Afferent Responses Research Articles

Insulin potentiates the response to capsaicin in dorsal root ganglion neurons in vitro and muscle afferents ex vivo in normal healthy rodents.

Systemic insulin administration evokes sympathoexcitatory actions, but the mechanisms underlying these observations are unknown. We reported that insulin sensitizes the response of thin-fibre primary afferents, as well as the dorsal root ganglion (DRG) that subserves them, to mechanical stimuli. However, little is known about the effects of insulin on primary neuronal responses to chemical stimuli. TRPV1, whose agonist is capsaicin (CAP), is widely expressed on chemically sensitive metaboreceptors and/or nociceptors. The aim of this investigation was to determine the effects of insulin on CAP-activated currents in small DRG neurons and CAP-induced action potentials in thin-fibre muscle afferents of normal healthy rodents. Additionally, we investigated whether insulin potentiates sympathetic nerve activity (SNA) responses to CAP. In whole-cell patch-clamp recordings from cultured mice DRG neurons in vitro, the fold change in CAP-activated current from pre- to post-application of insulin (n=13) was significantly (P<0.05) higher than with a vehicle control (n=14). Similar results were observed in single-fibre recording experiments ex vivo as insulin potentiated CAP-induced action potentials compared to vehicle controls (n=9 per group, P<0.05). Furthermore, insulin receptor blockade with GSK1838705 significantly suppressed the insulin-induced augmentation in CAP-activated currents (n=13) as well as the response magnitude of CAP-induced action potentials (n=9). Likewise, the renal SNA response to CAP after intramuscular injection of insulin (n=8) was significantly (P<0.05) greater compared to vehicle (n=9). The findings suggest that insulin potentiates TRPV1 responsiveness to CAP at the DRG and muscle tissue levels, possibly contributing to the augmentation in sympathoexcitation during activities such as physical exercise. KEY POINTS: Evidence suggests insulin centrally activates the sympathetic nervous system, and a chemical stimulus to tissues activates the sympathetic nervous system via thin fibre muscle afferents. Insulin is reported to modulate putative chemical-sensitive channels in the dorsal root ganglion neurons of these afferents. In the present study, it is demonstrated that insulin potentiates the responsiveness of thin fibre afferents to capsaicin at muscle tissue levels as well as at the level of dorsal root ganglion neurons. In addition, it is demonstrated that insulin augments the sympathetic nerve activity response to capsaicin in vivo. These data suggest that sympathoexcitation is peripherally mediated via insulin-induced chemical sensitization. The present study proposes a possible physiological role of insulin in the regulation of chemical sensitivity in somatosensory thin fibre muscle afferents.

CLINICAL AND NEUROPHYSIOLOGICAL PARALLELS OF THE BRACHIAL PLEXOPATHY IN THE STRUCTURE OF NEUROGENIC THORACIC OUTLET SYNDROME

The aim: Was assessment of the neurophysiological data and features of clinical picture in patients with neurogenic thoracic outlet syndrome (TOS). Materials and methods: 103 patients with upper extremity pain and/or paresthesia or hypotrophy, or a combination of these symptoms were examined. The examination algorithm included: cervical spine radiography, cervical spine and brachial plexuses magnetic resonance imaging (MRI), upper extremity soft tissues and vessels ultrasonic examination, stimulation electroneuromiography with F-waves registration. Results: Neurogenic TOS was diagnosed in 29 patients. A significant relationship between the following complaints and neurophysiological parameters was observed: pain, numbness during physical activity and decreased medial anrebrachial cutaneous nerve response amplitude by ≥25% compared to the contralateral side; hypothenar hypotrophy and decrease of ulnar nerve motor/sensory response amplitude; the 4-5th fingers hypoesthesia and decrease of ulnar nerve sensory response amplitude. Conclusions: Medial antebrachial cutaneous nerve amplitudes asymmetry indices of ≥25% or lack of response may be considered to be a marker of true neurogenic TOS.

Age is associated with reduced urethral pressure and afferent activity in rat.

