Circuit mechanisms for learning in the rodent Prefrontal cortex and their dysfunction in Schizophrenia

Flexible behavior, as shown by most mammals, requires continuous decision making where appropriate actions must be chosen from an array of available actions based on our current goals and prior experience. The medial prefrontal cortex (mPFC) is essential for selecting such appropriate actions and inhibiting inappropriate ones. The prefrontal cortex is not a homogenous structure but rather an agglomeration of sub-areas, which sub serve different functions. For example, the anterior cingulate is required for effort-based decision-making while the orbitofrontal cortex is essential for value based decision-making. However, the outcome of a decision making process is selection of a singular behavioral action or learning a new association. Hence, it would be reasonable to hypothesize that this selection would be a product of the combined output of the various prefrontal areas and the interactions among them. Thus to understand the neurobiological substrates of decision making one needs to explore the prefrontal cortex at two different levels: 1. The internal microcircuit and neuronal networks within individual prefrontal areas, and 2. Functional interactions among the prefrontal areas. The broad goal of my thesis was to use both of these approaches to study the prefrontal cortex of a well-established model organism (mouse) which has a relatively simple behavioral repertoire yet is evolutionarily complex enough to generalize my findings to higher order animals.
First, I focused my attention on the Prelimbic (PreL) and Infralimbic (IL) regions of the mouse medial prefrontal cortex (mPFC). These two areas have been studied most extensively among the rodent prefrontal areas. In several behavioral domains, the PreL and IL exert distinct and opposing, influences over behavior; in a PreL-Go/IL-NoGo manner. The most common examples of this complementary function are the expression and extinction of conditioned fear responses or drug seeking behavior. Furthermore, neuronal tuning studies have shown that the PreL neurons are tuned to the representation of goals in goal directed learning while the IL neurons appear to tune to alternative choices. I investigated how the PreL and IL cortices interact among each other to influence learning and selection of behavioral strategies. Such, interactions between IL and PreL or other prefrontal areas have not been studied in detail in the past with one notable exception. Research done by Ji and Neugebauer (2012) have shown that optogenetic activation of IL inhibits PreL pyramidal cells in vivo, implying an existence of feed-forward inhibition from the IL to PreL. I carried out selective chemogenetic silencing of PreL or IL during different sub phases of the Intra-dimension/ extra-dimension set shifting task (IEST) or trace learning and extinction to evaluate their individual contributions. My findings suggest that PreL promotes application of behavioral strategies or new learning corresponding to previously learnt associations while IL is required to learn alternative associations across different learning paradigms. Next, using viral mediated tracing techniques I show the existence of reciprocal layer5/6 derived IL↔PreL projections. Using selective unidirectional silencing/activation of these projections, I have shown that the ILPreL and PreLIL projections are required at different phases of learning. Unidirectional ILPreL projections are specifically required during IL mediated alternative learning (eg: extinction) and the PreL↔IL reciprocal projections are required +12-14h post learning to setup the role of IL in subsequent learning of alternative choices.
Prefrontal cortex dysfunction has been identified as a key neurobiological correlate of cognitive deficits associated with many neuropsychiatric disorders like Schizophrenia, Attention deficit/Hyperactivity disorder etc. Exploring the dysfunction of defined prefrontal neuronal networks and circuits in rodent models of neuropsychiatric disorders can be a different approach towards understanding decision making. In the second part of the thesis, I explored the dysfunction in the Parvalbumin (PV) interneuron network in a mouse model of Schizophrenia.
Parvalbumin interneurons have been shown to synchronize network activity, supporting different types of neuronal network oscillations, such as gamma and theta oscillation, ripple and spindle activity. Thereby, they play a significant role in the formation and consolidation of memories to support learning and behavior. Finally, dysfunction of the Parvalbumin interneuron system in the prefrontal cortex of human schizophrenia patients has emerged as a core substrate underlying the cognitive deficits in the disease. Thus, studying the dysfunction of the PV network in Schizophrenia not only provides a way to understanding their role in prefrontal function but also raise the possibility of developing a strategy to ameliorate the associated cognitive deficits. I first showed that the PV network in LgDel+/- animals fail to mature with respect to those of their wild type counterparts and remain stuck in an immature state, which is associated with altered neural synchrony in the gamma band and behavioral deficits. I further show that stimulation of the PreL PV neuron network within a specific window of treatment during early adulthood can rescue the dysfunctional PV network synchrony as well as behavioral deficits. In recent years, interactions between the hippocampus and prefrontal cortex (PFC) have emerged as key players in various cognitive and behavioral functions. Disruptions in hippocampal-prefrontal interactions have also been observed in psychiatric disease, most notably schizophrenia. I saw that long-term rescue of the PreL PV state and associated behavioral deficits in LgDel+/- mice can also be mediated through direct stimulation of the ventral hippocampal (vH) PV network. However if the rescue is targeted to PreL while preventing it in vH or vice versa, it fails to mediate any behavioral rescue in LgDel+/- mice. Thus suggesting that long-term rescue of the PV pathology and cognitive deficits in LgDel+/- animals requires a rescue of the entire hippocampal-prefrontal axis.