Immunocytochemical detection of cholecystokinin and corticotrophin-releasing hormone neuropeptides in the hypothalamic paraventricular nucleus of the jerboa (Jaculus orientalis): modulation by immobilisation stress

Abstract

The hypothalamic response to an environmental stress implicates the corticotrophin-releasing hormone (CRH) neuroendocrine system of the hypothalamic parvicellular paraventricular nucleus (PVN) in addition to other neuropeptides coexpressed within CRH neurones and controlling the hypothalamo-pituitary-adrenal (HPA) axis activity as well. Such neuropeptides are vasopressin, neurotensin and cholecystokinin (CCK). It has previously been demonstrated that the majority of the CRH neuronal population coexpresses CCK after a peripheral stress in rats. In the present study, we explored such neuroendocrine plasticity in the jerboa in captivity as another animal model. In particular, we studied CCK and CRH expression within the hypothalamic PVN by immunocytochemistry in control versus acute immobilisation stress-submitted jerboas. The results show that CCK- and CRH-immunoreactive neuronal systems are located in the hypothalamic parvicellular PVN. The number of CCK-immunoreactive neurones within the PVN was significantly increased (138% increase) in stressed animals compared to controls. Similarly, the number of CRH-containing neurones was higher in stressed jerboas (128%) compared to controls. These results suggest that the neurogenic stress caused by immobilisation stimulates CCK as well as CRH expression in jerboas, which correlates well with previous data obtained in rats using other stressors. The data obtained also suggest that, in addition to CRH, CCK is another neuropeptide involved in the response to stress in jerboa, acting by controlling HPA axis activity. Because CCK is involved in the phenotypical plasticity of CRH-containing neurones in response to an environmental stress, we also explored their coexpression by double immunocytochemistry within the PVN and the median eminence (i.e. the site of CRH and CCK corelease in the rat) following jerboa immobilisation. The results show that CCK is not coexpressed within CRH neurones in either control or stressed jerboa, suggesting differences between jerboas and rats in the neuroendocrine regulatory mechanisms of the stress response involving CRH and CCK. The adaptative physiological mechanisms to environmental conditions might vary from one mammal species to another.