A two-compartment in vitro model for studies of modulation of nociceptive transmission

A Microfluidic High-Capacity Screening Platform for Neurological Disorders

Compartmentalized cell cultures (CCCs) provide the possibility to study mechanisms of neurodegenerative diseases, such as spreading of misfolded proteins in Alzheimer's or Parkinson's disease or functional changes in, e.g., chronic pain, in vitro. However, many CCC devices do not provide the necessary capacity for identifying novel mechanisms, targets, or drugs in a drug discovery context. Here, we present a high-capacity cell culture microtiter microfluidic plate compliant with American National Standard Institute of the Society for Laboratory Automation and Screening (ANSI/SLAS) standards that allows to parallelize up to 96 CCCs/experimental units, where each experimental unit comprises three microchannel-connected compartments. The plate design allows the specific treatment of cells in individual compartments through the application of a fluidic barrier. Moreover, the compatibility of the plate with neuronal cultures was confirmed with rodent primary as well as human-induced pluripotent stem cell-derived neurons of the central or peripheral nervous system for up to 14 days in culture. Using immunocytochemistry, we demonstrated that the plate design restricts neuronal soma to individual compartments, while axons, but not dendrites, can grow through the connecting microchannels to neighboring compartments. In addition, we show that neurons are spontaneously active and, as deemed by the appearance of synchronous depolarizations in neighboring compartments, are synaptically coupled. In summary, the design of the microfluidic plate allows for both morphological and functional studies of neurological in vitro cultures with increased capacity to support identification of novel mechanisms, targets, or drugs.

The role of NaV channels in synaptic transmission after axotomy in a microfluidic culture platform

Voltage gated sodium channels are key players in aberrant pain signaling and sensitization of nociceptors after peripheral nerve injury. The extent to which sodium channel activity after injury contributes to synaptic transmission at the first pain synapse however remains unclear. To investigate the effect of axotomy on synaptic transmission between dorsal root ganglia neurons and dorsal horn neurons, we reconstructed the first pain synapse in a novel microfluidic based compartmentalized cell culture system, which recapitulates the connectivity of peripheral pain signaling. We show that following axotomy of the distal axons, inhibition of Na_V1.7 and Na_V1.8 sodium channels in incoming presynaptic DRG axons is no longer sufficient to block activation of these axons and the resulting synaptic transmission to dorsal horn neurons. We found that blockade of Na_V1.6 activity is highly effective in reducing activation of incoming axons contributing to synaptic transmission after axotomy of DRG neurons. The microfluidic culture system described here offers an in vitro platform to recapitulate and study the first pain synapse.

Compartmentalized Devices as Tools for Investigation of Human Brain Network Dynamics

Neuropsychiatric disorders have traditionally been difficult to study due to the complexity of the human brain and limited availability of human tissue. Induced pluripotent stem (iPS) cells provide a promising avenue to further our understanding of human disease mechanisms, but traditional 2D cell cultures can only provide a limited view of the neural circuits. To better model complex brain neurocircuitry, compartmentalized culturing systems and 3D organoids have been developed. Early compartmentalized devices demonstrated how neuronal cell bodies can be isolated both physically and chemically from neurites. Soft lithographic approaches have advanced this approach and offer the tools to construct novel model platforms, enabling circuit-level studies of disease, which can accelerate mechanistic studies and drug candidate screening. In this review, we describe some of the common technologies used to develop such systems and discuss how these lithographic techniques have been used to advance our understanding of neuropsychiatric disease. Finally, we address other in vitro model platforms such as 3D culture systems and organoids and compare these models with compartmentalized models. We ask important questions regarding how we can further harness iPS cells in these engineered culture systems for the development of improved in vitro models. Developmental Dynamics 248:65-77, 2019. © 2018 Wiley Periodicals, Inc.

