Mechanosensory encoding in ex vivo muscle-nerve preparations

Our objective was to evaluate an ex vivo muscle-nerve preparation used to study mechanosensory signalling by low threshold mechanosensory receptors (LTMRs). Specifically, we aimed to assess how well the ex vivo preparation represents in vivo firing behaviours of the three major LTMR subtypes of muscle primary sensory afferents, namely type Ia and II muscle spindle (MS) afferents and type Ib tendon organ afferents. Using published procedures for ex vivo study of LTMRs in mouse hindlimb muscles, we replicated earlier reports on afferent firing in response to conventional stretch paradigms applied to non-contracting, that is passive, muscle. Relative to in vivo studies, stretch-evoked firing for confirmed MS afferents in the ex vivo preparation was markedly reduced in firing rate and deficient in encoding dynamic features of muscle stretch. These deficiencies precluded conventional means of discriminating type Ia and II afferents. Muscle afferents, including confirmed Ib afferents were often indistinguishable based on their similar firing responses to the same physiologically relevant stretch paradigms. These observations raise uncertainty about conclusions drawn from earlier ex vivo studies that either attribute findings to specific afferent types or suggest an absence of treatment effects on dynamic firing. However, we found that replacing the recording solution with bicarbonate buffer resulted in afferent firing rates and profiles more like those seen in vivo. Improving representation of the distinctive sensory encoding properties in ex vivo muscle-nerve preparations will promote accuracy in assigning molecular markers and mechanisms to heterogeneous types of muscle mechanosensory neurons.

Isolation of human pluripotent stem cell-derived sensory neuron subtypes by immunopanning

Sensory neurons (SNs) detect a wide range of information from the body and the environment that is critical for homeostasis. There are three main subtypes of SNs: nociceptors, mechanoreceptors, and proprioceptors, which express different membrane proteins, such as TRKA, TRKB, or TRKC, respectively. Human pluripotent stem cell technology provides an ideal platform to study development and diseases of SNs, however there is not a viable method to isolate individual SN subtype for downstream analysis available. Here, we employ the method immunopanning to isolate each SN subtype. This method is very gentle and allows proper survival after the isolation. We use antibodies against TRKA, TRKB, and TRKC to isolate nociceptors, mechanoreceptors, and proprioceptors, respectively. We show that our cultures are enriched for each subtype and express their respective subtype markers. Furthermore, we show that the immunopanned SNs are electrically active and respond to specific stimuli. Thus, our method can be used to purify viable neuronal subtypes using respective membrane proteins for downstream studies.

Are superficial neuromasts proprioceptors underlying fast copulatory behavior?

In male Poeciliid fishes, the modified anal fin (i.e., gonopodium) and its axial and appendicular support are repositioned within the axial skeleton, creating a novel sexually dimorphic ano-urogenital region. During copulation, the relative location of the gonopodium is crucial for successful insemination. Therefore, the repositioning of these structures and organ relied on the reorganization of the efferent circuitry that controls spinal motor neurons innervating appendicular muscles critical for the movement of the gonopodium, including the fast and synchronous torque-trust motion during insemination attempts. Copulation occurs when a male positions himself largely outside a female's field of view, circumducts his gonopodium, and performs a rapid, complex maneuver to properly contact the female urogenital sinus with the distal tip of the gonopodium and transfers sperm. Although understanding of the efferent circuitry has significantly increased in the last 24 years, nothing is known about the cutaneous receptors involved in gonopodium movement, or how the afferent signals are processed to determine the location of this organ during copulation. Using Western mosquitofish, Gambusia affinis, as our model, we attempt to fill this gap in knowledge. Preliminary data showed cutaneous nerves and sensory neurons innervating superficial neuromasts surrounding the base of adult male gonopodium; those cutaneous nerves projected ventrally from the spinal cord through the 14th dorsal root ganglion and its corresponding ventral root towards the base and fin rays of the gonopodium. We asked what role the cutaneous superficial neuromasts play in controlling the positioning and timing of the gonopodium's fast and synchronous movements for effective sperm transfer. First, we found a greater number of superficial neuromasts surrounding the base of the male's gonopodium compared to the base of the female's anal fin. Second, we systemically removed superficial neuromasts surrounding the gonopodium base and observed significant impairment of the positioning and timing of gonopodial movements. Our findings provide a first step to supporting the following hypothesis: during radical reorganization of the Poeciliid body plan, superficial neuromasts have been partially co-opted as proprioceptors that allow the gonopodium to control precise positioning and timing during copulatory attempts.

