Characterisation of the non-5-HT3 high-affinity 'R' binding site for (R)-zacopride in brain and other tissues

Previous studies showed that whereas the potent 5-HT3 receptor antagonist (S)-[3H]zacopride only labels 5-HT3 receptor binding sites, the (R)-enantiomer, (R)-[3H]zacopride, labels these receptors and another class of high-affinity binding sites, named the R sites, in membranes from the rat cerebral cortex and NG 108-15 clonal cells (Kidd et al., Eur. J. Pharmacol. 211, 133, 1992). Further studies of R sites revealed that they existed not only in the cerebral cortex but also in various other areas of the rat brain and spinal cord. In addition, R sites were also found in post-mortem human brain tissues. Both in the rat and in man, the regional distribution of central R sites was markedly different from that of 5-HT3 receptors specifically labelled with (S)-[3H]zacopride. Under appropriate conditions for the specific labelling of R sites (with (R)-[3H]zacopride in the presence of 1.0 microM ondansetron to saturate 5-HT3 receptor binding sites--and 0.1 mM mianserin for the determination of non-specific binding), these R sites were also found in rat peripheral tissues (intestine > spleen > kidney > testicles = liver > adrenals > lung > heart). At least in the kidney and the liver, the pharmacological profile of R sites corresponded exactly to that found in NG 108-15 cells. R sites were also detected in membranes from C6 glioma cells and glial cells cultured from the whole cortex of new born rats. In contrast, no specific binding of (R)-[3H]zacopride to R sites could be found in membranes from N1E-115 neuroblastoma cells. Conversely, 5-HT3 receptors could be labelled by (S)-[3H]zacopride in the latter cells but not in C6 glioma and cultured glial cells. As expected from their glial location, the density of R sites increased in the rat hippocampus lesioned with kainic or ibotenic acid to induce local gliosis. In contrast, the density of hippocampal 5-HT3 receptors was unchanged in lesioned rats. Finally, the determination of the apparent molecular size of R sites by radiation inactivation gave a value (approximately 30 kDa) which was significantly lower than that of 5-HT3 receptor binding sites in the rat entorhinal cortex (40 kDa) and NG 108-15 cells (57 kDa). All these data clearly showed that R sites and 5-HT3 receptors are different molecular species. Whether R sites mediate the 5-HT3 receptor-unrelated actions of (R)-zacopride deserves further investigations.