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An investigation into the mechanisms of inhibition of calcium channel currents in cultured sensory neurones of the rat by guanine nucleotide analogues and (−)− baclofen
Author(s) -
Dolphin A.C.,
McGuirk S.M.,
Scott R.H.
Publication year - 1989
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.1476-5381.1989.tb11950.x
Subject(s) - forskolin , pertussis toxin , gtp' , chemistry , baclofen , guanosine , g protein , calcium , biophysics , calcium channel , agonist , biochemistry , medicine , endocrinology , biology , enzyme , signal transduction , receptor , organic chemistry
1 The mechanism of inhibition of calcium channel currents by the guanine nucleotide analogue guanosine 5′‐O‐3 thiotriphosphate (GTP‐γ‐S) and by the GABA B agonist (−)‐baclofen has been studied in cultured dorsal root ganglion neurones of the rat. The inhibition by GTP‐γ‐S is particularly characterized by an abolition of the transient component of calcium channel currents carried either by Ba 2+ (I Ba ) or by Ca 2+ (I Ca ). 2 The effect of agents increasing intracellular cyclic AMP levels has been examined. Neither internal cyclic AMP nor forskolin prevented the inhibition of I Ba by baclofen. Neither forskolin nor pretreatment of cells with cholera toxin prevented the inhibition of the transient component of I Ba by GTP‐γ‐S. However, both these treatments increased the amplitude of the sustained I Ba in the presence of GTP‐γ‐S. The ATP analogue adenosine imido‐diphosphate which inhibits many ATP requiring enzymes did not prevent the effect of GTP‐γ‐S although it reduced the amplitude of I Ba . 3 Baclofen (100 μ m ) produced a 22 ± 2% increase in inositol phosphate production in 30 s, whereas the increase produced by bradykinin (1 μ m ) was 70 + 14%. However, unlike baclofen, bradykinin did not inhibit I Ba or I Ca in these cells. 4 The effect of protein kinase C inhibitors was examined. Polymixin B (20 μ m in patch pipette) had no effect on the inhibition of I Ba by baclofen or GTP‐γ‐S. A higher concentration (100 μ m ) alone inhibited I Ba and no further inhibition by baclofen was observed. Neither H7 (50 μ m ) nor staurosporine (100 n m ), applied extracellularly, prevented the response to GTP‐γ‐S. 5 The protein kinase C activator di‐octanoyl glycerol (20 μ m ) did not inhibit I Ba . Arachidonic acid (100 μ m ) also produced no inhibition of I Ba . 6 In conclusion we have obtained no evidence that a second messenger system mediates the inhibition of calcium channel currents by GTP‐γ‐S or baclofen in dorsal root ganglion neurones. These results support the hypothesis that GABA B receptors are directly coupled to calcium channels by G proteins.