Abstract
We have investigated the effects of sigma ligands [1,3-di(2-tolyl)guanidine (DTG) and (+)-pentazocine] on the electrical activity of cultured frog pituitary melanotrope cells by using the patch-clamp technique. DTG and (+)-pentazocine (10 μM each) induced a reversible depolarization associated with an increase in membrane resistance and action potential firing. In voltage-clamp experiments, DTG and (+)-pentazocine elicited inward currents whose intensity augmented with membrane depolarization. The currents vanished or reversed between -90 and -100 mV, at values close to the K+ equilibrium potential (EK+ = -102 mV). DTG (2–500 μM) and (+)-pentazocine (0.2–200 μM) reduced the outward delayed rectifier K+ current [IK(V)] in a dose-dependent manner with EC50 of 64 and 37 μM, respectively. In contrast, naloxone (50 μM) and pirenzepine (10 μM) did not affect the sigma ligand-induced inhibition of IK (V). Addition of guanosine-5′-O-(3-thiophosphate) in the pipette solution irreversibly sustained the DTG-induced current whereas guanosine-5′-O-(2-thiodiphosphate) virtually suppressed the response. Cholera toxin-pretreatment (1 μg/ml; 18 hr) abolished the inward current and the inhibition of IK (V) induced by sigma ligands. In contrast, pretreatment with pertussis toxin (1 μg/ml; 18 hr) had no effect. Taken together, these data indicate that DTG and (+)-pentazocine activate the electrical activity of cultured frog melanotrope cells by reducing both a tonic K+ current and a voltage-dependent [IK (V)] K+conductance through the activation of a cholera toxin-sensitive G-protein.
Footnotes
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Send reprint requests to: Pr. Lionel Cazin, European Institute for Peptide Research (IFRMP No. 23), Laboratory of Cellular and Molecular Neuroendocrinology, INSERM U413, UA CNRS, University of Rouen, 76821 Mont-Saint-Aignan, France.
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↵1 This work was supported by grants from INSERM (U 413), the Institut de Recherche Jouveinal, the European Union (Human Capital and Mobility Programme; ERBCHRXCT920017) and the Conseil Régional de Haute-Normandie. O.S. was a recipient of a scholarship from the Fonds de la Recherche et de la Technologie (CIFRE program).
- Abbreviations:
- α-MSH
- α-melanocyte-stimulating hormone
- CTX
- cholera toxin
- DTG
- 1,3-di(2-tolyl)guanidine
- EGTA
- ethylene glycol-bis(β-aminoethylether)-N, N,N′,N′-tetraacetic acid
- GDPβS
- guanosine-5′-O-(2-thiodiphosphate)
- GTP
- guanosine triphosphate
- GTPγS
- guanosine-5′-O-(3-thiophosphate)
- HEPES
- N-(2-hydroxyethyl)piperazine-N′-(2-ethanesulfonic acid)
- IK(V)
- voltage-dependent delayed-rectifier outward potassium current
- NMDG
- N-methyl-d-glucamine
- NPY
- neuropeptide Y
- PCP
- phencyclidine
- (±)-SKF10047
- PTX, pertussis toxin
- (±)-N-allylnormetazocine.
- Received October 20, 1997.
- Accepted March 6, 1998.
- The American Society for Pharmacology and Experimental Therapeutics
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