Pharmacological Tools to Study the Role of Astrocytes in Neural Network Functions

Role of amygdala astrocytes in different phases of contextual fear memory

Growing evidence indicates a critical role of astrocytes in learning and memory. However, little is known about the role of basolateral amygdala complex (BLA-C) astrocytes in contextual fear conditioning (CFC), a paradigm relevant to understand and generate treatments for fear- and anxiety-related disorders. To get insights on the involvement of BLA-C astrocytes in fear memory, fluorocitrate (FLC), a reversible astroglial metabolic inhibitor, was applied at critical moments of the memory processing in order to target the acquisition, consolidation, retrieval and reconsolidation process of the fear memory. Adult Wistar male rats were bilaterally cannulated in BLA-C. Ten days later they were infused with different doses of FLC (0.5 or 1 nmol/0.5 µl) or saline before or after CFC and before or after retrieval. FLC impaired fear memory expression when administered before and shortly after CFC, but not one hour later. Infusion of FLC prior and after retrieval did not affect the memory. Our findings suggest that BLA-C astrocytes are critically involved in the acquisition/early consolidation of fear memory but not in the retrieval and reconsolidation. Furthermore, the extinction process was presumably not affected (considering that peri-retrieval administration could also affect this process).

Astrocytes mediate migraine-related intracranial meningeal mechanical hypersensitivity

ABSTRACT

The genesis of the headache phase in migraine with aura is thought to be mediated by cortical spreading depression (CSD) and the subsequent activation and sensitization of primary afferent neurons that innervate the intracranial meninges and their related large vessels. Yet, the exact mechanisms underlying this peripheral meningeal nociceptive response remain poorly understood. We investigated the relative contribution of cortical astrocytes to CSD-evoked meningeal nociception using extracellular single-unit recording of meningeal afferent activity and 2-photon imaging of cortical astrocyte calcium activity, in combination with 2 pharmacological approaches to inhibit astrocytic function. We found that fluoroacetate and l-α-aminoadipate, which inhibit astrocytes through distinct mechanisms, suppressed CSD-evoked afferent mechanical sensitization, but did not affect afferent activation. Pharmacological inhibition of astrocytic function, which ameliorated meningeal afferents' sensitization, reduced basal astrocyte calcium activity but had a minimal effect on the astrocytic calcium wave during CSD. We propose that calcium-independent signaling in cortical astrocytes plays an important role in driving the sensitization of meningeal afferents and the ensuing intracranial mechanical hypersensitivity in migraine with aura.

Involvement of astrocytes activation in orofacial hyperalgesia induced by experimental tooth movement

OBJECTIVE

The study aimed to investigate the involvement of astrocytes in the medullary dorsal horn (MDH) in the orofacial hyperalgesia induced by experimental tooth movement (ETM) and related mechanism.

MATERIALS AND METHODS

Experimental tooth movement was produced with nickel-titanium alloy closed-coil spring fixed between the left maxillary first molar and the left upper incisor. Fluorocitrate was administrated through medullary subarachnoid at 3 days after ETM. Pressure pain threshold (PPT) in masseter cutaneous area was measured. The expression of glial fibrillary acidic protein (GFAP) and c-Fos in MDH was measured using immunofluoroscence staining. The expression of interleukin-1β (IL-1β) and phosphorylated N-methyl-D-aspartic acid (NMDA) receptor subunit NR1 (p-NR1) was measured with Western blotting.

RESULTS

Experimental tooth movement-induced orofacial hyperalgesia from 1 to 9 days as the PPT was significantly reduced (P < .05). Immunofluoroscence staining showed that the expression of c-Fos in MDH was dramatically upregulated at 1 day and 3 days after ETM, while GFAP expression with both immunofluoroscence staining and Western blotting was significantly enhanced at 3 days and 7 days after ETM. Western blotting analysis indicated that the expression of IL-1β and p-NR1 in MDH was significantly enhanced at 3 days after ETM. Furthermore, we found that fluorocitrate administration at 3 days after ETM could markedly suppress the expression of c-Fos, GFAP, IL-1β and p-NR1 and attenuate the reduction of PPT induced by ETM.

