Acetylcholine (ACh) neurotransmission within the medial prefrontal cortex (mPFC) plays an important modulatory role to support mPFC-dependent cognitive functions. This role is mediated by ACh activation of its nicotinic (nAChR) and muscarinic (mAChR) classes of receptors, which are both present on mPFC layer VI pyramidal neurons. While the expression and function of nAChRs have been characterized thoroughly for rodent mPFC layer VI neurons during postnatal development, mAChRs have not been characterized in detail. We employed whole-cell electrophysiology with biocytin filling to demonstrate that mAChR function is greater during the juvenile period of development than in adulthood for both sexes. Pharmacological experiments suggest that each of the M1, M2, and M3 mAChR subtypes contributes to ACh responses in these neurons in a sex-dependent manner. Analysis of dendrite morphology identified effects of age more often in males, as the amount of dendrite matter was greatest during the juvenile period. Interestingly, a number of positive correlations were identified between the magnitude of ACh/mAChR responses and dendrite morphology in juvenile mice that were not present in adulthood. To our knowledge, this work describes the first detailed characterization of mAChR function and its correlation with neuron morphology within layer VI of the mPFC.

Neuron-derived 17β-estradiol (E2) alters synaptic transmission and plasticity in brain regions with endocrine and non-endocrine functions. Investigations into a modulatory role of E2 in synaptic activity and plasticity have mainly focused on the rodent hippocampal formation. In songbirds, E2 is synthesized by auditory forebrain neurons and promotes auditory signal processing and memory for salient acoustic stimuli; however, the modulatory effects of E2 on memory-related synaptic plasticity mechanisms have not been directly examined in the auditory forebrain. We investigated the effects of bidirectional E2 manipulations on synaptic transmission and long-term potentiation (LTP) in the rat primary auditory cortex (A1). Immunohistochemistry revealed widespread neuronal expression of the E2 biosynthetic enzyme aromatase in multiple regions of the rat sensory and association neocortex, including A1. In A1, E2 application reduced the threshold for in vivo LTP induction at layer IV synapses, whereas pharmacological suppression of E2 production by aromatase inhibition abolished LTP induction at layer II/III synapses. In acute A1 slices, glutamate and γ-aminobutyric acid (GABA) receptor-mediated currents were sensitive to E2 manipulations in a layer-specific manner. These findings demonstrate that locally synthesized E2 modulates synaptic transmission and plasticity in A1 and suggest potential mechanisms by which E2 contributes to auditory signal processing and memory.

Pyramidal neurons located within layer V of the medial prefrontal cortex drive cognitive circuits by integrating afferent signals and sending efferent projections to cortical and subcortical targets. This role is supported by cholinergic neurotransmission, which modulates pyramidal neuron excitability via postsynaptic nicotinic receptors. We employed whole-cell electrophysiology with neuron reconstruction in brain slices from mice of both sexes to demonstrate that medial prefrontal layer V pyramidal neurons comprise three subtypes that have distinct electrophysiological properties, receptor isoform-specific nicotinic responses, and projection targets. Burst-firing neurons may be sub-divided into subtypes having (i) α7 isoform nicotinic responses and projections to the contralateral cortex, or (ii) α7 and β2 isoform nicotinic responses and projections to the nucleus accumbens. Regular-firing neurons have β2 isoform nicotinic responses and projections to the ventromedial thalamus. These findings provide new insight into an isoform-specific mechanism by which cholinergic neurotransmission modulates distinct efferent projections from this cognitive brain region.

Neurotransmission mediated by acetylcholine at its nicotinic receptors (nAChRs) within the medial prefrontal cortex (mPFC) is critical for higher‐order cognitive tasks. Pyramidal neurons within layer 5 of the mPFC express nAChRs and form the primary signalling outputs from this brain region. These neurons may be categorized as either regular‐firing or initial burst‐firing, based on threshold responses to positive current injection. The objective of this study was to use whole‐cell electrophysiology and biocytin neuron labelling in young postnatal mice (age 15‐20 days) of both sexes to characterize the morphology and nAChR function within each of these neuron categories. When comparing the neuron categories, we first determined that regular‐firing neurons have a greater membrane resistance and spike adaptation ratio, and a lower magnitude post‐firing hyperpolarization. Regular‐firing neurons also exhibited a lower frequency of spontaneous excitatory postsynaptic potentials. Detailed morphological analysis determined that regular‐firing neurons have more apical dendrite matter distal to the soma and more basal dendrite matter proximal to the soma. Pharmacological experiments using receptor‐selective antagonists revealed that nicotinic responses are mediated by distinct nAChR isoforms that are expressed selectively on each of the two pyramidal neuron categories. Specifically, these experiments demonstrate that heteromeric α4β2* nAChRs are present on regular‐firing pyramidal neurons and homomeric α7 nAChRs are present on burst‐firing pyramidal neurons. Ongoing experiments aim to determine the projection targets for each type of pyramidal neuron within each sex. These novel findings suggest a detailed receptor‐based mechanism by which acetylcholine neurotransmission is regulated within output neurons of mPFC layer 5, which may impact the development of cognitive circuits involving this brain region.