Age‐related changes in the lower urinary tract (LUT) can affect the coordination of reflexes and increase the incidence of bladder disorders in elderly. This study examines the age‐related loss of urethral signaling capability by measuring the afferent activity directly. We find that less urethral pressure develops in response to fluid flow in old rats compared to young rats and that pressure and flow evoke less urethral afferent activation. These findings are consistent with our previous study demonstrating that the urethra‐to‐bladder reflex, which is required for efficient voiding, becomes weaker with age. We measured the pudendal afferent response in young (4–7 months) and old (18–24 months) rats to fluid flow in the urethra across a range of flow rates. We used paraffin embedding and hematoxylin and eosin staining to quantify age‐related changes in the sensory branch of the pudendal nerve. Urethral afferent signaling in response to the same urethral flow rates was weaker in older animals. That is, the sensitivity of urethra afferents to flow decreased with age, and higher flow rates were required in older animals to recruit urethra afferents. There was also a reduction in the myelin thickness of pudendal afferents in old rats, which is a possible contributing factor to the sensory activity. Furthermore, the same flow rates evoked less pressure in the urethras of old animals, indicating there is an age‐related change of the urethral tissue that reduces the pressure stimulus to which these afferents respond. These results help characterize the underlying changes in LUT system with age.

Median amplitude and frequency analysis of sensory nerve responses to intraepidermal stimulation

BackgroundIn clinical practice, small myelinated sensory fibers conveying pain and other sensations, Aδ-fibers, cannot be examined with available nerve conduction study techniques. New methodEquipment available in clinical neurophysiology laboratories is used to record from human sensory nerves multiple averaged responses to non-painful stimulation of intraepidermal nerves. Ten averaged responses are analyzed in all possible pair combinations with an algorithm applied to a 0.45 ms period of amplitude and frequency (power spectrum). The median of the algorithms is compared to control data to identify potentials generated as response to intraepidermal stimulation. ResultsMedian analysis of the algorithm applied to amplitude and frequency of multiple record pairs identifies potentials with conduction velocities of Aδ-fibers. The analysis of frequency (power spectrum) adds data to the analysis of amplitude. Median analysis of multiple record pairs yields more data than analysis of one pair of alternate averages with the same algorithms. Comparison with existing method(s)At present, analysis of one pair of alternate average records with an algorithm is the only method to identify Aδ-fiber generated potentials. Median analysis of the same algorithm applied to the amplitude of multiple record pairs increases the number of Aδ-fiber generated potentials identified. Neither median analysis of amplitude nor frequency of multiple records pairs has ever been used for conduction studies of nerve fibers, including Aδ-fibers. ConclusionsStimulation, recording and data analysis methods used in this study can be applied in the clinical EMG laboratory to identify the conduction velocities of Aδ-fibers in human sensory nerves.

Prior Acoustic Trauma Alters Type II Afferent Activity in the Mouse Cochlea.

Auditory stimuli travel from the cochlea to the brainstem through type I and type II cochlear afferents. While type I afferents convey information about the frequency, intensity, and timing of sounds, the role of type II afferents remains unresolved. Limited recordings of type II afferents from cochlear apex of prehearing rats reveal they are activated by widespread outer hair cell stimulation, ATP, and by the rupture of nearby outer hair cells. Altogether, these lines of evidence suggest that type II afferents sense loud, potentially damaging levels of sound. To explore this hypothesis further, calcium imaging was used to determine the impact of acoustic trauma on the activity of type II cochlear afferents of young adult mice of both sexes. Two known marker genes (Th, Drd2) and one new marker gene (Tac1), expressed in type II afferents and some other cochlear cell types, drove GCaMP6f expression to reveal calcium transients in response to focal damage in the organ of Corti in all turns of the cochlea. Mature type II afferents responded to acute photoablation damage less often but at greater length compared with prehearing neurons. In addition, days after acoustic trauma, acute photoablation triggered a novel response pattern in type II afferents and surrounding epithelial cells, delayed bursts of activity occurring minutes after the initial response subsided. Overall, calcium imaging can report type II afferent responses to damage even in mature and noise-exposed animals and reveals previously unknown tissue hyperactivity subsequent to acoustic trauma.

