Functional evidence that the nucleus of the hippocampal commissure shows an earlier activation from a stressor than the paraventricular nucleus: Implication of an additional structural component of the avian hypothalamo-pituitary-adrenal axis

Despite extensive data addressing the regulation of the hypothalamo-pituitary-adrenal (HPA) axis in vertebrates, the neuroendocrine regulation of stress in birds remains incomplete. The paraventricular nucleus (PVN) contains the key neuropeptides, corticotropin releasing hormone (CRH) and arginine vasotocin (AVT), containing neurons. However, another population of CRH neurons was recently identified in a septal nucleus called the nucleus of the hippocampal commissure (NHpC). Therefore, the current study investigated changes in gene expression of CRH and AVT in the PVN and CRH in the NHpC, as well as changes in plasma corticosterone concentrations following a stressor, food deprivation. In the NHpC, a rapid increase in CRH mRNA levels was observed as early as 2 h, while relative CRH mRNA expression in the PVN increased thereafter from 4 to 12 h of food deprivation. On the other hand, relative mRNA levels of AVT in the PVN were not observed until 8 h and significantly increased at 12 and 24 h following food deprivation. Furthermore, at the level of the anterior pituitary, relative expression of proopiomelanocortin transcripts followed gene expression patterns of CRH and AVT in the brain. In the absence of food, the pattern of plasma CORT showed an initial rise at 2 h and a fourfold increase was measured at 4 h that was sustained through 24 h. Taken together, results from this study suggest that (1) CRH neurons in the NHpC appear to be the first responsive neurons to stress stimuli compared to those in the PVN, (2) CRH is predominantly functional in the early phase of stress while AVT is involved in the later phase of the stress period and (3) in birds, CRH neurons in the NHpC appear to be part of the classical HPA axis.