Reduced tonic inhibition in the dentate gyrus contributes to chronic stress-induced impairments in learning and memory.

It is well established that stress impacts the underlying processes of learning and memory. The effects of stress on memory are thought to involve, at least in part, effects on the hippocampus, which is particularly vulnerable to stress. Chronic stress induces hippocampal alterations, including but not limited to dendritic atrophy and decreased neurogenesis, which are thought to contribute to chronic stress-induced hippocampal dysfunction and deficits in learning and memory. Changes in synaptic transmission, including changes in GABAergic inhibition, have been documented following chronic stress. Recently, our laboratory demonstrated shifts in EGABA in CA1 pyramidal neurons following chronic stress, compromising GABAergic transmission and increasing excitability of these neurons. Interestingly, here we demonstrate that these alterations are unique to CA1 pyramidal neurons, since we do not observe shifts in EGABA following chronic stress in dentate gyrus granule cells. Following chronic stress, there is a decrease in the expression of the GABAA receptor (GABAA R) δ subunit and tonic GABAergic inhibition in dentate gyrus granule cells, whereas there is an increase in the phasic component of GABAergic inhibition, evident by an increase in the peak amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs). Given the numerous changes observed in the hippocampus following stress, it is difficult to pinpoint the pertinent contributing pathophysiological factors. Here we directly assess the impact of a reduction in tonic GABAergic inhibition of dentate gyrus granule cells on learning and memory using a mouse model with a decrease in GABAA R δ subunit expression specifically in dentate gyrus granule cells (Gabrd/Pomc mice). Reduced GABAA R δ subunit expression and function in dentate gyrus granule cells is sufficient to induce deficits in learning and memory. Collectively, these findings suggest that the reduction in GABAA R δ subunit-mediated tonic inhibition in dentate gyrus granule cells contributes, at least in part, to deficits in learning and memory associated with chronic stress. These findings have significant implications regarding the pathophysiological mechanisms underlying impairments in learning and memory associated with stress and suggest a role for GABAA R δ subunit containing receptors in dentate gyrus granule cells. © 2016 Wiley Periodicals, Inc.

Impact of inhibitory constraint of interneurons on neuronal excitability.

Tonic inhibition is thought to dampen the excitability of principal neurons; however, little is known about the role of tonic GABAergic inhibition in interneurons and the impact on principal neuron excitability. In many brain regions, tonic GABAergic inhibition is mediated by extrasynaptic, δ-subunit-containing GABAA receptors (GABAARs). In the present study we demonstrate the importance of GABAAR δ-subunit-mediated tonic inhibition in interneurons. Selective elimination of the GABAAR δ-subunit from interneurons was achieved by crossing a novel floxed Gabrd mouse model with GAD65-Cre mice (Gabrd/Gad mice). Deficits in GABAAR δ-subunit expression in GAD65-positive neurons result in a decrease in tonic GABAergic inhibition and increased excitability of both molecular layer (ML) and stratum radiatum (SR) interneurons. Disinhibition of interneurons results in robust alterations in the neuronal excitability of principal neurons and decreased seizure susceptibility. Gabrd/Gad mice have enhanced tonic and phasic GABAergic inhibition in both CA1 pyramidal neurons and dentate gyrus granule cells (DGGCs). Consistent with alterations in hippocampal excitability, CA1 pyramidal neurons and DGGCs from Gabrd/Gad mice exhibit a shift in the input-output relationship toward decreased excitability compared with those from Cre(-/-) littermates. Furthermore, seizure susceptibility, in response to 20 mg/kg kainic acid, is significantly decreased in Gabrd/Gad mice compared with Cre(-/-) controls. These data demonstrate a critical role for GABAAR δ-subunit-mediated tonic GABAergic inhibition of interneurons on principal neuronal excitability and seizure susceptibility.

Alterations in brain metabolism during the first year of HIV infection.