Gastric neurons in the nucleus tractus solitarius are selective to the orientation of gastric electrical stimulation

Objective. Gastric electrical stimulation (GES) is a bioelectric intervention for gastroparesis, obesity, and other functional gastrointestinal disorders. In a potential mechanism of action, GES activates the nerve endings of vagal afferent neurons and induces the vago-vagal reflex through the nucleus tractus solitarius (NTS) in the brainstem. However, it is unclear where and how to stimulate in order to optimize the vagal afferent responses. Approach. To address this question with electrophysiology in rats, we applied mild electrical currents to two serosal targets on the distal forestomach with dense distributions of vagal intramuscular arrays (IMAs) that innervated the circular and longitudinal smooth muscle layers. During stimulation, we recorded single and multi-unit responses from gastric neurons in NTS and evaluated how the recorded responses depended on the stimulus orientation and amplitude. Main results. We found that NTS responses were highly selective to the stimulus orientation for a range of stimulus amplitudes. The strongest responses were observed when the applied current flowed in the same direction as the IMAs in parallel with the underlying smooth muscle fibers. Our results suggest that gastric neurons in NTS may encode the orientation-specific activity of gastric smooth muscles relayed by vagal afferent neurons. Significance. This finding suggests that the orientation of GES is critical to effective engagement of vagal afferents and should be considered in light of the structural phenotypes of vagal terminals in the stomach.

Neural Recruitment During Conventional, Burst, and 10-kHz Spinal Cord Stimulation for Pain

Spinal cord stimulation (SCS) is a popular neurostimulation therapy for severe chronic pain. To improve stimulation efficacy, multiple modes are now used clinically, including conventional, burst, and 10-kHz SCS. Clinical observations have produced speculation that these modes target different neural elements and/or work via distinct mechanisms of action. However, in humans, these hypotheses cannot be conclusively answered via experimental methods. Therefore, we utilized computational modeling to assess the response of primary afferents, interneurons, and projection neurons to conventional, burst, and 10-kHz SCS. We found that local cell thresholds were always higher than afferent thresholds, arguing against direct recruitment of these local cells. Furthermore, although we observed relative threshold differences between conventional, burst, and 10-kHz SCS, the recruitment order was the same. Finally, contrary to previous reports, axon collateralization produced complex changes in activation thresholds of primary afferents. These results motivate future work to contextualize clinical observations across SCS paradigms. PerspectiveThis article presents the first computational modeling study to investigate neural recruitment during conventional, burst, and 10-kilohertz spinal cord stimulation for chronic pain within a single modeling framework. The results provide insight into these treatments’ unknown mechanisms of action and offer context to interpreting clinical observations.

Dynamic Heterogeneity Shapes Patterns of Spiral Ganglion Activity.

Neural response properties that typify primary sensory afferents are critical to fully appreciate because they establish and, ultimately represent, the fundamental coding design used for higher-level processing. Studies illuminating the center-surround receptive fields of retinal ganglion cells, for example, were ground-breaking because they determined the foundation of visual form detection. For the auditory system, a basic organizing principle of the spiral ganglion afferents is their extensive electrophysiological heterogeneity establishing diverse intrinsic firing properties in neurons throughout the spiral ganglion. Moreover, these neurons display an impressively large array of neurotransmitter receptor types that are responsive to efferent feedback. Thus, electrophysiological diversity and its neuromodulation are a fundamental encoding mechanism contributed by the primary afferents in the auditory system. To place these features into context, we evaluated the effects of hyperpolarization and cAMP on threshold level as indicators of overall afferent responsiveness in CBA/CaJ mice of either sex. Hyperpolarization modified threshold gradients such that distinct voltage protocols could shift the relationship between sensitivity and stimulus input to reshape resolution. This resulted in an "accordion effect" that appeared to stretch, compress, or maintain responsivity across the gradient of afferent thresholds. cAMP targeted threshold and kinetic shifts to rapidly adapting neurons, thus revealing multiple cochleotopic properties that could potentially be independently regulated. These examples of dynamic heterogeneity in primary auditory afferents not only have the capacity to shift the range, sensitivity, and resolution, but to do so in a coordinated manner that appears to orchestrate changes with a seemingly unlimited repertoire.SIGNIFICANCE STATEMENT How do we discriminate the more nuanced qualities of the sound around us? Beyond the basics of pitch and loudness, aspects, such as pattern, distance, velocity, and location, are all attributes that must be used to encode acoustic sensations effectively. While higher-level processing is required for perception, it would not be unexpected if the primary auditory afferents optimized receptor input to expedite neural encoding. The findings reported herein are consistent with this design. Neuromodulation compressed, expanded, shifted, or realigned intrinsic electrophysiological heterogeneity to alter neuronal responses selectively and dynamically. This suggests that diverse spiral ganglion phenotypes provide a rich substrate to support an almost limitless array of coding strategies within the first neural element of the auditory pathway.

