During normal brain operations, cortical neurons are subjected to continuous cholinergic modulations. In vitro studies have indicated
that, in addition to affecting general cellular excitability, acetylcholine also modulates synaptic transmission. Whether these cholinergic
mechanisms lead to a modulation of functional connectivity in vivo is not yet known. Herein, the effects were studied of an iontophoretic
application 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-correlations. These findings are consistent with cholinergic agonists primarily affecting the excitability of cortical neurons rather
than the strength of cortical connections. However, when changes of cross-correlations occurred, they were usually not correlated with
concomitant changes in average firing rates nor with changes in autocorrelations, which suggests a secondary cholinergic effect on
specific cortico-cortical or thalamo-cortical connections.