LABELING OF 5-HT(3) RECEPTOR RECOGNITION SITES IN THE RAT-BRAIN USING THE 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 and whole rat brain sections was assessed in an attempt to further investigate the binding of agonists to the 5-HT3 receptor. In crude homogenates of rat cortex, no reproducible specific [H-3]mCPBG (1.0 nM) binding (defined by either 10 muM granisetron, 100 muM 5-HT or 100 nM 'cold' mCPBG) was detected. Using autoradiographic techniques, in rat hindbrain sections, [H-3]mCPBG (1.0 nM) labelled a differential distribution of specific binding sites (defined by the inclusion of granisetron, 1.0 muM). Specific binding was only detected within the dorsal vagal complex (nucleus tractus solitarius, area postrema and dorsal motor nucleus of the vagus nerve). An identical distribution of specific binding was detected in adjacent sections incubated with the selective 5-HT3 receptor radioligand, [H-3](S)-zacopride (0.5 nM; non-specific binding defined by the inclusion of granisetron, 1.0 muM). No reproducible specific [H-3]mCPBG (1.0 nM) binding (defined by the inclusion of granisetron, 1.0 muM) was detected within the rat forebrain. In contrast, [H-3](S)-zacopride (0.5 nM) labelled specific sites (defined by the inclusion 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 radioligand appears to mask the specific binding in regions which do not express relatively high densities of the 5-HT3 receptor.