Neuronal Gonadotrophin-Releasing Hormone (GnRH) and Astrocytic Gonadotrophin Inhibitory Hormone (GnIH) Immunoreactivity in the Adult Rat Hippocampus.

Gonadotrophin-releasing hormone (GnRH) and gonadotrophin inhibitory hormone (GnIH) are neuropeptides secreted by the hypothalamus that regulate reproduction. GnRH receptors are not only present in the anterior pituitary, but also are abundantly expressed in the hippocampus of rats, suggesting that GnRH regulates hippocampal function. GnIH inhibits pituitary gonadotrophin secretion and is also expressed in the hippocampus of a songbird; its role outside of the reproductive axis is not well established. In the present study, we employed immunohistochemistry to examine three forms of GnRH [mammalian GnRH-I (mGnRH-I), chicken GnRH-II (cGnRH-II) and lamprey GnRH-III (lGnRH-III)] and GnIH in the adult rat hippocampus. No mGnRH-I and cGnRH-II+ cell bodies were present in the hippocampus. Sparse mGnRH-I and cGnRH-II+ fibres were present within the CA1 and CA3 fields of the hippocampus, along the hippocampal fissure, and within the hilus of the dentate gyrus. No lGnRH-III was present in the rodent hippocampus. GnIH-immunoreactivity was present in the hippocampus in cell bodies that resembled astrocytes. Males had more GnIH+ cells in the hilus of the dentate gyrus than females. To confirm the GnIH+ cell body phenotype, we performed double-label immunofluorescence against GnIH, glial fibrillary acidic protein (GFAP) and NeuN. Immunofluorescence revealed that all GnIH+ cell bodies in the hippocampus also contained GFAP, a marker of astrocytes. Taken together, these data suggest that GnRH does not reach GnRH receptors in the rat hippocampus primarily via synaptic release. By contrast, GnIH might be synthesised locally in the rat hippocampus by astrocytes. These data shed light on the sites of action and possible functions of GnRH and GnIH outside of the hypothalamic-pituitary-gonadal axis.

Effects of nutritional stress during different developmental periods on song and the hypothalamic-pituitary-adrenal axis in zebra finches.

In songbirds, developmental stress affects song learning and production. Altered hypothalamic-pituitary-adrenal (HPA) axis function resulting in elevated corticosterone (CORT) may contribute to this effect. We examined whether developmental conditions affected the association between adult song and HPA axis function, and whether nutritional stress before and after nutritional independence has distinct effects on song learning and/or vocal performance. Zebra finches (Taeniopygia guttata) were raised in consistently high (HH) or low (LL) food conditions until post-hatch day (PHD) 62, or were switched from high to low conditions (HL) or vice versa (LH) at PHD 34. Song was recorded in adulthood. We assessed the response of CORT to handling during development and to dexamethasone (DEX) and adrenocorticotropic hormone (ACTH) challenges during adulthood. Song learning and vocal performance were not affected by nutritional stress at either developmental stage. Nutritional stress elevated baseline CORT during development. Nutritional stress also increased rate of CORT secretion in birds that experienced stress only in the juvenile phase (HL group). Birds in the LL group had lower CORT levels after injection of ACTH compared to the other groups, however there was no effect of nutritional stress on the response to DEX. Thus, our findings indicate that developmental stress can affect HPA function without concurrently affecting song.