Noradrenergic effects on rat visual cortex: single-cell microiontophoretic studies of alpha-2 adrenergic receptors

The catecholamine noradrenaline has been proposed to modulate the excitability of cortical neurons, and such a regulation may be mediated by specific adrenergic receptors. We characterized, using electrophysiological recordings, the types of responses of single cells in the rat visual cortex (areas 17 and 18) to the iontophoretic application of adrenergic agents. For the majority of spontaneous and visually-driven cells sampled, noradrenaline decreased the firing frequency, and in some cases of visually-driven cells could increase the signal/noise ratio. These effects were also documented after the application of the alpha-2 adrenergic agonists clonidine and oxymetazoline, and could be reduced or blocked by a previous ejection of the specific alpha-2 antagonist idazoxan. The present study supports a role for alpha-2 adrenoceptors in the modulation of sensory inputs to the visual cortex.