Activity-dependent sensory signal processing in mechanically responsive slowly conducting meningeal afferents

Electrical matrix stimulation suppresses acute itch independently of activation of sleeping nociceptors

INTRODUCTION

Itch can be reduced by pain. Activation of sleeping nociceptors (CMi) is a crucial mechanism for the peripheral component of intense and long-lasting pain. Thus, activation of CMi might be especially effective in itch reduction. Electrical stimulation using sinusoidal pulses activates CMi with tolerable pain intensity, whereas short rectangular pulses with low intensity do not. In humans, histaminergic itch is mediated by histamine-sensitive CMi, whereas other pruritogens activate polymodal nociceptors (CM).

METHODS

In a psychophysical approach in a balanced crossover repeated-measures design in healthy volunteers, we activated nociceptors by two different electrical stimulation paradigms via a matrix electrode: 4 Hz sinusoidal pulses that activate C-nociceptors including CMi or 4 Hz rectangular stimuli to activate nociceptors excluding CMi. After 5-min stimulation, itch was induced by either histamine iontophoresis or application of cowhage spicules. Itch ratings were assessed via a numerical rating scale (NRS).

RESULTS

Electrical 4 Hz sine wave stimulation (0.1 mA) with low pain ratings of 1.5 (NRS; 0-10) induced an axon reflex erythema (3 cm2 ), indicating activation of CMi, whereas rectangular 0.2 ms pulses (average 0.91 mA) with the same pain rating did not. Both electrical stimulation paradigms reduced itch magnitude over time evoked by either histamine or cowhage to a similar extent. Peak maximum itch evoked by histamine was reduced by both stimulation paradigms, but not cowhage maximum itch.

DISCUSSION

Since electrical stimulation with the rectangular pulse paradigm reduces itch to a similar extent as the sine wave stimulation paradigm, the input of CMi is not necessarily required for itch suppression. The input of A-fibres and polymodal nociceptors, similarly, as also achieved by scratching, seems to be sufficient for both forms of chemically evoked itch.

SIGNIFICANCE

Since activation of CMi does not provide additional benefit for itch suppression, spinal pain pathways transmitted via CM versus CMi have differential effects on itch-processing circuits. This is important knowledge for using electrical matrix stimulation as itch suppressor since activation of sleeping nociceptors either requires significantly painful stimulation paradigms or specialized stimulation paradigms as sinusoidal pulses. An alternative approach using half-sine wave pulses with low pain intensity activating specifically polymodal nociceptors to suppress itch via matrix electrode stimulation may be considered.

Activation of Meningeal Afferents Relevant to Trigeminal Headache Pain after Photothrombotic Stroke Lesion: A Pilot Study in Mice

Stroke can be followed by immediate severe headaches. As headaches are initiated by the activation of trigeminal meningeal afferents, we assessed changes in the activity of meningeal afferents in mice subjected to cortical photothrombosis. Cortical photothrombosis induced ipsilateral lesions of variable sizes that were associated with contralateral sensorimotor impairment. Nociceptive firing of mechanosensitive Piezo1 channels, activated by the agonist Yoda1, was increased in meningeal afferents in the ischemic hemispheres. These meningeal afferents also had a higher maximal spike frequency at baseline and during activation of the mechanosensitive Piezo1 channel by Yoda1. Moreover, in these meningeal afferents, nociceptive firing was active during the entire induction of transient receptor potential vanilloid 1 (TRPV1) channels by capsaicin. No such activation was observed on the contralateral hemi-skulls of the same group of mice or in control mice. Our data suggest the involvement of mechanosensitive Piezo1 channels capable of maintaining high-frequency spiking activity and of nociceptive TRPV1 channels in trigeminal headache pain responses after experimental ischemic stroke in mice.

Decoding Neuropathic Pain: Can We Predict Fluctuations of Propagation Speed in Stimulated Peripheral Nerve?

To understand neural encoding of neuropathic pain, evoked and resting activity of peripheral human C-fibers are studied via microneurography experiments. Before different spiking patterns can be analyzed, spike sorting is necessary to distinguish the activity of particular fibers of a recorded bundle. Due to single-electrode measurements and high noise contamination, standard methods based on spike shapes are insufficient and need to be enhanced with additional information. Such information can be derived from the activity-dependent slowing of the fiber propagation speed, which in turn can be assessed by introducing continuous "background" 0.125-0.25 Hz electrical stimulation and recording the corresponding responses from the fibers. Each fiber's speed propagation remains almost constant in the absence of spontaneous firing or additional stimulation. This way, the responses to the "background stimulation" can be sorted by fiber. In this article, we model the changes in the propagation speed resulting from the history of fiber activity with polynomial regression. This is done to assess the feasibility of using the developed models to enhance the spike shape-based sorting. In addition to human microneurography data, we use animal in-vitro recordings with a similar stimulation protocol as higher signal-to-noise ratio data example for the models.

