Development of Genetic Strategies to study a novel disinhibitory system in the cortex - Summary Neocortical layer 1 (L1), the “crowning mystery” of David Hubel, is the most superficial layer of the cortex. It is a major target of high order cortico-cortical and thalamocortical projections that carry “top-down” or “feedback” information, such as behavioral saliency, expectations, predictions, and memories. This top-down information is integrated with bottom-up sensory input received by the pyramidal cells (PCs), the output neurons of the cortex, to generate percepts and drive context-dependent behavior. A detailed understanding of how top-down and bottom-up signals are integrated is necessary to understand functions such as sensory perception, learning and decision making. This is also of significant medical relevance, as disruptions of this integration are thought to underlie various neurocognitive disorders such as autism and schizophrenia. L1 is unique among neocortical layers in that it lacks excitatory neurons but instead contains the distal (tuft) dendrites of the pyramidal cells (PCs) located in deeper layers. These dendritic compartments receive the diverse top-down projections arriving in L1, allowing their integration with the feedforward sensory input arriving at the basal dendrites of the PCs. L1 also contains a complex specialized population of GABAergic interneurons (INs) the only neurons in this layer. L1 INs modulate how top-down information is relayed to the tuft dendrites of the PCs. However, the precise circuit mechanisms through which these INs regulate the processing of top-down signals is poorly understood. Unlike INs in other layers, which have been the focus of extensive research, L1 INs are distinct, and their functional roles are less well characterized. This knowledge gap has persisted largely because of the lack of molecular genetic tools that have facilitated the study of neuronal circuits in other layers. Recent efforts by us and others have provided genetic reagents that facilitate the identification and manipulation of three of the four IN populations of L1: the NDNF neurogliaform cells (NGFCs) and canopy cells, as well as to VIP INs, an interneuron population that has low abundance in L1, but is highly enriched in L2/3 and contains prominent dendrites in L1. However, we still lack access to the fourth IN population, called a7 INs due to their prominent expression of a7 nicotinic receptors (a7 nAChRs). a7 INs are also unique to L1, are key targets of long-range projections of L1 and we hypothesize they define a novel L1 disinhibitory system. To advance our understanding of signal integration in the neocortex we are studying signal processing in L1 in an associative learning task, for which we plan to submit a BRAIN Circuits Projects R01. To prepare for this submission, in this R34 Targeted BRAIN Circuits Planning Project we plan to develop tools to gain genetic access to the L1 a7 INs. In Aim 1, we propose several strategies to develop mice to target the a7 IN population (Aim 1A) and utilize these mice to investigate the efferent connectivity of a7 INs unto L2/3 and L5 PCs and INs (Aim 1B). In Aim 2, we generate a conditional knockout of the a7 nAChR in L1 a7 INs to obtain support for the hypothesis that the fast cholinergic modulation of these neurons by these receptors is essential for associative learning in our behavior.
