Endogenous nociceptin / orphanin FQ system involvement in hypothalamic-pituitary-adrenal axis responses: relevance to models of inflammation.

Nociceptin/orphanin FQ (N/OFQ) peptide and its receptor (NOP) function in the neuromodulation of anxiety, stress and hypothalamic-pituitary-adrenal (HPA) axis activity. We investigated the endogenous NOP system using the selective NOP antagonist, UFP-101, during the HPA axis response to bacterial endotoxin, lipopolysaccharide (LPS). Although i.c.v. N/OFQ (1 microg/rat) had no significant effect on LPS-induced (250 microg/rat i.p) plasma corticosterone release at 30 or 60 min post-i.c.v. injection, i.c.v. UFP-101 (1 microg/rat)/LPS significantly attenuated plasma adrenocorticotrophic hormone and corticosterone at the 30-min time-point compared to i.c.v saline (0.9%)/LPS. Parvocellular paraventricular nucleus (PVN) corticotrophin-releasing factor (CRF) and corticotrophic pro-opiomelanocortin (POMC), but not parvocellular PVN arginine vasopressin (AVP), mRNA expression was significantly increased by LPS compared to non-LPS control. Intracerebroventricular UFP-101/LPS treatment was associated with increased POMC mRNA expression 4 h after injection and a clear trend towards increased parvocellular CRF mRNA. Furthermore, i.c.v. UFP-101 was selectively associated with an LPS-induced increase in parvocellular AVP mRNA, an effect that was absent in the i.c.v saline/LPS group. To determine whether LPS challenge was associated with compensatory changes in N/OFQ precursor or NOP receptor mRNAs, in a separate study, we undertook reverse transcriptase-polymerase chain reaction analysis of preproN/OFQ and NOP transcripts. In support of an endogenous role for central N/OFQ in inflammatory stress, we found that LPS significantly increased preproN/OFQ transcript expression in the hypothalamus 4 h after injection compared to the saline control. No changes in preproN/OFQ mRNA level in the hippocampus or basal forebrain (including bed nucleus of stria terminalis) were seen, albeit at 4 h. LPS was associated with a significant attenuation of NOP mRNA in the basal forebrain at 4 h, possibly as a compensatory response to increased N/OFQ release. Although the exact mechanisms require elucidation, the findings obtained in the present study provide evidence indicating that the endogenous NOP system is involved in the acute HPA axis response to immune challenge.

Organizational role for pubertal androgens on adult hypothalamic-pituitary-adrenal sensitivity to testosterone in the male rat.

The inhibitory effect of androgens on the hypothalamic-pituitary-adrenal (HPA) axis in basal and stress conditions in adult male rats is well documented. Major sex-related neuroendocrine changes take place during puberty. There is a robust rise in production and secretion of gonadal steroids, which is thought to underlie numerous neural and behavioural changes brought on after puberty. The present study investigated the effect of the pubertal rise in gonadal steroid levels on the subsequent adult corticosterone profile, particularly the sensitivity of the adult HPA axis to testosterone. Animals were castrated either prepubertally (28 days) or in adulthood (11 weeks) and adult animals were subsequently treated with subcutaneous implants containing either testosterone or cholesterol. Using an automated blood sampling system, blood was collected from each freely moving, conscious rat every 10 min (i) over a 24 h period; (ii) in response to 10 min of noise stress, and (iii) following an immunological challenge with lipopolysaccharide (LPS). Analysis revealed that testosterone treatment did not significantly affect overall corticosterone release over the 24 h period in adult animals castrated before puberty in contrast to animals castrated in adulthood in which testosterone significantly suppressed corticosterone secretion. Following either a noise stress or LPS injection, testosterone treatment did not affect the hypothalamic or adrenal stress response in animals castrated prepubertally. Testosterone significantly suppressed the corticotrophin-releasing hormone and arginine vasopressin mRNA as well as the corticosterone response to LPS in castrated animals that had had their testes intact over puberty. These data provide evidence that puberty is a critical organizational period during which rising levels of gonadal steroids programme the sensitivity of the adult HPA axis to gonadal steroids in adulthood.

Chronic citalopram treatment does not sensitize the adrenal gland to ACTH (1-24) in rats.