Similarities and Differences Between Vestibular and Cochlear Systems - A Review of Clinical and Physiological Evidence.

The evoked response to repeated brief stimuli, such as clicks or short tone bursts, is used for clinical evaluation of the function of both the auditory and vestibular systems. One auditory response is a neural potential — the Auditory Brainstem Response (ABR) — recorded by surface electrodes on the head. The clinical analogue for testing the otolithic response to abrupt sounds and vibration is the myogenic potential recorded from tensed muscles — the vestibular evoked myogenic potential (VEMP). VEMPs have provided clinicians with a long sought-after tool — a simple, clinically realistic indicator of the function of each of the 4 otolithic sensory regions. We review the basic neural evidence for VEMPs and discuss the similarities and differences between otolithic and cochlear receptors and afferents. VEMPs are probably initiated by sound or vibration selectively activating afferent neurons with irregular resting discharge originating from the unique type I receptors at a specialized region of the otolithic maculae (the striola). We review how changes in VEMP responses indicate the functional state of peripheral vestibular function and the likely transduction mechanisms allowing otolithic receptors and afferents to trigger such very short latency responses. In section “ELECTROPHYSIOLOGY” we show how cochlear and vestibular receptors and afferents have many similar electrophysiological characteristics [e.g., both generate microphonics, summating potentials, and compound action potentials (the vestibular evoked potential, VsEP)]. Recent electrophysiological evidence shows that the hydrodynamic changes in the labyrinth caused by increased fluid volume (endolymphatic hydrops), change the responses of utricular receptors and afferents in a way which mimics the changes in vestibular function attributed to endolymphatic hydrops in human patients. In section “MECHANICS OF OTOLITHS IN VEMPS TESTING” we show how the major VEMP results (latency and frequency response) follow from modeling the physical characteristics of the macula (dimensions, stiffness etc.). In particular, the structure and mechanical operation of the utricular macula explains the very fast response of the type I receptors and irregular afferents which is the very basis of VEMPs and these structural changes of the macula in Menière’s Disease (MD) predict the upward shift of VEMP tuning in these patients.

Sensory adaptation at ribbon synapses in the zebrafish lateral line.

Adaptation is used by sensory systems to adjust continuously their sensitivity to match changes in environmental stimuli. In the auditory and vestibular systems, the release properties of glutamate-containing vesicles at the hair-cell ribbon synapses play a crucial role in sensory adaptation, thus shaping the neural response to sustained stimulation. How ribbon synapses regulate the release of glutamate and how they modulate afferent responses in vivo is still largely unknown. Here, we have used two-photon imaging and electrophysiology to investigate the synaptic transfer characteristics of the hair cells in the context of sensory adaptation in live zebrafish. Prolonged and repeated water-jet stimulation of the hair-cell stereociliary bundles caused adaptation of the action potential firing rate elicited in the afferent neurons. By monitoring glutamate at ribbon synapses using time-lapse imaging, we identified two kinetically distinct release components: a rapid response that was exhausted within 50-100 ms and a slower and sustained response lasting the entire stimulation. After repeated stimulations, the recovery of the fast component followed a biphasic time course. Depression of glutamate release was largely responsible for the rapid firing rate adaptation recorded in the afferent neurons. However, postsynaptic Ca2+ responses had a slower recovery time course than that of glutamate release, indicating that they are also likely to contribute to the afferent firing adaptation. Hair cells also exhibited a form of adaptation during inhibitory bundle stimulations. We conclude that hair cells have optimised their synaptic machinery to encode prolonged stimuli and to maintain their sensitivity to new incoming stimuli.

