Stimulation of the Dorsal Root Ganglion using an Injectrode

Objective: The goal of this work was to compare afferent fiber recruitment by dorsal root ganglion (DRG) stimulation using an injectable polymer electrode (Injectrode®) and a more traditional cylindrical metal electrode. Approach: We exposed the L6 and L7 DRG in four cats via a partial laminectomy or burr hole. We stimulated the DRG using an Injectrode or a stainless steel electrode using biphasic pulses at three different pulse widths (80, 150, 300 μs) and pulse amplitudes spanning the range used for clinical DRG stimulation. We recorded antidromic evoked compound action potentials (ECAPs) in the sciatic, tibial, and common peroneal nerves using nerve cuffs. We calculated the conduction velocity of the ECAPs and determined the charge-thresholds and recruitment rates for ECAPs from Aɑ, Aβ, and Aδ fibers. We also performed electrochemical impedance spectroscopy measurements for both electrode types. Main Results: The Injectrode had similar or lower ECAP thresholds relative to the stainless steel electrode across all primary afferents (Aɑ, Aβ, Aδ) and pulse widths; charge-thresholds increased with wider pulse widths. Thresholds for generating ECAPs from Aβ fibers were 100.0 ± 32.3 nC using the stainless steel electrode, and 90.9 ± 42.9 nC using the Injectrode. The ECAP thresholds from the Injectrode were consistent over several hours of stimulation. The rate of recruitment was similar between the Injectrodes and stainless steel electrode and decreased with wider pulse widths. Significance: The Injectrode can effectively excite primary afferents when used for DRG stimulation within the range of parameters used for clinical DRG stimulation. The Injectrode can be implanted through minimally invasive techniques while achieving similar neural activation to conventional electrodes, making it an excellent candidate for future DRG stimulation and neuroprosthetic applications.

Vagus nerve stimulation activates two distinct neuroimmune circuits converging in the spleen to protect mice from kidney injury

Acute kidney injury is highly prevalent and associated with high morbidity and mortality, and there are no approved drugs for its prevention and treatment. Vagus nerve stimulation (VNS) alleviates inflammatory diseases including kidney disease; however, neural circuits involved in VNS-induced tissue protection remain poorly understood. The vagus nerve, a heterogeneous group of neural fibers, innervates numerous organs. VNS broadly stimulates these fibers without specificity. We used optogenetics to selectively stimulate vagus efferent or afferent fibers. Anterograde efferent fiber stimulation or anterograde (centripetal) sensory afferent fiber stimulation both conferred kidney protection from ischemia–reperfusion injury. We identified the C1 neurons–sympathetic nervous system–splenic nerve–spleen–kidney axis as the downstream pathway of vagus afferent fiber stimulation. Our study provides a map of the neural circuits important for kidney protection induced by VNS, which is critical for the safe and effective clinical application of VNS for protection from acute kidney injury.

Axodendritic versus axosomatic cochlear efferent termination is determined by afferent type in a hierarchical logic of circuit formation

Hearing involves a stereotyped neural network communicating cochlea and brain. How this sensorineural circuit assembles is largely unknown. The cochlea houses two types of mechanosensory hair cells differing in function (sound transmission versus amplification) and location (inner versus outer compartments). Inner (IHCs) and outer hair cells (OHCs) are each innervated by a distinct pair of afferent and efferent neurons: IHCs are contacted by type I afferents receiving axodendritic efferent contacts; OHCs are contacted by type II afferents and axosomatically terminating efferents. Using an Insm1 mouse mutant with IHCs in the position of OHCs, we discover a hierarchical sequence of instructions in which first IHCs attract, and OHCs repel, type I afferents; second, type II afferents innervate hair cells not contacted by type I afferents; and last, afferent fiber type determines if and how efferents innervate, whether axodendritically on the afferent, axosomatically on the hair cell, or not at all.