Developing an In Vitro Model to Screen Drugs for Nerve Regeneration

Peripheral nerve injuries (PNI) have a high prevalence and can be debilitating, resulting in life‐long loss or disturbance in end‐organ function, which compromises quality of life for patients. Current therapies use microsurgical approaches but there is the potential for enhancing recovery through other therapeutic modalities such as; cell‐based conduits, gene therapy and small molecules. A number of molecular targets and drugs which have the potential to improve nerve regeneration have been identified, however, there are challenges associated with moving therapies toward clinical translation. Due to the lack of detailed knowledge about the pro‐regenerative effect of potential drug treatments, there is a need for effective in vitro models to screen compounds to inform future pre‐clinical and clinical studies. The interaction between regenerating neurites and supporting Schwann cells is a key feature of the nerve environment, therefore, in vitro models that mimic this cellular association are useful tools. In this study, we have investigated various cell culture models, including simple monolayer systems and more complex 3D‐engineered co‐cultures, as models for use in PNI drug development. Anat Rec, 301:1628–1637, 2018. © 2018 The Authors. The Anatomical Record published by Wiley Periodicals, Inc. on behalf of American Association of Anatomists.

Integrating Mass Spectrometry with Microphysiological Systems for Improved Neurochemical Studies

Microphysiological systems, often referred to as "organs-on-chips", are in vitro platforms designed to model the spatial, chemical, structural, and physiological elements of in vivo cellular environments. They enhance the evaluation of complex engineered biological systems and are a step between traditional cell culture and in vivo experimentation. As neurochemists and measurement scientists studying the molecules involved in intercellular communication in the nervous system, we focus here on recent advances in neuroscience using microneurological systems and their potential to interface with mass spectrometry. We discuss a number of examples - microfluidic devices, spheroid cultures, hydrogels, scaffolds, and fibers - highlighting those that would benefit from mass spectrometric technologies to obtain improved chemical information.

μNeurocircuitry: Establishing in vitro models of neurocircuits with human neurons

Neurocircuits in the human brain govern complex behavior and involve connections from many different neuronal subtypes from different brain regions. Recent advances in stem cell biology have enabled the derivation of patient-specific human neuronal cells of various subtypes for the study of neuronal function and disease pathology. Nevertheless, one persistent challenge using these human-derived neurons is the ability to reconstruct models of human brain circuitry. To overcome this obstacle, we have developed a compartmentalized microfluidic device, which allows for spatial separation of cell bodies of different human-derived neuronal subtypes (excitatory, inhibitory and dopaminergic) but is permissive to the spreading of projecting processes. Induced neurons (iNs) cultured in the device expressed pan-neuronal markers and subtype specific markers. Morphologically, we demonstrate defined synaptic contacts between selected neuronal subtypes by synapsin staining. Functionally, we show that excitatory neuronal stimulation evoked excitatory postsynaptic current responses in the neurons cultured in a separate chamber.

Acute Pain Management

Acute pain management is improving steadily over the past few years, but training and professional education are still lacking in many professions. Untreated or undertreated acute pain could have detrimental effects on the patient in terms of comfort and recovery from trauma or surgery. Acute undertreated pain can decrease a patient's vascular perfusion, increase oxygen demand, suppress the immune system, and possibly risk increased incidence of venous thrombosis. Although acute postoperative pain needs to be managed aggressively, patients are most vulnerable during this period for developing adverse effects, and therefore, patient assessment and careful drug therapy evaluation are necessary processes in therapeutic planning. Acute pain management requires careful and thorough initial assessment and follow-up reassessment in addition to frequent dosage adjustments, and managing analgesic induced side effects. Analgesic selection and dosing must be based on the patient's past and recent analgesic exposure. There is no single acute pain management regimen that is suitable for all patients. Analgesics must be tailored to the individual patient.

Chronic pregabalin inhibits synaptic transmission between rat dorsal root ganglion and dorsal horn neurons in culture

In this study, we have examined the properties of synaptic transmission between dorsal root ganglion (DRG) and dorsal horn (DH) neurons, placed in co-culture. We also examined the effect of the anti-hyperalgesic gabapentinoid drug pregabalin (PGB) at this pharmacologically relevant synapse. The main method used was electrophysiological recording of excitatory post synaptic currents (EPSCs) in DH neurons. Synaptic transmission between DRG and DH neurons was stimulated by capsaicin, which activates transient receptor potential vanilloid-1 (TRPV1) receptors on small diameter DRG neurons. Capsaicin (1 μM) application increased the frequency of EPSCs recorded in DH neurons in DRG-DH co-cultures, by about 3-fold, but had no effect on other measured properties of the EPSCs. There was also no effect of capsaicin in the absence of co-cultured DRGs. Application of PGB (100 μM) for 40–48 h caused a reduction in the capsaicin-induced increase in EPSC frequency by 57%. In contrast, brief preincubation of PGB had no significant effect on the capsaicin-induced increase in EPSC frequency. In conclusion, this study shows that PGB applied for 40–48 h, but not acute application inhibits excitatory synaptic transmission at DRG-DH synapses, in response to nociceptive stimulation, most likely by a presynaptic effect on neurotransmitter release from DRG presynaptic terminals.