Responses of cutaneous C-fiber afferents and spinal microglia after hindlimb cast immobilization in rats

Previous studies have shown that persistent limb immobilization using a cast increases nociceptive behavior to somatic stimuli in rats. However, the peripheral neural mechanisms of nociception remain unclear. Using single-fiber electrophysiological recordings in vitro, we examined the general characteristics of cutaneous C-fiber afferents in the saphenous nerve and their responsiveness to mechanical and heat stimuli in a rat model of immobilization-induced pain by subjecting the rats to hindlimb cast immobilization for 4 weeks. The mechanical response of C-fibers appeared to increase in the model; however, statistical analysis revealed that neither the response threshold nor the response magnitude was altered. The general characteristics and heat responses of the C-fibers were not altered. The number of microglia and cell diameters significantly increased in the superficial dorsal horn of the lumbar spinal cord. Thus, activated microglia-mediated spinal mechanisms are associated with the induction of nociceptive hypersensitivity in rats after persistent cast immobilization.

Decoy bypass for appetite suppression in obese adults: role of synergistic nutrient sensing receptors GPR84 and FFAR4 on colonic endocrine cells

ObjectiveColonic enteroendocrine cells (EECs) store and release potent anorectic hormones that are key regulators of satiety. EECs express multiple nutrient sensing receptors, particularly for medium-chain fatty acids (MCFAs): GPR84 and...

Human low-threshold mechanoafferent responses to pure changes in friction controlled using an ultrasonic haptic device

The forces that are developed when manipulating objects generate sensory cues that inform the central nervous system about the qualities of the object’s surface and the status of the hand/object interaction. Afferent responses to frictional transients or slips have been studied in the context of lifting/holding tasks. Here, we used microneurography and an innovative tactile stimulator, the Stimtac, to modulate both the friction level of a surface, without changing the surface or adding a lubricant, and, to generate the frictional transients in a pure and net fashion. In three protocols, we manipulated: the frictional transients, the friction levels, the rise times, the alternation of phases of decrease or increase in friction to emulate grating-like stimuli. Afferent responses were recorded in 2 FAIs, 1 FAII, 2 SAIs and 3 SAIIs from the median nerve of human participants. Independently of the unit type, we observed that: single spikes were generated time-locked to the frictional transients, and that reducing the friction level reduced the number of spikes during the stable phase of the stimulation. Our results suggest that those frictional cues are encoded in all the unit types and emphasize the possibility to use the Stimtac device to control mechanoreceptor firing with high temporal precision.

A differential participation of voltage gated sodium channels is related to the selective efficacy of carbamazepine in trigeminal neuralgia

This study is based on the clinical observations that a subpopulation of patients suffering from classical trigeminal neuralgia are responsive to the anti-seizure, voltage-gated Na+ channel (VGSC)-blocking drugs carbamazepine (CBZ) and oxcarbamazepine while these drugs have failed to demonstrate efficacy in clinical trials with other neuropathic pain conditions, even those associated with nerve compression injuries. Furthermore, with preclinical evidence of afferent subpopulation specific responses to injury, we hypothesized that the therapeutic selectivity of carbamazepine for TN reflects the unique combination of injury-induced changes in VGSCs in the trigeminal nerve. We found that potency and efficacy of CBZ-induced inhibition of the compound action potential (CAP) was increased following chronic constriction injury (CCI) of the infraorbital nerve (ION), but not of the sciatic nerve (SN).. The potency of CBZ-induced block of the CAP evoked in human spinal nerves was comparable to the SN from rats, but was more potent in the block of the CAP evoked in human trigeminal nerves. Among different VGSCs blockers evaluated, the efficacy of ICA121431 (Nav 1.1 and 1.3 blocker) was significantly greater in the inhibition of the Ab and C components of the ION CAP compared to the SN CAP from the sham group, and CCI of the ION but not SN was associated with an increase in efficacy and potency of ICA121431 on the inhibition of the Ab and C components of the CAP. A CCI-induced increase in Nav 1.1-like immunoreactivity was detected with western blot of protein extracted from the ION, while a CCI-induced increase in Nav 1.3-like immunoreactivity was detected in the SN. Finally, local administration of ICA121431 on the ION alleviated mechanical hypersensitivity in rats with ION-CCI. Our results are consistent with our initial hypothesis and suggest that the therapeutic selectivity of CBZ is related to a differential regulation of the VGSC. This work was supported by generous donations from TN patients and family members as well as NIH grant R01NS064988.

