Abstract

The technique of microelectrophoresis was used in order to compare the actions of noradrenaline and dopamine on single cortical neurones in the rat. Both excitatory and depressant responses could be evoked by both catecholamines; every neurone studied responded in the same direction to the two drugs. In the case of both excitatory and depressant responses, noradrenaline consistently appeared to be more potent than dopamine; this potency difference was presumably of biological origin, since the transport number of dopamine was somewhat higher than that of noradrenaline. Excitatory responses to both catecholamines could be antagonized by the α-adrenoceptor antagonist. phenoxybenzamine, and by the neuroleptics, haloperidol and α-flupenthixol. However, phenoxybenzamine had a more pronounced effect on responses to noradrenaline than on responses to dopamine, whereas the neuroleptics showed a greater antagonistic effect on responses to dopamine than on responses to noradrenaline. Responses to acetylcholine were not affected by the antagonists. β-Flupenthixol was a much less effective and a less specific antagonist of responses to dopamine than was α-flupenthixol. These findings suggest that excitatory responses of cortical neurones to the catecholamines may be mediated by two populations of receptors: α-adrenoceptors and excitatory dopamine receptors.

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