Cholinergic activation and tonic excitation induce persistent gamma oscillations in mouse somatosensory cortex in vitro

1. Concomitant application of the cholinergic agonist carbachol and nanomol ar doses of kainate can elicit persistent gamma frequency oscillations in a ll layers of the mouse somatosensory cortex in vitro. Receptor pharmacology with bath-applied antagonists indicated that oscillatory network activity depended crucially on the participation of cholinergic muscarinic, (S)-alph a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate and GAB A, receptors. 2. The timing of action potentials and the occurrence of excitatory as well as inhibitory postsynaptic events was highly correlated with the phasic ch ange of extracellularly recorded population activity. Firing probability wa s lowest during the peak negativity of IPSPs and gradually increased during their ensuing decay. In conjunction with the effect of a barbiturate to de crease the frequency of gamma oscillations, this suggests a crucial role of IPSPs in phasing the suprathreshold activity of principal neurons. 3. At nearby (<1 mm) sites contained within any given cortical layer, oscil latory extra- and intracellular activity was highly synchronous with no app arent phase lag. However, interlaminar mapping experiments demonstrated a p hase reversal of both extra- and intracellularly recorded activity near the lower border of thalamo-recipient layer 4, thus corroborating findings tha t have been obtained in vivo. 4. In conclusion, a modest increase of tonic excitatory drive in conjunctio n with the activation of cholinergic muscarinic receptors can elicit persis tent gamma frequency network oscillations in the rodent somatosensory corte x. These findings (re)emphasize the role of the cholinergic ascending syste m in the cortical processing of sensory information.