Functional tetrodotoxin-resistant Na(+) channels are expressed presynaptically in rat dorsal root ganglia neurons

The tetrodotoxin-resistant (TTX-R) voltage-gated Na(+) channels Na(v)1.8 and Na(v)1.9 are expressed by a subset of primary sensory neurons and have been implicated in various pain states. Although recent studies suggest involvement of TTX-R Na(+) channels in sensory synaptic transmission and spinal pain processing, it remains unknown whether TTX-R Na(+) channels are expressed and function presynaptically. We examined expression of TTX-R channels at sensory synapses formed between rat dorsal root ganglion (DRG) and spinal cord (SC) neurons in a DRG/SC co-culture system. Immunostaining showed extensive labeling of presynaptic axonal boutons with Na(v)1.8- and Na(v)1.9-specific antibodies. Measurements using the fluorescent Na(+) indicator SBFI demonstrated action potential-induced presynaptic Na(+) entry that was resistant to tetrodotoxin (TTX) but was blocked by lidocaine. Furthermore, presynaptic [Ca(2+)](i) elevation in response to a single action potential was not affected by TTX in TTX-resistant DRG neurons. Finally, glutamatergic synaptic transmission was not inhibited by TTX in more than 50% of synaptic pairs examined; subsequent treatment with lidocaine completely blocked these TTX-resistant excitatory postsynaptic currents. Taken together, these results provide evidence for presynaptic expression of functional TTX-R Na(+) channels that may be important for shaping presynaptic action potentials and regulating transmitter release at the first sensory synapse.

Single-cell electroporation of adult sensory neurons for gene screening with RNA interference mechanism

RNA interference appears as a technique of choice to identify gene candidate or to evaluate gene function. To date, a main problem is to achieve high transfection efficiencies on native cells such as adult neurons. In addition, transfection on organ or mass culture does not allow to approach the cellular diversity. Dorsal root ganglia are composed with several cell types to convey somato-sensory sensations. Single-cell electroporation is the most recent method of transfection that allows the introduction into cells, not only dyes or drugs, but also large molecules such plasmid DNA expression constructs. In the present study, the application of the RNA interference technique with the use of single-cell electroporation was evaluated in primary culture of adult sensory neurons. With the use of fluorescent dextran as a co-transfectant, we first determined the non-specific siRNA concentration leading to cell death. Efficacy of siRNA at the non-toxic concentration was demonstrated at the protein level by extinction of GFP fluorescence in actin-GFP neurons and by the inhibition of the intracellular Cl- concentration increase following activation of the membrane co-transporter Na+-K+-2Cl- in regenerating axotomized sensory neurons. Altogether, these data show that delivery of siRNAs by single-cell electroporation leads to the induction of functional RNA interference.

Visualizing sensory transmission between dorsal root ganglion and dorsal horn neurons in co-culture with calcium imaging

Sensory information is conveyed to the central nervous system by primary afferent neurons within dorsal root ganglia (DRG), which synapse onto neurons of the dorsal horn of the spinal cord. This synaptic connection is central to the processing of both sensory and pain stimuli. Here, we describe a model system to monitor synaptic transmission between DRG neurons and dorsal horn neurons that is compatible with high-throughput screening. This co-culture preparation comprises DRG and dorsal horn neurons and utilizes Ca(2+) imaging with the indicator dye Fura-2 to visualize synaptic transmission. Addition of capsaicin to co-cultures stimulated DRG neurons and led to activation of dorsal horn neurons as well as increased intracellular Ca(2+) concentrations. This effect was dose-dependent and absent when DRG neurons were omitted from the culture. NMDA receptors are a critical component of synapses between DRG and dorsal horn neurons as MK-801, a use-dependent non-competitive antagonist, prevented activation of dorsal horn neurons following capsaicin treatment. This model system allows for rapid and efficient analysis of noxious stimulus-evoked Ca(2+) signal transmission and provides a new approach both for investigating synaptic transmission in the spinal cord and for screening potential analgesic compounds.

