A PHARMACOLOGICAL COMPARISON OF [H-3] GRANISETRON BINDING-SITES IN BRAIN AND PERIPHERAL-TISSUES OF THE MOUSE

The affinities of a range of structurally diverse 5-HT3 receptor agoni sts and antagonists for [H-3]-granisetron binding sites have been meas ured in membrane homogenates prepared from central and peripheral tiss ues of the mouse. By comparing the a affinities of compounds across th ese tissues, the question of whether intra-species 5-HT3 receptor subt ypes exist in the mouse has been addressed. In entorhinal cortex and b rainstem, [H-3]-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 a nd 7 +/- 2 fmol (mg protein)(-1) respectively; mean +/- SEM; n = 3). I n distal and proximal colon, the specific binding of [H-3]-granisetron was best fitted to a 2-site model. K-d values obtained for the high a ffinity site were similar to those obtained in brain tissue (distal co lon: 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. T he K-d value (calculated from the high affinity site in the 2-site mod el) 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 resp ectively. For all tissues the estimated affinity of the low affinity s ite was variable, and B-max values could not be reliably calculated. E xtensive comparative studies performed with 17 different 5-HT3 recepto r agonists and antagonists in the five tissues did not reveal differen ces 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 significantl y lower affinity for colon than brain binding sites. Also, MDL72222, 2 -methyl-5-HT, GR80284, 1-(m-chlorophenyl)-biguanide, metoclopramide, a nd granisetron had significantly lower affinity for the salivary gland binding sites than the brain binding sites. In an attempt to replicat e these observations, we conducted a second study using the compounds which had shown the largest inter-tissue differences in affinity keepi ng as many variables as possible constant. Simultaneous comparative as says on entorhinal cortex, colon and salivary gland homogenates taken from the same mice showed that the differences that were apparent in t he initial comparative study were not maintained. In conclusion, we ca n find no clear evidence for the existence of tissue-specific subtypes of the 5-HT3 high affinity binding site for [H-3]-granisetron in the mouse in the tissues tested. However, a low affinity binding site for [H-3]-granisetron was detected in peripheral tissues.