Peripherally restricted viral challenge elevates extracellular glutamate and enhances synaptic transmission in the hippocampus

Intermittent systemic exposure to lipopolysaccharide-induced inflammation disrupts hippocampal long-term potentiation and impairs cognition in aging male mice

Age-related cognitive decline, a common component of the brain aging process, is associated with significant impairment in daily functioning and quality of life among geriatric adults. While the complexity of mechanisms underlying cognitive aging are still being elucidated, microbial exposure and the multifactorial inflammatory cascades associated with systemic infections are emerging as potential drivers of neurological senescence. The negative cognitive and neurobiological consequences of a single pathogen-associated inflammatory experience, such as that modeled through treatment with lipopolysaccharide (LPS), are well documented. Yet, the brain aging impacts of repeated, intermittent inflammatory challenges are less well studied. To extend the emerging literature assessing the impact of infection burden on cognitive function among normally aging mice, here, we repeatedly exposed adult mice to intermittent LPS challenges during the aging period. Male 10-month-old C57BL6 mice were systemically administered escalating doses of LPS once every two weeks for 2.5 months. We evaluated cognitive consequences using the non-spatial step-through inhibitory avoidance task, and both spatial working and reference memory versions of the Morris water maze. We also probed several potential mechanisms, including cortical and hippocampal cytokine/chemokine gene expression, as well as hippocampal neuronal function via extracellular field potential recordings. Though there was limited evidence for an ongoing inflammatory state in cortex and hippocampus, we observed impaired learning and memory and a disruption of hippocampal long-term potentiation. These data suggest that a history of intermittent exposure to LPS-induced inflammation is associated with subtle but significantly impaired cognition among normally aging mice. The broader impact of these findings may have important implications for standard of care involving infections in aging individuals or populations at-risk for dementia.

Influenza A Virus (H1N1) Infection Induces Microglial Activation and Temporal Dysbalance in Glutamatergic Synaptic Transmission

Influenza A virus (IAV) causes respiratory tract disease and is responsible for seasonal and reoccurring epidemics affecting all age groups. Next to typical disease symptoms, such as fever and fatigue, IAV infection has been associated with behavioral alterations presumably contributing to the development of major depression. Previous experiments using IAV/H1N1 infection models have shown impaired hippocampal neuronal morphology and cognitive abilities, but the underlying pathways have not been fully described. In this study, we demonstrate that infection with a low-dose non-neurotrophic H1N1 strain of IAV causes ample peripheral immune response followed by a temporary blood-brain barrier disturbance. Although histological examination did not reveal obvious pathological processes in the brains of IAV-infected mice, detailed multidimensional flow cytometric characterization of immune cells uncovered subtle alterations in the activation status of microglial cells. More specifically, we detected an altered expression pattern of major histocompatibility complex classes I and II, CD80, and F4/80 accompanied by elevated mRNA levels of CD36, CD68, C1QA, and C3, suggesting evolved synaptic pruning. To closer evaluate how these profound changes affect synaptic balance, we established a highly sensitive multiplex flow cytometry-based approach called flow synaptometry. The introduction of this novel technique enabled us to simultaneously quantify the abundance of pre- and postsynapses from distinct brain regions. Our data reveal a significant reduction of VGLUT1 in excitatory presynaptic terminals in the cortex and hippocampus, identifying a subtle dysbalance in glutamatergic synapse transmission upon H1N1 infection in mice. In conclusion, our results highlight the consequences of systemic IAV-triggered inflammation on the central nervous system and the induction and progression of neuronal alterations. IMPORTANCE Influenza A virus (IAV) causes mainly respiratory tract disease with fever and fatigue but is also associated with behavioral alterations in humans. Here, we demonstrate that infection with a low-dose non-neurotrophic H1N1 strain of IAV causes peripheral immune response followed by a temporary blood-brain barrier disturbance. Characterization of immune cells uncovered subtle alterations in the activation status of microglia cells that might reshape neuronal synapses. We established a highly sensitive multiplex flow cytometry-based approach called flow synaptometry to more closely study the synapses. Thus, we detected a specific dysbalance in glutamatergic synapse transmission upon H1N1 infection in mice. In conclusion, our results highlight the consequences of systemic IAV-triggered inflammation on the central nervous system and the induction and progression of neuronal alterations.

