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Agonist interactions with 5‐HT 3 receptor recognition sites in the rat entorhinal cortex labelled by structurally diverse radioligands
Author(s) -
Barnes J.M.,
Barnes N.M.,
Costall B.,
Jagger S.M.,
Naylor R.J.,
Robertson D.W.,
Roe S.Y.
Publication year - 1992
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.1476-5381.1992.tb14283.x
Subject(s) - quipazine , granisetron , entorhinal cortex , agonist , chemistry , 5 ht3 receptor , receptor , endocrinology , 5 ht receptor , medicine , stereochemistry , serotonin , hippocampus , biology , biochemistry , antiemetic , vomiting
1 The pharmacological properties of 5‐HT 3 receptor recognition sites labelled with [ 3 H]‐(S)‐zacopride, [ 3 H]‐LY278,584, [ 3 H]‐granisetron and [ 3 H]‐GR67330 in membranes prepared from the rat entorhinal cortex were investigated to assess the presence of cooperativity within the 5‐HT 3 receptor complex. 2 In rat entorhinal cortex homogenates, [ 3 H]‐(S)‐zacopride, [ 3 H]‐LY278,584, [ 3 H]‐granisetron and [ 3 H]‐GR67330 labelled homogeneous densities of recognition sites (defined by granisetron, 10 μ m ) with high affinity ( B max = 75 ± 5, 53 ± 5, 92 ± 6 and 79 ± 6 fmol mg −1 protein, respectively; p K d = 9.41 ± 0.04, 8.69 ± 0.14, 8.81 ± 0.06 and 10.14 ± 0.04 for [ 3 H]‐(S)‐zacopride, [ 3 H]‐LY278,584, [ 3 H]‐granisetron and [ 3 H]‐GR67330, respectively, n = 3–8). 3 Quipazine and granisetron competed for the binding of each of the radioligands in the rat entorhinal cortex preparation at low nanomolar concentrations (pIC 50 ; quipazine 9.38–8.51, granisetron 8.62–8.03), whilst the agonists, 5‐hydroxytryptamine (5‐HT), phenylbiguanide (PBG) and 2‐methyl‐5‐HT competed at sub‐micromolar concentrations (pIC 50 ; 5‐HT 7.16–6.42, PBG 7.52–6.40, 2‐methyl‐5‐HT 7.38–6.09). 4 Competition curves generated with increasing concentrations of quipazine, PBG, 5‐HT and 2‐methyl‐5‐HT displayed Hill coefficients greater than unity when the 5‐HT 3 receptor recognition sites in the entorhinal cortex preparation were labelled with [ 3 H]‐LY278,584, [ 3 H]‐granisetron and [ 3 H]‐GR67330. These competing compounds displayed Hill coefficients of around unity when the sites were labelled with [ 3 H]‐(S)‐zacopride. Competition for the binding of [ 3 H]‐(S)‐zacopride, [ 3 H]‐LY278,584, [ 3 H]‐granisetron and [ 3 H]‐GR67330 by granisetron generated Hill coefficients around unity. 5 The nature of the interaction of competing compounds (quipazine, granisetron, PBG, 5‐HT, 2‐methyl‐5‐HT) for the [ 3 H]‐(S)‐zacopride binding site in the rat entorhinal cortex preparation was not altered by the removal of the Krebs ions or the addition of the monoamine oxidase inhibitor, pargyline, to the HEPES/Krebs buffer. 6 In conclusion, the present studies provide further evidence towards the presence of cooperativity within the 5‐HT 3 receptor macromolecule and indicate that either [ 3 H]‐(S)‐zacopride labels a different site on the receptor complex from [ 3 H]‐LY278,584, [ 3 H]‐granisetron or [ 3 H]‐GR67330, or it binds in such a manner as to prevent the conformatory change in the receptor protein responsible for the cooperative binding of agonists (and quipazine).