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Galanin synaptic input onto gonadotropin-releasing hormone (GnRH) neuronal cell bodies was analysed in female mice using the presynaptic vesicle-specific protein, synaptophysin (Syn) as a marker. In the first experiment, forebrain sections from normal ovariectomized ovarian steroid-primed mice exhibiting a surge of luteinizing hormone were processed for immunohistochemical labelling for GnRH, synaptophysin, galanin and Fos. Two representative sections from each brain, one passing through the anterior septum (anterior section) and the other through the organum vasculosum lamina terminalis-preoptic area (posterior section), were analysed under the confocal microscope. None of the GnRH cells analysed in the anterior sections were Fos immunoreactive (IR) or received input from galanin-IR fibres. In contrast, the majority of GnRH cells in the posterior sections analysed were Fos-positive. The number of galanin synapses onto the Fos-positive GnRH cells was significantly higher than that in the Fos-negative cells in this area of the brain, even though the number of Syn-IR appositions was comparable to each other. Transplantation of preoptic area (POA) into the third cerebral ventricle of hypogonadal (HPG) mice corrects deficits in the reproductive system. In the second experiment, synaptic input to GnRH cells was compared between HPG/POA mice with (functional graft) or without (nonfunctional graft) gonadal development. The mean numbers of Syn-IR appositions and galanin synapses per GnRH cell and the proportion of GnRH cells with galanin input were significantly higher in the functional than in the nonfunctional grafts. The results suggest that galanin can act directly on the GnRH cell bodies and may have an important regulatory role on the GnRH system.