Calcium Homeostasis in Parvalbumin DRG Neurons is Altered After Sciatic Nerve Crush and Sciatic Nerve Transection Injuries

Reflex abnormalities mediated by proprioceptive sensory neurons after peripheral nerve injury (PNI) can limit functional improvement, leaving patients with disability that affects their quality of life. We examined post-injury calcium transients in a subpopulation of DRG neurons consisting primarily of proprioceptors to determine whether alterations in calcium homeostasis are present in proprioceptors, as has been documented in other DRG neurons after PNI. Using transgenic mice, we restricted expression of the calcium indicator GCaMP6s to DRG neurons containing parvalbumin (PV). Mice of both sexes were randomly assigned to sham, sciatic nerve crush, or sciatic nerve transection and resuture conditions. Calcium transients were recorded from ex-vivo preparations of animals at one of three post-surgery time points: 1-3 days, 7-11 days, and after 60 days of recovery. Results demonstrated that the post-PNI calcium transients of PV DRG neurons are significantly different than sham. Abnormalities were not present during the acute response to injury (1-3 days), but transients were significantly different than sham at the recovery stage where axon regeneration is thought to be underway (7-11 days). During late-stage recovery (60 days post-injury), disturbances in the decay time course of calcium transients in transection animals persisted, whereas parameters of transients from crush animals returned to normal. These findings identify a deficit in calcium homeostasis in proprioceptive neurons, which may contribute to the failure to fully recover proprioceptive reflexes after PNI. Significant differences in the calcium transients of crush versus transection animals after reinnervation illustrate calcium homeostasis alterations are distinctive to injury type.

Dual modulatory effects on feedback from a proprioceptor in the crustacean stomatogastric nervous system

Neuromodulatory actions that change the properties of proprioceptors or the muscle movements to which they respond necessarily affect the feedback provided to the central network. Here we further characterize the responses of the gastropyloric receptor 1 (GPR1) and gastropyloric receptor 2 (GPR2) neurons in the stomatogastric nervous system of the crab Cancer borealis to movements and contractions of muscles, and we report how neuromodulation modifies those responses. We observed that the GPR1 response to contractions of the gastric mill 4 muscle (gm4) was absent, or nearly so, when the neuron was quiescent but robust when it was spontaneously active. We also found that the effects of four neuromodulatory substances (GABA, serotonin, proctolin, and TNRNFLRFamide) on the GPR1 response to muscle stretch were similar to those previously reported for GPR2. Finally, we showed that an excitatory action on gm4 due to proctolin combined with an inhibitory action on GPR2 due to GABA can allow for larger muscle contractions without increased proprioceptive feedback.NEW & NOTEWORTHY We report that the combination of GABA and the peptide proctolin increases contraction of a stomatogastric muscle while decreasing the corresponding response of the proprioceptor that reports on it. These results suggest a general mechanism by which muscle movements can be modified while sensory feedback is conserved, one that may be particularly well suited for providing flexibility to central pattern generator networks.

Neural Coding of Leg Proprioception in Drosophila

Animals rely on an internal sense of body position and movement to effectively control motor behavior. This sense of proprioception is mediated by diverse populations of mechanosensory neurons distributed throughout the body. Here, we investigate neural coding of leg proprioception in Drosophila, using in vivo two-photon calcium imaging of proprioceptive sensory neurons during controlled movements of the fly tibia. We found that the axons of leg proprioceptors are organized into distinct functional projections that contain topographic representations of specific kinematic features. Using subclass-specific genetic driver lines, we show that one group of axons encodes tibia position (flexion/extension), another encodes movement direction, and a third encodes bidirectional movement and vibration frequency. Overall, our findings reveal how proprioceptive stimuli from a single leg joint are encoded by a diverse population of sensory neurons, and provide a framework for understanding how proprioceptive feedback signals are used by motor circuits to coordinate the body.

[Effect of electroacupuncture on proprioception in cynomolgus monkeys after unilateral anterior cruciate ligament injury]

OBJECTIVE

To study the effect of electroacupuncture on proprioception in cynomolgus monkeys after unilateral anterior cruciate ligament (ACL) injury.

