Projected pain from noxious heat stimulation of an exposed peripheral nerve--a case report

Human heat sensation: A randomized crossover trial

Sensing of noxious heat has been reported to be mediated by TRPV1, TRPA1, TRPM3, and ANO1 in mice, and this is redundant so that the loss of one receptor is at least partially compensated for by others. We have established an infusion-based human heat pain model. Heat-induced pain probed with antagonists for the four receptors did not match the redundancy found in mice. In healthy participants, only TRPV1 contributes to the detection of noxious heat; none of the other three receptors are involved. TRPV1 inhibition reduced the pain at all noxious temperatures, which can also be seen as an increase in the temperature that causes a particular level of pain. However, even if the TRPV1-dependent shift in heat detection is about 1°C, at the end of the temperature ramp to 52°C, most heat-induced pain remains unexplained. This difference between species reopens the quest for the molecular safety net for the detection of noxious heat in humans.

The formalin test does not probe inflammatory pain but excitotoxicity in rodent skin

The most widely used formalin test to screen antinociceptive drug candidates is still apostrophized as targeting inflammatory pain, in spite of strong opposing evidence published. In our rat skin-nerve preparation ex vivo, recording from all classes of sensory single-fibers (n = 32), 30 units were transiently excited by formaldehyde concentrations 1-100 mM applied to receptive fields (RFs) for 3 min, C and Aδ-fibers being more sensitive (1-30 mM) than Aβ-fibers. From 30 mM on, ~1% of the concentration usually injected in vivo, all RFs were defunctionalized and conduction in an isolated sciatic nerve preparation was irreversibly blocked. Thus, formaldehyde, generated a state of 'anesthesia dolorosa' in the RFs in so far as after a quiescent interphase all fibers with unmyelinated terminals developed a second phase of vigorous discharge activity which correlated well in time course and magnitude with published pain-related behaviors. Sural nerve filament recordings in vivo confirmed that higher formalin concentrations (> 42 mM) have to be injected to the skin to induce this second phase of discharge. Patch-clamp and calcium-imaging confirmed TRPA1 as the primary transducer of formaldehyde (10 mM) effects on mouse sensory neurons. However, stimulated CGRP release from isolated skin of TRPA1+/+ and TRPA1-/- mice showed a convergence of the saturating concentration-response curves at 100 mM formaldehyde, which did not occur with nerve and trachea preparations. Finally, skin-nerve recordings from C and Aδ-fibers of TRPA1-/- mice revealed a massive reduction in formaldehyde (30 mM)-evoked discharge. However, the remaining activity was still biphasic, thus confirming additional unspecific excitotoxic actions of the fixative that diffuses along still excitable axons as previously published. The multiplicity of formaldehyde's actions requires extensive discussion and literature review, leading to a fundamental reevaluation of the formalin test.

Origins of antidromic activity in sensory afferent fibers and neurogenic inflammation

Neurogenic inflammation results from the release of biologically active agents from the peripheral primary afferent terminal. This release reflects the presence of releasable pools of active product and depolarization-exocytotic coupling mechanisms in the distal afferent terminal and serves to alter the physiologic function of innervated organ systems ranging from the skin and meninges to muscle, bone, and viscera. Aside from direct stimulation, this biologically important release from the peripheral afferent terminal can be initiated by antidromic activity arising from five anatomically distinct points of origin: (i) afferent collaterals at the peripheral-target organ level, (ii) afferent collaterals arising proximal to the target organ, (iii) from mid-axon where afferents lacking myelin sheaths (C fibers and others following demyelinating injuries) may display crosstalk and respond to local irritation, (iv) the dorsal root ganglion itself, and (v) the central terminals of the afferent in the dorsal horn where local circuits and bulbospinal projections can initiate the so-called dorsal root reflexes, i.e., antidromic traffic in the sensory afferent.