HDAC1 Mediates High Salt Impaired Endothelial NO Signaling

High salt diet (HS) impairment of endothelial nitric oxide (NO) signaling has been described in human and rodents. The mechanisms which mediate this dysfunction remain unknown. We and others have previously shown histone deacetylase 1 (HDAC1) decreases endothelial NO production. We therefore sought to investigate the mechanisms by which HS initiates endothelial dysfunction and if it is HDAC1-dependent. We first investigated if HS induced endothelial dysfunction is acutely regulated by a circulating factor. Male Sprague Dawley rats were fed normal salt diet (NS; 0.49% NaCl) or 2 weeks of HS (4.0% NaCl). Afferent arteriole vasoconstrictor responses to the NOS inhibitor, L-NAME, were monitored using the in vitro, blood perfused juxtamedullary nephron preparation. Arterioles from HS fed rats perfused with blood from HS fed rats demonstrated blunted constriction to L-NAME (96 ± 2% of Con) compared to arterioles from NS fed rats perfused with blood from NS fed rats (76.2 ± 2% of Con; P=0.0017). Perfusing arterioles from NS fed rats with blood HS fed rats blunted constriction to L-NAME (88 ± 1% of Con) compared to perfusing with blood from NS fed rats (P=0.0003), but not to the same extent as arterioles from HS fed rats perfused with blood from HS fed rats (P=0.0095). HS arterioles perfused with blood from NS fed rats had similar L-NAME-induced constriction (75 ± 2% of Con) compared to arterioles from NS fed rats perfused with blood from NS fed rats (P=0.9107). In short, the impaired constriction to L-NAME is dependent up on the diet of the blood donor and not the diet of the arteriole donor suggesting HS impairs endothelial function through an acute-acting circulating factor. This was not due to increased plasma arginase activity (NS: 7.96 ± 3.73 U/L vs HS: 5.42 ± 1.83 U/L P=0.5813), changes in plasma tetrahydrobiopterin (NS: 65.09 ± 11.57 pmol/ml vs HS: 53.54 ± 5.59 pmol/ml, P=0.311) or changes in plasma dihydrobiopterin (NS: 98.18 ± 7.84 pmol/ml vs HS: 87.45 ± 13.28 pmol/ml, P=0.530). We then investigated if HS increased endothelial HDAC1 activity. Renal endothelial cells were isolated from NS and HS fed rats and incubated in plasma from the same rat. HDAC1 activity was defined as the MS-275 (HDAC1 specific inhibitor) inhibitable portion of total HDAC activity. We found HS significantly increased renal endothelial HDAC1 activity (NS: 0.38 ± 0.03 pmol/min vs HS:0.94 ± 0.19 pmol/min, P=0.01). Furthermore, in vitro HDAC1 inhibition with 300 nM MS-275 restored the arteriolar vasoconstrictor responses in HS rats (79 ± 3% of Con) to values similar to NS fed rats (P=0.969) and to NS rats + MS-275 (79 ± 1% of Con; P>0.998). When plasma from HS fed rats was combined with erythrocytes from NS fed rats and perfused through arterioles from NS fed rats, constriction to L-NAME was blunted (85 ± 3% of Con; P=0.0129). Under these same conditions, HDAC1 inhibition restored constriction (75.7 ± 0.95 % of Con) similar to controls (P=0.979). We conclude that HS disruption of renal microvascular NO signaling is initiated by a circulating factor(s) that increases endothelial HDAC1 activity.