CONCLUSION

Astrocyte activation in MDH is involved in the mechanical hyperalgesia, and the subsequent upregulated IL-1β and overexpression of p-NR1 may participate in this process.

Inhibition of Reactive Astrocytes with Fluorocitrate Ameliorates Learning and Memory Impairment Through Upregulating CRTC1 and Synaptophysin in Ischemic Stroke Rats

Ischemic stroke often causes motor and cognitive deficits. Deregulated glia gap junction communication, which is reflected by increased protein levels of glial fibrillary acidic protein (GFAP) and connexin 43 (Cx43), has been observed in ischemic hippocampus and has been associated with cognitive impairment in animal stroke models. Here, we tested the hypothesis that reactive astrocytes-mediated loss of synaptophysin (SYP) and CREB-regulated transcription coactivator 1 (CRTC1) contribute to dysfunction in glia gap junction communication and memory impairment after ischemic stroke. Male Sprague-Dawley rats were subjected to a 90-min middle cerebral artery occlusion (MCAO) with 7-day reperfusion. Fluorocitrate (1 nmol), the reversible inhibitor of the astrocytic tricarboxylic acid cycle, was injected into the right lateral ventricle of MCAO rats once every 2 days starting immediately before reperfusion. The Morris water maze was used to assess memory in conjunction with western blotting and immunostaining to detect protein expression and distribution in the hippocampus. Our results showed that ischemic stroke caused significant memory impairment accompanied by increased protein levels of GFAP and Cx43 in hippocampal tissue. In addition, the levels of several key memory-related important proteins including SYP, CRTC1, myelin basic protein and high-mobility group-box-1 were significantly reduced in the hippocampal tissue. Of note, inhibition of reactive astrocytes with fluorocitrate was shown to significantly reverse the above noted changes induced by ischemic stroke. Taken together, our findings demonstrate that inhibiting reactive astrocytes with fluorocitrate immediately before reperfusion may protect against ischemic stroke-induced memory impairment through the upregulation of CRTC1 and SYP.

Investigating brain d-serine: Advocacy for good practices

The last two decades have witnessed remarkable advance in our understanding the role of d-amino acids in the mammalian nervous system: from the unknown, to known molecules with unknown functions, to potential central players in health and disease. d-Amino acids have emerged as an important class of signaling molecules. In particular, the exploration of the roles of d-serine in brain physiopathology is a vibrant field that is growing at an accelerating pace. However, disentangling the functions of a chiral molecule in a complex chemical matrice as the brain requires specific measurement and detection methods but is also a challenging task as many molecular tools and models investigators are using can lead to confounded observations. Thus, study of d-amino acids demands accurate methodologies and specific controls, and these have often been lacking. Here we outline best practices for d-amino acid research, with a special emphasis on d-serine. We hope these concepts help move the field to greater rigor and reproducibility, allowing the field to advance.

Clinical and experimental aspects of breathing modulation by inflammation

Neuroinflammation is produced by local or systemic alterations and mediated mainly by glia, affecting the activity of various neural circuits including those involved in breathing rhythm generation and control. Several pathological conditions, such as sudden infant death syndrome, obstructive sleep apnea and asthma exert an inflammatory influence on breathing-related circuits. Consequently breathing (both resting and ventilatory responses to physiological challenges), is affected; e.g., responses to hypoxia and hypercapnia are compromised. Moreover, inflammation can induce long-lasting changes in breathing and affect adaptive plasticity; e.g., hypoxic acclimatization or long-term facilitation. Mediators of the influences of inflammation on breathing are most likely proinflammatory molecules such as cytokines and prostaglandins. The focus of this review is to summarize the available information concerning the modulation of the breathing function by inflammation and the cellular and molecular aspects of this process. I will consider: 1) some clinical and experimental conditions in which inflammation influences breathing; 2) the variety of experimental approaches used to understand this inflammatory modulation; 3) the likely cellular and molecular mechanisms.

