Correlating the pharmacology and molecular biology of opioid receptors. Cloning and antisense mapping a kappa 3-related opiate receptor

Dimerization of morphine and orphanin FQ/nociceptin receptors: generation of a novel opioid receptor subtype

Although orphanin FQ/nociceptin (OFQ/N) receptors are a member of the opioid receptor family of receptors, they bind traditional opioids with very poor affinity. We now demonstrate that mu opioid receptors can physically associate with OFQ/N receptors, resulting in a complex with a unique binding selectivity profile. Immunoprecipitation of epitope-tagged OFQ/N receptors co-precipitates mu receptors. When the two receptors were co-expressed in CHO cells, [3H]OFQ/N retained its high binding affinity for its receptor. However, co-expression of the two receptors increased by up to 250-fold the affinity of a series of opioids in [3H]OFQ/N binding assays. This enhanced affinity was limited to agonists with high affinity for mu receptors. Selective kappa(1) and delta opioids did not lower binding. Despite the dramatic increase in affinity for the opioid agonists in co-expressing cells, the opioid antagonists naloxone and diprenorphine failed to compete [3H]OFQ/N binding.

Role of endogenous opioid peptides in mediating progesterone-induced disruption of the activation and transmission stages of the GnRH surge induction process

How progesterone blocks the E2-induced GnRH surge in females is not known. In this study we assessed whether the endogenous opioid peptides (EOPs) that mediate progesterone negative feedback on pulsatile GnRH secretion also mediate the blockade of the GnRH surge. We treated ovariectomized ewes with physiological levels of E2 and progesterone to stimulate and block the GnRH surge, respectively, using LH secretion as an index of GnRH release. A pilot study confirmed that blocking opioidergic neurotransmission with the opioid receptor antagonist, naloxone (NAL; 1 mg/kg.h, i.v.), could prevent the suppression of pulsatile LH secretion by progesterone in our model. By contrast, antagonizing EOP receptors with NAL did not restore LH surges in ewes in which the E2-induced GnRH surge was blocked by progesterone treatment during the E2-dependent activation stage (Exp 1) of the GnRH surge induction process. However, in ewes treated with progesterone during the E2-independent transmission stage (Exp 2), NAL partially restored blocked LH surges, as indicated by increased fluctuations in LH that, in some cases, resembled LH surges. We conclude, therefore, that the EOPs that mediate progesterone negative feedback on pulsatile GnRH secretion are not involved in blockade of activation of the E2-induced GnRH surge by progesterone, but do appear to be part of the mechanism by which progesterone disrupts the transmission stage.

Functional blockade of opioid analgesia by orphanin FQ/nociceptin

Orphanin FQ/nociceptin (OFQ/N) is a recently identified neuropeptide with high affinity for the orphan opioid receptor. OFQ/N blocked morphine analgesia in mice in a dose-dependent manner, as well as the analgesic actions of [D-Pen2, D-Pen5]enkephalin (DPDPE), morphine-6 beta-glucuronide, trans-3,4-dichloro-N-[2-(1-pyrrolindinyl)-cyclohexyl]-benzeneac eta mide, methane sulfonate hydrate (U50,488H) and naloxone benzoylhydrazone. These actions are anti-analgesic, because OFQ/N also blocked clonidine analgesia and OFQ/N was inactive against the inhibition of gastrointestinal transit by morphine. Although OFQ/N was quite potent in these paradigms, two truncated forms, OFQ/N(1-11) and OFQ/N(1-7), were inactive. An antisense oligodeoxynucleotide targeting the first coding exon of KOR-3, the mouse homolog of the orphan opioid receptor, effectively prevented the anti-opioid actions of OFQ/N, confirming the importance of the orphan opioid receptor in this action.