DIFFERENTIAL-EFFECTS OF ACETYLCHOLINE ON NEURONAL-ACTIVITY AND INTERACTIONS IN THE AUDITORY-CORTEX OF THE GUINEA-PIG

During normal brain operations, cortical neurons are subjected to cont inuous cholinergic modulations. In vitro studies have indicated that, in addition to affecting general cellular excitability, acetylcholine also modulates synaptic transmission. Whether these cholinergic mechan isms lead to a modulation of functional connectivity in vivo is not ye t known. Herein, the effects were studied of an iontophoretic applicat ion of acetylcholine and of the muscarinic agonist, carbachol, on the ongoing activity and co-activity of neurons simultaneously recorded in the auditory cortex of the anaesthetized guinea-pig. Iontophoresis of cholinergic agonists mainly affected the spontaneous firing rates of auditory neurons, affected autocorrelations less (in most cases their central peak areas were reduced), and rarely affected cross-correlatio ns. These findings are consistent with cholinergic agonists primarily affecting the excitability of cortical neurons rather than the strengt h of cortical connections. However, when changes of cross-correlations occurred, they were usually not correlated with concomitant changes i n average firing rates nor with changes in autocorrelations, which sug gests a secondary cholinergic effect on specific cortico-cortical or t halamo-cortical connections.