Vesicle‐Released Glutamate Maintains Muscle Spindle Afferent Excitability in Adult Mice

The molecular mechanisms by which Group Ia and II muscle spindle (MS) afferents transduce muscle movement into action potentials is not fully understood, although the rapidly adapting PIEZO2 mechanically sensitive cation channel is essential. Mechanosensation in many types of sensory neurons relies on PIEZO2, suggesting that unique response properties are mediated by the complement of molecular mediators found in the different sensory neurons. Synaptic-like vesicles containing glutamate are released from the MS afferent receptor ending in a stretch and calcium dependent manner, suggesting a way to couple depolarization and increased intracellular calcium via PIEZO2 with additional depolarizing current via glutamate. Here we test the hypothesis that vesicle-released glutamate is necessary for maintaining MS afferent excitability especially during the static phase of stretch. We isolated the extensor digitorum longus muscle and the sciatic nerve from adult mice (8-12 wks old) and compared identified MS afferent firing rates during ramp-and-hold stretch and vibration before and after the addition of glutamate and an inhibitor of glutamate packaging into vesicles (xanthurenic acid; XA). Glutamate addition led to a significant increase in MS afferent firing rate during the end of stretch (1mM; n=12 in control and glutamate groups), while blocking glutamate release with XA significantly decreased firing (3mM; n=17), with 5 afferents exhibiting a complete absence of firing. Firing rates during the dynamic phase of stretch and sinusoidal vibrations were not as affected by drug treatment. However, we noticed heterogeneity in MS afferent responses, with some afferents not responding to the drug treatments. To confirm our pharmacological findings, we used a transgenic mouse line with 1 or 2 copies of the vesicular glutamate 1 transporter gene (VGLUT1; B6.129X1-Sic17a7tm1Edw/MmcD; Fremeau, 2004). VGLUT1+/- afferents (n=14) had significantly lower firing rates during the end of stretch than afferents from wildtype littermates (WT; n=14), however firing rates during vibration were not significantly different. Importantly, 3 of 14 VGLUT1+/- afferents could not maintain firing throughout the entire 4 s stretch, something never seen in WT afferents. These results support our hypothesis that glutamate is critical to maintaining MS afferent firing during maintained stretch. Future studies will explore which glutamate receptor subtype and signaling pathway mediates this effect. A better understanding of MS afferent mechanotransduction could define targets for therapeutic intervention during diseases with abnormal afferent excitability.

Interactions of Whisking and Touch Signals in the Rat Brainstem.

Perception is an active process, requiring the integration of both proprioceptive and exteroceptive information. In the rat's vibrissal system, a classical model for active sensing, the relative contribution of the two information streams was previously studied at the peripheral, thalamic, and cortical levels. Contributions of brainstem neurons were only indirectly inferred for some trigeminal nuclei according to their thalamic projections. The current work addressed this knowledge gap by performing the first comparative study of the encoding of proprioceptive whisking and exteroceptive touch signals in the oralis (SpVo), interpolaris (SpVi), and paratrigeminal (Pa5) brainstem nuclei. We used artificial whisking in anesthetized male rats, which allows a systematic analysis of the relative contribution of the proprioceptive and exteroceptive information streams along the ascending pathways in the absence of motor or cognitive top-down modulations. We found that (1) neurons in the rostral and caudal parts of the SpVi convey whisking and touch information, respectively, as predicted by their thalamic projections; (2) neurons in the SpVo encode both whisking and (primarily) touch information; and (3) neurons of the Pa5 encode a complex combination of whisking and touch information. In particular, the Pa5 contains a relatively large fraction of neurons that are inhibited by active touch, a response observed so far only in the thalamus. Overall, our systematic characterization of afferent responses to active touch in the trigeminal brainstem approves the hypothesized functions of SpVi neurons and presents evidence that SpVo and Pa5 neurons are involved in the processing of active vibrissal touch.SIGNIFICANCE STATEMENT The present work constitutes the first comparative study of the encoding of proprioceptive (whisking) and exteroceptive (touch) information in the rat's brainstem trigeminal nuclei, the first stage of vibrissal processing in the CNS. It shows that (1) as expected, the rostral and caudal interpolaris neurons convey primarily whisking and touch information, respectively; (2) the oralis nucleus, whose function was previously unknown, encodes both whisking and (primarily) touch touch information; (3) a subtractive computation, reported at the thalamic level, already occurs at the brainstem level; and (4) a novel afferent pathway probably ascends via the paratrigeminal nucleus, encoding both proprioceptive and exteroceptive information.

