The inhibitory action of monoamines on lateral geniculate neurones

1. L ‐Noradrenaline (NA), dopamine, 5‐hydroxytryptamine (5‐HT) and lysergide were administered iontophoretically to neurones in the dorsal lateral geniculate nucleus of the cat and the responses to these drugs recorded. 2. Many neurones were depressed by the monoamines and lysergide. 3. This depression was manifested by a reduction or abolition of the effects of optic nerve or visual stimulation, by a failure in some instances of an antidromically propagating spike to invade the cell soma, and by a depression of the excitant effects of L ‐glutamate and acetylcholine. 4. Although there was considerable variation in the magnitude of the depressant effects of the monoamines, dopamine was found to be slightly more potent than 5‐HT and NA. Neurones which were inhibited by catecholamines did not always respond to 5‐HT and lysergide and vice versa. 5. The inhibitory actions of monoamines on lateral geniculate neurones are comparable with those that have been recorded in other structures of the central nervous system. 6. Lysergide did not antagonize the action of 5‐HT. 7. Catecholamines and 5‐HT are present in nerve terminals in the lateral geniculate nucleus and the findings of this study suggest that they act as inhibitory transmitters.