Afferent-specific modulation of excitatory synaptic transmission by acetylcholine and serotonin in the prelimbic cortex

Acetylcholine (ACh) and serotonin (5-hydroxytryptamine, or 5-HT) differentially regulate the excitability of pyramidal neurons in the mouse prelimbic (PL) cortex according to their long-distance projections. We tested for afferent- and/or target-specific modulation of glutamate release by these neuromodulators in two key afferents to the PL cortex: commissural (COM) projections from the contralateral hemisphere and projections from the mediodorsal nucleus (MDN) of the thalamus. Using an ex-vivo optogenetic approach, we mapped the connectivity of COM and MDN afferents in intratelencephalic (IT) and extratelencephalic (ET) layer 5 projection neurons and tested for afferent-selective neuromodulation of synaptic transmission. Dual whole-cell recordings in pairs of IT and ET neurons found that COM afferents monosynaptically target both IT and ET neurons, but that MDN afferents selectively target IT neurons. Both afferents exhibited similar, and target-independent, short-term synaptic plasticity (facilitation), but were differentially modulated by ACh and 5-HT. COM transmission was suppressed strongly by ACh and moderately by 5-HT, while MDN inputs to IT neurons were largely unaffected by either neuromodulator. Cholinergic suppression of COM transmission was blocked by atropine or the M4-preferring antagonist VU602841, but enhanced by the M4 positive allosteric modulator VU0467154. Serotonergic suppression of COM transmission was mildly reduced in the presence of SB216691, a 5-HT _1B receptor antagonist. Chemogenetic inhibition of hM4Di-expressing COM terminals mimicked the suppressive effects of ACh and 5-HT on synaptic transmission. These data demonstrate that, while ACh and 5-HT differentially regulate IT and ET excitability, they are similarly selective in regulating COM synaptic transmission in the PL cortex.