Spectrum of myelinated pulmonary afferents (III) cracking intermediate adapting receptors

Conventional one-sensor theory (one afferent fiber connects to a single sensor) categorizes the bronchopulmonary mechanosensors into the rapidly adapting receptors (RARs), slowly adapting receptors (SARs), or intermediate adapting receptors (IARs). RARs and SARs are known to sense the rate and magnitude of mechanical change, respectively; however, there is no agreement on what IARs sense. Some investigators believe that the three types of sensors are actually one group with similar but different properties and IARs operate within that group. Other investigators (majority) believe IARs overlap with the RARs and SARs and can be classified within them according to their characteristics. Clearly, there is no consensus on IARs function. Recently, a multiple-sensor theory has been advanced in which a sensory unit may contain many heterogeneous sensors, such as both RARs and SARs. There are no IARs. Intermediate adapting unit behavior results from coexistence of RARs and SARs. Therefore, the unit can sense both rate and magnitude of changes. The purpose of this review is to provide evidence that the multiple-sensor theory better explains sensory unit behavior.

Estrogen re-enhanced prenatal stress-related visceral sensitization and pain regulation

Background: Visceral pain is one of the most common sign of irritable bowel syndrome (IBS). Chronic stress during pregnancy may increase visceral pain sensitivity of offspring in a sexdependent way. Combining adult stress in offspring will increase this sensitivity. Based on the evidence implicating estrogen exacerbates visceral hypersensitivity in female rodents in pre-clinical models, we predicted that chronic prenatal stress (CPS) plus chronic adult stress (CAS) will maximize visceral pain sensitivity; and estrogen plays an important role in this hyperalgesia.Methods: The CPS plus CAS rodent model was established in which the balloon was used to distend colorectum. Meanwhile, the single fiber recording in vivo and patch-clamp experiments in vitro were used to monitor neuronal activity. The RT-PCR, Western Blot, and Immunofluorescence were used to study the effects of CPS and CAS on colon primary afferent sensitivity and molecular or transmission changes. We use Ovariectomy and Letrozole to treate female rats respectively in order to assess the role of estrogen in female-specific enhanced primary afferent sensitization. Letrozole mainly used to reduce estrogen levels.Results: As predicted, CPS significantly increased single unit afferent fiber activity in L6-S2 dorsal roots in response. Activity was further enhanced by CAS. And the activity in offspring females was significantly greater than the males. Besides, the excitability of colon-projecting dorsal root ganglion (DRG) neurons increases in CPS + CAS rats that was associated with a decrease in transient A-type K+ current. Letrozole treatment decreases the colon DRG neuron excitability in females by decreasing the estrogen levels. Conclusions: This study adds to the growing evidence for the development of chronic stress induced visceral hypersensitivity in female, which involves estrogen-dependent sensitization of primary afferent colon neurons. Understanding this neurophysiological mechanisms will spur the development of female pain specific therapies.

Optogenetic activation of afferent pathways in brain slices and modulation of responses by volatile anesthetics

ABSTRACTAnesthetics influence consciousness in part via their actions on thalamocortical circuits. However, the extent to which volatile anesthetics affect distinct cellular and network components of these circuits remains unclear. Ex vivo brain slices provide a means by which investigators may probe discrete components of complex networks and disentangle potential mechanisms underlying the effects of volatile anesthetics on evoked responses. To isolate potential cell type- and pathway-specific drug effects in brain slices, investigators must be able to independently activate afferent fiber pathways, identify non-overlapping populations of cells, and apply volatile anesthetics to tissue in aqueous solution. In this protocol, we describe methods to measure optogenetically-evoked responses to two independent afferent pathways to neocortex in ex vivo brain slices. We record extracellular responses to assay network activity and conduct targeted whole-cell patch clamp recordings in somatostatin- and parvalbumin-positive interneurons. We also describe a means by which to deliver physiologically relevant concentrations of isoflurane via artificial cerebral spinal fluid to modulate cellular and network responses.SUMMARYEx vivo brain slices can be used to study the effects of volatile anesthetics on evoked responses to afferent inputs. We employ optogenetics to independently activate thalamocortical and corticocortical afferents to non-primary neocortex, and we modulate synaptic and network responses with isoflurane.

