文摘
Increasing evidence indicates that the 5-HT3 receptor antagonist R(+)-zacopride labels an additional site in brain tissue that is not sensitive to 5-HT (non-5-HT R(+)-zacopride site, R(+)-site). Since the levels of R(+)-sites in the brain are relatively low, the present studies explored the use of []R(+)-zacopride to label the R(+)-site; the incorporation of an [] atom considerably increasing the specific activity of the radioligand relative to []R(+)-zacopride that has been utilised previously. Competition experiments with []R(+)-zacopride (1.0 nM) binding to rat whole brain homogenates, in the presence of the 5-HT3 receptor antagonist granisetron (1.0 μM), identified that R(+)-zacopride and prazosin bound to two sites (pIC50: 7.59 and 5.28, respectively, for R(+)-zacopride; 6.75 and 4.42, respectively, for prazosin) whereas S(−)-zacopride and mianserin possessed relatively low affinity (pIC50: 4.37 and 3.80, respectively) while (−)sulpiride and 5-HT failed to compete for []R(+)-zacopride binding at concentrations up to 10 μM. Autoradiographic radioligand binding studies using []R(+)-zacopride (0.5 nM) identified a heterogeneous distribution of specific binding sites (defined by unlabelled R(+)-zacopride, 1.0 μM) throughout the rat brain. In the presence of a saturating concentration of granisetron (1.0 μM), highest levels of specific []R(+)-zacopride binding sites (defined by R(+)-zacopride, 1.0 μM; R(+)-site), were detected in the olfactory tubercle, thalamus, corpus callosum, colliculus, dorsal and median raphe nucleus, spinal cord and the pons (8.0–13.0 fmol/mg). Moderate densities of R(+)-sites were located in the striatum, nucleus accumbens, substantia nigra, ventral tegmental area, globus pallidus, septal nuclei, frontal cortex and cerebellum (2.0–7.9 fmol/mg). In the hippocampus, amygdala and cortical areas, R(+)-site levels were low but detectable (0.1–1.9 fmol/mg). []R(+)-zacopride labelled R(+)-sites were also detected in some rat peripheral tissues, for instance kidney cortex, adrenal gland and liver (2.4–6.8 fmol/mg). The present results indicate that specific non-5-HT []R(+)-zacopride sites are heterogeneously distributed throughout the rat brain and are expressed in various peripheral tissues.