We have further characterized the 5-HT3 receptors in rat and rabbit tissues by evaluating the binding of the 5-HT3 receptor ligand, [H-3]GR67330 to homogenates of rabbit ileum, rat ileum and rat brain (entorhinal cortex). In each tissue specific [H-3]GR67330 binding represented a single saturable, high affinity site (K(d) = 0.14, 0.18, 0.076 nM in rabbit ileum, rat ileum and rat brain respectively). The densities of sites present in rabbit and rat ileum were similar to that present in rat brain (B(max) = 63, 47, 72 fmol/mg protein in rabbit ileum, rat ileum and rat brain respectively). In each tissue, 5-HT3 receptor agonists and antagonists potently competed for [H-3]GR67330 binding. Derived inhibition constants were similar in rat ileum and brain. However marked differences in IC50s were apparent for rabbit ileum compared with rat brain or ileum. These were most apparent with agonists. Thus. mCPBG [1-(meta-chlorophenylbiguanide)], phenylbiguanide, 5-HT and 2-methyl 5-HT were at least 5 times less potent to inhibit [H-3]GR67330 binding in rabbit ileum that rat brain. The most pronounced differences were evident with phenylbiguanide and mCPBG which were approximately 70 and approximately 300 times less potent in the rabbit ileum respectively compared with the rat tissues. These differences were unlikely to be due to depletion effects because tissue combination experiments (rabbit ileum and rat brain) yielded biphasic inhibition curves for phenylbiguanide with affinities for each component similar to those in the individual tissues. Antagonist affinities also varied between the rabbit and rat tissues, although less markedly. Amongst the antagonists, the most marked differences were apparent with SDZ 206-830 and quipazine each being approximately 10 times less potent to inhibit binding to rabbit than rat tissue. Hill coefficients for inhibition of binding varied with tissue. In rat brain, as previously described for [H-3]GR67330. Hill coefficients for agonist (and quipazine) inhibition of binding were greater than unity. This was less marked in rat and rabbit ileum tissues. The present studies provide further evidence for species variation in 5-HT3 receptors.
