Number of parvocellular (A) and magnocellular (B) AVP positive neurons were counted within each region (12 sections per region) of the PVN and averaged per animal for SHC (n=8 per region) and CSC (n=78 per region). receptor protein expression was down-regulated. Relative pituitary pro-opiomelanocortin and arginine vasopressin (AVP) receptor 1b (AVPR-1b) protein expression, FS (6 min)-induced ACTH secretion in dexamethasone-blocked mice, and the number of AVP positive magnocellular and parvocellular neurons in the paraventricular hypothalamic nucleus (PVN) was unaffected following CSC. Taken together, the data of the present study indicate Ascomycin that 19 days of CSC result in pituitary hyperactivity, under both basal and acute heterotypic stress conditions. Although further studies have to assess this in detail, an increased number of pituitary corticotrophs together with unaffected relative pituitary AVPR-1b and decreased CRH-R1 protein expression following CSC suggests that pituitary hyperdrive is mediated by newly formed corticotrophs that are more sensitive to AVP than CRH. Moreover, our data indicate that changes in PVN AVP and negative feedback inhibition seem not to play a major role in pituitary hyperactivity following CSC. == Introduction == The acute activation of the hypothalamus-pituitary-adrenal (HPA) axis in response Rabbit polyclonal to PNLIPRP2 to stressful stimuli represents an important mechanism to promote survival, whereas, in contrast, a prolonged increase in Ascomycin plasma glucocorticoid (GC) concentrations has deleterious consequences for an organism, amongst others promoting intestinal and affective disorders (for review see[1],[2],[3]). Therefore, it is beneficial for the long term health of an individual to habituate possibly fast to a prolonged and not life threatening homotypic stressor[4],[5], and to be sensitized to subsequent heterotypic and possibly dangerous challenges[6],[7],[8]. Although it is generally accepted that these habituation/sensitization phenomena do not occur if the chronic stressor is of social nature (for review see[9]), we just recently provided first evidence that this holds not true for the chronic subordinate colony housing (CSC, 19 days) paradigm, a pre-clinically validated chronic psychosocial stress paradigm relevant for human affective and somatic disorders[10],[11]. CSC compared with single-housed control (SHC) mice show unaffected basal morning plasma corticosterone (CORT) levels, despite significantly enlarged adrenals[12], and a more pronounced CORT response to an acute heterotypic stressor (elevated platform (EPF), 5 min). Interestingly, and this is in contrast to studies describing habituation to the same (homotypic) stressor and sensitization to a novel (heterotypic) stressor to be mediated mainly at the level of the pituitary gland (for review see[13]), in CSC mice adrenal mechanisms seem to play the major role. This was indicated by the fact that SHC and CSC mice did not differ in plasma adrenocorticotropic hormone (ACTH) concentrations 5 min after EPF exposure[12]. However, as basal plasma ACTH concentrations have never been assessed following CSC and as pituitary weight was increased in CSC compared with SHC mice 8 d after termination of CSC exposure[14], it can’t be ruled out at this stage that pituitary mechanisms are not at least partly involved in the observed HPA axis adaptation/sensitization processes seen following 19 days of CSC exposure. To test this hypothesis was the aim of the current study. Pituitary ACTH production and release are influenced by various stimuli, with Ascomycin the neuropeptides corticotropin releasing hormone (CRH) and, to a lesser extent, arginine vasopressin (AVP) from the hypothalamus representing the main ACTH secretagogues (for review see[13],[15]). CRH neurosecretory cells are located in the dorsal medial parvocellular subdivision of the paraventricular nucleus (PVN) of the hypothalamus, with about 50% of the cells coexpressing AVP[16],[17]. The axons of these neurons project to the external zone of the median eminence from where CRH and AVP are released into the pituitary portal circulation in order to drive ACTH production/secretion in/from pituitary corticotroph cells[18],[19]upon binding to CRH receptor 1 (CRH-R1) or AVP receptor 1b (AVP-R1b), respectively. ACTH, via the blood stream, subsequently reaches the adrenal cortex and induces the production and secretion of GC (for review see[15]) which, in turn, bind to both mineralocorticoid.