Abstract
On account of the postulated existence of 5-HT 3 receptor subtypes, the respective physicochemical and pharmacological properties of specific binding sites for the potent 5-HT 3 antagonist [ 3H]zacopride were compared using membranes from the rat posterior cortex or neuroblastoma-glioma NG 108-15 clonal cells. In both membrane preparations, [ 3H]zacopride bound to a single class of specific sites with a K d close to 0.5 nM. However, the B max value in NG 108-15 cell membranes (970 ± 194 mol/ mg protein) was approximately 50 times larger than that in cortical membranes (19 ± 2 mol/mg protein). The specific binding of [ 3H]zacopride was equally affected by temperature, pH and molarity of the assay medium, and equally insensitive to thiol- and disulfide-reagents ( N-ethylmaleimide, p-chloro-mercuribenzene sulfonic acid, dithiothreitol) and GTP in cortical as well as NG 108-15 cell membranes. Determination of the molecular size of [ 3H]zacopride specific binding sites by radiation inactivation yielded values close to 35 kDa for both membrane preparations. Finally, a highly significant positive correlation ( r = 0.979) was found between the respective p K i values of 34 different drugs for their inhibition of [ 3H]zacopride specific binding to cortical or NG 108-15 cell membranes. Among them, the most potent was S(−)zacopride (p K i = 9.55), followed by BRL 43964, ICS 205-930, quipazine, R(+)zacopride, GR 38032F and MDL 72222. Atyplcal antidepressants (mianserin, amoxapine) and neuroleptics (clotiapine, loxapine and clozapine) were active in rather low concentrations (p K i < 6.5), suggesting that recognition of 5-HT 3 sites might be relevant to part of the in vivo effects of these drugs. Such identical physico-chemical and pharmacological properties of [ 3H]zacopride specific binding in cortical and NG 108-15 cell membranes strongly suggest that the same 5-HT 3 receptor (subtype ?) exists in these two preparations.
Published Version
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