Migration of both uninfected and infected monocytes into the brain during acute HIV infection likely initiates metabolic changes that can be observed with magnetic resonance spectroscopy (MRS). Herein, we measured changes in brain metabolism during the first year of HIV infection and examined the relationship of these metabolite levels to CD16+ monocyte populations measured in the blood. MRS was performed on nine HIV+ subjects identified during acute HIV infection and nine seronegative control subjects. HIV+ subjects were examined within 90 days of an indeterminate Western blot, then again 2 and 6 months later, during early infection. Blood samples were collected for plasma viral RNA and monocyte subset quantification. HIV+ subjects were identified with acute viral ailment and did not display severe cognitive deficits such as dementia or minor cognitive motor disorder. Changes in lipid membrane metabolism (choline levels) in the frontal cortex and white matter were observed during the initial year of HIV infection. Greater numbers of CD16+ monocytes were associated with lower N-acetylaspartate levels and higher choline levels in the brain. These results suggest that HIV infection induces metabolic changes in the brain early during infection and that these changes may be related to monocyte dynamics in the periphery.

Factor analysis of proton MR spectroscopic imaging data in HIV infection: metabolite-derived factors help identify infection and dementia.

PURPOSE: To develop a relevant pathophysiologic model of human immunodeficiency virus (HIV)-associated dementia by studying regional variations in metabolite levels measured with magnetic resonance (MR) spectroscopic imaging and their relationship to immunologic measures and cognitive dysfunction. MATERIALS AND METHODS: This was a HIPAA-compliant, institutional review board-approved study involving written informed consent. Distributions of N-acetylaspartate (NAA), choline (Cho), and creatine (Cr) concentrations in 94 subjects (20 seronegative controls and 74 HIV-positive subjects; 34 of the HIV-positive subjects having HIV-associated dementia; 63 men, 31 women; mean age, 40 years) were determined with proton (hydrogen 1 [(1)H]) MR spectroscopic imaging. HIV-positive subjects underwent neuropsychological testing and blood and cerebrospinal fluid (CSF) analysis. Factor analysis was utilized to determine associations between metabolites across regions. Analysis of variance and t tests were used to isolate differences between cohorts. RESULTS: A "Cho factor" differentiated seronegative controls from HIV-infected cohorts, indicating elevated Cho levels across deep gray and white matter regions of HIV-positive individuals. An "NAA factor" differentiated those with dementia from those without and correlated best with psychomotor and executive function tests. A "Cr factor" indicated Cr elevations correlated with CSF monocyte chemoattractant protein-1 levels. NAA and Cr factor scores were strongly weighted to metabolite changes in white matter regions. CONCLUSION: These results highlight the importance of white matter involvement in HIV-associated dementia and support the current pathogenesis model of glial cell proliferation in HIV infection, denoted by regional Cho elevations, and neuronal dysfunction and/or death, denoted by NAA decreases, associated with dementia. Factor analysis of MR spectroscopic imaging data is a useful method for determining regional metabolic variations in HIV infection and its neuropsychological correlates.

In vivo proton magnetic resonance spectroscopy reveals region specific metabolic responses to SIV infection in the macaque brain.

BACKGROUND: In vivo proton magnetic resonance spectroscopy (1H-MRS) studies of HIV-infected humans have demonstrated significant metabolic abnormalities that vary by brain region, but the causes are poorly understood. Metabolic changes in the frontal cortex, basal ganglia and white matter in 18 SIV-infected macaques were investigated using MRS during the first month of infection. RESULTS: Changes in the N-acetylaspartate (NAA), choline (Cho), myo-inositol (MI), creatine (Cr) and glutamine/glutamate (Glx) resonances were quantified both in absolute terms and relative to the creatine resonance. Most abnormalities were observed at the time of peak viremia, 2 weeks post infection (wpi). At that time point, significant decreases in NAA and NAA/Cr, reflecting neuronal injury, were observed only in the frontal cortex. Cr was significantly elevated only in the white matter. Changes in Cho and Cho/Cr were similar across the brain regions, increasing at 2 wpi, and falling below baseline levels at 4 wpi. MI and MI/Cr levels were increased across all brain regions. CONCLUSION: These data best support the hypothesis that different brain regions have variable intrinsic vulnerabilities to neuronal injury caused by the AIDS virus.

