Muscarinic modulation of firing mode of layer pyramidal neurons in the cat motor cortex

The effects of muscarinic receptor agonist carbamylcholine chloride (carbachol) on the firing mode of layer II/III pyramidal neurons were studied in acute slices of motor cortex from adult cats. Layer II/III pyramidal neurons in the cat motor cortex showed regular-spiking with apparent adaptation in firing frequency or doublet-or-burst firing patterns in response to long depolarizing pulses. Bath application of carbachol (20μM) increased the firing rate of both regular-spiking and doublet-or-burst firing neurons. In regular-spiking neurons, the adaptation in firing frequency was partially eliminated by carbachol, resulting in an elongated train of action potentials. This elongation of spike trains was correlated with the blockage of slow afterhyperpolarization (sAHP), suggesting that muscarinic modulation of I_AHP contributed to the effect. In doublet-or-burst firing neurons, although the overall firing frequency was increased, the initial high frequency doublet or burst firing was suppressed by carbachol. These results indicated that activation of cholinergic pathways in the motor cortex enhances the responsiveness of layer II/III pyramidal neurons to stimuli.