Dendritic speeding of synaptic potentials in an auditory brainstem principal neuron

Principal cells of the medial nucleus of the trapezoid body (MNTB) in the mammalian auditory brainstem receive most of their strong synaptic inputs directly on the cell soma. However, these neurons also grow extensive dendrites during the first four postnatal weeks. What are the functional roles of these dendrites? We studied the morphology and growth of the dendrites in the mouse MNTB using both electron microscopy and confocal fluorescence imaging from postnatal day 9 (P9; pre-hearing) to P30. The soma of principal cells sprouted 1 to 3 thin dendrites (diameter ~ 1.5 microns) by P21 to P30. Each dendrite bifurcated into 2-3 branches and spanned an overall distance of about 80 to 200 microns. By contrast, at P9-11 the soma had 1 to 2 dendrites that extended for only 25 microns on average. Patch clamp experiments revealed that the growth of dendrites during development correlates with a progressive decrease in the input resistance, whereas acute removal of dendrites during brain slicing leads to higher input resistances. Accordingly, recordings of excitatory postsynaptic potentials (EPSPs) evoked by afferent fiber stimulation show that EPSP decay is faster in P21-24 neurons with intact dendrites than in neurons without dendrites. This dendritic speeding of the EPSP reduces the decay time constant 5-fold, which will impact significantly synaptic current summation and the ability to fire high-frequency spike trains. These data suggest a novel role for dendrites in auditory brainstem neurons: the speeding of EPSPs for faster and more precise output signal transfer.Significance StatementAuditory circuits that compute sound localization express different types of specialized synapses. Some are capable of fast, precise and sustained synaptic transmission. As the paradigm example, principal cells of the MNTB receive a single calyx-type nerve terminal on their soma and this large excitatory synapse produces fast and brief supra-threshold EPSPs that can trigger trains of high frequency spikes. However, these neurons also extend thin and long dendrites with unknown function. We examined the relationship between dendritic morphology, passive electrical properties and EPSP waveform. We found that more mature neurons with intact dendrites have lower input resistances and short EPSP waveforms, ideally suited for conveying precise timing information, whereas immature neurons with shorter dendrites and higher input resistance have longer lasting EPSPs.

Regulation of Noise-Induced Loss of Serotonin Transporters with Resveratrol in a Rat Model Using 4-[18F]-ADAM/Small-Animal Positron Emission Tomography

Serotonin (5-HT) plays a crucial role in modulating the afferent fiber discharge rate in the inferior colliculus, auditory cortex, and other nuclei of the ascending auditory system. Resveratrol, a natural polyphenol phytoalexin, can inhibit serotonin transporters (SERT) to increase synaptic 5-HT levels. In this study, we investigated the effects of resveratrol on noise-induced damage in the serotonergic system. Male Sprague-Dawley rats were anaesthetized and exposed to an 8-kHz tone at 116 dB for 3.5 h. Resveratrol (30 mg/kg, intraperitoneal injection [IP]) and citalopram (20 mg/kg, IP), a specific SERT inhibitor used as a positive control, were administered once a day for four consecutive days, with the first treatment occurring 2 days before noise exposure. Auditory brainstem response testing and positron emission tomography (PET) with N,N-dimethyl-2-(2-amino-4-[18F]fluorophenylthio)benzylamine (4-[18F]-ADAM, a specific radioligand for SERT) were used to evaluate functionality of the auditory system and integrity of the serotonergic system, respectively, before and after noise exposure. Finally, immunohistochemistry was performed 1 day after the last PET scan. Our results indicate that noise-induced serotonergic fiber loss occurred in multiple brain regions including the midbrain, thalamus, hypothalamus, striatum, auditory cortex, and frontal cortex. This noise-induced damage to the serotonergic system was ameliorated in response to treatment with resveratrol and citalopram. However, noise exposure increased the hearing threshold in the rats regardless of drug treatment status. We conclude that resveratrol has protective effects against noise-induced loss of SERT.