title: Mu-opioid and GABA(B) receptors modulate different types of Ca2+ currents in rat nodose ganglion neurons.
abstract: Whole-cell patch-clamp recordings were obtained from nodose ganglion neurons acutely dissociated from 10-30-day-old rats to characterize the Ca2+ channel types that are modulated by GABA(B) and mu-opioid receptors. Five components of high-threshold current were distinguished on the basis of their sensitivity to blockade by omega-conotoxin GVIA, nifedipine, omega-agatoxin IVA and omega-conotoxin MVIIC. Administration of the mu-opioid agonist H-Tyr-D-Ala-Gly-Phe(N-Me)-Gly-ol (0.3-1 mM) or the GABA(B) agonist baclofen in saturating concentrations suppressed high-threshold Ca2+ currents by 49.9+/-2.4% (n=69) and 18.7+/-2.1% (n=35), respectively. The inhibition by H-Tyr-D-Ala-Gly-Phe(N-Me)-Gly-ol exceeded that by baclofen in virtually all neurons that responded to both agonists (67%), and occlusion experiments revealed that responses to mu-opioid and GABA(B) receptor activation were not linearly additive. In addition, administration of staurosporine, a non-selective inhibitor of protein kinase A and C, did not affect the inhibitory responses to either agonist or prevent the occlusion of baclofen-induced current inhibition by H-Tyr-D-Ala-Gly-Phe(N-Me)-Gly-ol. Blockade of N-type channels by omega-conotoxin GVIA eliminated current suppression by baclofen in all cells tested (n=11). Mu-opioid-induced inhibition in current was abolished by omega-conotoxin GVIA in 12 of 30 neurons tested, but was only partially reduced in the remaining 18 neurons. In the latter cells administration of omega-agatoxin IVA reduced, but did not eliminate the mu-opioid sensitive current component that persisted after blockade of N-type channels. This residual component of mu-opioid-sensitive current was blocked completely by omega-conotoxin MVIIC in nine neurons, whereas responses to H-Tyr-D-Ala-Gly-Phe(N-Me)-Gly-ol were still recorded in the remaining cells after administration of these Ca2+ channel toxins and nifedipine. Dihydropyridine-sensitive (L-type) current was not affected by activation of mu-opioid or GABA(B) receptors in any of the neurons. These data indicate that in nodose ganglion neurons mu-opioid receptors are negatively coupled to N-, P- and Q-type channels as well as to a fourth, unidentified toxin-resistant Ca2+ channel. In contrast, GABA(B) receptors are coupled only to N-type channels. Furthermore, the results do not support a role for either protein kinase C or A in the modulatory pathway(s) coupling mu-opioid and GABA(B) receptors to Ca2+ channels, but rather lend credence to the notion that the signalling mechanisms utilized by these two receptors might simply compete for inhibitory control of a common pool of N-type channels.
authors.affiliation: Department of Physiology, University of Michigan Medical School, Ann Arbor 48109-0622, USA.
authors.forename: K I
authors.initials: KI
authors.lastname: Rusin
authors.forename: H C
authors.initials: HC
authors.lastname: Moises
mesh_terms: D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester / Q000494:pharmacology; D000701:Analgesics, Opioid / Q000494:pharmacology; D000818:Animals; D001418:Baclofen / Q000031:analogs & derivatives / Q000494:pharmacology; D002104:Cadmium / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002120:Calcium Channel Agonists / Q000494:pharmacology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000502:physiology; D020746:Calcium Channels, L-Type; D020864:Calcium Channels, N-Type; D004305:Dose-Response Relationship, Drug; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D004745:Enkephalins / Q000494:pharmacology; D018755:GABA Agonists / Q000494:pharmacology; D018756:GABA Antagonists / Q000494:pharmacology; D009270:Naloxone / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D009419:Nerve Tissue Proteins / Q000502:physiology; D009474:Neurons / Q000737:chemistry / Q000378:metabolism*; D009543:Nifedipine / Q000494:pharmacology; D009620:Nodose Ganglion / Q000737:chemistry / Q000166:cytology* / Q000378:metabolism; D018408:Patch-Clamp Techniques; D010455:Peptides / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D018080:Receptors, GABA-B / Q000502:physiology*; D017450:Receptors, Opioid, mu / Q000819:agonists / Q000037:antagonists & inhibitors / Q000502:physiology*; D013111:Spider Venoms / Q000494:pharmacology; D020867:omega-Agatoxin IVA; D020866:omega-Conotoxin GVIA; D020960:omega-Conotoxins
chemical_list: D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; D000701:Analgesics, Opioid; D001418:Baclofen; D002104:Cadmium; D002118:Calcium; D002120:Calcium Channel Agonists; D002121:Calcium Channel Blockers; D015220:Calcium Channels; D020746:Calcium Channels, L-Type; D020864:Calcium Channels, N-Type; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D004745:Enkephalins; D018755:GABA Agonists; D018756:GABA Antagonists; D009270:Naloxone; D009292:Narcotic Antagonists; D009419:Nerve Tissue Proteins; D009543:Nifedipine; D010455:Peptides; D018080:Receptors, GABA-B; D017450:Receptors, Opioid, mu; D013111:Spider Venoms; D020867:omega-Agatoxin IVA; D020866:omega-Conotoxin GVIA; D020960:omega-Conotoxins; C075851:omega-conotoxin-MVIIC; C080629:saclofen