We have previously reported that rats exposed chronically to citalopram are able to elicit a corticosterone but not adrenocorticotropic hormone (ACTH) response to restraint stress. Thus we proposed the hypothesis that the corticosterone response to restraint in citalopram-treated rats was maintained due to increased adrenal sensitivity to lower ACTH levels. To test this hypothesis, we intravenously injected ACTH (1-24) to rats (dose 3 ng/rat) exposed to citalopram through minipump infusion for 14 days and to control rats (no citalopram). ACTH significantly increased plasma corticosterone levels in both control and citalopram treated rats over a period of 120 min. There was no significant difference in plasma corticosterone between citalopram treated rats and control rats at any time point. Therefore we conclude that, under these experimental conditions, citalopram does not appear to sensitize the rodent adrenal gland to ACTH, and that other mechanisms may be responsible for the ACTH/corticosterone disconnection.

The nociceptin/orphanin FQ antagonist UFP-101 differentially modulates the glucocorticoid response to restraint stress in rats during the peak and nadir phases of the hypothalamo-pituitary-adrenal axis circadian rhythm.

The involvement of nociceptin (N/OFQ) and the nociceptin/orphanin FQ peptide (NOP) receptor in behavior associated with stress and anxiety has been established but their role in the regulation of the hypothalamo-pituitary-adrenal (HPA) axis under conditions of stress has not been fully investigated. We used the selective NOP receptor antagonist UFP-101 to examine the contribution of endogenous N/OFQ to HPA axis control under conditions of restraint stress in the morning and the evening. We found that in the morning during the HPA axis circadian nadir rats exposed to restraint stress in both the presence and absence of UFP-101 exhibited significantly elevated plasma corticosterone at 30 min post-i.c.v. injection compared to the home cage control group. Additionally, rats treated with UFP-101 and exposed to restraint had significantly elevated corticosterone levels at 60 min post-i.c.v. injection compared to all other treatment groups. Interestingly, while there was a significant increase in the expression of CRF mRNA in the paraventricular nucleus (PVN) of rats exposed to restraint stress only, there was no comparable increase in those co-treated with UFP-101. There was no change in the expression of AVP or POMC mRNA in any of the treatment groups. In contrast, when carried out in the evening we observed significantly elevated plasma corticosterone in the vehicle-treated restraint group only at 30 min post-i.c.v. injection. There was no significant difference between the UFP-101-treated restraint group and either of the home cage control groups or the vehicle-treated restraint group. Additionally, in contrast to the morning study, UFP-101 did not prolong glucocorticoid release at the 60 min time-point. These results demonstrate for the first time a differential effect of UFP-101 on restraint stress-induced HPA axis activity characterized by significant prolongation of stress-induced activity in the morning but no significant effect on the response to restraint in the evening.

The nociceptin receptor antagonist [Nphe1,Arg14,Lys15]nociceptin/orphanin FQ-NH2 blocks the stimulatory effects of nociceptin/orphanin FQ on the HPA axis in rats.

Nociceptin/orphanin FQ (N/OFQ) is an opioid-related peptide that stimulates corticosterone release after i.c.v. administration in non-stressed rats. We employed in situ hybridization histochemistry to investigate N/OFQ-stimulated activation of the HPA axis at the hypothalamic and pituitary level. We have demonstrated that N/OFQ-induced activation of the HPA axis is mediated via the central N/OFQ peptide receptor (NOP) using the recently described selective NOP antagonist [Nphe(1),Arg(14),Lys(15)]nociceptin/orphanin FQ-NH(2) (UFP-101). We found that, at 30 min post-i.c.v. injection, N/OFQ dose-dependently increased plasma adrenocorticotrophin hormone and corticosterone compared with the vehicle-injected controls. N/OFQ (1.0 microg) significantly increased CRF mRNA but not AVP mRNA within the parvocellular hypothalamic paraventricular nucleus compared with the control group, and significantly increased pro-opiomelanocortin (POMC) mRNA in the anterior pituitary. While UFP-101 (1.0 microg) alone had no significant effect on plasma corticosterone concentration it blocked the effect of N/OFQ (1.0 microg) on plasma corticosterone levels when compared with N/OFQ administered alone. UFP-101 also blocked the N/OFQ-induced increase in CRF mRNA and POMC mRNA. These results demonstrate that centrally administered N/OFQ activates the HPA axis via up-regulation of CRF and POMC mRNA and stimulation of corticosterone release in rats. Further, we have demonstrated for the first time that the selective NOP receptor antagonist UFP-101 blocks these effects indicating that N/OFQ-induced HPA axis activation is mediated via central NOP receptors.