Differential delayed responses of arginine vasotocin and its receptors in septo-hypothalamic brain structures and anterior pituitary that sustain hypothalamic pituitary-adrenal (HPA) axis functions during acute stress

Recently, we proposed that corticotropin releasing hormone (CRH) neurons in the nucleus of hippocampal commissure (NHpC), located in the septum, function as a part of the traditional hypothalamic pituitary-adrenal (HPA) axis in avian species. CRH and its receptor, CRHR1, are regulated differently in the NHpC compared to the paraventricular nucleus (PVN) following feed deprivation (FD). Therefore, we followed up our work by examining arginine vasotocin (AVT), the other major ACTH secretagogue, and its receptors, V1aR and V1bR, gene expression during FD stress in the NHpC, PVN, and ventral mediobasal hypothalamus/median eminence (MBHv/ME). The objectives were to 1) identify AVT perikarya, fibers and its two major receptors, V1aR and V1bR, in the NHpC, PVN, and MBHv/ME using immunohistochemistry, 2) determine the effect of stress on AVT, V1aR and V1bR mRNA expression in the same three brain structures, NHpC, PVN, and MBHv/ME; and, 3) ascertain the expression pattern of V1aR and V1bR mRNA in the anterior pituitary and measure plasma stress hormone, corticosterone (CORT), concentration following FD stress. Male chicks (Cobb 500), 14 days of age, were divided into six groups (10 birds/treatment) and subjected to different times of FD stress: (Control, 1 h, 2 h, 3 h, 4 h, and 8 h). For each bird, blood, brain, and anterior pituitary were sampled and frozen immediately. The NHpC, PVN, and MBHv/ME were micro-dissected for RT-PCR. Data were analyzed using one-way ANOVA followed by Tukey Kramer HSD test using a significance level of p < 0.05. Perikarya of AVT neurons were identified in the PVN but not in the NHpC nor MBHv/ME, and only V1aR-immunoreactivity (ir) was observed in the three structures, however, gene expression data for AVT and its two receptors were obtained in all structures. Both AVT and V1aR mRNA are expressed and increased significantly in the PVN following FD stress (p < 0.01). For the first time, V1bR mRNA was documented in the avian brain and specifically shown upregulated in the NHpC and PVN (p < 0.01) following stress. Additionally, delayed significant gene expression of AVT and its receptors in the PVN showed a positive feedback relationship responsible for maintaining CORT release. In contrast, a significant downregulation of AVT mRNA and upregulation of V1aR mRNA occurred in the NHpC (p < 0.01) during FD showing a negative feedback relationship between AVT and its receptors, V1aR and V1bR. Within the MBHv/ME and anterior pituitary, a gradual increase of AVT mRNA in PVN as well as MBHv/ME was associated with significant upregulation of V1bR (p < 0.01) and downregulation of V1aR (p < 0.01) in both MBHv/ME and anterior pituitary indicating AVT regulates its receptors differentially to sustain CORT release and control overstimulation of the anterior pituitary during a stress response.