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Nociceptin receptor coupling to a potassium conductance in rat locus coeruleus neurones in vitro
Author(s) -
Connor Mark,
Vaughan Christopher W.,
Chieng Billy,
Christie Macdonald J.
Publication year - 1996
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1996.tb16080.x
Subject(s) - nociceptin receptor , chemistry , locus coeruleus , reversal potential , opioid receptor , medicine , endocrinology , antagonist , opioid peptide , biophysics , opioid , receptor , patch clamp , biology , biochemistry , central nervous system
1 In this study we have examined the effects of nociceptin, an endogenous ligand for the opioid‐like receptor ORL 1 , 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 EC 50 of 90 nM. Neither the potency nor the maximal effect of nociceptin was altered in the presence of the peptidase inhibitors, bestatin (20 μ m ) or thiorphan (2 μ m ). 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 μ m ) occluded the current produced by simultaneous application of high concentrations of Met‐enkephalin (10 μ m ), (3 μ m ) somatostatin and UK 14304 (3 μ m ), indicating that nociceptin activated the same conductance as μ‐opioid and somatostatin receptors and α 2 ‐adrenoceptors. 5 The actions of nociceptin were weakly antagonized by the opioid antagonist, naloxone, with p K b ′s estimated from 2 cells of −4.23 and −4.33. The μ‐opioid antagonist, CTAP (D‐Phe‐Cys‐Tyr‐D‐Trp‐Arg‐Pen‐Thr‐NH 2 , 1 μ m ), the opioid antagonist, nalorphine (30 μ m ), or the somatostatin antagonist, CPP ( cyclo (7‐aminoheptanoyl‐Phe‐D‐Tip‐Lys‐Thr[Bzl]) 3 μ m ) did not affect the nociceptin‐induced current. 6 Dynorphin A (3 μ m ), another putative endogenous ligand for ORL 1 , caused a robust outward current in locus coeruleus neurones that was, however, completely antagonized by moderate concentrations of naloxone (300 nM‐1 μ m ). 7 Continuous application of nociceptin (3 μ m ) resulted in a decrease of the outward current to a steady level of 70% of the maximum response with a t 1/2 of 120s. Desensitization was largely homologous because simultaneous application of Met‐enkephalin (30 μ m ) 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 μ m ) 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 μ m ) 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 1 receptor. Dynorphin A does not appear to be a ligand for ORL 1 in rat locus coeruleus neurones.