Acute corneal epithelial debridement unmasks the corneal stromal nerve responses to ocular stimulation in rats: implications for abnormal sensations of the eye

It is widely accepted that the mechanisms for transducing sensory information reside in the nerve terminals. Occasionally, however, studies have appeared demonstrating that similar mechanisms may exist in the axon to which these terminals are connected. We examined this issue in the cornea, where nerve terminals in the epithelial cell layers are easily accessible for debridement, leaving the underlying stromal (axonal) nerves undisturbed. In isoflurane-anesthetized rats, we recorded extracellularly from single trigeminal ganglion neurons innervating the cornea that are excited by ocular dryness and cooling: low-threshold (<2°C cooling) and high-threshold (>2°C) cold-sensitive plus dry-sensitive neurons playing possible roles in tearing and ocular pain. We found that the responses in both types of neurons to dryness, wetness, and menthol stimuli were effectively abolished by the debridement, indicating that their transduction mechanisms lie in the nerve terminals. However, some responses to the cold, heat, and hyperosmolar stimuli in low-threshold cold-sensitive plus dry-sensitive neurons still remained. Surprisingly, the responses to heat in approximately half of the neurons were augmented after the debridement. We were also able to evoke these residual responses and follow the trajectory of the stromal nerves, which we subsequently confirmed histologically. The residual responses always disappeared when the stromal nerves were cut at the limbus, suggesting that the additional transduction mechanisms for these sensory modalities originated most likely in stromal nerves. The functional significance of these residual and enhanced responses from stromal nerves may be related to the abnormal sensations observed in ocular disease.NEW & NOTEWORTHY In addition to the traditional view that the sensory transduction mechanisms exist in the nerve terminals, we report here that the proximal axons (stromal nerves in the cornea from which these nerve terminals originate) may also be capable of transducing sensory information. We arrived at this conclusion by removing the epithelial cell layers of the cornea in which the nerve terminals reside but leaving the underlying stromal nerves undisturbed.

Regulation of alternative VEGF-A mRNA splicing is a therapeutic target for analgesia

Vascular endothelial growth factor-A (VEGF-A) is best known as a key regulator of the formation of new blood vessels. Neutralization of VEGF-A with anti-VEGF therapy e.g. bevacizumab, can be painful, and this is hypothesized to result from a loss of VEGF-A-mediated neuroprotection. The multiple vegf-a gene products consist of two alternatively spliced families, typified by VEGF-A₁₆₅a and VEGF-A₁₆₅b (both contain 165 amino acids), both of which are neuroprotective. Under pathological conditions, such as in inflammation and cancer, the pro-angiogenic VEGF-A₁₆₅a is upregulated and predominates over the VEGF-A₁₆₅b isoform.

We show here that in rats and mice VEGF-A₁₆₅a and VEGF-A₁₆₅b have opposing effects on pain, and that blocking the proximal splicing event – leading to the preferential expression of VEGF-A₁₆₅b over VEGF₁₆₅a – prevents pain in vivo. VEGF-A₁₆₅a sensitizes peripheral nociceptive neurons through actions on VEGFR2 and a TRPV1-dependent mechanism, thus enhancing nociceptive signaling. VEGF-A₁₆₅b blocks the effect of VEGF-A₁₆₅a.

After nerve injury, the endogenous balance of VEGF-A isoforms switches to greater expression of VEGF-A_xxxa compared to VEGF-A_xxxb, through an SRPK1-dependent pre-mRNA splicing mechanism. Pharmacological inhibition of SRPK1 after traumatic nerve injury selectively reduced VEGF-A_xxxa expression and reversed associated neuropathic pain. Exogenous VEGF-A₁₆₅b also ameliorated neuropathic pain.

We conclude that the relative levels of alternatively spliced VEGF-A isoforms are critical for pain modulation under both normal conditions and in sensory neuropathy. Altering VEGF-A_xxxa/VEGF-A_xxxb balance by targeting alternative RNA splicing may be a new analgesic strategy.

•

The different vegf-a splice variants, VEGF-A₁₆₅a and VEGF-A₁₆₅b have pro- and anti-nociceptive actions respectively.

•

Pro-nociceptive actions of VEGF-A₁₆₅a are dependent on TRPV1.

•

Alternative pre-mRNA splicing underpins peripheral sensitization by VEGF-A isoforms in normal and neuropathic animals.