Peripheral Nerve Injury-Induced Astrocyte Activation in Spinal Ventral Horn Contributes to Nerve Regeneration

Accumulating evidences suggest that peripheral nerve injury (PNI) may initiate astrocytic responses in the central nervous system (CNS). However, the response of astrocytes in the spinal ventral horn and its potential role in nerve regeneration after PNI remain unclear. Herein, we firstly illustrated that astrocytes in the spinal ventral horn were dramatically activated in the early stage following sciatic nerve injury, and these profiles were eliminated in the chronic stage. Additionally, we found that the expression of neurotrophins, including brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), and neurotrophin-3 (NT-3), also accompanied with astrocyte activation. In comparison with the irreversible transected subjects, astrocyte activation and the neurotrophic upregulation in the early stage were more drastic in case the transected nerve was rebridged immediately after injury. Furthermore, administering fluorocitrate to inhibit astrocyte activation resulted in decreased neurotrophin expression in the spinal ventral horn and delayed axonal regeneration in the nerve as well as motor function recovery. Overall, the present study indicates that peripheral nerve injury can initiate astrocyte activation accompanied with neurotrophin upregulation in the spinal ventral horn. The above responses mainly occur in the early stage of PNI and may contribute to nerve regeneration and motor function recovery.

In Vivo Small Molecule Delivery to the Optic Nerve in a Rodent Model

Small molecule delivery to the optic nerve would allow for exploration of molecular and cellular pathways involved in normal physiology and optic neuropathies such as glaucoma, and provide a tool for screening therapeutics in animal models. We report a novel surgical method for small molecule drug delivery to the optic nerve head (ONH) in a rodent model. In proof-of-principle experiments, we delivered cytochalasin D (Cyt D; a filamentous actin inhibitor) to the junction of the superior optic nerve and globe in rats to target the actin-rich astrocytic cytoskeleton of the ONH. Cyt D delivery was quantified by liquid chromatography and mass spectrometry of isolated optic nerve tissue. One day after Cyt D delivery, anterior ONH filamentous actin bundle content was significantly reduced as assessed by fluorescent-tagged phalloidin labeling, relative to sham delivery. Anterior ONH nuclear counts and axon-specific beta-3 tubulin levels, as well as peripapillary retinal ganglion cell layer nuclear counts were not significantly altered after Cyt D delivery relative to sham delivery. Lastly, the surgical delivery technique caused minimal observable axon degeneration up to 10 days post-surgery. This small molecule delivery technique provides a new approach to studying optic neuropathies in in vivo rodent models.

Pharmacological Tools to Activate Microglia and their Possible use to Study Neural Network Patho-physiology

BACKGROUND

Microglia are the resident immunocompetent cells of the CNS and also constitute a unique cell type that contributes to neural network homeostasis and function. Understanding microglia cell-signaling not only will reveal their diverse functions but also will help to identify pharmacological and non-pharmacological tools to modulate the activity of these cells.

METHODS

We undertook a search of bibliographic databases for peer-reviewed research literature to identify microglial activators and their cell-specificity. We also looked for their effects on neural network function and dysfunction.

RESULTS

We identified several pharmacological targets to modulate microglial function, which are more or less specific (with the proper control experiments). We also identified pharmacological targets that would require the development of new potent and specific modulators. We identified a wealth of evidence about the participation of microglia in neural network function and their alterations in pathological conditions.

CONCLUSION

The identification of specific microglia-activating signals provides experimental tools to modulate the activity of this heterogeneous cell type in order to evaluate its impact on other components of the nervous system, and it also helps to identify therapeutic approaches to ease some pathological conditions related to microglial dysfunction.