- Late postnatal developement as a model for aging
- Synaptic and cognitive development in NMDA receptor chimeric mice
- Neuronal silencing the mouse hippocampus using the fly allatostatin receptor
- Homeostatic plasticity in rat hippocampal slices
- The use of anti-stress gene therapy to minimize the lasting impact of chronic developmental stress on adult cognition
- Explaining false positive in situ hybridization
- Controlling the transition from attentive to automatic navigation
- Necessity for metabolism in memory maintenance in tardigrades
Late postnatal development of memory as a model for aging
Episodic memories are memories of one's personal experience. The hippocampus is the primary brain structure involved in forming episodic memories. How hippocampal neurons operate to encode episodic memories is presently unclear, but appears to involve activity-dependent functional alterations at excitatory synapses, the communication points between neurons. Episodic memories are not formed until late in the postnatal period across mammalian species, suggesting delayed development of the hippocampus. As such, the developmental emergence of episodic memory presents a valuable model for understanding the neuronal properties in the hippocampus that are critical for memory formation.
We have found that excitatory synapses in the rat hippocampus are weak in their ability to activate postsynaptic neurons until the end of the third postnatal week. This is the same age at which hippocampal-dependent learning and memory abilities are first observed in this species, suggesting that insufficient synaptic excitation limits memory formation or retrieval. Drugs that cause excitatory synaptic responses to last longer also increase postsynaptic activation and enhance synaptic plasticity in adult rats. We have delivered the same drugs to juvenile animals and examined their ability to perform hippocampal-dependent maze tasks. We showed that prolonging excitatory synaptic responses permitted animals under three weeks of age to navigate in a Y-maze as if they were over three weeks of age (Blair et al., 2013). The pharmacological improvement in navigation was associated with larger network activation and facilitation of synaptic plasticity induction recorded in hippocampal slices.
Interestingly, the same memory abilities that develop last in the postnatal period are the first to collapse in aging and the same drugs used in our experiments have been used to improve memory in elderly humans. As such, we are applying the information gathered in the current studies to help explain memory loss in aging (this work is funded by the National Institute on Aging).
Synaptic and cognitive development in NMDA receptor chimeric mice
Neural network development and information processing in the brain both require synaptic plasticity. As circuits in the mammalian forebrain mature, synaptic plasticity is adjusted to better suit information processing. Disruption of this process has been implicated in various neurodevelopmental disorders, including autism spectrum disorders. In the rodent hippocampus, this transition happens late in postnatal development, culminating in the emergence of hippocampal-dependent learning and memory abilities at the end of the third postnatal week. This research aims to explore the molecular determinants of the developmental alterations in synaptic plasticity and emergence of cognitive abilities.
Forebrain glutamatergic N-methyl-D-aspartate receptors (NMDARs) exist primarily as quatramers with two NR1 and two NR2 subunits. Auxiliary NR2 subunits regulate numerous facets of receptor function. Conductance regulating domains exist in the extracellular amino and transmembrane regions while synaptic targeting and intracellular signaling domains exist in the intracellular carboxy terminus. At hippocampal Schaffer collateral (SC-CA1) synapses, NMDARs contain predominantly NR2B subunits during early postnatal development and NR2A subunits after the end of the third postnatal week. As such, the developmental NR2 subunit switch produces numerous changes in NMDAR function. Interestingly, the shift in NMDAR composition parallels developmental changes in the ability to induce activity-dependent synaptic plasticity and completion of the shift marks the onset of adult-like spatial navigation.
We have generated transgenic mice that express NMDARs with chimeric NR2 subunits at SC-CA1 synapses, which allows for separation of the NR2-dependent conductance and intracellular signaling properties. We are currently conducting research the molecular, physiological, and behavioral level to better understand which NMDAR properties are most closely related to the age-related changes in synaptic plasticity and in learning and memory abilities.
Neuronal silencing the mouse hippocampus using the fly allatostatin receptor
Your brain is made up of billions of neurons each having thousands of communication contact sites. As such, the complexity of brain circuitry is amazing complex. This complexity makes it difficult to draw precise relationships between neuron function and behavior. Traditionally, this has been attempted with lesion studies to eliminate small regions of the brain prior to maze training. Unfortunately, virtually all lesions procedures lack the precision to eliminate specific subpopulations of neurons interspersed with other neuron types and are permanent. Recent advances in genetic tools have produced the ability to functionally silence highly specific subsets of neurons in a reversible manner. This allows for non-invasive circuit analysis in awake behaving mice. One of these methods involves expression of the fly allatostatin receptor (AlstR). When activated, this receptor maintains neurons in a hyperpolarized state, in essence, functionally removing them from ongoing activity. We are examining the impact of AlstR expression and activation in specific sets of hippocampal neurons to better relate their function to specific aspects of hippocampal-dependent learning and memory abilities.
