Nociceptin receptor coupling to a potassium conductance in rat locus coeruleus neurones in vitro

Abstract

1 In this study we have examined the effects of nociceptin, an endogenous ligand for the opioid-like receptor ORLI, on the membrane properties of rat locus coeruleus (LC) neurones in vitro, using intracellular and whole cell patch clamp recording. 2 When locus coeruleus neurones were voltage clamped to -60 mV, application of nociceptin caused an outward current in all cells examined (n = 49), with an EC50 of 90 nm. Neither the potency nor the maximal effect of nociceptin was altered in the presence of the peptidase inhibitors, bestatin (20 gM) or thiorphan (2 pM). 3 The outward currents caused by nociceptin in 2.5 mM extracellular K+ reversed polarity at -123 mV, more negative than the predicted K+ reversal potential of -105 mV. Increasing extracellular K+ to 6.5 mM resulted in a shift of the reversal potential of + 25 mV, a shift consistent with a K+ conductance. The conductance activated by nociceptin showed mild inward rectification.4 Application of a high concentration of nociceptin (3 gM) occluded the current produced by simultaneous application of high concentrations of Met-enkephalin (10 gM), (3 uM) somatostatin and UK 14304 (3 gM), indicating that nociceptin activated the same conductance as p-opioid and somatostatin receptors and a2-adrenoceptors. 6 Dynorphin A (3 gM), another putative endogenous ligand for ORL,, caused a robust outward current in locus coeruleus neurones that was, however, completely antagonized by moderate concentrations of naloxone (300 nM-1 gM). 7 Continuous application of nociceptin (3 gM) resulted in a decrease of the outward current to a steady level of 70% of the maximum response with a t1/2 of 120s. Desensitization was largely homologous because simultaneous application of Met-enkephalin (30 gM) during the desensitized period of the nociceptin response resulted in an outward current that was 92% of control responses to Met-enkephalin in the same cells. Conversely, continuous application of Met-enkephalin (30 gM) resulted in a decrease of Met-enkephalin current to a steady level that was 54% of the initial current. During this desensitized period application of nociceptin (3 gM) resulted in a current that was 78% of the control responses to nociceptin in the same cells. 8 Thus nociceptin potently activates an inwardly rectifying K+ conductance in locus coeruleus neurones, with a pharmacological profile consistent with activation of the ORL, receptor. Dynorphin A does not appear to be a ligand for ORLI in rat locus coeruleus neurones.

Opioid Agonists Have Different Efficacy Profiles for G Protein Activation, Rapid Desensitization, and Endocytosis of Mu-opioid Receptors

“…8, 17, and 76) and the fact that morphine promotes desensitization but not internalization could be interpreted as evidence for MOP desensitization of inhibition of I Ca occurring via a mechanism unrelated to the internalization pathway. Agonist-stimulated endocytosis and recycling of MOP is thought to be regulated by phosphorylation of residues in the C-terminal region of the receptor by GRKs (12,13,77), whereas acute desensitization of MOP might involve phosphorylation of a residue in the second intracellular loop (63). A differential ability to induce phosphorylation of these residues could provide a structural basis for the apparent separation between the efficacy for desensitization and internalization evident with morphine.…”

Buprenorphine: A Unique Drug with Complex Pharmacology

“…Interestingly, this fourth member of the opioid receptor family is also coupled to the same second messenger systems [9,10,22,42,57,61,66,70,78,97]. Thus, activation of the ORL-1 receptor leads to inhibition of the enzyme adenylate cyclase [61,78] and calcium channel conductance [10,42,57].…”

“…Thus, activation of the ORL-1 receptor leads to inhibition of the enzyme adenylate cyclase [61,78] and calcium channel conductance [10,42,57]. Stimulation of the ORL-1 receptor also activates inwardly rectifying potassium channels [9,97].…”

Comparison of the effects of [Phe1Ψ(CH2‐NH)Gly2]Nociceptin (1–13)NH2 in rat brain, rat vas deferens and CHO cells expressing recombinant human nociceptin receptors

“…Despite these observations, the cellular actions of NC are similar to those of opioids. NC activates the NC-receptor to either inhibit adenylyl cyclase (Meunier et al, 1995;Reinscheid et al, 1995) and/or voltage-gated calcium channels (Connor et al, 1996b;Abdulla & Smith, 1997) or to stimulate an inwardly rectifying potassium conductance (Matthes et al, 1996;Connor et al, 1996a;Vaughan & Christie, 1996). These actions are expected to reduce neuronal excitability and neurotransmitter release.…”