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The effects of microiontophoretically applied noradrenaline, dopamine, and 5-hydroxytryptamine were investigated on spontaneously firing neurones of rat cerebral cortex. It was observed that noradrenaline invariably produced an excitation of the firing and similar responses were seen with dopamine. However, 5-hydroxytryptamine had a consistent depressant effect on the firing rate. When applied on the same neurones, noradrenaline and dopamine produced almost similar excitatory responses and had a similar time course of action. On the other hand, neurones which were excited by noradrenaline and dopamine were inhibited by 5-hydroxytryptamine. While excitatory responses of noradrenaline and dopamine were equally affected by the α-adrenergic blocking agent phentolamine hydrochloride, 5-hydroxytryptamine induced depressant responses were unaffected by its antagonist methysergide. The results suggest that noradrenaline has an excitatory effect on the cortical neurones of rats. Dopamine seems to act in the same way and possibly on the same receptors, whereas 5-hydroxytryptamine appears to have a predominantly inhibitory role.
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