Date of Award

1-1-2013

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

College/School/Department

Department of Psychology

Program

Behavioral Neuroscience

Content Description

1 online resource (xvi, 268 pages) : illustrations (some color)

Dissertation/Thesis Chair

Ewan C McNay

Committee Members

James Neely, Christine Wagner

Keywords

Glucose, GluT4, Hippocampus, Insulin, Memory, Pharmacology, Hippocampus (Brain), Sprague Dawley rats

Subject Categories

Neuroscience and Neurobiology

Abstract

Although a great deal of research has elucidated both localization and function of many glucose transporters (GluTs) throughout the brain, the function of brain insulin-responsive glucose transporter-4 (GluT4) remains unclear (McEwen & Reagan, 2004). Because type 2 diabetes mellitus (T2DM; insulin resistance, i.e., impaired insulin signaling) is a risk factor for Alzheimer's disease (AD), and because the signaling mechanisms insulin uses to mediate hippocampal processes and memory are unclear, a major purpose of this dissertation was to determine whether hippocampal GluT4, which is responsible for insulin's glucoregulatory and canonical effects in the periphery, is necessary for insulin's effects in the hippocampus (Green et al., 2005; Grillo, Piroli, Hendry, & Reagan, 2009b; Ho et al., 2004; Irie et al., 2008; Janson et al., 2004; Julien et al., 2010; Launer, 2009; Luchsinger, Tang, Stern, Shea, & Mayeux, 2001; McNay et al.; Peila, Rodriguez, & Launer, 2002; Vannucci et al., 1998c). In this dissertation I provided functional analyses of hippocampal GluT4 and several of its effectors. I showed experimental evidence that GluT4 is involved in hippocampal function and memory, and I examined the relationship between GluT4 and insulin signaling in the hippocampus. The animal model used was juvenile male Sprague-Dawley rats and the major experimental approaches included behavioral pharmacology, behavioral testing, and neurochemical and metabolic analyses. In Chapter 1, because recent research has identified amyloid-β 1-42 oligomers (i.e., a key AD pathogen in the hippocampus) as inhibitors of insulin signaling in the hippocampus (De Felice et al., 2009b; Zhao et al., 2008a), I hypothesized that amyloid-β 1-42 oligomers would impair hippocampal insulin signaling and GluT4, thus leading to impaired hippocampal glucose metabolism and memory. I demonstrated that amyloid-β 1-42 oligomers decreased translocation of plasma membrane (PM) GluT4 in the hippocampus (with no effect on translocation of other hippocampal GluTs), impaired hippocampal glucose utilization during cognitive load (but not in its absence), and induced spatial working memory impairments (as measured by spontaneous alternation, SA). These data were consistent with previous reports that insulin signaling is a key component of hippocampally-dependent memory (McNay et al., 2010; McNay & Recknagel, 2011; Zhao & Alkon, 2001; Zhao, Chen, Quon, & Alkon, 2004). The correlation between impaired memory and decreased GluT4 translocation to the PM observed in Chapter 1 suggested that GluT4 (but not other GluTs) might normally be activated during hippocampally-dependent memory formation to mediate memory formation. In Chapter 2, I demonstrated spatial working memory (i.e., SA) and acquisition of avoidance memory increased translocation of GluT4 to the PM relative to other GluTs, consistent with the hypothesis that GluT4 is activated during memory formation. Retrieval of a successfully consolidated memory did not affect GluT4 trafficking. These data indicate that GluT4 might be involved in acquisition or consolidation of memory (i.e., memory formation). These data led to the next hypothesis that blockade of hippocampal GluT4 activity (via direct blockade of GluT4 at the PM) would impair memory to the same extent as blocking insulin signaling would (i.e., suggesting that insulin's effects on hippocampal function require GluT4). In Chapter 3, I used acute pharmacological inhibition of hippocampal GluT4 and tested rats for SA performance; I did not observe an effect of hippocampal GluT4 inhibition on SA performance when levels of hippocampal insulin were at baseline. However, GluT4 inhibition