The labelled-lines principle of the somatosensory physiology might explain the phantom limb phenomenon

From bristle to brain: embryonic development of topographic projections from basiconic sensilla in the antennal nervous system of the locust Schistocerca gregaria

The antennal flagellum of the locust S. gregaria is an articulated structure bearing a spectrum of sensilla that responds to sensory stimuli. In this study, we focus on the basiconic-type bristles as a model for sensory system development in the antenna. At the end of embryogenesis, these bristles are found at fixed locations and then on only the most distal six articulations of the antenna. They are innervated by a dendrite from a sensory cell cluster in the underlying epithelium, with each cluster directing fused axons topographically to an antennal tract running to the brain. We employ confocal imaging and immunolabeling to (a) identify mitotically active sense organ precursors for sensory cell clusters in the most distal annuli of the early embryonic antenna; (b) observe the subsequent spatial appearance of their neuronal progeny; and (c) map the spatial and temporal organization of axon projections from such clusters into the antennal tracts. We show that early in embryogenesis, proliferative precursors are localized circumferentially within discrete epithelial domains of the flagellum. Progeny first appear distally at the antennal tip and then sequentially in a proximal direction so that sensory neuron populations are distributed in an age-dependent manner along the antenna. Autotracing reveals that axon fasciculation with a tract is also sequential and reflects the location and age of the cell cluster along the most distal annuli. Cell cluster location and bristle location are therefore represented topographically and temporally within the axon profile of the tract and its projection to the brain.

Models for discriminating image blur from loss of contrast

Observers can discriminate between blurry and low-contrast images (Morgan, 2017). Wang and Simoncelli (2004) demonstrated that a code for blur is inherent to the phase relationships between localized pattern detectors of different scales. To test whether human observers actually use local phase coherence when discriminating between image blur and loss of contrast, we compared phase-scrambled chessboards with unscrambled chessboards. Although both stimuli had identical amplitude spectra, local phase coherence was disrupted by phase-scrambling. Human observers were required to concurrently detect and identify (as contrast or blur) image manipulations in the 2 × 2 forced-choice paradigm (Nachmias & Weber, 1975; Watson & Robson, 1981) traditionally considered to be a litmus test for “labelled lines” (i.e. detection mechanisms that can be distinguished on the basis of their preferred stimuli). Phase scrambling reduced some observers’ ability to discriminate between blur and a reduction in contrast. However, none of our observers produced data consistent with Watson and Robson's most stringent test for labeled lines, regardless whether phases were scrambled or not. Models of performance fit significantly better when (a) the blur detector also responded to contrast modulations, (b) the contrast detector also responded to blur modulations, or (c) noise in the two detectors was anticorrelated.

Delta Opioid Receptor Expression and Function in Primary Afferent Somatosensory Neurons

The functional diversity of primary afferent neurons of the dorsal root ganglia (DRG) generates a variety of qualitatively and quantitatively distinct somatosensory experiences, from shooting pain to pleasant touch. In recent years, the identification of dozens of genetic markers specifically expressed by subpopulations of DRG neurons has dramatically improved our understanding of this diversity and provided the tools to manipulate their activity and uncover their molecular identity and function. Opioid receptors have long been known to be expressed by discrete populations of DRG neurons, in which they regulate cell excitability and neurotransmitter release. We review recent insights into the identity of the DRG neurons that express the delta opioid receptor (DOR) and the ion channel mechanisms that DOR engages in these cells to regulate sensory input. We highlight recent findings derived from DORGFP reporter mice and from in situ hybridization and RNA sequencing studies in wild-type mice that revealed DOR presence in cutaneous mechanosensory afferents eliciting touch and implicated in tactile allodynia. Mechanistically, we describe how DOR modulates opening of voltage-gated calcium channels (VGCCs) to control glutamatergic neurotransmission between somatosensory neurons and postsynaptic neurons in the spinal cord dorsal horn. We additionally discuss other potential signaling mechanisms, including those involving potassium channels, which DOR may engage to fine tune somatosensation. We conclude by discussing how this knowledge may explain the analgesic properties of DOR agonists against mechanical pain and uncovers an unanticipated specialized function for DOR in cutaneous mechanosensation.