Peripheral viral challenge increases c-fos level in cerebral neurons

Peripheral viral infection can substantially alter brain function. We have previously shown that intraperitoneal (i.p.) injection of a viral mimetic, polyinosinic-polycytidylic acid (PIC), engenders hyperexcitability of cerebral neurons. Because neuronal activity is invariably associated with their expression of the Cfos gene, the present study was undertaken to determine whether PIC challenge also increases neuronal c-fos protein level. Female C57BL/6 mice were i.p. injected with PIC, and neuronal c-fos was analyzed in the motor cortex by immunohistochemistry. PIC challenge instigated a robust increase in the number of c-fos-positive neurons. This increase reached approximately tenfold over control at 24 h. Also, the c-fos staining intensity of individual neurons increased. AMG-487, a specific inhibitor of the chemokine receptor CXCR3, profoundly attenuated the accumulation of neuronal c-fos, indicating the activation of CXCL10/CXCR3 axis as the trigger of the process. Together, these results show that the accumulation of c-fos is a viable readout to assess the response of cerebral neurons to peripheral PIC challenge, and to elucidate the underlying molecular mechanisms.

Glutamate levels and perfusion in pons during migraine attacks: A 3T MRI study using proton spectroscopy and arterial spin labeling

Migraine is a complex disorder, involving peripheral and central brain structures, where mechanisms and site of attack initiation are an unresolved puzzle. While abnormal pontine neuronal activation during migraine attacks has been reported, exact implication of this finding is unknown. Evidence suggests an important role of glutamate in migraine, implying a possible association of pontine hyperactivity to increased glutamate levels. Migraine without aura patients were scanned during attacks after calcitonin gene-related peptide and sildenafil in a double-blind, randomized, double-dummy, cross-over design, on two separate study days, by proton magnetic resonance spectroscopy and pseudo-continuous arterial spin labeling at 3T. Headache characteristics were recorded until 24 h after drug administrations. Twenty-six patients were scanned during migraine, yielding a total of 41 attacks. Cerebral blood flow increased in dorsolateral pons, ipsilateral to pain side during attacks, compared to outside attacks (13.6%, p = 0.009). Glutamate levels in the same area remained unchanged during attacks (p = 0.873), while total creatine levels increased (3.5%, p = 0.041). In conclusion, dorsolateral pontine activation during migraine was not associated with higher glutamate levels. However, the concurrently increased total creatine levels may suggest an altered energy metabolism, which should be investigated in future studies to elucidate the role of pons in acute migraine.

Tumor Necrosis Factor-Alpha Alters Electrophysiological Properties of Rabbit Hippocampal Neurons

Previous studies have shown tumor necrosis factor-alpha (TNF-α) may impact neurodegeneration in Alzheimer's disease (AD) by regulating amyloid-β and tau pathogenesis. However, it is unclear whether TNF-α has a role in a cholesterol-fed rabbit model of AD or TNF-α affects the electrophysiological properties of rabbit hippocampus. This study was designed to investigate whether long-term feeding of cholesterol diet known to induce AD pathology regulates TNF-α expression in the hippocampus and whether TNF-α would modulate electrophysiological properties of rabbit hippocampal CA1 neurons. TNF-α ELISA showed dietary cholesterol increased hippocampal TNF-α expression in a dose-dependent manner. Whole-cell recordings revealed TNF-α altered the membrane properties of rabbit hippocampal CA1 neurons, which was characterized by a decrease in after-hyperpolarization amplitudes; Field potential recordings showed TNF-α inhibited long-term potentiation but did not influence presynaptic function. Interestingly, TNF-α did not significantly affect the after-hyperpolarization amplitudes of hippocampal CA1 neurons from cholesterol fed rabbits compared to normal chow fed rabbits. In conclusion, dietary cholesterol generated an in vivo model of chronic TNF-α elevation and TNF-α may underlie the learning and memory changes previously seen in the rabbit model of AD by acting as a bridge between dietary cholesterol and brain function and directly modulating the electrophysiological properties of hippocampal CA1 neurons.

Neuronal CXCL10/CXCR3 Axis Mediates the Induction of Cerebral Hyperexcitability by Peripheral Viral Challenge