METHODS

Twenty-seven cynomolgus monkeys were randomized equally into 3 groups, namely unilateral ACL injury with electroacupuncture group, unilateral ACL injury model group and blank control group. One week after modeling, the monkeys in electroacupuncture group were treated daily for 12 weeks with electroacupuncture at the acupoints Wei Yang, Yin Gu, Xi Yangguan and Qu Quan. At 4, 8, and 12 weeks during the treatment, the changes in the somatosensory evoked potentials (SEPs) and motor nerve conduction velocity (MCV) of the nerves of the ACL were examined with neural electrophysiological method; the changes in the morphology and the total and variable number of the proprioceptors in the ACL were observed with gold chloride staining. RESULTS; In the mokeys in the model group, the incubation period of the SEPs and MCV on the injured side of the knee were significantly extended and the amplitudes were decreased with the passage of time (P<0.05). Intervention with electroacupuncture significantly reduced the incubation period and increased the amplitudes of the SEPs and MCV (P<0.05). The total number of the proprioceptors in the ACL was significantly decreased and the variable number of the proprioceptors in ACL was increased with time in the monkeys in the model group (P<0.05); electroacupuncture intervention obviously slowed the reduction rate of total number of the proprioceptors (P<0.05) but without affecting the variable number of the proprioceptors (P>0.05). CoONCLUSION: ACL injury causes attenuation of the proprioception on the injury side, and electroacupuncture intervention can produce a positive effect on the proprioception in cynomolgus monkeys.

State-dependent sensorimotor gating in a rhythmic motor system

Sensory feedback influences motor circuits and/or their projection neuron inputs to adjust ongoing motor activity, but its efficacy varies. Currently, less is known about regulation of sensory feedback onto projection neurons that control downstream motor circuits than about sensory regulation of the motor circuit neurons themselves. In this study, we tested whether sensory feedback onto projection neurons is sensitive only to activation of a motor system, or also to the modulatory state underlying that activation, using the crab Cancer borealis stomatogastric nervous system. We examined how proprioceptor neurons (gastropyloric receptors, GPRs) influence the gastric mill (chewing) circuit neurons and the projection neurons (MCN1, CPN2) that drive the gastric mill rhythm. During gastric mill rhythms triggered by the mechanosensory ventral cardiac neurons (VCNs), GPR was shown previously to influence gastric mill circuit neurons, but its excitation of MCN1/CPN2 was absent. In this study, we tested whether GPR effects on MCN1/CPN2 are also absent during gastric mill rhythms triggered by the peptidergic postoesophageal commissure (POC) neurons. The VCN and POC pathways both trigger lasting MCN1/CPN2 activation, but their distinct influence on circuit feedback to these neurons produces different gastric mill motor patterns. We show that GPR excites MCN1 and CPN2 during the POC-gastric mill rhythm, altering their firing rates and activity patterns. This action changes both phases of the POC-gastric mill rhythm, whereas GPR only alters one phase of the VCN-gastric mill rhythm. Thus sensory feedback to projection neurons can be gated as a function of the modulatory state of an active motor system, not simply switched on/off with the onset of motor activity.NEW & NOTEWORTHY Sensory feedback influences motor systems (i.e., motor circuits and their projection neuron inputs). However, whether regulation of sensory feedback to these projection neurons is consistent across different versions of the same motor pattern driven by the same motor system was not known. We found that gating of sensory feedback to projection neurons is determined by the modulatory state of the motor system, and not simply by whether the system is active or inactive.

Dicer maintains the identity and function of proprioceptive sensory neurons

Neuronal cell identity is established during development and must be maintained throughout an animal's life (Fishell G, Heintz N. Neuron 80: 602-612, 2013). Transcription factors critical for establishing neuronal identity can be required for maintaining it (Deneris ES, Hobert O. Nat Neurosci 17: 899-907, 2014). Posttranscriptional regulation also plays an important role in neuronal differentiation (Bian S, Sun T. Mol Neurobiol 44: 359-373, 2011), but its role in maintaining cell identity is less established. To better understand how posttranscriptional regulation might contribute to cell identity, we examined the proprioceptive neurons in the dorsal root ganglion (DRG), a highly specialized sensory neuron class, with well-established properties that distinguish them from other neurons in the ganglion. By conditionally ablating Dicer in mice, using parvalbumin (Pvalb)-driven Cre recombinase, we impaired posttranscriptional regulation in the proprioceptive sensory neuron population. Knockout (KO) animals display a progressive form of ataxia at the beginning of the fourth postnatal week that is accompanied by a cell death within the DRG. Before cell loss, expression profiling shows a reduction of proprioceptor specific genes and an increased expression of nonproprioceptive genes normally enriched in other ganglion neurons. Furthermore, although central connections of these neurons are intact, the peripheral connections to the muscle are functionally impaired. Posttranscriptional regulation is therefore necessary to retain the transcriptional identity and support functional specialization of the proprioceptive sensory neurons.NEW & NOTEWORTHY We have demonstrated that selectively impairing Dicer in parvalbumin-positive neurons, which include the proprioceptors, triggers behavioral changes, a lack of muscle connectivity, and a loss of transcriptional identity as observed through RNA sequencing. These results suggest that Dicer and, most likely by extension, microRNAs are crucially important for maintaining proprioception. Additionally, this study hints at the larger question of how neurons maintain their functional and molecular specificity.