Analysis of Nociceptive Information Encoded in the Temporal Discharge Patterns of Cutaneous C-Fibers

The generation of pain signals from primary afferent neurons is explained by a labeled-line code. However, this notion cannot apply in a simple way to cutaneous C-fibers, which carry signals from a variety of receptors that respond to various stimuli including agonist chemicals. To represent the discharge patterns of C-fibers according to different agonist chemicals, we have developed a quantitative approach using three consecutive spikes. By using this method, the generation of pain in response to chemical stimuli is shown to be dependent on the temporal aspect of the spike trains. Furthermore, under pathological conditions, gamma-aminobutyric acid resulted in pain behavior without change of spike number but with an altered discharge pattern. Our results suggest that information about the agonist chemicals may be encoded in specific temporal patterns of signals in C-fibers, and nociceptive sensation may be influenced by the extent of temporal summation originating from the temporal patterns.

Dorsal Root Ganglion (DRG) Stimulation in the Treatment of Phantom Limb Pain (PLP)

OBJECTIVES

Phantom limb pain (PLP) is a neuropathic condition in which pain is perceived as arising from an amputated limb. PLP is distinct from, although associated with, pain in the residual limb and nonpainful phantom sensations of the missing limb. Its treatment is extremely challenging; pharmaceutical options, while commonly employed, may be insufficient or intolerable. Neuromodulatory interventions such as spinal cord stimulation have generated mixed results and may be limited by poor somatotopic specificity. It was theorized that dorsal root ganglion (DRG) neuromodulation may be more effective.

MATERIALS AND METHODS

Patients trialed a DRG neurostimulation system for their PLP and were subsequently implanted if results were positive. Retrospective chart review was completed, including pain ratings on a 100-mm visual analogue scale (VAS) and patient-reported outcomes.

RESULTS

Across eight patients, the average baseline pain rating was 85.5 mm. At follow-up (mean of 14.4 months), pain was rated at 43.5 mm. Subjective ratings of quality of life and functional capacity improved. Some patients reduced or eliminated pain medications. Patients reported precise concordance of the paresthesia with painful regions, including in their phantom limbs; in one case, stimulation eliminated PLP as well as nonpainful phantom sensations. Three patients experienced a diminution of pain relief, despite good initial outcomes.

CONCLUSIONS

DRG neuromodulation may be an effective tool in treating this pain etiology. Clinical outcomes in this report support recent converging evidence suggesting that the DRG may be the site of PLP generation and/or maintenance. Further research is warranted to elucidate mechanisms and optimal treatment pathways.

Tactile, thermal, and electrical thresholds in patients with and without phantom limb pain after traumatic lower limb amputation

Purpose

To examine whether there is central sensitization in patients with phantom limb pain (PLP) after traumatic limb amputation.

Methods

Seventeen patients after unilateral lower limb amputation secondary to trauma were enrolled. Ten patients had chronic PLP, while the other seven patients had no PLP. Tactile-sensation threshold, cold- and warm-sensation thresholds, cold- and heat-pain thresholds, electrical-sensation threshold (EST), and electrical-pain threshold on the distal residual limb and the symmetrical site on the sound limb were measured in all tested patients. Their thresholds were compared within the PLP and non-PLP group, and between the groups.

Results

The novel findings included: 1) electrical-pain threshold was only decreased in the sound limb in the PLP group and there was no difference between two limbs in the non-PLP group, suggesting central sensitization in patients with PLP; and 2) EST was increased on the affected limb as compared to the sound limb within the PLP group, but there were no significant differences in EST between the PLP and non-PLP group. There were in general no significant differences in other tested thresholds within the groups and between groups.

Conclusion

Our results demonstrate central sensitization in the patients with PLP after traumatic limb amputation.