Peripheral infections can potently exacerbate neuropathological conditions, though the underlying mechanisms are poorly understood. We have previously demonstrated that intraperitoneal (i.p.) injection of a viral mimetic, polyinosinic-polycytidylic acid (PIC) induces a robust generation of CXCL10 chemokine in the hippocampus. The hippocampus also features hyperexcitability of neuronal circuits following PIC challenge. The present study was undertaken to determine the role of CXCL10 in mediating the development of hyperexcitability in response to PIC challenge. Briefly, young female C57BL/6 mice were i.p. injected with PIC, and after 24 h, the brains were analyzed by confocal microscopy. CXCL10 staining of neuronal perikarya and a less intense staining of the neuropil was observed in the hippocampus and cortex. CXCL10 staining was also evident in a subpopulation of astrocytes, whereas microglia were CXCL10 negative. CXCR3, the cognate receptor of CXCL10 was present exclusively on neurons, indicating that the CXCL10/CXCR3 axis operates through an autocrine/paracrine neuronal signaling. Blocking cerebral CXCR3 through intracerebroventricular injection of a specific inhibitor, AMG487, abrogated PIC challenge-induced increase in basal synaptic transmission and long-term potentiation (LTP), as well as the reduction of paired-pulse facilitation (PPF), in the hippocampus. The PIC-mediated abolishment of hippocampal long-term depression (LTD) was also restored after administration of AMG487. Moreover, CXCR3 inhibition attenuated seizure hypersensitivity induced by PIC challenge. The efficacy of AMG487 strongly strengthens the notion that CXCL10/CXCR3 axis mediates the induction of cerebral hyperexcitability by PIC challenge.

Adiponectin Knockout Mice Display Cognitive and Synaptic Deficits

Adiponectin is an adipokine that has recently been under investigation for potential neuroprotective effects in various brain disorders including Alzheimer's disease, stroke, and depression. Adiponectin receptors (AdipoR1 and AdipoR2) are found throughout various brain regions, including the hippocampus. However, the role of these receptors in synaptic and cognitive function is not clear. Therefore, the goal of the current study was to evaluate synaptic and cognitive function in the absence of adiponectin. The current study utilized 12-month-old adiponectin knockout (APN-KO) mice and age-matched controls to study cognitive and hippocampal synaptic alterations. We determined that AdipoR1 and AdipoR2 are present in the synaptosome, with AdipoR2 displaying increased presynaptic vs. postsynaptic localization, whereas AdipoR1 was enriched in both the presynaptic and postsynaptic fractions. APN-KO mice displayed cognitive deficits in the novel object recognition (NOR) and Y-maze tests. This was mirrored by deficits in long-term potentiation (LTP) of the hippocampal Schaefer collateral pathway in APN-KO mice. APN-KO mice also displayed a reduction in basal synaptic transmission and an increase in presynaptic release probability. Deficits in LTP were rescued through hippocampal slice incubation with the adiponectin receptor agonist, AdipoRon, indicating that acute alterations in adiponectin receptor signaling influence synaptic function. Along with the deficits in LTP, altered levels of key presynaptic and postsynaptic proteins involved in glutamatergic neurotransmission were observed in APN-KO mice. Taken together, these results indicate that adiponectin is an important regulator of cognition and synaptic function in the hippocampus. Future studies should examine the role of specific adiponectin receptors in synaptic processes.

Effects of sildenafil and calcitonin gene-related peptide on brainstem glutamate levels: a pharmacological proton magnetic resonance spectroscopy study at 3.0 T

BACKGROUND

Studies involving human pharmacological migraine models have predominantly focused on the vasoactive effects of headache-inducing drugs, including sildenafil and calcitonin gene-related peptide (CGRP). However, the role of possible glutamate level changes in the brainstem and thalamus is of emerging interest in the field of migraine research bringing forth the need for a novel, validated method to study the biochemical effects in these areas.

METHODS

We applied an optimized in vivo human pharmacological proton (¹H) magnetic resonance spectroscopy (MRS) protocol (PRESS, repetition time 3000 ms, echo time 37.6-38.3 ms) at 3.0 T in combination with sildenafil and CGRP in a double-blind, placebo-controlled, randomized, double-dummy, three-way cross-over design. Seventeen healthy participants were scanned with the ¹H-MRS protocol at baseline and twice (at 40 min and 140 min) after drug administration to investigate the sildenafil- and CGRP-induced glutamate changes in both brainstem and thalamus.

RESULTS

The glutamate levels increased transiently in the brainstem at 40-70 min after sildenafil administration compared to placebo (5.6%, P = 0.039). We found no sildenafil-induced glutamate changes in the thalamus, and no CGRP-induced glutamate changes in the brainstem or thalamus compared to placebo. Both sildenafil and CGRP induced headache in 53%-62% of participants. We found no interaction in the glutamate levels in the brainstem or thalamus between participants who developed sildenafil and/or CGRP-induced headache as compared to participants who did not.

CONCLUSIONS

The transient sildenafil-induced glutamate change in the brainstem possibly reflects increased excitability of the brainstem neurons. CGRP did not induce brainstem or thalamic glutamate changes, suggesting that it rather exerts its headache-inducing effects on the peripheral trigeminal pain pathways.

