Modulatory effects of catecholamines on neurons of the rat visual cortex: single-cell iontophoretic studies

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Abstract
The catecholamines noradrenaline and dopamine have been proposed as neuromodulators of cortical neuron excitability, and such a regulation could be mediated by specific adrenergic and dopaminergic receptors. We characterized electrophysiologically some of the types of responses to the iontophoretic application of adrenergic and dopaminergic agonists and antagonists on single cells in the rat visual cortex (areas occipital 1 monocular or Oc 1 M and occipital 1 binocular or Oc 1 B). For the majority of spontaneously active and visual cortical cells, noradrenaline and dopamine decreased the firing frequency. In the case of visually driven (synaptically activated) neurons, background firing was the main component of the response to be inhibited by the administration of noradrenaline, clonidine, and oxymetazoline, leading to an enhancement of the signal-to-noise ratio. Since these effects could be reduced or blocked by a previous ejection of the specific α2-antagonist idazoxan, the findings support a role for α2-adrenergic receptors in the transmission of sensory inputs to the visual cortex. These effects were not found with the mixed α-adrenergic agonist phenylephrine nor with the β-agonist isoproterenol. Finally, the use of the inhibitory amino acid GABA rules out a simple hyperpolarizing response as the mechanism underlying noradrenaline modulatory effects in the cerebral cortex.Key words: visual cortex, noradrenaline, dopamine, iontophoresis, neuromodulators.