Sex, drugs and pain control

A study finds that pain hypersensitivity in male and female mice is differentially dependent on microglia and T cells, and describes a sex-specific response to microglia-targeted pain treatments

Controlling litter effects to enhance rigor and reproducibility with rodent models of neurodevelopmental disorders

Research with rodents is crucial for expanding our understanding of genetic and environmental risk factors for neurodevelopmental disorders (NDD). However, there is growing concern about the number of animal studies that are difficult to replicate, potentially undermining the validity of results. These concerns have prompted funding agencies and academic journals to implement more rigorous standards in an effort to increase reproducibility in research. However, these standards fail to address a major source of variability in rodent research brought on by the “litter effect,” the fact that rodents from the same litter are phenotypically more similar to one other than rodents from different litters of the same strain. We show that the litter effect accounts for 30–60% of the variability associated with commonly studied phenotypes, including brain, placenta, and body weight. Moreover, we show how failure to control for litter-to-litter variation can mask a phenotype in Chd8V986*/+ mice that model haploinsufficiency of CHD8, a high-confidence autism gene. Thus, if not properly controlled, the litter effect has the potential to negatively influence rigor and reproducibility of NDD research. While efforts have been made to educate scientists on the importance of controlling for litter effects in previous publications, our analysis of the recent literature (2015–2020) shows that the vast majority of NDD studies focused on genetic risks, including mutant mouse studies, and environmental risks, such as air pollution and valproic acid exposure, do not correct for litter effects or report information on the number of litters used. We outline best practices to help scientists minimize the impact of litter-to-litter variability and to enhance rigor and reproducibility in future NDD studies using rodent models

Enhanced behavioral responses to cold stimuli following CGRPα sensory neuron ablation are dependent on TRPM8

BACKGROUND: Calcitonin gene-related peptide-α (CGRPα) is a classic marker of peptidergic nociceptive neurons and is expressed in myelinated and unmyelinated dorsal root ganglia (DRG) neurons. Recently, we found that ablation of Cgrpα-expressing sensory neurons reduced noxious heat sensitivity and enhanced sensitivity to cold stimuli in mice. These studies suggested that the enhanced cold responses were due to disinhibition of spinal neurons that receive inputs from cold-sensing/TRPM8 primary afferents; although a direct role for TRPM8 was not examined at the time.RESULTS: Here, we ablated Cgrpα-expressing sensory neurons in mice lacking functional TRPM8 and evaluated sensory responses to noxious heat, cold temperatures, and cold mimetics (acetone evaporative cooling and icilin). We also evaluated thermoregulation in these mice following an evaporative cold challenge. We found that ablation of Cgrpα-expressing sensory neurons in a Trpm8-/- background reduced sensitivity to noxious heat but did not enhance sensitivity to cold stimuli. Thermoregulation following the evaporative cold challenge was not affected by deletion of Trpm8 in control or Cgrpα-expressing sensory neuron-ablated mice.CONCLUSIONS: Our data indicate that the enhanced behavioral responses to cold stimuli in CGRPα sensory neuron-ablated mice are dependent on functional TRPM8, whereas the other sensory and thermoregulatory phenotypes caused by CGRPα sensory neuron ablation are independent of TRPM8

Optimized filter set and viewing conditions for the S65T mutant of GFP in living cells

Investigators are increasingly taking advantage of the autonomous fluorescence from the green fluorescent protein (GFP) to detect the cellular and subcellular location of GFP-fusion proteins within living cells (3,8–10,13,14). Work with these fusion constructs has shed new light on the kinetics underlying fundamental biological processes such as mitosis (9). In an attempt to study how a novel rat brain kinesin-related motor protein (rbKRP1) functions within living cells, we have transfected BHK and PC12 cells with a cDNA encoding the serine 65 ® threonine 65 (S65T) GFP mutant (6) fused to the amino terminus of rbKRP1 (S65T GFP–rbKRP1). We use the S65T GFP mutant since it emits four times as much light as the wild-type GFP (6), and it can enhance our ability to detect localized kinesin motor molecules

A deep neural network to assess spontaneous pain from mouse facial expressions

Grimace scales quantify characteristic facial expressions associated with spontaneous pain in rodents and other mammals. However, these scales have not been widely adopted largely because of the time and effort required for highly trained humans to manually score the images. Convoluted neural networks were recently developed that distinguish individual humans and objects in images. Here, we trained one of these networks, the InceptionV3 convolutional neural net, with a large set of human-scored mouse images. Output consists of a binary pain/no-pain assessment and a confidence score. Our automated Mouse Grimace Scale integrates these two outputs and is highly accurate (94%) at assessing the presence of pain in mice across different experimental assays. In addition, we used a novel set of “pain” and “no pain” images to show that automated Mouse Grimace Scale scores are highly correlated with human scores (Pearson’s r = 0.75). Moreover, the automated Mouse Grimace Scale classified a greater proportion of images as “pain” following laparotomy surgery when compared to animals receiving a sham surgery or a post-surgical analgesic. Together, these findings suggest that the automated Mouse Grimace Scale can eliminate the need for tedious human scoring of images and provide an objective and rapid way to quantify spontaneous pain and pain relief in mice