Saint John's wort, an herbal inducer of the cytochrome P4503A4 isoform, may alleviate symptoms of Willis-Ekbom's disease

OBJECTIVE

Certain drug classes alleviate the symptoms of Willis-Ekbom's disease, whereas others aggravate them. The pharmacological profiles of these drugs suggest that drugs that alleviate Willis-Ekbom's disease inhibit thyroid hormone activity, whereas drugs that aggravate Willis-Ekbom's disease increase thyroid hormone activity. These different effects may be secondary to the opposing actions that drugs have on the CYP4503A4 enzyme isoform. Drugs that worsen the symptoms of the Willis-Ekbom's disease inhibit the CYP4503A4 isoform, and drugs that ameliorate the symptoms induce CYP4503A4. The aim of this study is to determine whether Saint John's wort, as an inducer of the CYP4503A4 isoform, diminishes the severity of Willis-Ekbom's disease symptoms by increasing the metabolism of thyroid hormone in treated patients.

METHODS

In an open-label pilot trial, we treated 21 Willis-Ekbom's disease patients with a concentrated extract of Saint John's wort at a daily dose of 300 mg over the course of three months.

RESULTS

Saint John's wort reduced the severity of Willis-Ekbom's disease symptoms in 17 of the 21 patients.

CONCLUSION

Results of this trial suggest that Saint John's wort may benefit some Willis-Ekbom's disease patients. However, as this trial was not placebo-controlled, the extent to which Saint John's wort is effective as a Willis-Ekbom's disease treatment will depend on future, blinded placebo-controlled studies.

Transient Willis-Ekbom's disease (restless legs syndrome) during pregnancy may be caused by estradiol-mediated dopamine overmodulation

Willis-Ekbom's disease (WED), formerly called restless legs syndrome, is more common in pregnant than in non-pregnant women, implying that the physiological and biochemical changes during pregnancy influence its development. During pregnancy, many hormone levels undergo significant changes, and some hormones significantly increase in activity and can interfere with other hormones. For example, the steroid hormone estradiol interferes with the neuroendocrine hormone dopamine. During pregnancy, the activity of the thyroid axis is enhanced to meet the increased demand for thyroid hormones during this state. Dopamine is a neuroendocrine hormone that diminishes the levels of thyrotropin and consequently of thyroxine, and one of the roles of the dopaminergic system is to counteract the activity of thyroid hormones. When the activity of dopamine is not sufficient to modulate thyroid hormones, WED may occur. Robust evidence in the medical literature suggests that an imbalance between thyroid hormones and the dopaminergic system underpins WED pathophysiology. In this article, we present evidence that this imbalance may also mediate transient WED during pregnancy. It is possible that the main hormonal alteration responsible for transient WED of pregnancy is the excessive modulation of dopamine release in the pituitary stalk by estradiol. The reduced quantities of dopamine then cause decreased modulation of thyrotropin, leading to enhanced thyroid axis activity and subsequent WED symptoms. Iron deficiency may also be a predisposing factor for WED during pregnancy, as it can both diminish dopamine and increase thyroid hormone.

Restless legs syndrome in subjects with a knee prosthesis: evidence that symptoms are generated in the periphery

OBJECTIVE

There are no data adressing the prevalence of restless legs syndrome in subjects who have knee prosthesis. Therefore, we conducted a cross-sectional survey of subjects who underwent knee prosthesis surgery.

METHOD

A total of 107 subjects (30 male, 77 female) were interviewed over the telephone regarding restless legs syndrome symptoms. If the patients exhibited symptoms of the syndrome, we conducted face-to-face interviews. Lastly, a therapeutic test with pramipexole was proposed for each subject.

RESULTS

In our cohort, 7 males (23%) and 30 females (39%) had restless legs syndrome. Of these, 6 males and 23 females were submitted to face-to-face-interview. Of the males, 5 (83%) had restless legs after the knee surgery-exclusively in the operated leg- and reported no family restless legs history. One man had a prior case of bilateral restless legs syndrome, a positive family history and claimed exacerbation of symptoms in the operated leg. Among the females, 16 (69%) had restless legs prior to surgery. A total of 10 female patients reported bilateral symptoms, with fewer symptoms in the operated leg, while 6 displayed a worse outcome in the operated leg. The 7 females (31%) without restless legs prior to surgery and without a family history experienced symptoms only in the operated leg. All subjects responded favorably to the pramipexole therapeutic test.

CONCLUSION

Our results suggest that secondary unilateral restless legs syndrome may ensue from knee prosthesis surgery and that the symptoms are generated in the peripheral nervous system.