Homeostatic plasticity in rat hippocampal slices
Homeostasis is a compensatory change in a system that occurs in response to changes in the environment to maintain a stable condition. Homeostatic plasticity refers to the changes that neuron networks undergo to maintain an optimal level of excitability given a consistent change in input. Currently, we examine the homeostatic synaptic plasticity that occurs in area CA1 of the hippocampal slice upon surgical removal of input area CA3. We previously found that removal of area CA3 area reduces Schaffer collateral input to area CA1 and that glutamatergic Schaffer collateral synapses display a compensatory homeostatic increase in synaptic strength. Currently, we are applying immunohistochemistry to determine if the increase in synaptic strength is supported by a greater number of postsynaptic AMPA receptors. In parallel we are performing optical imaging to spatially characterize the functional homeostatic alterations.
The use of anti-stress gene therapy to minimize the lasting impact of chronic developmental stress on adult cognition
When neonatal and juvenile animals are exposed to severe stress, numerous changes in hormonal and neural function are produced, many lasting into adulthood. A primary mediator of the lasting effects of early life stress is impaired feedback regulation of the hypothalamic-pituitary-adrenal axis and hypersecretion of the major stress hormones, glucocorticoids (GCs). This hypersecretion impairs neuronal function, blunts memory, and predisposes neurons to additional damage and disease. In contrast, another steroidal hormone, estrogen, has been shown to be a neuroprotective agent that can rescue both spatial and non-spatial learning impairments due to chronic stress. Given the similarity in the structure of GC and estrogen receptors, it is possible to use molecular biology to convert the endangering GC effects into protective estrogen signaling after early life stress. We test this notion by applying gene therapy to express a chimeric steroidal receptor protein (ER/GR) composed of the glucocorticoid receptor (GR) ligand binding domain and the estrogen receptor (ER) DNA binding domain. We hypothesize that the neural and cognitive deficits seen in rodents due to early life stress will be mitigated by expression of the ER/GR chimeric receptor.
Explaining false positive in situ hybridization
The ability to visualize RNA in tissue sections is highly informative because it yields anatomical information about gene expression. In situ hybridization is a process by which specific RNA species are tagged with visible markers and is commonly used in neuroscience research. One relatively quick and easy method for visualizing tagged RNA species is through enzymatic production of a colored precipitate (the enzyme is first bound to the RNA of interest). While simple to perform, this colorimetric method is vulnerable to false positives (colored neurons that do not contain the RNA of interest). We have found that improper hybridization temperature produces false positive signals and are currently working out the mechanisms by which this happens.
Controlling the transition from attentive to automatic navigation
Learning a novel task enlists a naïve attention-based mode of learning which underlies performance. With repeated task experience, task accuracy increases and the mode underlying performance frequently transitions to that of automation (habit). These transitional stages, as they pertain to spatial navigation in a rodent model, rely on distinct brain regions; namely the dorsal hippocampus, which mediates attentive allocentric spatial strategies, and the dorsolateral striatum, which mediates stimulus-response based strategies. We investigate this transition to clarify 1) how these brain regions dynamically control behavior over repeated experiences, and 2) the environmental demands that modulate this control. To this end, we have modified a basic plus maze that permits attentive and/or automatic strategies to be employed within the same task and subject. Moreover, by utilizing current neuroscience techniques that measure brain activity and plasticity (including slice electrophysiology and whole mount in situ hybridization), in concert with genetic neuronal silencing systems, we can learn about this transition on multiple levels, from molecules to neural systems to behaviors.
Necessity for metabolism in memory maintenance in tardigrades
Memory refers to the storage of information across time. Historically, information storage in a biological system has been seen as requiring either a persistent structural or functional change at the cellular level. Whether a memory relies on a structural or functional alteration in neurons is a difficult question to address in most model systems as one cannot fully separate structure from function at the cellular and molecular levels. In this regard, tardigrades of the genus hypsibius (water bears) become an important neuroscience subject. Tardigrades are microscopic multicellular organisms with a nervous system. They are special organisms in that they can survive absolute zero and a complete vacuum by taking on a "tun" state in which metabolism ceases. Tardigrades can be used to separate the structural and functional contributions to memory because all neuronal dynamics can be stopped and restarted during the storage phase. We train tardigrades in chemoaversive or chemoattractive tasks and then dessicate them and store them at ultracold temperatures. Upon revival, we test them to see if they remember what they learned prior to being freeze dried.