Differential actions of acute and chronic citalopram on the rodent hypothalamic-pituitary-adrenal axis response to acute restraint stress.

Serotonin re-uptake inhibitors (SSRIs) can affect the basal activity of the hypothalamic-pituitary-adrenal (HPA) axis in rats. A single injection of citalopram has been shown to stimulate the HPA axis while repeated administration leads to attenuation of the corticosterone response to the SSRI. The purpose of this work was to investigate the rodent HPA axis response to restraint stress, following acute and chronic treatment with the SSRI citalopram. We have demonstrated that a single injection of citalopram is able to prolong acute restraint-induced increases in plasma levels of corticosterone and adrenocorticotrophin (ACTH). This is possibly mediated by arginine vasopressin (AVP) in the parvocellular cells of the paraventricular nucleus (pPVN), as treatment with citalopram or restraint alone did not increase AVP mRNA in pPVN while the combination of treatments resulted in a significant increase in AVP mRNA in the pPVN. In contrast, the increase in corticotrophin-releasing factor (CRF) mRNA in the pPVN in response to acute restraint stress was not altered by citalopram. Oxytocin (OT) mRNA was also increased in the magnocellular PVN (mPVN) by the solo treatments of citalopram and restraint, and was not further enhanced by the dual treatment of restraint and citalopram. Chronic treatment with citalopram did not alter basal plasma levels of corticosterone or ACTH. However, the ACTH response to acute restraint was attenuated following chronic citalopram treatment. AVP mRNA in the pPVN was significantly elevated in response to chronic citalopram compared with saline controls suggesting an effect mediated through the AVP subset of pPVN neurones. The CRF mRNA response to acute restraint was not altered in rats treated chronically with citalopram. OT mRNA was not enhanced in the mPVN following chronic infusion of citalopram but was increased by acute restraint stress. We conclude from these data that both acute and chronic citalopram treatment has the potential to alter the rodent response to acute restraint stress. These effects appear to be regulated by the AVP-containing subset of CRF neurons in the pPVN and thus suggest that parvocellular AVP may have an important role in mediating the actions of SSRIs.