Nociceptin/Orphanin FQ Content is Decreased in Forebrain Neurones During Acute Stress D. P. Devine,  M. T. Hoversten,y Y. Ueday and H. Akily  Department of Psychology, University of Florida, Gainesville, FL, USA. yMental Health Research Institute, University of Michigan, Ann Arbor, MI, USA. Key words: stress, HPA axis, ACTH, corticosterone, opioid. Abstract We examined the effects of acute and chronic stress on neurotransmission of nociceptin/orphanin FQ (N/OFQ) in a variety of brain regions. Four groups of rats were exposed to chronic variable stress, and/or a single acute stress before decapitation. Group 1 served as unstressed controls. The rats in group 2 (chronic stress/no acute stress) were exposed to a 10-day regimen of chronic stress (two unpredictable stressors per day). These rats were decapitated 20 h after the last stressor. The rats in group 3 (no chronic stress/acute stress) were not exposed to chronic stress, but they were restrained for 30 min prior to decapitation. The rats in group 4 (chronic stress/acute stress) were chronically stressed for 10 days, and were then restrained prior to decapitation. Trunk blood was collected, and plasma adrenocorticotrophic hormone (ACTH) and corticosterone (CORT) were assayed by radioimmunoassay (RIA). The rats’ brains were dissected, and N/OFQ content was measured by RIA in a variety of brain regions, and in spinal cord. Chronic stress exposure altered the hormonal responses to the acute stress exposure. In the rats that were exposed to chronic stress without acute stress (group 2), N/OFQ content did not differ from the content of the unstressed controls in any of the dissected brain regions. In the two groups that were stressed acutely just before decapitation (groups 3 and 4), N/OFQ content was decreased by 25–30% in the basal forebrain. Accordingly, the neuronal content of N/OFQ is decreased in basal forebrain neurones during acute stress exposure. In light of our previous finding that N/OFQ administration increases circulating ACTH and CORT concentrations, and augments hormonal responses to an acute stressor, the current finding raises the possibility that endogenous N/OFQ participates in neuronal regulation of hormonal responses to acute stress exposure. Nociceptin/orphanin FQ (N/OFQ) is a 17-amino acid peptide that is evolutionarily related to the opioid family of neuropeptide transmitters (1, 2). It exhibits a high degree of amino acid sequence homology with the opioid peptides (especially dynor- phin A 117 ), but does not bind to the m-, d- and k-opioid receptor types. Rather, it binds saturably and with high affinity to the NOP receptor (formerly known as ORL1 or LC132). NOP is a member of the super family of Gi protein-coupled receptors that contain seven trans-membrane alpha helices. It is negatively linked to adenylate cyclase, activates inward rectifying K þ channels and inhibits N-type Ca 2þ channels (3–10). NOP exhibits a high degree of amino acid sequence homology with the cloned m-, d- and k- opioid receptors, particularly in the trans-membrane and cytosolic domains (approximately 85% homology in the intracellular loop between trans-membrane 5 and 6) (9). However, despite the structural and functional similarities between NOP and the opioid receptors, NOP does not selectively bind prototypical opioid agonists or antagonists (3, 4, 8–10). Accordingly, N/OFQ and NOP represent an opioid-related neurotransmitter system that has the potential to mediate physiological and behavioural actions distinct from those that are mediated by the opioid system. The range of physiological and behavioural actions that are mediated by the N/OFQ-NOP system has not been fully characterized. N/OFQ and NOP (and their respective mRNAs) are ubiqui- tously expressed in mammalian brain and spinal cord (11–13), and are particularly abundant within a variety of limbic structures (3, 4, 8, 10–16). Furthermore, functional activity of N/OFQ has been demonstrated in these limbic structures by means of [ 35 S]GTPgS autoradiography (17–19). This neuroanatomical and physiologi- cal association between the N/OFQ system and limbic sites raises the possibility that N/OFQ neurotransmission may participate in processing of emotional stimuli. We recently reported that intracerebroventricular (i.c.v.) microinjections of N/OFQ produce increases in plasma adreno- corticotrophic hormone (ACTH) and corticosterone (CORT) con- centrations in unstressed rats. Furthermore, the i.c.v. N/OFQ Journal of Neuroendocrinology, 2003, Vol. 15, 69–74 # 2003 Blackwell Publishing Ltd Correspondence to: Darragh P. Devine, Behavioral Neuroscience Program, University of Florida, Department of Psychology, PO Box 112250, Gainesville, FL 32611-2250, USA (e-mail: dpdevine@ufl.edu).