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Acetylcholine (ACh) is a neurotransmitter well-known to play essential roles in modulating sensory and higher cognitive functions. The basal forebrain is a major source of ACh to the neocortex and its activity is heightened during cognitive processes. Activation of muscarinic acetylcholine receptors (mAChR) has several functional consequences in the neocortex. At the behavioral level, it is required for attention maintenance and working memory. At the circuit level, it facilitates long term potentiation and enhances the coding capacity of a population of neurons. At the cellular level, it increases neuronal excitability by modulating a constellation of ionic currents. It also allows the cell to maintain its activity beyond the stimulus, a phenomenon termed “persistent activity”, which is often seen as the cellular hallmark of working memory. In spite of the extensive observations made with respect to mAChR activation at different levels, the cellular mechanisms of these phenomena are not well understood. In this work, I used electrophysiological, optogenetic, and imaging methods to investigate the cellular basis of various phenomena of mAChR activation.In the first part of my results, I investigated the ionic mechanism of persistent activity triggered by depolarizing stimuli following mAChR activation, and found that Ether-a-Go-Go Related Gene (ERG) Potassium channel was down-modulated post-stimuli, which, in turn, depolarized the cell and increased neuronal excitability. ERG blockers abolished persistent firing in temporal and prefrontal association cortices. Next, I investigated what ionic current was responsible for the increase in excitability upon mAChR activation. With optogenetic tools, I stimulated cholinergic fibers originating from the basal forebrain in the neocortex, and found that mAChR activation increased neuronal excitability during the stimulus by down-modulating a component of spike frequency adaptation (SFA) mediated by ERG. I found that SFA reduced by mAChR activation is responsible for the decrease in inter-neuronal correlative activity previously observed in vivo. Thus, a variety of neurodynamic phenomena of mAChR activation can be traced back to its modulation of the biophysical properties of neurons.

The prefrontal cortex (PFC) mediates many executive functions including working memory, behavioral flexibility, decision making, and social cognition. The normal functioning of the PFC depends strongly on regulation by neuromodulators. Dysfunction of the PFC leads to deficits in these abilities and is a major factor in many neuropsychiatric disorders including schizophrenia and autism. Layer 5 (L5) of the PFC, a major site of dopaminergic modulation, is thought to play an important role in regulating these higher order processes. Indeed, many neurological disorders such as schizophrenia have been associated with imbalances in dopamine neurotransmission. Prefrontal dopamine D2 receptors (D2Rs), the primary target for first generation antipsychotic drugs, specifically play a major role in tasks that are disrupted in schizophrenia. One major hypothesis is that excessive D2R activation within the PFC contributes to many of the cognitive impairments associated with schizophrenia. This dissertation investigates the mechanisms of D2R-mediated neuromodulation within the PFC and abnormalities within PFC circuitry and neuromodulation in mouse models of schizophrenia and autism. We have previously described a phenomenon whereby D2R activation elicits afterdepolarization (ADPs) in subcortically-projecting (SC) pyramidal neurons within L5 of the PFC. Results presented in Chapter II of this dissertation show that this unusual physiological phenomenon, in which D2Rs enhance cellular excitability dependent on synaptic input, is mediated at the cellular level through the recruitment of signaling pathways associated with Gs, rather than Gi-associated mechanisms that have classically been ascribed to D2Rs.In Chapter III, I discuss differences in D2R neuromodulation in mice with a dominant-negative mutation in disrupted in schizophrenia 1 (DISC1). DISC1 dysfunction has been associated with schizophrenia and causes deficits in working memory. Current studies, it has been shown that there is an upregulation of D2R expression and/or activity in DISC1 mouse models. Interestingly, DISC1 is also implicated in the regulation of cAMP. Here, I will show that these mice lack the quinpirole-induced Gi-independent ADP. The medial prefrontal cortex (mPFC), particularly deep layer projection neurons, has also been implicated as a potential locus for Autism Spectrum Disorder (ASD) pathology. Previous work from our lab has shown that social exploration preferentially recruits mPRC D2R positive pyramidal neurons and that this recruitment is attenuated in three etiologically distinct mouse models of autism. In Chapter IV of this dissertation, we investigate the affects of T-brain-1 (Tbr1) selectively within layer 5 and layer 6 cortical neurons. We found that Tbr1 function is required to maintain many aspects of layer 6 identity, Tbr1 layer6 mutant neurons transform towards the identity of L5 neurons, including their transcriptome, dendritic pattern, and physiological properties. Tbr1 layer5 mutant neurons become a more homogeneous population transforming towards the identity of D2R expressing pyramidal neurons. Both Tbr1 layer6 and Tbr1 layer5 mutants have reduced excitatory and inhibitory synaptic density as well as reduced spontaneous EPSCs and IPSCs. We also present data suggesting that loss of Tbr1 function in layer 6 leads to increased anxiety and aggressive behavior whereas loss of Tbr1 function in layer 5 leads to decreased social behaviors, phenotypes consistent with different aspects of observed ASD patient behavioral characteristics.