New neurons are continually added to the granule cell (GC) layer of the dentate gyrus during adulthood. Within 2 weeks, over 50% of these granule cells die, with most surviving GCs incorporating into the dentate circuit. Although the functional significance of dentate neurogenesis is unclear, treatments that reduce GC neurogenesis impair associative memory, and associative learning tasks increase the survival of new GCs. Thus the regulation of GC survival appears important for associative learning. Our preliminary studies show low-frequency stimulation of the perforant pathway, the principal glutamatergic projection to the dentate gyrus, confers complete survival of immature GC neurons 8 days of age. Furthermore, subsequent activation of the same perforant path fibers 1 week later induces the immediate-early gene (IEG) Zif268 in immature neurons. Our primary hypothesis is afferent activity promotes GC survival, most likely via GABAa receptor activation. This, in turn, initiates rapid development of immature GCs into functional, mature GCs that display NMDA receptors and synaptic plasticity as reflected in the activity-dependent expression of the IEG Zif268. To test these hypotheses, we will use adult rats with permanently implanted electrodes in the perforant pathway and dentate gyrus. The effects of perforant path stimulation on GC neurogenesis is assessed by administering bromodeoxyuridine (BrdU), a thymidine analog incorporated only in dividing cells, in conjunction with unbiased stereology and fluorescent immuno-cytochemistry to quantify BrdU-positive cells, IEG protein expression and markers of GC development (progenitor, immature and mature). In the studies of Specific Aim 1, we will determine the `critical period'after mitosis during which perforant path stimulation can confer survival of newly- generated GC neurons. We also will explore the factor(s) that are provided by afferent activity that confer GC survival, and determine whether local or systemic administration of GABAa, AMPA, or NMDA receptor antagonists block GC survival mediated by perforant path stimulation.
In Specific Aim 2, we will determine if conferring survival of 8 day old GCs also accelerates their functional development This will be assessed by determining the earliest time point when NR2B NMDAR subunits and Zif268, an IEG protein implicated in synaptic plasticity, can be expressed following afferent-induced GC survival when compared with unstimulated GCs of the same age. We also will explore possible factors provided by afferent activity that are crucial for Zif268 expression, and determine if GABAa, AMPA, or NMDA receptor antagonists block Zif268 expression in surviving GCs.
In Specific Aim 3, we will determine if increasing or depleting the population of 8-day old, survival-competent GCs facilitates or impairs performance in spatial or episodic memory tasks. These studies will contribute to understanding stem cell survival, crucial for treating neurological disorders, and will facilitate the PI's development goal of obtaining mainstream funding to maintain a viable research/teaching laboratory environment at a minority institution.
An understanding neurogenesis in the adult dentate gyrus will provide information regarding how the functional development of immature granule cells is regulated by synaptic activity in vivo. The factors necessary for neuronal survival and appropriate targeting of neurons generated from adult stem cells remain unknown, limiting the ability of transplanted cells to be used as a treatment for various diseases- such as Alzheimer's disease, Parkinson's disease, ischemia, and brain trauma - that involve a loss of neurons. As regulating neuronal stem cell survival is essential to achieve the goal of treatment with stem cells, these studies will provide crucial information on how neuronal stem cells survive and incorporate into extant neural circuits, and address a major caveat in stem cell therapy.
|Gonzalez, Jossina; Morales, Isaiah S; Villarreal, Desiree M et al. (2014) Low-frequency stimulation induces long-term depression and slow onset long-term potentiation at perforant path-dentate gyrus synapses in vivo. J Neurophysiol 111:1259-73|
|Martinez, Carlo O; Do, Viet H; Derrick, Brian E (2011) Endogenous opioid peptides contribute to associative LTP in the hippocampal CA3 region. Neurobiol Learn Mem 96:207-17|