Probing a natural example of “regeneration” in the mammalian hippocampus
The goal of this project is to understand the molecules that control the integration of newborn neurons into the circuitry of the adult hippocampus. The dentate gyrus is one of the two places in the adult brain where neurogenesis persists throughout adult life. These new neurons have been implicated in synaptic plasticity in the adult brain and may also play an important role in diseases such as depression, epilepsy, stroke, and traumatic brain injury. Neurogenesis is very sensitive to functional perturbations, being increased by physiological stimuli such as exercise as well as by pathological conditions including seizures and ischemia. Perhaps more importantly in our view, the integration of these newborn neurons provides a unique window into the formidable barriers to synaptogenesis, dendritic outgrowth and circuit formation in the adult nervous system. In terms of the potential for stem cells and cell replacement approaches in the nervous system, these barriers are perhaps more daunting than even cell differentiation and survival. We would like to use the success of this natural “regeneration” to understand the molecules that prevent/enhance synaptic development and circuit formation in the adult nervous system.
To approach this problem, we first made use of a unique transgenic mouse in which newborn neurons are transiently labeled with a fluorescent marker (green fluorescent protein, GFP). We used these mice to examine the development of dendrites and synapses, which revealed several distinct stages—an early stage in which dendrites are limited to the inner molecular layer of the dentate gyrus and synapses are exclusively GABAergic. We also showed that seizures accelerate the integration of newborn neurons. Our current approach is to use viral-mediated gene transfer to specifically up- or down-regulate molecules in either newborn or mature granule cells to examine their role in circuit formation. We assess the results using a variety of cellular and molecular techniques including single cell physiology in brain slices, measures of gene expression, and confocal imaging of hippocampal tissue.
We are using the olfactory system to examine the role of microcircuits in the function of this sensory system. We are interested in understanding how the unique synaptic architecture of the bulb shapes the highly organized incoming sensory information, i.e. what aspects of the circuit control detection and discrimination of odors. Our current experiments focus on a set of 1000 neuropil structures in the olfactory bulb called glomeruli (the red spherical structures in the image.) Neurons within a glomerulus receive specific sensory input and show highly synchronized activity. Our experiments have revealed both slow and fast coordination of activity in glomeruli involving slow modulatory receptors such as metabotropic glutamate receptor 1, as well as rapid action potential synchronization involving the gap junction molecule, connexin 36, on distal dendrites. We are also examining the functional architecture of the olfactory cortex using paired whole-cell recording in principal neurons and interneurons of the anterior olfactory cortex and piriform cortex.