Magnetic resonance spectroscopy study of oxygen therapy in ischemic stroke.

BACKGROUND AND PURPOSE: Recent studies suggest that normobaric oxygen therapy (NBO) is neuroprotective in acute ischemic stroke. METHODS: We performed multivoxel magnetic resonance spectroscopic imaging and diffusion/perfusion MRI in patients with stroke treated with NBO or room air. Imaging was performed before, during, and after therapy. RESULTS: Voxel-based analysis showed excellent correlation between apparent diffusion coefficient values, lactate, and N-acetyl-aspartate levels at all time points. Lactate decreased during NBO and increased post-NBO. N-acetyl-aspartate decreased in patients receiving room air but not in NBO-treated patients. CONCLUSIONS: These data suggest that NBO improves aerobic metabolism and preserves neuronal integrity in the ischemic brain.

Effects of acute and chronic ethanol exposure on the behavior of adult zebrafish (Danio rerio).

The zebrafish has been a popular subject of embryology and genetic research for the past three decades. Recently, however, the interest in its neurobiology and behavior has also increased. Nevertheless, compared to other model organisms, e.g., rodents, zebrafish behavior is understudied and very few behavioral paradigms exist for mutation or drug screening purposes. Alcoholism is one of the biggest and costliest diseases whose mechanisms are not well understood. Model organisms such as the zebrafish may be utilized in this line of research. Previously, we investigated the effects of acute ethanol exposure on adult zebrafish using four behavioral paradigms and employing manual quantification methods. Here, we study the effects of chronic ethanol exposure and analyze how it modifies the effects of acute ethanol treatment. We employ a videotracking-based automated quantification method in a predator model paradigm and show that this method is capable of detecting an avoidance reaction that is ameliorated by higher doses of ethanol, a potential anxiolytic effect. Importantly, we also demonstrate that chronic, two week long, exposure to ethanol results in significant adaptation to this substance in adult zebrafish. Overall, our results suggest that zebrafish will be an appropriate subject for high throughput screening applications aimed at the analysis of the mechanisms and pharmacology of acute and chronic ethanol induced changes in the vertebrate brain.

The impact of tonic GABAA receptor-mediated inhibition on neuronal excitability varies across brain region and cell type.

The diversity of GABAA receptor (GABAAR) subunits and the numerous configurations during subunit assembly give rise to a variety of receptors with different functional properties. This heterogeneity results in variations in GABAergic conductances across numerous brain regions and cell types. Phasic inhibition is mediated by synaptically-localized receptors with a low affinity for GABA and results in a transient, rapidly desensitizing GABAergic conductance; whereas, tonic inhibition is mediated by extrasynaptic receptors with a high affinity for GABA and results in a persistent GABAergic conductance. The specific functions of tonic versus phasic GABAergic inhibition in different cell types and the impact on specific neural circuits are only beginning to be unraveled. Here we review the diversity in the magnitude of tonic GABAergic inhibition in various brain regions and cell types, and highlight the impact on neuronal excitability in different neuronal circuits. Further, we discuss the relevance of tonic inhibition in various physiological and pathological contexts as well as the potential of targeting these receptor subtypes for treatment of diseases, such as epilepsy.

Loss of Gabrd in CRH neurons blunts the corticosterone response to stress and diminishes stress-related behaviors.

