Acetyl-RYYRIK-NH2, a purported orphanin FQ antagonist, does not block the orphanin FQ-induced prolactin secretory response in female rats

Anti-nociceptive and anti-allodynic effects of a high affinity NOP hexapeptide [Ac-RY(3-Cl)YRWR-NH2] (Syn 1020) in rodents

There has been a flurry of activity to develop agonists and antagonists for the member of the opioid receptor family, NOP receptor (also known as ORL1), in part to understand its role in pain. Modifications of a hexapeptide originally identified from a combinatorial library have led to the discovery of a high affinity hexapeptide agonist Ac-RY(3-Cl)YRWR-NH2 (Syn 1020). In the following experiments we characterized the anti-nociceptive effects of Syn 1020 in the tail-flick model of acute pain and the diabetic neuropathy model of chronic pain in mice and rats, respectively. Acute antinociception was assessed using the tail-flick assay in mice in which animals received intracerebroventricular (i.c.v.) or subcutaneous (s.c.) injections of Syn 1020 alone or with morphine and were tested for tail-flick latencies. In the chronic pain model, diabetic neuropathy was induced by injections of streptozotocin in rats. Tactile allodynia was measured, with von Frey hair filaments, following intraperitoneal (i.p.) injections of Syn 1020 or gabapentin (positive control). In mice, i.c.v. injections of Syn 1020 did not have any pro- or anti-nociceptive effects, however, Syn 1020 reversed morphine antinociception with a similar potency as N/OFQ (the natural ligand to NOP). S.c. injections of Syn 1020 in mice also produced analgesic effects. In rats, i.p, injections of Syn 1020 produced anti-allodynic effects. Thus, Syn 1020, a NOP receptor directed peptide, administered systemically has anti-nociceptive activity in both acute and chronic pain models in mice and rats respectively.

Orphanin FQ/nociceptin is a physiological regulator of prolactin secretion in female rats

Orphanin FQ/nociceptin (OFQ/N), the most recently identified endogenous opioid peptide, stimulates prolactin secretion in both male and female rats. OFQ/N, however, did not elicit this stimulatory effect through the mu-, delta-, or kappa-opiate receptor subtype. The role OFQ/N plays in prolactin regulation under physiological conditions and its mechanism of action are not known. The purpose of these studies was to determine the physiological significance and pharmacological specificity of the prolactin secretory response to OFQ/N. In addition, the role of the tuberoinfundibular dopaminergic (TIDA) neurons in mediating this response was examined. Opioid receptor-like-1 (ORL-1) receptors were blocked by pretreatment with compound B (Comp B), a purported OFQ/N antagonist, or receptor synthesis was disrupted by pretreatment with ORL-1 receptor antisense oligonucleotides. The prolactin secretory response to OFQ/N administration in diestrous females was measured. Furthermore, the suckling-induced prolactin response was also determined after Comp B pretreatment. TIDA neuronal activity was quantified in diestrous female rats to determine whether OFQ/N stimulates prolactin release by inhibiting TIDA neurons. OFQ/N significantly inhibited the TIDA neurons by 1 min, preceding the prolactin secretory response. Both Comp B and antisense pretreatment blocked the stimulatory effects of OFQ/N on prolactin release, and Comp B abolished the suckling-induced prolactin response. These studies indicate that OFQ/N is a potent stimulus for prolactin secretion in female rats and that it mediates this effect by rapid and transient inhibition of TIDA neuronal activity. Furthermore, OFQ/N plays a physiologically significant role in the regulation of prolactin secretion during lactation, and it mediates its effects via actions at the ORL-1 receptor subtype.

Lack of involvement of dopamine and serotonin during the orphanin FQ/Nociceptin (OFQ/N)-induced prolactin secretory response

The purpose of these studies was to examine possible mechanisms of Orphanin FQ/Nociceptin (OFQ/N)-induced prolactin release. We investigated the involvement of the dopaminergic neurons by quantifying DOPAC:DA levels in the median eminence and neurointermediate lobe following central administration of OFQ/N to female Sprague-Dawley rats. To specifically determine the involvement of the tuberoinfundibular dopaminergic neurons, immunocytochemical studies were conducted to visualize c-fos protein expression in the arcuate nucleus following central administration of OFQ/N. In addition, the role of serotonergic activation was examined in dose response studies using the selective serotonin antagonist ritansarin and the nonselective antagonist metergoline. Finally, the pharmacological specificity of the prolactin response was examined by pretreating animals with [Nphe1] NC (1-13)NH2, a drug reported to antagonize OFQ/N effects. The results of these studies indicate that the increase in prolactin release following central administration of OFQ/N does not inhibit tuberoinfundibular, tuberohypophyseal or periventricular hypophysial dopaminergic neuronal activity at 10 min after drug administration, a time when prolactin levels were significantly elevated. Furthermore, serotonergic activation is not involved since pharmacological blockade of serotonergic receptors did not alter the prolactin secretory response to OFQ/N. NC (1-13)NH2 did not antagonize the stimulatory effects of OFQ/N on prolactin secretion. The neural effects of OFQ/N on dopaminergic neuronal activity may occur following a different time course than that of the prolactin increase.

Endogenous opiates and behavior: 2003

This paper is the 26th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2003 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Structure-activity studies on high affinity NOP-active hexapeptides

Nociceptin/orphanin FQ (N/OFQ) is a 17 amino acid peptide that is the endogenous ligand for the G-protein coupled receptor ORL1 (NOP), a member of the opioid receptor family. Although it is clear that this receptor system is involved in a variety of physiologic functions, including analgesia, the precise actions of N/OFQ remain largely uncharacterized. One reason for this has been limited number of high-affinity ligands to NOP, and particularly the lack of availability of useful specific antagonists. Herein, we describe the pharmacologic activity of a series of modified amino acid containing modifications of the hexapeptide Ac-RYYRWR-NH2, with high affinity for NOP. These compounds were tested for binding affinity using [3H]N/OFQ binding to human NOP in CHO cells, and functional activity by measuring stimulation of [35S]GTPgammaS-binding in CHO cell membranes. These studies suggest that each Arg of the hexapeptide is required to maintain high-binding affinity. The peptide maintains high affinity if the Tyr2 or Tyr3 are modified, but at least one of these residues must maintain its hydroxyl group or there is a large decrease in intrinsic activity of the peptide.