Cholinergic activation of corticofugal circuits in the mature mouse prefrontal cortex.

In layer 5 of the neocortex, ACh promotes cortical output to the thalamus and brainstem by preferentially enhancing the postsynaptic excitability of pyramidal tract (PT) neurons relative to neighboring intratelencephalic (IT) neurons. Less is known about how ACh regulates the excitatory synaptic drive of IT and PT neurons. To address this question, spontaneous excitatory postsynaptic potentials (sEPSPs) were recorded in pairs of IT and PT neurons in slices of prelimbic cortex from adult female and male mice. ACh (20 μM) enhanced sEPSP amplitudes, frequencies, rise-times, and half-widths preferentially in PT neurons, an effect blocked by the muscarinic acetylcholine receptor antagonist atropine (1 μM). When challenged with pirenzepine (1 μM), an antagonist selective for M1-type muscarinic receptors, ACh failed to enhance excitatory drive to PT neurons and instead reduced sEPSP frequencies. The cholinergic increase in sEPSP amplitudes and frequencies in PT neurons was not sensitive to blockade of GABAergic receptors with gabazine (10 μM) and CGP52432 (2.5 μM), but was blocked by tetrodotoxin (1 μM), suggesting that ACh enhances action-potential-dependent excitatory synaptic transmission in PT neurons. ACh also preferentially promoted synchronous sEPSPs in pairs of PT neurons relative to shuffled data across pairs. Finally, selective chemogenetic silencing of hM4Di-expressing PT neurons with clozapine-N-oxide (5 μM) was sufficient to block cholinergic enhancement of sEPSP amplitudes and frequencies in PT neurons. These data demonstrate that, in addition to enhancing the postsynaptic excitability of PT neurons, M1 receptor activation promotes corticofugal output by amplifying recurrent excitation within PT neuron networks.