Stimulus-dependent muscarinic effects on evoked unit activity in the rat barrel cortex

The cerebral cortex receives a prominent cholinergic innervation, which is thought to play an important role in the regulation of its normal function. Electrophysiological studies have shown that activation of muscarinic cholinergic receptors results in a marked enhancement of excitatory stimuli onto cortical neurons. In the present study, we examined the effects of acetylcholine (ACh) and its muscarinic agonists (applied by pressure injection) on the response components of individual cortical neurons in layers IV and V of the rat somatosensory cortex in identified barrels (C2 and C3). It was found that the muscarinic agonists could modify the evoked unit activity (in most cases, they caused units to respond to previously minimally effective whisker stimuli), but the modulatory effect was highly dependent on the stimulus parameters, and in most cases, the effect was limited to only one component of 'on' or 'off' responses consisting of 2-4 spikes.