contacts between galanin terminals and GnRH perikarya needs to be confirmed by electron microscopy. As in rodents and primates, galanin could mediate the positive feedback of oestradiol on GnRH neurones during the preovulatory surge in ewes.