Abstract
1. The technique of microelectrophoresis was used to compare the actions of octopamine, noradrenaline and dopamine on single cortical neurones of the rat. 2. Octopamine both excited and depressed neurones of the cortex. Frequently cells depressed by noradrenaline were excited by octopamine; occasionally the converse was true. The time courses of action of the two amines also differed. Dopamine-elicited excitations were observed, but also were not correlated with octopamine-elicited effects. 3. When octopamine and noradrenaline both caused depressant effects, octopamine frequently was of less apparent potency than noradrenaline. When these amines were excitatory, octopamine appeared at least as and sometimes more potent than noradrenaline. 4. Octopamine was only weakly effective on cortical neurones identified by antidromic stimulation of the pyramidal tract, or synaptically excited by stimulation of the ventrobasal thalamus. 5. alpha-Flupenthixol and propranolol were without effect on octopamine-elicited changes in firing rate at doses which were effective in blocking the actions of dopamine and noradrenaline respectively. Metoclopramide did not block the actions of any of the three agonists, but had strong effects of its own. 6. The results suggest that receptors sensitive to octopamine, and which appear to be pharmacologically distinct from those previously categorized as noradrenaline and dopamine receptors, may exist on central neurones of the rat.