prevented the spatial working memory (i.e., SA) enhancing and metabolic effects (i.e., glucose utilization) of intrahippocampal insulin administration (i.e., supra-baseline levels of hippocampal insulin). In light of previous work showing that inhibition of hippocampal insulin signaling impaired SA (McNay et al., 2010), my data extend previous findings by demonstrating that hippocampal GluT4 activation is necessary for the memory effects of supra-baseline levels of insulin, but not essential to the effects of insulin on SA when hippocampal insulin levels are kept at baseline. Furthermore, I showed that insulin's effects on hippocampal neuronal glucose utilization require functional GluT4 activity, demonstrating that the canonical insulin signaling pathway (i.e., insulin activating GluT4 translocation and glucose uptake) is likely present in the hippocampus. In Chapter 4, I connected several studies that showed T2DM is a risk factor for AD (Ben-Romano et al., 2003; Green et al., 2005; Ho et al., 2004; Hresko & Hruz, 2011; Irie et al., 2008; Janson et al., 2004; Julien et al., 2010; Launer, 2009; Luchsinger et al., 2001; McNay et al.; Peila et al., 2002; Rudich et al., 2003; Vyas, Koster, Tzekov, & Hruz, 2010b) and that hippocampal insulin resistance may emerge during AD and HIV-associated neurocognitive disorder (HAND). Because long-term inhibition of hippocampal GluT4 has been hypothesized to occur in HAND, I hypothesized that long-term inhibition of hippocampal GluT4 activity would produce an AD-like condition in rats, as measured by decreased insulin signaling, increased cell death, and amyloid-β pathology, all within the hippocampus. I first showed that long-term hippocampal GluT4 inhibition led to a dose-dependent increase in SA performance. These results were unexpected given the previous SA results, which showed that impaired SA performance was associated with decreased GluT4 activity or translocation. I then extended these findings to show that STM was increased following long-term hippocampal GluT4 inhibition, but long-term memory (LTM) was impaired. The STM enhancement was likely induced by increased glutamatergic activity and glucose utilization in the hippocampus, whereas the LTM impairment was likely mediated by decreased hippocampal brain-derived neutrophic factor. I did not observe an effect on extra-hippocampally-mediated behaviors, consistent with the high expression of GluT4 in the hippocampus (Alquier et al., 2006; and unpublished data) Also, I discovered that chronic brain GluT4 inhibition failed to produce AD-like pathology (i.e., there was no change in amyloid processing between groups) nor an insulin resistant-like state in the brain (which would have been detected by decreased PM GluT4 and decreased activity in insulin signaling proteins within the hippocampus, neither of which occurred). This suggests that chronic hippocampal GluT4 inhibition does not produce the same effects as chronic hippocampal insulin signaling inhibition, as observed in T2DM (i.e., AD-like pathology and decreased insulin signaling). Taken together, these findings suggest the following: First, although the effects of supra-baseline levels of hippocampal insulin required GluT4, the effects at baseline insulin levels did not, suggesting that baseline insulin mediates hippocampal memory differently from supra-baseline levels (i.e., perhaps by enhancing glucose utilization). Second, memory training increased GluT4 translocation to the PM in the hippocampus, which was unexpected given that inhibition of GluT4 during memory training had no effect on SA behavior. Third, unexpectedly, blocking chronic hippocampal GluT4 did not impair (and indeed enhanced) STM, an effect which appeared to be associated with hippocampal adaption to upregulated glucose supply mechanisms in response to this blockage; intriguingly, LTM processes were impaired. Collectively, these data offer insights into the distinct hippocampal molecular pathways underlying GluT4 activity in the hippocampus as well as providing insight into the neural (and specifically, hippocampal) impact of diseases including T2DM, AD, and HAND.

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