A pharmacological comparison of [3H]-granisetron binding sites in brain and peripheral tissues of the mouse

The affinities of a range of structurally diverse 5-HT₃ receptor agonists and antagonists for ³H]-granisetron binding sites have been measured in membrane homogenates prepared from central and peripheral tissues of the mouse. By comparing the affinities of compounds across these tissues, the question of whether intea-species 5-HT₃ receptor subtypes exist in the mouse has been addressed.In entorhinal cortex and brainstem, ³H]-granisetron bound to a single high affinity saturable binding site (K_d 0.47 ± 0.14 and 0.60 ± 0.05 nM; B _max 20 ± 6 and 7 ± 2 fmol (mg protein)^–1 respectively; mean ±SEM; n = 3). In distal and proximal colon, the specific binding of ³H]-granisetron was best fitted to a 2-site model. K_d values obtained for the high affinity site were similar to those obtained in brain tissue (distal colon: 0.47 ± 0.09 nM, n = 4; proximal colon: 0.39 ± 0.09 nM, n = 4). In salivary gland, 2-sites were evident in 2 out of 4 experiments. The K_d value (calculated from the high affinity site in the 2-site model) was approximately 10-fold less than in brain or colon (3.3 ± 1.1 nM, n = 4). B _max values were 7 ± 2, 4 ± 1 and 71 ± 16 fmol (mg protein)^–1 for distal colon, proximal colon and salivary gland respectively. For all tissues the estimated affinity of the low affinity site was variable, and B _max values could not be reliably calculated.Extensive comparative studies performed with 17 different 5-HT₃ receptor agonists and antagonists in the five tissues did not reveal differences in affinity for any compound between the entorhinal cortex and the brainstem nor between the two regions of the colon. However, MDL72222, R-zacopride, d-tubocurarine, and GR80284 apparently had significantly lower affinity for colon than brain binding sites. Also, MDL72222, 2-methyl-5-HT, GR80284, 1-(m-chlorophenyl)-biguanide, metoclopramide, and granisetron had significantly lower affinity for the salivary gland binding sites than the brain binding sites. In an attempt to replicate these observations, we conducted a second study using the compounds which had shown the largest inter-tissue differences in affinity keeping as many variables as possible constant. Simultaneous comparative assays on entorhinal cortex, colon and salivary gland homogenates taken from the same mice showed that the differences that were apparent in the initial comparative study were not maintained. In conclusion, we can find no clear evidence for the existence of tissue-specific subtypes of the 5-HT₃ high affinity binding site for ³H]-granisetron in the mouse in the tissues tested. However, a low affinity binding site for ³H]-granisetron was detected in peripheral tissues.