Homeostatic plasticity in mouse visual cortex

The primary visual cortex (V1) is one of the most extensively studied areas of the mammalian brain, not only because it is crucial for our understanding of human vision but also because it has become a model system for studying cellular processes underlying plasticity, learning and memory. Visual experience during a so-called critical period in early childhood shapes the way neurons in V1 respond to visual stimuli throughout life, and any conditions that put one eye at a disadvantage (such as cross-eyes or a cataract) can cause amblyopia ('lazy eye') if not corrected in time. Up to 4% of the population suffer from this condition. In recent years research (including in our own labs) has increasingly employed mice in an effort to discover the cellular and molecular mechanisms of the underlying processes, using monocular eyelid suture ('monocular deprivation') as the standard paradigm to challenge visual cortex plasticity. Studies have identified the involvement of different mechanisms for different lengths of monocular deprivation and at different ages. One of these is 'homeostatic plasticity', an important ability of neurons to regulate their excitability in order to maintain stable network activity, balancing the effects of long-term synaptic potentiation and depression. This is thought to be mediated by a process called synaptic scaling. We have previously discovered evidence for the operation of this mechanism following monocular deprivation in juvenile but not adult mouse visual cortex. We also showed that the GluA1 subunit of the AMPA receptor which is the main mediator of excitatory transmission in the visual cortex is important for homeostatic plasticity.
In addition to monocular deprivation, homeostatic plasticity can be triggered by placing animals in complete darkness, presumably because this depresses overall cortical activity dramatically. Dark exposure has recently been shown to promote recovery from monocular deprivation in adolescent rats and cats and may therefore present an opportunity to treat amblyopia beyond the end of the critical period. The latest work in our lab demonstrates that just a few days of dark exposure restore plasticity in V1 of mice which have been monocular deprived beyond the end of the critical period.
Here we address key questions regarding the cellular mechanisms of homeostatic plasticity in juvenile and adult mice, as well as of the effects of dark exposure on plasticity. We shall investigate,
1) By which mechanism does dark exposure promote plasticity in the adult visual cortex? Does it enable the GluA1 dependent homeostatic plasticity that operates in the juvenile cortex, or does it simply enhance existing mechanisms of adult plasticity (such as long-term potentiation)?
2) Is the restoration of visual cortex plasticity in adult mice by means of dark exposure behaviourally significant? In other words, do mice perform better on visually guided tasks, using the previously deprived eye, after a period of dark exposure?
3) Which of the cells in V1 mediate the GluA1 dependent plasticity? Using molecular biology techniques we shall inhibit the production of GluA1 in each of 3 main classes of cells (excitatory pyramidal neurons, parvalbumin positive inhibitory neurons and astrocytes) in turn to assess whether plasticity in response to monocular deprivation in young mice is affected.
4) Does homeostatic plasticity occur at all in adult visual cortex, and if not through synaptic scaling then through which alternative mechanism?

The primary visual cortex (V1) is extensively studied as a model system for cellular processes underlying plasticity, learning and memory. Work in our lab as well as others has elucidated the key mechanisms by which the mouse visual cortex responds to experiential modifications such monocular deprivation (MD) and dark rearing (DR). We have presented clear evidence that the response to MD during the critical period is mechanistically different between the critical period (where it involves GluA1 and TNFa dependent homeostatic plasticity) and in adulthood (where it involves a CaMKII dependent LTP-like process). DR during the critical period also triggers homeostatic plasticity. In our most recent work we have shown that a brief period of DR in adulthood enables an otherwise absent recovery from the effects of long-term MD. Here we address 4 key questions arising from our work, using a combination of optical imaging, two photon imaging, optogenetics and tests of visually guided behaviour.
1) Does DR in adulthood restore the juvenile, GluA1 dependent mechanism of ocular dominance (OD) plasticity, or does it enhance the existing LTP-like mechanism? This will be addressed by studying mice lacking in either one of the four key molecular players, GluA1, TNFa, CaMKII or BDNF.
2) Is the enhancement of OD plasticity by DR correlated with an improvement of sensory and/or motor function? We will image neurons in V1 while animals will perform previously trained visual discrimination tasks.
3) How exactly is GluA1 involved in homeostatic potentiation in the visual cortex? We will test how knock-down of GluA1 in each of three key cell types (excitatory pyramidal cells, parvalbumin positive inhibitory cells and astrocytes) affects plasticity.
4) Does homeostasis occur in adult V1 during plasticity? We will test whether the recently described layer 6 mediated gain control plays a role in this by optogenetically manipulating the level of activity of layer 6 excitatory neurons.

The research will add to the growing body of knowledge about how the brain adapts to an ever-changing sensory environment and about the cellular mechanisms underlying plasticity, learning and memory. While the most immediate beneficiaries will be other researchers with an interest in cortical plasticity there are a number of likely beneficiaries outside this group. Foremost among these will be ophthalmologists and optometrists specialising in amblyopia and its treatment: although this is a basic research proposal, an understanding of how dark exposure facilitates recovery from the effects of monocular deprivation in adulthood may lead to new approaches of amblyopia therapy in adolescent or adult humans. Similarly, there are currently no drug treatments of amblyopia available, but any molecular pathways that are identified in this project as routes to enhancing plasticity in the adult brain could provide a target for the pharmaceutical industry to exploit. In order to translate findings at cellular level back to outcomes of behavioural relevance (e.g. human vision) functional readouts such as those proposed here are equally important. Applications would not necessarily be limited to vision but could include e.g. drugs for memory enhancement.
Many neurodevelopmental and neuropsychiatric diseases such as Fragile X or schizophrenia involve defects and malfunctions at the cellular and synaptic level. A better understanding of the cellular processes involved in synaptic plasticity under normal conditions will ultimately increase the chances of understanding and fighting disease processes that disrupt developmental plasticity, learning and memory. Therefore, this research will have beneficiaries both in the academic and commercial sector who work on conditions in which developmental plasticity is disrupted: the techniques employed in this proposal could be adapted readily for assaying plasticity in a variety of those conditions (such as Fragile X). This could in turn lead to testing of potential cures. The increasing importance of 'applied neuroscience' is exemplified by a special issue of the leading journal Current Biology to which the PI has contributed an article [Sengpiel F (2014). Plasticity of the visual cortex and treatment of amblyopia. Current Biology 24:R936-R940].
Cardiff University provides the ideal environment for exploiting the outcomes of basic neuroscience research for translational purposes. The Neuroscience & Mental Health Research Institute has the explicit aim to bring neuroscientists from different disciplines together (Biosciences, Medicine, Psychology and Optometry & Vision Sciences) and to facilitate translating advances in basic neuroscience research into greater understanding, diagnosis and treatment of neurological mental illness. Our collaborator Dr Adam Ranson is a member of staff of the Neuroscience & Mental Health Research Institute and the techniques developed in this proposal will be of immediate benefit to the institute and its remit.
Furthermore, the wider public can benefit from this research through public engagement activities. Many members of the public are fascinated by neuroscience and want to know more about how the brain works. The PI is the public engagement leader at Cardiff University's Neuroscience & Mental Health Research Institute, organising public lectures, speaking about neuroscience research on local radio and in public events, and educating both children and adults during Brain Awareness Week and throughout the year, including the Brain Games at the National Museum of Wales.