Proprioceptive feedback modulates coordinating information in a system of segmentally distributed microcircuits

The system of modular neural circuits that controls crustacean swimmerets drives a metachronal sequence of power-stroke (PS, retraction) and return-stroke (RS, protraction) movements that propels the animal forward efficiently. These neural modules are synchronized by an intersegmental coordinating circuit that imposes characteristic phase differences between these modules. Using a semi-intact preparation that left one swimmeret attached to an otherwise isolated central nervous system (CNS) of the crayfish, Pacifastacus leniusculus, we investigated how the rhythmic activity of this system responded to imposed movements. We recorded extracellularly from the PS and RS nerves that innervated the attached limb and from coordinating axons that encode efference copies of the periodic bursts in PS and RS axons. Simultaneously, we recorded from homologous nerves in more anterior and posterior segments. Maintained retractions did not affect cycle period but promptly weakened PS bursts, strengthened RS bursts, and caused corresponding changes in the strength and timing of efference copies in the module's coordinating axons. Changes in these efference copies then caused changes in the phase and duration, but not the strength, of PS bursts in modules controlling neighboring swimmerets. These changes were promptly reversed when the limb was released. Each swimmeret is innervated by two nonspiking stretch receptors (NSSRs) that depolarize when the limb is retracted. Voltage clamp of an NSSR changed the durations and strengths of bursts in PS and RS axons innervating the same limb and caused corresponding changes in the efference copies of this motor output.

Vestibular nucleus neurons respond to hindlimb movement in the decerebrate cat

The vestibular nuclei integrate information from vestibular and proprioceptive afferents, which presumably facilitates the maintenance of stable balance and posture. However, little is currently known about the processing of sensory signals from the limbs by vestibular nucleus neurons. This study tested the hypothesis that limb movement is encoded by vestibular nucleus neurons and described the changes in activity of these neurons elicited by limb extension and flexion. In decerebrate cats, we recorded the activity of 70 vestibular nucleus neurons whose activity was modulated by limb movements. Most of these neurons (57/70, 81.4%) encoded information about the direction of hindlimb movement, while the remaining neurons (13/70, 18.6%) encoded the presence of hindlimb movement without signaling the direction of movement. The activity of many vestibular nucleus neurons that responded to limb movement was also modulated by rotating the animal's body in vertical planes, suggesting that the neurons integrated hindlimb and labyrinthine inputs. Neurons whose firing rate increased during ipsilateral ear-down roll rotations tended to be excited by hindlimb flexion, whereas neurons whose firing rate increased during contralateral ear-down tilts were excited by hindlimb extension. These observations suggest that there is a purposeful mapping of hindlimb inputs onto vestibular nucleus neurons, such that integration of hindlimb and labyrinthine inputs to the neurons is functionally relevant.

C. elegans multi-dendritic sensory neurons: morphology and function

PVD and FLP sensory neurons envelope the body of the C. elegans adult with a highly branched network of thin sensory processes. Both PVD and FLP neurons are mechanosensors. PVD is known to mediate the response to high threshold mechanical stimuli. Thus PVD and FLP neurons are similar in both morphology and function to mammalian nociceptors. To better understand the function of these neurons we generated strains lacking them. Behavioral analysis shows that PVD and FLP regulate movement under normal growth conditions, as animals lacking these neurons demonstrate higher dwelling behavior. In addition, PVD--whose thin branches project across the body-wall muscles--may have a role in proprioception, as ablation of PVD leads to defective posture. Moreover, movement-dependent calcium transients are seen in PVD, a response that requires MEC-10, a subunit of the mechanosensory DEG/ENaC channel that is also required for maintaining wild-type posture. Hence, PVD senses both noxious and innocuous signals to regulate C. elegans behavior, and thus combines the functions of multiple mammalian somatosensory neurons. Finally, strong mechanical stimulation leads to inhibition of egg-laying, and this response also depends on PVD and FLP neurons. Based on all these results we suggest that noxious signals perceived by PVD and FLP promote an escape behavior consisting of increased speed, reduced pauses and reversals, and inhibition of egg-laying.