Selective labeling of pre-synaptic receptors by 3H-dopamine, 3H-apomorphine and 3H-clonidine; labeling of post-synaptic sites by 3H-neuroleptics.

To test the hypothesis that nM concentrations of ³H-dopamine, ³H-apomorphine and ³H-clonidine prefer pre-synaptic sites, while ³H-neuroleptics and ³H-dihydroergocryptine prefer post-synaptic sites, we tested catecholaminergic agonists and antagonists on the binding of these radio-ligands to calf caudate tissue. 1) Dopamine agonists (apomorphine, NPA and bromocryptine) inhibited ³H-spiperone binding, but not ³H-dopamine binding, in direct correlation to their clinical potencies. 2) Dopamine agonists inhibited ³H-apomorphine binding at concentrations identical to those causing pre-synaptic cardio-inhibition. 3) The IC₅₀ values for ³H-dihydroergocryptine binding of alpha-adrenoceptor drugs did not correlate with the pre-synaptic IC₅₀ values for affecting noradrenaline release; those for ³H-clonidine did. The 3 findings are compatible with the working hypothesis.