The hypothalamic-pituitary-adrenal (HPA) axis is under tight regulation by strong GABAergic inhibition onto corticotropin-releasing hormone (CRH) neurons in the paraventricular nucleus (PVN) of the hypothalamus. CRH neurons receive two forms of GABAergic inhibition, phasic and tonic, but the specific roles of these two types of signaling have not yet been studied in this cell type. Our lab recently demonstrated a role for the GABAAR δ subunit in the tonic GABAergic regulation of CRH neurons. Using a floxed Gabrd mouse model established in our laboratory, we generated mice in which the GABAAR δ subunit is selectively removed from CRH neurons (Gabrd/Crh mice), resulting in a loss of tonic GABAergic inhibition in these neurons. Interestingly, the loss of this tonic GABAergic constraint did not significantly alter basal levels of corticosterone (CORT). However, the loss of the GABAAR δ subunit in CRH neurons blunted the CORT response to stress, likely due to the loss of the disinhibitory effect of GABA following acute stress. This blunting of HPA axis reactivity was associated with a decrease in depression-like and anxiety-like behaviors. Exogenous CORT was sufficient to increase anxiety-like and depression-like behaviors in Gabrd/Crh mice. Together, these results show the importance of the GABAAR δ subunit in the regulation of CRH neurons, and thus the HPA axis, and demonstrate that dysregulation of CRH neurons alters stress-related behaviors.

Metabolic markers of neuronal injury correlate with SIV CNS disease severity and inoculum in the macaque model of neuroAIDS.

In vivo MR spectroscopy (MRS) studies have shown reductions in NAA/Cr levels in patients with severe neurocognitive deficits due to AIDS dementia complex (ADC), also known as neuroAIDS. The relationship between the cellular changes within the brain during neuroAIDS and the role of NAA/Cr as a metabolic marker remains unclear. In order to clarify the relationship between NAA/Cr and disease severity we utilized the simian immunodeficiency virus (SIV)/macaque model of encephalitis. High-field proton MRS was performed on extracted metabolites from frontal cortex tissue samples of 29 rhesus macaques (6 healthy, 23 moribund with AIDS). Neuropathologic determination of encephalitis severity for each animal was completed and was found to correlate with NAA/Cr levels. Decreases in Glu/Cr and GABA/Cr may indicate that both excitatory and inhibitory neurons are affected. Highly significant correlations between NAA/Cr, Glu/Cr, and GABA/Cr were observed. These neuronal metabolites were also decreased in the absence of classical SIV encephalitis (SIVE). At any disease classification, animals inoculated with SIVmac251 were found to have lower levels of NAA/Cr than animals inoculated with SIVmac239. In considering therapy for neuroAIDS the findings here support prevention of the encephalitic process, but suggest that suppressing the formation of multinucleated giant cells alone would be insufficient to prevent neuronal injury.

Factor analysis reveals differences in brain metabolism in macaques with SIV/AIDS and those with SIV-induced encephalitis.

MRS has often been used to study metabolic processes in the HIV-infected brain. However, it remains unclear how changes in individual metabolites are related to one another in this context of virus-induced central nervous system dysfunction. We used factor analysis (FA) to identify patterns of metabolite distributions from an MRS study of healthy macaques and those infected with simian immunodeficiency virus (SIV) which were moribund with AIDS. FA summarized the correlations from nine metabolites into three main factors. Factor 3 identified patterns that discern healthy animals from those with SIV/AIDS. Factor 2 was able to differentiate between animals that had encephalitis and those moribund with AIDS but lacking encephalitis. Specifically, Factor 2 was able to distinguish animals with moderate to severe encephalitis from animals with mild or no encephalitis as well as uninfected controls. FA not only confirmed the involvement of neuronal metabolites (N-acetylaspartate and glutamate) in disease severity, but also detected changes in creatine and myo-inositol that have not been observed in the SIV macaque model previously. These results suggest that the divergent pathways of N-acetylaspartate and creatine in this disease may enable the commonly reported ratio N-acetylaspartate/creatine to be a more sensitive marker of disease severity.