Interferon-gamma induces characteristics of central sensitization in spinal dorsal horn neurons in vitro

Hyperexcitability of spinal dorsal horn neurons, also known as 'central sensitization', is a component of pain associated with pathological conditions in the nervous system. The aim of the present study was to analyze if the pro-inflammatory cytokine, interferon-gamma (IFN-gamma), which can be released for extended periods of time in the nervous system during inflammatory and infectious events, can alter synaptic activity in dorsal horn neurons and thereby contribute to such hyperexcitability. Treatment of cultured dorsal horn neurons with IFN-gamma for 2 weeks resulted in a significantly reduced clustering of alpha-amino-3-hydroxy-5-methylisoxazole (AMPA) receptor subunit 1 (GluR1) that was dependent on nitric oxide. The neurons displayed an increased frequency and amplitude of excitatory postsynaptic currents (EPSCs) upon IFN-gamma treatment. Treated dorsal horn neurons also exhibited increased responsiveness to stimulation of dorsal root ganglia (DRG) axons in a two-compartment model. Furthermore, disinhibition by the GABA(A) receptor antagonist picrotoxin (PTX) significantly increased EPSC frequency and induced bursting in untreated cultures but did not significantly increase the frequency in treated neurons, which displayed bursting even without PTX. GABA(A) agonists reduced activity more strongly in treated cultures and immunochemical staining for GABA(A) receptors showed no difference from controls. Since GluR1-containing AMPA receptors (AMPARs) occur predominantly on inhibitory neurons in the dorsal horn, we suggest that the IFN-gamma-mediated increase in spontaneous activity and responsiveness to DRG axon stimulation, decrease in sensitivity to PTX and tendency for EPSC bursting result from a reduced expression of GluR1 on these neurons and not from a reduction in active GABA(A) receptors in the network. IFN-gamma thereby likely causes disinhibition of synaptic activity and primary afferent input in the dorsal horn, which consequently results in central sensitization.

In vitro development of a tissue-engineered model of peripheral nerve regeneration to study neurite growth

A unique tissue-engineered model of peripheral nerve regeneration was developed in vitro to study neurite outgrowth. Mouse dorsal root ganglia neurons were seeded on a collagen sponge populated with human endothelial cells and/or human fibroblasts. Addition of nerve growth factor (NGF; 10 ng/ml) was not required for sensory neurons survival but was necessary to promote neurite outgrowth, as assessed by immunostaining of the 150 kDa neurofilament. A vigorous neurite elongation was detected inside the reconstructed tissue after 14 and 31 days of neurons culture, reaching up to 770 microm from day 14. Axons were often observed closely associated with the capillary-like tubes reconstructed in the model, in a similar pattern as in the human dermis. The presence of endothelial cells induced a significant increase of the neurite elongation after 14 days of culture. The addition of human keratinocytes totally avoided the twofold decrease in the amount of neurites observed between 14 and 31 days in controls. Besides the addition of NGF, axonal growth did not necessitate B27 supplement or glial cell coculture to be promoted and stabilized for long-term culture. Thus, this model might be a valuable tool to study the effect of various cells and/or attractive or repulsive molecules on neurite outgrowth in vitro.

Expanding the biological basis of tinnitus: crossmodal origins and the role of neuroplasticity

Tinnitus is most often initiated by modality specific otopathologic disturbances affecting peripheral and central auditory pathways. However, there is growing evidence indicating that the anatomical location generating tinnitus occurs at sites different from the initial pathology. Support for this notion is found in individuals where tinnitus can be triggered or modulated by inputs from other sensory modalities or sensorimotor systems (somatosensory, somatomotor, visual-motor). The use of functional imaging methods combined with psychophysics, detailed physical examinations and questionnaire-based assessments has reinforced and validated these observations. Available data suggest that tinnitus-related crossmodal interactions are more common than previously anticipated. This communication reviews these advancements and suggests that a relatively broad multimodal network of neurons is involved in generating and sustaining the tinnitus perception in some forms of the disorder. Also implicated as part of the tinnitus experience are interactions within large-scale neural networks subserving attention, cognition, and emotion. Incorporating this knowledge into contemporary psychophysiological models will help facilitate the conceptualization of this phantom perception in a more comprehensive manner.