Partial nerve injury induces electrophysiological changes in conducting (uninjured) nociceptive and nonnociceptive DRG neurons: Possible relationships to aspects of peripheral neuropathic pain and paresthesias

Partial nerve injury leads to peripheral neuropathic pain. This injury results in conducting/uninterrupted (also called uninjured) sensory fibres, conducting through the damaged nerve alongside axotomised/degenerating fibres. In rats seven days after L5 spinal nerve axotomy (SNA) or modified-SNA (added loose-ligation of L4 spinal nerve with neuroinflammation-inducing chromic-gut), we investigated a) neuropathic pain behaviours and b) electrophysiological changes in conducting/uninterrupted L4 dorsal root ganglion (DRG) neurons with receptive fields (called: L4-receptive-field-neurons). Compared to pretreatment, modified-SNA rats showed highly significant increases in spontaneous-foot-lifting duration, mechanical-hypersensitivity/allodynia, and heat-hypersensitivity/hyperalgesia, that were significantly greater than after SNA, especially spontaneous-foot-lifting. We recorded intracellularly in vivo from normal L4/L5 DRG neurons and ipsilateral L4-receptive-field-neurons. After SNA or modified-SNA, L4-receptive-field-neurons showed the following: a) increased percentages of C-, Ad-, and Ab-nociceptors and cutaneous Aa/b-low-threshold mechanoreceptors with ongoing/spontaneous firing; b) spontaneous firing in C-nociceptors that originated peripherally; this was at a faster rate in modified-SNA than SNA; c) decreased electrical thresholds in A-nociceptors after SNA; d) hyperpolarised membrane potentials in A-nociceptors and Aa/b-low-threshold-mechanoreceptors after SNA, but not C-nociceptors; e) decreased somatic action potential rise times in C- and A-nociceptors, not Aa/b-low-threshold-mechanoreceptors. We suggest that these changes in subtypes of conducting/uninterrupted neurons after partial nerve injury contribute to the different aspects of neuropathic pain as follows: spontaneous firing in nociceptors to ongoing/spontaneous pain; spontaneous firing in Aa/b-low-threshold-mechanoreceptors to dysesthesias/paresthesias; and lowered A-nociceptor electrical thresholds to A-nociceptor sensitization, and greater evoked pain.

Axonal thermosensitivity and mechanosensitivity of cutaneous afferent neurons

We hypothesized that cutaneous afferent myelinated fibers (A-fibers) and afferent unmyelinated fibers (C-fibers) respond to the same natural stimuli applied to their axons as to their terminals in the skin. In anesthetized rats, activity was recorded from afferent axons in strands isolated proximally from the sural nerve. Mechanical, cold or heat stimuli were applied to the skin or along a 15-mm length of the distal sural nerve. One-hundred and eighteen A-fibers and 109 C-fibers were characterized by their conduction velocity and/or shape of their action potentials, and by their responses to natural stimulation of the skin. Then, these fibers were tested for their responses to the same stimuli applied to the nerve. In some cases, the nerve was crushed distally after the nerve fibers had been characterized by their responses to physiological stimulation of the skin, and the responses to stimuli applied to the nerve proximal to the lesion were tested again. Almost all non-nociceptive cold-sensitive (type 1) C-fibers (97%) could be activated by cold stimuli applied to the nerve. Of nociceptive cold-sensitive (type 2) C-fibers, 39% were activated by cold stimuli applied to the nerve. Furthermore, 34% of heat-sensitive C-fibers could be activated by heating the nerve. In contrast, only 2-4% of mechanosensitive A-fibers and C-fibers responded to mechanical stimuli applied to the nerve. In conclusion, cold and heat sensitivity of cutaneous afferent neurons is not restricted to their terminals in the skin, but often extends along the axons in the nerve. Mechanosensitivity is restricted to the afferent endings in the skin.

Visceral organ cross-sensitization - an integrated perspective

Viscero-somatic referral and sensitization has been well documented clinically and widely investigated, whereas viscero-visceral referral and sensitization (termed cross-organ sensitization) has only recently received attention as important to visceral disease states. Because second order neurons in the CNS have been extensively shown to receive convergent input from different visceral organs, it has been assumed that cross-organ sensitization arises by the same convergence-projection mechanism as advanced for viscero-somatic referral and sensitization. However, increasing evidence also suggests participation of peripheral mechanisms to explain referral and sensitization. We briefly summarize behavioral, morphological and physiological support of and focus on potential mechanisms underlying cross-organ sensitization.