Interrogation of the role of transient interneuron circuits in the development of normal sensory activity in vivo.

A major obstacle to our understanding of brain function is the sheer diversity and number of cells that contribute to even the simplest of behaviours. One approach aims to tackle this problem by investigating the rules that govern the developing brain. The reasoning being that if we can understand the biology going on at early stages - processes that lay the foundations for the amazing processing power of our brains - then we can perhaps extrapolate them to the more complex adult brain. A further advantage of this approach is that it allows us to study and dissect the relative contribution of genetics and environment on brain formation, both of which combine to sculpt an individual's unique perception of the world. Yet even at these formative stages, amongst all the millions of nerve cells jostling for position there remains the question as to how an individual nerve cell in the developing brain know where to go and which other nerve cells it should form connections to? One particular cell type - local, inhibitory nerve cells termed interneurons, face a monumental task as their origin is completely distinct from their eventual site of function. To get round this challenge it appears that the migration and integration of these cells into the correct brain circuits is governed largely by genetic events. In essence they represent 'hard-wired' components of the circuit. Ongoing studies in the lab suggest that this is the case. We have used the power of genetics to interrogate the contribution of interneurons to emergent brain circuits and through these studies have revealed that certain subtypes of interneuron act as a scaffold for the developing brain, constraining the impact of the environment so that no matter what our early life experience are, should we have the right genetic programmes, our brain will end up with being 'normal'. Moreover just like a scaffold these particular interneuron circuits are transient and so we cannot study them in the adult brain. Unfortunately from then on the story is still largely incomplete and we have a limited knowledge of what function these interneurons actually have in the newborn brain. We believe that resolving this is critically important because this activity lays the foundation for all of our subsequent cognitive development. As such we want to determine (1) the exact contribution of interneurons to formative brain activity in the normal brain. We want to see if our scaffold interneurons change function as the brain develops; (2) we want to test if this scaffold is critical. We will achieve this by blocking formation of the scaffold by using genetics to promote the mature brain state earlier in development; before (3) altering directly the performance of the transient network at discrete stages through development and assessing the consequences on network maturation. Our belief is that the latter will demonstrate the importance of this early transient interneuron circuit to normal brain development.

Inhibitory interneurons of the cerebral cortex are fundamental not only to normal adult cognitive function but have also been proposed to play a critical role in circuit formation. In recent experiments we have taken advantage of an approach that combines developmental genetics, physiology and optogenetics to probe the contribution of individual interneuron subtypes to the emergent networks. A key finding from these studies has been the identification of a number of transient interneuron circuits in the early postnatal neocortex. These include one pathway mediated by somatostatin-positive (SST+) interneurons in infragranular layers of neocortex that appears critical for the timely acquisition of thalamocortical input and that is absent in a mouse model (transgenic over-expression of neuregulin-1) wherein local inhibition mediated by PV+ interneurons is promoted. To better understand the contribution of such pathways and resolve how different sources of inhibition sculpt emergent brain function we propose to use a range of optical and electrophysiological approaches in vivo to resolve the contribution of GABAergic interneuron subtypes to early oscillatory activity in primary somatosensory cortex of first wild-type and second Nrg1-overexpressing transgenic animals. Our objectives are 4-fold: (1) to identify the contribution of different interneuron subtypes to normal development; (2) determine if Nrg1 transgenic animal exhibit altered levels of interneuron recruitment in vivo; (3) to assess how imbalances in translaminar versus local inhibition alter early postnatal oscillations and if so, how interneurons contribute to altered activity; (4) determine the temporal requirement for each interneuron subtypes using conditional cell silencing strategies including chemo-genetics. The sum of these experiments should provide additional insight into early circuit events and identify how discrete population of inhibitory interneuron underpin emergent normal cognitive function.

The current proposal represents a continuation of the work originally funded by the MRC (MR/K/004387/1); a study that used developmental genetics and laser scanning photostimulation (LSPS) to map the circuitry of the developing neocortex in normal and mutant brains. During this investigation we identified a number of early transient circuits that are distinct from those present in the adult brain. We found a hitherto unknown pathway present during the critical period plasticity which dictates the timeline for normal sensory integration and is absent in the more mature circuits. The intention is to now explore this circuit in vivo and directly test the requirement for this early pathway in the establishment of normal sensory awareness. At a basic level this represents an investigation into the importance of 'nature versus nurture' in brain development and as such will greatly inform our understanding of cortical circuits through life. Our investigation will identify how specific interneuron subtypes - that act as a scaffold for emergent mature connections - contribute to network activity and ultimately better understand the importance of these early circuits in the development of the normal brain. As such the initial impact of our findings will be felt by the immediate neuroscience community focused on neurodevelopment, GABAergic interneurons and more generally circuitry of the juvenile and adult cerebral cortex. This will include researchers working in both basic and clinical neuroscience who investigate normal and dysfunctional cognitive performance. One of the core advances of this proposal - which we believe will have an impact of future research across these fields - is to meld in vivo physiology with LSPS to provide a link between network activity and synaptic integration at the cellular level. In addition, while we routinely exploit developmental genetics to identify cortical interneuron subtypes, the thought is to further extend this strategy to selectively silencing and interrogation of the role of interneurons subtypes to early behaviour. Such approaches have not been exploited previously in neonates as such our results will act as a proof of principle for further investigation.
We envisage that our data will promote better understanding of the aetiology of a wide range of neurological conditions (including autism, schizophrenia and bipolar disorder) in which GABAergic interneuron synaptic dysfunction have been implicated. Our recent data suggest that an appreciation of transient interneuron circuits will be necessary if we are to resolve the causative factors for such conditions. Moreover it suggests that the deficits that underpin these conditions in neonatal circuits might be quite distinct from the phenotype observed in juvenile/adults. Identification of early circuit dynamics will advance our understanding and could lead to (1) a better appreciation of pre-symptomatic events and (2) the development of alternative strategies to target early circuit abnormalities. To maximize the potential impact of our findings we will develop closer collaboration with colleagues in the Dept. of Psychiatry and in Clinical Neuroscience to consider how best to exploit and disseminate our basic science findings within the clinical arena. This is a priority area for our future engagement given the impact such neurological disorders have on the community across the lifespan of the individual concerned. There is a real need to place our understanding of environmental and genetic factors linked to such disorders within a better model of circuit development. As such this is an area where we believe the research proposed will have its greatest longer term impact.