Müller glia are the predominant glial cell types in the retina that play a critical role in development and neuronal support. In adult mammalian retinas Müller glia are mitotically quiescent. However in other organisms, such as zebrafish, the Müller glia re-enter the cell cycle, proliferate and differentiate into new neurons upon retinal damage to replace lost ones. The goal of this study was to promote Müller glial proliferation and regenerate neurons in the adult mammalian retina. p27 is a tumor suppressor and plays a role in controlling cell proliferation. To test if the loss of p27 promotes Müller glial proliferation in adult retinas, we have generated Müller glia-specific p27 knockout mouse. Using tamoxifen-inducible, CreER/lox system to control timing of p27 deletion in Müller glia our Western blot data shows that daily injections of tamoxifen for four days efficiently eliminated p27 expression in adult Müller glia. When p27 expression is lost, adult Müller glia appeared to proliferate. Our data suggest that p27 plays a role in keeping adult Müller glia from re-entering the cell cycle. The controlled re-differentiation of Müller glia and proliferation into new neurons could be used to cure diseases involving retinal damage or degeneration like macular degeneration.

The brain effortlessly carries out the daunting task of visual perception by utilizing millions of neurons to process various aspects of the visual scene. The systematic arrangement of these cells across the primary visual cortex (V1) can be described by a set of superimposed maps that convey the orientation, scale, location and eye of origin of the image features that best excite the neurons. Current theory suggests these maps are arranged such that all combinations of stimulus parameters are covered equally and continuously in local regions across the visual cortex. This idea is appealing because it corresponds to the intuition that we should be able to see the features of a simple shape equally well at any rotation and at a variety of distances, and it is important because it places a constraint on the architecture of the cortex. To examine whether this constraint fits with other known anatomical and physiological measurements, we have conducted computer simulations to measure the evenness of coverage in large-scale, biologically plausible models of V1. We presented a large combination of visual stimuli to the model and computed a coverage metric, c', defined in the literature as the standard deviation divided by the mean of cortical activity across all stimuli. Taken together, c' and the associated map of neuronal activity in stimulus space reveal the coverage uniformity of map variable combinations. We found that our original models had a striking imbalance in activity across spatial scale, which counterintuitively arose from the assumption of a uniform change in neuronal receptive field (RF) size across the visual cortex. In these models, there were equal numbers of neurons with small and large RFs. Based on our observations, we are developing an improved model that uses a nonlinear distribution of RF sizes in an attempt to preserve uniform coverage.

Understanding how the brain processes information using neural circuits remains a challenging task. For example, although neurons in the mammalian visual cortex are known to be specialized for processing particular visual features, such as motion, color or depth, little progress has been made on elucidating the underlying circuit diagrams. To address this problem, we have developed a system with a Java-based user interface that can apply optimization strategies to rapidly test competing models against large amounts of neurophysiological data, parallelizing the computational load across a cluster of workstations, to uncover plausible circuit models. In order to demonstrate the effectiveness of this system, we are applying it to understand the function of motion-sensitive neurons in the primate visual cortex. These neurons are direction selective (DS), meaning that they respond well to a visual stimulus moving in one direction but not in others. Previous studies demonstrated that this response also changes with stimulus speed: as the visual world moves faster, the neurons appear to operate on a shorter timescale. Current mathematical models for DS neurons, which are based on filters that change position linearly with time, predict the response to some stimuli but do not account for the ability of the neuronal response to adapt to changes in speed. We hypothesize that a model using a filter that changes position nonlinearly with time will better explain the experimental data. To discover this filter, we are building several candidate models and will apply our optimization system to test their performance against our own experimental data. We will test these nonlinear filter models against an alternative that uses multiple linear filters, each tuned for a different velocity. If our approach can improve upon current models for DS neurons, it may offer neuroscientists a powerful tool for revealing neural circuits that underlie visual perception.

Nociception is the ability to detect noxious stimuli. Without nociceptors organisms would not be capable of sensing pain, a sensation that induces an avoidance response to minimize injury. Noxious stimuli, such as unusual thermal, chemical, and mechanical stimulation, are detected by groupings of peripheral neurons within the trigeminal ganglion and dorsal root ganglion. This is a screen that aims to identify zebrafish mutants that display abnormal nociceptive responses. We have developed a high throughput locomotive response screen, which we use to screen the third generation progeny of adult males, who have been treated with a mutagen. Following Mendelian genetics, we expect ¼ of the F3 generation to display a recessive phenotype. Any mutant nociceptive phenotype (i.e. decreased locomotive response to noxious heat or chemical stimuli) should be seen in this quarter of the population. Our preliminary data demonstrates that this screen is feasible. Upon identification of mutants we will characterize the observed phenotype by temperature and chemical based choice testing. We will use these mutants to identify genes involved in the regulation of stimulus detection and the development of nociceptive neuronal circuits. The long-term goal of this screen is to increase the understanding of nociception, which will aid in the search for new novel analgesics.