LABELING OF 5-HT(3) RECEPTOR RECOGNITION SITES IN THE RAT-BRAIN USINGTHE AGONIST RADIOLIGAND [H-3] META-CHLOROPHENYLBIGUANIDE

The binding of the tritiated derivative of the 5-HT3 receptor agonist meta-chlorophenylbiguanide ([H-3]mCPBG) to rat cortical homogenates an d whole rat brain sections was assessed in an attempt to further inves tigate the binding of agonists to the 5-HT3 receptor. In crude homogen ates of rat cortex, no reproducible specific [H-3]mCPBG (1.0 nM) bindi ng (defined by either 10 muM granisetron, 100 muM 5-HT or 100 nM 'cold ' mCPBG) was detected. Using autoradiographic techniques, in rat hindb rain sections, [H-3]mCPBG (1.0 nM) labelled a differential distributio n of specific binding sites (defined by the inclusion of granisetron, 1.0 muM). Specific binding was only detected within the dorsal vagal c omplex (nucleus tractus solitarius, area postrema and dorsal motor nuc leus of the vagus nerve). An identical distribution of specific bindin g was detected in adjacent sections incubated with the selective 5-HT3 receptor radioligand, [H-3](S)-zacopride (0.5 nM; non-specific bindin g defined by the inclusion of granisetron, 1.0 muM). No reproducible s pecific [H-3]mCPBG (1.0 nM) binding (defined by the inclusion of grani setron, 1.0 muM) was detected within the rat forebrain. In contrast, [ H-3](S)-zacopride (0.5 nM) labelled specific sites (defined by the inc lusion of granisetron, 1.0 muM) in some limbic brain structures (e.g. cerebral cortex, hippocampus, amygdala). These studies indicate that [ H-3]mCPBG labels the 5-HT3 receptor in rat brain tissue. However, the relatively high level of non-specific binding associated with this rad ioligand appears to mask the specific binding in regions which do not express relatively high densities of the 5-HT3 receptor.