Peripheral viral challenge triggers hippocampal production of inflammatory proteins

Peripheral viral infections increase seizure propensity and intensity in susceptible individuals. We have modeled this comorbidity by demonstrating that intraperitoneal (ip) injection of the conventional viral mimetic, polyinosinic-polycytidylic acid (PIC), renders the brain hypersusceptible to seizures induced by kainic acid (KA). At the molecular level, the hippocampus, which is the ictal site of KA-induced seizures, exhibits upregulated expression of messages encoding several inflammatory genes. Here, we profiled temporal expression of these genes at the protein level. Briefly, eight-week old female C57BL/6 mice were ip injected with 12 mg/kg of PIC and inflammatory proteins were quantified in the hippocampus and blood by ELISA. We found a robust but transient increase in blood concentration of IL-6, CXCL10, CCL2, CXCL9, CCL7 and CCL12 six hours after PIC challenge. CXCL1, IL1β, TNFα and CXCL2 featured a moderate increase. However, only four chemokines were increased in the hippocampus. CXCL10 showed the highest increase 6-12 h after PIC challenge, and its level dwindled to the baseline by 48 h. CXCL1, CXCl9 and CXCL2 were also transiently elevated but their maximal values were by an order of magnitude lower than the values for CXCL10. These results indicate that CXCL10 is the primary inflammatory protein generated in the hippocampus in response to PIC challenge, and that this chemokine may drive the development of seizure hypersusceptibility. In addition, the hippocampus featured a protracted increase in the levels of anaphylatoxins C3a and C5a, indicating the activation of the complement cascades.

Using Enzyme-based Biosensors to Measure Tonic and Phasic Glutamate in Alzheimer's Mouse Models

Neurotransmitter disruption is often a key component of diseases of the central nervous system (CNS), playing a role in the pathology underlying Alzheimer's disease, Parkinson's disease, depression, and anxiety. Traditionally, microdialysis has been the most common (lauded) technique to examine neurotransmitter changes that occur in these disorders. But because microdialysis has the ability to measure slow 1-20 minute changes across large areas of tissue, it has the disadvantage of invasiveness, potentially destroying intrinsic connections within the brain and a slow sampling capability. A relatively newer technique, the microelectrode array (MEA), has numerous advantages for measuring specific neurotransmitter changes within discrete brain regions as they occur, making for a spatially and temporally precise approach. In addition, using MEAs is minimally invasive, allowing for measurement of neurotransmitter alterations in vivo. In our laboratory, we have been specifically interested in changes in the neurotransmitter, glutamate, related to Alzheimer's disease pathology. As such, the method described here has been used to assess potential hippocampal disruptions in glutamate in a transgenic mouse model of Alzheimer's disease. Briefly, the method used involves coating a multi-site microelectrode with an enzyme very selective for the neurotransmitter of interest and using self-referencing sites to subtract out background noise and interferents. After plating and calibration, the MEA can be constructed with a micropipette and lowered into the brain region of interest using a stereotaxic device. Here, the method described involves anesthetizing rTg(TauP301L)4510 mice and using a stereotaxic device to precisely target sub-regions (DG, CA1, and CA3) of the hippocampus.

Peripheral viral challenge elevates extracellular glutamate in the hippocampus leading to seizure hypersusceptibility

Peripheral viral infections increase seizure propensity and intensity in susceptible individuals. We have modeled this comorbidity by demonstrating that the acute phase response instigated by an intraperitoneal (i.p.) injection of a viral mimetic, polyinosinic-polycytidylic acid (PIC), induces protracted hypersusceptibility to kainic acid-induced seizures. We have further demonstrated that PIC challenge robustly increases the level of tonic extracellular glutamate and neuronal excitability in the hippocampus. This study was undertaken to determine a relationship between tonic glutamate and seizure susceptibility following PIC challenge. Briefly, glutamate-sensing microelectrodes were permanently implanted into the CA1 of 8-week-old female C57BL/6 mice. Following a 3-day recovery, acute phase response was induced by i.p. injection of 12 mg/kg of PIC, while saline-injected mice served as controls. Tonic glutamate was measured at 1, 2, 3 and 4 days after PIC challenge. PIC challenge induced an approximately fourfold increase in tonic glutamate levels measured after 24 h. The levels gradually declined to the baseline values within 4 days. Twenty-four hours after PIC challenge, the mice featured an approximately threefold increase in cumulative seizure scores and twofold increase in the duration of status epilepticus induced by subcutaneous injection of 12 mg/kg of kainic acid. Seizure scores positively correlated with pre-seizure tonic glutamate. Moreover, seizures resulted in a profound (76%) elevation of extracellular glutamate in the CA1 of PIC-challenged but not saline-injected mice. Our results implicate the increase in extracellular glutamate as a mediator of seizure hypersusceptibility induced by peripheral viral challenge.