Mechanosensitive meningeal nociception via Piezo channels: Implications for pulsatile pain in migraine?

BACKGROUND

Recent discovery of mechanosensitive Piezo receptors in trigeminal ganglia suggested the novel molecular candidate for generation of migraine pain. However, the contribution of Piezo channels in migraine pathology was not tested yet. Therefore, in this study, we explored a potential involvement of Piezo channels in peripheral trigeminal nociception implicated in generation of migraine pain.

METHODS

We used immunohistochemistry, calcium imaging, calcitonin gene related peptide (CGRP) release assay and electrophysiology in mouse and rat isolated trigeminal neurons and rat hemiskulls to study action of various stimulants of Piezo receptors on migraine-related peripheral nociception.

RESULTS

We found that essential (35%) fraction of isolated rat trigeminal neurons responded to chemical Piezo1 agonist Yoda1 and about a half of Yoda1 positive neurons responded to hypo-osmotic solution (HOS) and a quarter to mechanical stimulation by focused ultrasound (US). In ex vivo hemiskull preparation, Yoda1 and HOS largely activated persistent nociceptive firing in meningeal branches of trigeminal nerve. By using our novel cluster analysis of pain spikes, we demonstrated that 42% of fibers responded to Piezo1 agonist and 20% of trigeminal fibers were activated by Yoda1 and by capsaicin, suggesting expression of Piezo receptors in TRPV1 positive peptidergic nociceptive nerve fibers. Consistent with this, Yoda1 promoted the release of the key migraine mediator CGRP from hemiskull preparation.

CONCLUSION

Taken together, our data suggest the involvement of mechanosensitive Piezo receptors, in particular, Piezo1 subtype in peripheral trigeminal nociception, which provides a new view on mechanotransduction in migraine pathology and suggests novel molecular targets for anti-migraine medicine.

Evidence for a spinal involvement in temporal pain contrast enhancement

Spatiotemporal filtering and amplification of sensory information at multiple levels during the generation of perceptual representations is a fundamental processing principle of the nervous system. While for the visual and auditory system temporal filtering of sensory signals has been noticed for a long time, respective contrast mechanisms within the nociceptive system became only recently subject of investigations, mainly in the context of offset analgesia (OA) subsequent to noxious stimulus decreases. In the present study we corroborate in a first experiment the assumption that offset analgesia involves a central component by showing that an OA-like effect accounting for 74% of a corresponding OA reference can be evoked by decomposing the stimulus offset into two separate box-car stimuli applied within the same dermatome but to separate populations of primary afferent neurons. In order to draw conclusions about the levels of the CNS at which temporal filtering of nociceptive information takes place during OA we investigate in a second experiment neuronal activity in the spinal cord during a painful thermal stimulus offset employing high-resolution fMRI in healthy volunteers. Pain-related BOLD responses in the spinal cord were significantly reduced during OA and their time course followed widely behavioral hypoalgesia, but not the thermal stimulation profile. In summary, the results suggest that temporal pain contrast enhancement during OA comprises a central mechanism and this mechanism becomes already effective at the level of the spinal cord.

Hunting for origins of migraine pain: cluster analysis of spontaneous and capsaicin-induced firing in meningeal trigeminal nerve fibers

Trigeminal nerves in meninges are implicated in generation of nociceptive firing underlying migraine pain. However, the neurochemical mechanisms of nociceptive firing in meningeal trigeminal nerves are little understood. In this study, using suction electrode recordings from peripheral branches of the trigeminal nerve in isolated rat meninges, we analyzed spontaneous and capsaicin-induced orthodromic spiking activity. In control, biphasic single spikes with variable amplitude and shapes were observed. Application of the transient receptor potential vanilloid 1 (TRPV1) agonist capsaicin to meninges dramatically increased firing whereas the amplitudes and shapes of spikes remained essentially unchanged. This effect was antagonized by the specific TRPV1 antagonist capsazepine. Using the clustering approach, several groups of uniform spikes (clusters) were identified. The clustering approach combined with capsaicin application allowed us to detect and to distinguish "responder" (65%) from "non-responder" clusters (35%). Notably, responders fired spikes at frequencies exceeding 10 Hz, high enough to provide postsynaptic temporal summation of excitation at brainstem and spinal cord level. Almost all spikes were suppressed by tetrodotoxin (TTX) suggesting an involvement of the TTX-sensitive sodium channels in nociceptive signaling at the peripheral branches of trigeminal neurons. Our analysis also identified transient (desensitizing) and long-lasting (slowly desensitizing) responses to the continuous application of capsaicin. Thus, the persistent activation of nociceptors in capsaicin-sensitive nerve fibers shown here may be involved in trigeminal pain signaling and plasticity along with the release of migraine-related neuropeptides from TRPV1 positive neurons. Furthermore, cluster analysis could be widely used to characterize the temporal and neurochemical profiles of other pain transducers likely implicated in migraine.