The Troubled Touch of Autism

A study finds that deficits in touch-sensing somatosensory neurons contribute to social interaction and anxiety phenotypes in mouse models of autism and Rett syndrome. These findings suggest that some core symptoms of autism might originate from aberrant development or function of the peripheral nervous system. A study finds that deficits in touch-sensing somatosensory neurons contribute to social interaction and anxiety phenotypes in mouse models of autism and Rett syndrome. These findings suggest that some core symptoms of autism might originate from aberrant development or function of the peripheral nervous system

Gene length matters in neurons

A recent study by Gabel etal. (2015) found that Mecp2, the gene mutated in Rett syndrome, represses long (>100 kb) genes associated with neuronal physiology and connectivity by binding to methylated CA sites in DNA. This study adds to a growing body of literature implicating gene length and transcriptional mechanisms in neurodevelopmental and neurodegenerative disorders. A recent study by Gabel etal. (2015) found that Mecp2, the gene mutated in Rett syndrome, represses long (> 100 kb) genes associated with neuronal physiology and connectivity by binding to methylated CA sites in DNA. This study adds to a growing body of literature implicating gene length and transcriptional mechanisms in neurodevelopmental and neurodegenerative disorders

CGRPα-expressing sensory neurons respond to stimuli that evoke sensations of pain and itch

Calcitonin gene-related peptide (CGRPα, encoded by Calca) is a classic marker of nociceptive dorsal root ganglia (DRG) neurons. Despite years of research, it is unclear what stimuli these neurons detect in vitro or in vivo. To facilitate functional studies of these neurons, we genetically targeted an axonal tracer (farnesylated enhanced green fluorescent protein; GFP) and a LoxP-stopped cell ablation construct (human diphtheria toxin receptor; DTR) to the Calca locus. In culture, 10-50% (depending on ligand) of all CGRPα-GFP-positive (+) neurons responded to capsaicin, mustard oil, menthol, acidic pH, ATP, and pruritogens (histamine and chloroquine), suggesting a role for peptidergic neurons in detecting noxious stimuli and itch. In contrast, few (2.2±1.3%) CGRPα-GFP+ neurons responded to the TRPM8-selective cooling agent icilin. In adult mice, CGRPα-GFP+ cell bodies were located in the DRG, spinal cord (motor neurons and dorsal horn neurons), brain and thyroid-reproducibly marking all cell types known to express Calca. Half of all CGRPα-GFP+ DRG neurons expressed TRPV1, ~25% expressed neurofilament-200, <10% contained nonpeptidergic markers (IB4 and Prostatic acid phosphatase) and almost none (<1%) expressed TRPM8. CGRPα-GFP+ neurons innervated the dorsal spinal cord and innervated cutaneous and visceral tissues. This included nerve endings in the epidermis and on guard hairs. Our study provides direct evidence that CGRPα+ DRG neurons respond to agonists that evoke pain and itch and constitute a sensory circuit that is largely distinct from nonpeptidergic circuits and TRPM8+/cool temperature circuits. In future studies, it should be possible to conditionally ablate CGRPα-expressing neurons to evaluate sensory and non-sensory functions for these neurons

Improving pain assessment in mice and rats with advanced videography and computational approaches

Accurately measuring pain in humans and rodents is essential to unravel the neurobiology of pain and discover effective pain therapeutics. However, given its inherently subjective nature, pain is nearly impossible to objectively assess. In the clinic, patients can articulate their pain experience using questionnaires and pain scales but self-reporting can be unreliable due to various psychological and social influences or difficulties for some patients to verbalize their experience (eg, infants, toddlers, and those with neurodevelopmental disorders). At the bench, these challenges are even more daunting as researchers rely on the behaviors of rodents to measure pain or pain relief. Given this, there is a growing realization among pain researchers, clinicians, and funding entities that these traditional approaches of assessing pain in rodents may be flawed. Importantly, these flaws may have contributed to several failed drugs that initially showed promise as analgesics and point toward inconsistencies in our understanding of basic pain neurobiology

Enhanced histamine-induced itch in diacylglycerol kinase iota knockout mice

Subsets of small-diameter dorsal root ganglia (DRG) neurons detect pruritogenic (itch-causing) and algogenic (pain-causing) stimuli and can be activated or sensitized by chemical mediators. Many of these chemical mediators activate receptors that are coupled to lipid hydrolysis and diacylglycerol (DAG) production. Diacylglycerol kinase iota (DGKI) can phos-phorylate DAG and is expressed at high levels in small-diameter mouse DRG neurons. Given the importance of these neurons in sensing pruritogenic and algogenic chemicals, we sought to determine if loss of DGKI impaired responses to itch- or pain-producing stimuli. Using male and female Dgki-knockout mice, we found that in vivo sensitivity to histamine—but not other pruritogens—was enhanced. In contrast, baseline pain sensitivity and pain sensitization following inflammatory or neuropathic injury were equivalent between wild type and Dgki-/- mice. In vitro calcium responses in DRG neurons to histamine was enhanced, while responses to algogenic ligands were unaffected by Dgki deletion. These data suggest Dgki regulates sensory neuron and behavioral responses to histamine, without affecting responses to other pruritogenic or algogenic agents