Cell excitation enhances muscarinic cholinergic responses in rat association cortex

The cerebral cortex receives a prominent cholinergic innervation which is thought to play an important role in regulating its normal function. Electrophysiological studies have shown that activation of cholinergic receptors results in a marked enhancement of excitatory stimuli onto cortical neurons and it has been suggested that this effect is secondary to the blockade of several voltage- and calcium-dependent potassium conductances in these cells. It is reported here that, in addition to these effects, activation of muscarinic receptors in the prefrontal cortex elicits the appearance of a slow calcium-dependent inward current in response to the generation of action potentials. This inward aftercurrent produces a slowly decaying depolarizing afterpotential which, when activated by stimulation of the cell, can summate with the carbachol-induced depolarization greatly increasing its magnitude. As a result the ability of muscarinic receptor to elicit a depolarization and excite cells in this region can be dramatically potentiated by evoked cell activation. This effect expands the range of mechanisms by which muscarinic receptors can facilitate excitatory inputs and provides a mechanism by which the association of brief excitatory stimuli to cholinergic stimulation can selectively enhance muscarinic responses among discrete cell populations in the cerebral cortex.