Cortical pathways and synaptic mechanisms for texture discrimination learning in rodents

Humans mainly use their visual system to understand and interpret the world. It is therefore often difficult for us to imagine the world of touch in any detail or understand its importance. Yet if we were unable to recognise objects that we pick up from the way they feel, we would surely recognise the wealth of information yielded up by this vital sense. Touch is most often appreciated when there is no light and we are feeling our way in the dark, or when we are looking for an object we cannot see in a pocket or a bag. Firefighters rely on their tactile sense when exploring smoke filled rooms; surgeons need to relearn how to feel and grasp objects when they wear surgical gloves (which alter the tactile experience). In these cases, the spatial arrangement of small protuberances and depressions on the surface of an object and the way they yield, or are deflected in time, in other words the texture of the surface, give us a great deal of information about the object's identity. Think of how you might distinguish an old (paper) and a new (plastic) note in your pocket purely by touch.

This grant is aimed at understanding how texture information is processed in the brain and how the brain adapts and learns to attribute meaning to particular textures. To do this we will study texture processing in the rodent brain. Rodents are nocturnal animals and are therefore highly reliant on tactile information for identifying objects in their environment. Rodents are experts at touch. In laboratory tasks, we have found that they preferentially use their whiskers to identify different textures. Rodents have a highly stylised array of 40 large whiskers on either side of the snout that they can move back on forth (or whisk), effectively to palpate objects and recognise them. Remarkably, they are able to distinguish between surfaces that differ in particle size by just 18um, a distance that is orders of magnitude smaller than the spacing between whiskers on the face. We can teach the animals to associate a reward with a particular texture and then discover which areas of the brain are involved by silencing those brain areas during the exploration. We can also record neuronal activity from the same brain regions and discover how the neurones encode texture information in those places we suspect to be involved. So far, work on higher order touch processing has generally tended to be conducted in monkeys. If we could establish where the processing streams are located in rodents, there is a possibility that work would proceed at a faster pace in this field and fewer monkeys might need to be studied.

The second major part of our study concerns how and where changes occur in the brain when animals learn about new textures or attribute meaning to familiar textures. We can test this idea by asking the rodents to learn to distinguish between two similar textures to acquire a reward. While they are learning, we can image the synaptic connections between neurones in the brain that might be involved. Synapses connect neurones together and allow them to communicate with one another. Specifically, we can see whether new connections are formed and whether they correlate with the memory of the texture. To do this we can make a small window in the brain and make the neurones of interest fluoresce by producing GFP (green fluorescent protein). We can view the dendritic spines (which are one half of the synapse) on the neurones using a 2-photon microscope. We can view whether new dendritic spines are produced when the animal learns. We can also eliminate or erase those spines using a molecular probe that selectively infiltrates new spines and see whether the new spines are indeed necessary for learning the texture discrimination. This will help us understand the physical basis of learning and memory in general.

We aim to understand (i) the functional anatomy of texture processing in the rodent cortex (ii) how information is coded at different stages in the pathway (iii) how and where structural plasticity is used to learn a texture discrimination and (iv) when cortical activity is required for memory consolidation. It is known that texture information from the whisker system is partly represented as timing information in S1 cortex and that the hippocampus responds to somatosensory stimuli. Less is known about the route by which information transfer occurs between the two locations and less still about neuronal encoding at different stages in the pathway. Here, we will use bilateral virally-delivered DREADDs to inactivate specific cortical areas in our hypothesised pathway between SI and Hippocampus, a pathway that is homologous to the primate ventral stream. In combination, we will use simultaneous multisite recordings from neurones in the putative ventral stream to monitor the transformation of coding from one area to the next and its adaptation during texture discrimination learning. To understand where and how plasticity enables learning and whether the same cortical structures are involved, we will study structural plasticity in different cortical layers of S1 and, using retrograde labelling, S1 neurones projecting to ventral stream structures. We will study whether new synapse formation and stable dendritic spine enlargement occurs during discrimination learning and how well these factors are correlated with the discrimination performance in individual animals. We will use a novel molecular probe directed to label newly formed dendritic spines to erase the newly formed spines and thereby determine if there is a causal relationship between new spine formation and the learned discrimination. Finally, we will determine when and where the consolidation period for memory formation occurs by inactivating specific areas with DREADDs after the learning has taken place.

While this project constitutes fundamental research and will initially impact understanding of learning, memory and cortical information processing, we have longer-term plans to ensure the studies (1) on learning and memory to have an impact on understanding mental health conditions and dementia (impacting the healthy ageing stratgic priority) and (2) on cortical information processing have an impact on the 3Rs strategic priority.

(1) Creating a method for understanding learning deficits at the synaptic level: Our plan is first to discover what cortical areas are necessary for texture discrimination, what actually changes at the levels of neuronal activity and the synapse and then to discover when these changes occur, including the time period immediately after learning and later time periods during sleep. This will then open up the possibility of using mouse genetics models to understand neurodevelopmental conditions in which learning is affected, such as in autism, fragile X and ADHD. With these particular conditions, there is also a modality specific relevance to the impact, because they involve tactile defensiveness and an aversion to touch that impairs social interaction. We will also have created a system for understanding memory problems such as Alzheimers and by using transgenic models of the disease and for investigating the role played by sleep in learning and memory, which can be pursued further by using circadian clock mutants. Sleep is often disrupted in both neuropsychiatric and dementia conditions. In this way we aim to impact areas of psychiatric health and problems encountered with ageing such as dementia. We have a track record in these areas, having elucidated a critical period for synapse development, principally relevant to schizophrenia (but possibly other psychiatric conditions as well) (Greenhill et al. 2015 Science 24;349(6246):424-7) and in identifying a major modifier of brain plasticity relevant to AIDS-related dementia (Zhou et al. 2016 eLife 5), which we are pursuing as a possible treatment of some forms of plasticity deficit. To promote the system of investigation we use here to the neuroscience and neuropsychiatric community, we will publish our findings in major journals in the field and give talks at international scientific meetings such as the annual Society for Neuroscience meeting. To promote the impact of our method on psychiatric and dementia conditions we will interact with psychiatrists at the Neuroscience and Mental Health Research Institute at Cardiff University (and more broadly analogous groups nationally and internationally) to create collaborations to understand, at the mechanistic level, why the mutations they identify in humans create learning and memory deficits. We can do so by creating equivalent mutant strains or by introducing equivalent effects by siRNA knockdown. These studies will require further grant applications and funding, but will build on the methods established in this study.

(2) Understanding higher order information processing in the cortex: We anticipate that one of the impacts in this area will be to help refine questions regarding ventral stream processing in the brain. Understanding of ventral stream processing is only really established in primates at present. If it is possible to establish that there are ventral stream structures in the rodent homologous to those in the primate, progress can be made in this lower sentient species, using all the genetic, optogenetic and chemogenetic tools available in the mouse before before returning to the primate with more specific and refined questions. In this way, this project will lead to an impact on the 3Rs. We will promote this impact through contacts with somatosensory cortex primate researchers directly at neuroscience meetings, by talking to colleagues studying primate somatosensory cortex in labs known to us and by publications in journals read by this important research community.