Investigating the role of the thalamic nucleus reuniens in relaying prefrontal cortex input to the hippocampus

A great deal of knowledge gained from fundamental neuroscience research has come from studying the circuitry of specific brain regions, yet most of our functional knowledge comes from behavioural studies and functional imaging experiments carried out in intact animals. If the overall aim of neuroscience is to understand how physiological processes at the level of the neuron give rise to cognition and complex behaviours, then we must bridge the gap between cellular neuroscience and behaviour, by studying how different brain regions interact. This is a problem that the current project seeks to address: we aim to study the cellular circuitry that allows brain regions to synchronise their activity across long distances. Specifically, we will study how the prefrontal cortex, a region linked to executive control and planning, can control activity in the hippocampus, a structure that is essential for memory and spatial navigation.

We will address this problem through use of cutting edge genetic approaches for studying circuit function, combined with established neurophysiological methods. Input from the prefrontal cortex is relayed to the hippocampus via the thalamic nucleus reuniens, and we will study its connections to the hippocampus using optogenetic methods. For our project, the optogenetic methods will involve using viruses to deliver genes to the nucleus reuniens to allow the expression of proteins that allow one to control the activity of neurons using light. This will enable us to record the activity of individual neurons in the hippocampus and then use light pulses to determine whether these neurons receive input from the nucleus reuniens, and what the function of this input will be. The majority of these experiments will be carried out using reduced brain slice preparations to allow us to determine the identity of neurons that receive connections from the nucleus reuniens. In a second set of experiments, we will also record the activity of the hippocampus in intact, anaesthetised animals to allow us to determine how inputs from the nucleus reuniens can affect activity in the hippocampal network.

Neurons can be split into two broad categories: excitatory cells that cause another neurons to become more active, and inhibitory cells that reduce the activity of other neurons. The nucleus reuniens is located in the thalamus, a structure in the brain that generally relays excitatory information from one brain region to another. In our pilot data, we found evidence to suggest that, very unexpectedly, the nucleus reuniens does not target excitatory neurons in the hippocampus. To determine whether this observation, made using physiological methods, is indeed accurate, we will carry out a set of experiments using advanced rabies tracing methods. These rabies tracing methods make it possible to visualise all direct connections that an individual group of neurons receive, so we will be able to view all direct inputs to excitatory cells in the hippocampus to confirm whether or not they do receive direct input from the nucleus reuniens.

This project has the potential to make a timely and significant contribution to neuroscience: interactions between the prefrontal cortex and hippocampus are important for working memory and goal-directed behaviour. Communication between the prefrontal cortex and the hippocampus is disrupted in both psychiatric conditions such as schizophrenia, and neurodegenerative disorders such as Parkinson's disease. Understanding the circuitry through which these regions interact is an important step that must be taken before we can move on to design effective, focused treatments for these conditions.

Neuroscience has made great strides in understanding connectivity within specific neural circuits but it is becoming increasingly clear that many psychiatric disorders are caused by dysfunction not only within local circuits, but also by a breakdown in communication between different regions. Interactions between the prefrontal cortex and hippocampal formation are essential for cognition and working memory. Most interactions between the prefrontal and hippocampal circuits are mediated by the thalamic nucleus reuniens (NRe); disruption of this pathway impairs working memory and goal-directed spatial navigation. However, little is known of the cellular circuitry that links these regions together, which is the problem we will address.

Our overall aim is to understand how the prefrontal cortex, acting via the nucleus reuniens, can control activity in the hippocampal network. We will approach this through use of cutting edge viral circuit-mapping and optogenetic methods, combined with in vitro and in vivo electrophysiology. We will use optogenetic methods and in vitro slice electrophysiology to map the inputs from NRe to the hippocampus, where NRe afferents terminate in CA1, with fibres restricted to SL-M. Using transgenic mice expressing GFP in various interneuron subtypes, we will determine which groups of neurons receive NRe input and compare this with input from the entorhinal cortex, which also terminates in SL-M, and with a recently described direct projection from the anterior cingulate cortex to both CA1 and CA3. Building on preliminary electrophysiological data suggesting that CA1 pyramidal cells receive no input from NRe, we will use monosynaptic rabies tracing methods to confirm that CA1 pyramidal cells receive no direct input from NRe. We will use in vitro and in vivo models of neuronal oscillations to study how activation of NRe inputs to CA1 can alter hippocampal network dynamics, and how it compares with activation of other external inputs to CA1.

Beyond the academic beneficiaries of our research, this project has the potential to bring economic and societal benefits as well. Disruption of communication between the prefrontal cortex and the hippocampus is associated with a number of psychiatric disorders. The most widely known of these is schizophrenia, but prefrontal - hippocampal interactions have also been implicated in other conditions such as PTSD and OCD. Abnormalities in the thalamus have been reported in some schizophrenic patients, so by providing a better understanding of the normal function of our circuit, this project has the potential to provide new insights into therapeutic treatments for this disorder.

This is important because, while often alleviating positive symptoms in psychotic disorders such as schizophrenia, all drug treatments for psychiatric disorders (both typical and atypical antipsychotics) have significant side effects as they target neurotransmitter receptors that are distributed widely throughout the brain. It is estimated that 3% of the UK population suffers from psychosis at some point in their life, so by providing new avenues to explore for novel therapies, this research has the potential to bring both societal impact through improved treatment, and economic impacts through improved revenue for the pharmaceutical companies that can provide new treatment. One of our future aims is to establish industrial collaborations with pharmaceutical companies to explore the nucleus reuniens as a potential target for psychiatric disorders.