Vesicle-released glutamate is necessary to maintain muscle spindle afferent excitability but not dynamic sensitivity in adult mice.

Muscle spindle afferents are slowly adapting low threshold mechanoreceptors that report muscle length and movement information critical for motor control and proprioception. The rapidly adapting cation channel PIEZO2has been identified as necessary for muscle spindle afferent stretch sensitivity, although the properties of this channel suggest that additional molecular elements are necessary for mediating the complex slowly adapting response of muscle spindle afferents. We report that glutamate increases muscle spindle afferent static sensitivity in an ex vivo mouse muscle nerve preparation, although blocking glutamate packaging into vesicles by the sole vesicular glutamate transporter, VGLUT1, either pharmacologically or by transgenic knockout of one allele of VGLUT1 decreases muscle spindle afferent static but not dynamic sensitivity. Our results confirm that vesicle-released glutamate is an important contributor to maintained muscle spindle afferent excitability and may suggest a therapeutic target for normalizing muscle spindle afferent function. Muscle spindle afferents are slowly adapting low threshold mechanoreceptors that have both dynamic and static sensitivity to muscle stretch. The exact mechanism by which these neurons translate muscle movement into action potentials is not well understood, although the PIEZO2mechanically sensitive cation channel is essential for stretch sensitivity. PIEZO2 is rapidly adapting, suggesting the requirement for additional molecular elements to maintain firing during stretch. Spindle afferent sensory endings contain glutamate-filled synaptic-like vesicles that are released in a stretch- and calcium-dependent manner. Previous work has shown that glutamate can increase and a phospholipase-D coupled metabotropic glutamate antagonist can abolish firing during static stretch. Here, we test the hypothesis that vesicle-released glutamate is necessary for maintaining muscle spindle afferent excitability during static but not dynamic stretch. To test this hypothesis, we used a mouse muscle-nerve ex vivo preparation to measure identified muscle spindle afferent responses to stretch and vibration. In C57BL/6 adult mice, bath applied glutamate significantly increased the firing rate during the plateau phase of stretch but not during the dynamic phase of stretch. Blocking the packaging of glutamate into vesicles by the sole vesicular glutamate transporter, VGLUT1, either with xanthurenic acid or by using a transgenic mouse with only one copy of the VGLUT1 gene (VGLUT1+/- ), decreased muscle spindle afferent firing during sustained stretch but not during vibration. Our results suggest a model of mechanotransduction where calcium entering the PIEZO2 channel can cause the release of glutamate from synaptic-like vesicles, which then helps to maintain afferent depolarization and firing.

Constraints on the deformation of the vibrissa within the follicle

Nearly all mammals have a vibrissal system specialized for tactile sensation, composed of whiskers growing from sensor-rich follicles in the skin. When a whisker deflects against an object, it deforms within the follicle and exerts forces on the mechanoreceptors inside. In addition, during active whisking behavior, muscle contractions around the follicle and increases in blood pressure in the ring sinus will affect the whisker deformation profile. To date, however, it is not yet possible to experimentally measure how the whisker deforms in an intact follicle or its effects on different groups of mechanoreceptors. The present study develops a novel model to predict vibrissal deformation within the follicle sinus complex. The model is based on experimental results from a previous ex vivo study on whisker deformation within the follicle, and on a new histological analysis of follicle tissue. It is then used to simulate whisker deformation within the follicle during passive touch and active whisking. Results suggest that the most likely whisker deformation profile is “S-shaped,” crossing the midline of the follicle right below the ring sinus. Simulations of active whisking indicate that an increase in overall muscle stiffness, an increase in the ratio between deep and superficial intrinsic muscle stiffness, and an increase in sinus blood pressure will all enhance tactile sensitivity. Finally, we discuss how the deformation profiles might map to the responses of primary afferents of each mechanoreceptor type. The mechanical model presented in this study is an important first step in simulating mechanical interactions within whisker follicles.

Crosstalk opposing view: Independent fusimotor control of muscle spindles in humans: there is little to gain.

Crosstalk opposing view: Independent fusimotor control of muscle spindles in humans: there is little to gain.