Normative Preclinical Algesiometry Data on the von Frey and Radiant Heat Paw-Withdrawal Tests: An Analysis of Data from More Than 8,000 Mice Over 20 Years.

The measurement of withdrawal to experimenter-delivered mechanical stimuli (von Frey test) and to heat stimuli (radiant heat paw-withdrawal or Hargreaves' test) applied to the hind paws is ubiquitous in preclinical pain research, but no normative values for the most-common applications of these tests have ever been published. We analyzed a retrospective data set of withdrawal thresholds or latencies in 8,150 mice in which these measures were taken using replicate determinations, before and after injection of inflammatory substances or experimental nerve damage producing pain hypersensitivity, totaling 97,332 measurements. All mice were tested in the same physical laboratory over a 20-year period using similar equipment and procedures. We nonetheless find evidence of large interindividual variability, affected by tester, genotype, mouse sex, tester sex, replicate order, and injury. These factors are discussed, and we believe that these normative data will serve as a useful reference for expected values in preclinical pain testing. PERSPECTIVE: This article presents a retrospective analysis of a large data set of mouse von Frey and radiant heat paw-withdrawal (Hargreaves' test) measurements collected in a single laboratory over 20 years. In addition to serving as a normative guide, sources of variability are identified including genotype, tester, and sex.

The stress regulator FKBP51: a novel and promising druggable target for the treatment of persistent pain states across sexes.

It is well established that FKBP51 regulates the stress system by modulating the sensitivity of the glucocorticoid receptor to stress hormones. Recently, we have demonstrated that FKBP51 also drives long-term inflammatory pain states in male mice by modulating glucocorticoid signalling at spinal cord level. Here, we explored the potential of FKBP51 as a new pharmacological target for the treatment of persistent pain across the sexes. First, we demonstrated that FKBP51 regulates long-term pain states of different aetiologies independently of sex. Deletion of FKBP51 reduced the mechanical hypersensitivity seen in joint inflammatory and neuropathic pain states in female and male mice. Furthermore, FKBP51 deletion also reduced the hypersensitivity seen in a translational model of chemotherapy-induced pain. Interestingly, these 3 pain states were associated with changes in glucocorticoid signalling, as indicated by the increased expression, at spinal cord level, of the glucocorticoid receptor isoform associated with glucocorticoid resistance, GRß, and increased levels of plasma corticosterone. These pain states were also accompanied by an upregulation of interleukin-6 in the spinal cord. Crucially, we were able to pharmacologically reduce the severity of the mechanical hypersensitivity seen in these 3 models of persistent pain with the unique FKBP51 ligand SAFit2. When SAFit2 was combined with a state-of-the-art vesicular phospholipid gel formulation for slow release, a single injection of SAFit2 offered pain relief for at least 7 days. We therefore propose the pharmacological blockade of FKBP51 as a new approach for the treatment of persistent pain across sexes, likely in humans as well as rodents.

Complex regulation of the regulator of synaptic plasticity histone deacetylase 2 in the rodent dorsal horn after peripheral injury.

Histone deacetylases (HDACs), HDAC2 in particular, have been shown to regulate various forms of learning and memory. Since cognitive processes share mechanisms with spinal nociceptive signalling, we decided to investigate the HDAC2 expression in the dorsal horn after peripheral injury. Using immunohistochemistry, we found that spinal HDAC2 was mainly seen in neurons and astrocytes, with neuronal expression in naïve tissue 2.6 times greater than that in astrocytes. Cysteine (S)-nitrosylation of HDAC2 releases HDAC2 gene silencing and is controlled by nitric oxide (NO). A duration of 48 h after intraplantar injection of complete Freund's adjuvant, there was an ipsilateral increase in the most important NO-producing enzyme in pain states, nitric oxide synthase (nNOS), accompanied by an increase in HDAC2 S-nitrosylation. Moreover, a subset of nNOS-positive neurons expressed cFos, a known target of HDAC2, suggesting that derepression of cFos expression following HDAC2 S-nitrosylation might occur after noxious stimulation. We saw no change in global HDAC2 expression in both short- and long-term pain states. However, HDAC2 was increased in astrocytes 7 days after neuropathic injury suggesting that HDAC2 might inhibit astrocytic gene expression in neuropathic pain states. All together, our results indicate that the epigenetic regulation of transcriptional programmes in the dorsal horn after injury is cell specific. Moreover, the prominent role of NO in persistent pain states suggests that HDAC2 S-nitrosylation could play a crucial role in the regulation of gene expression leading to hypersensitivity. Our manuscript describes for the first time the regulation of the memory regulator histone deacetylase 2 (HDAC2) in the superficial dorsal horn of adult rats following peripheral injury. Our cell-specific approach has revealed a complex pattern of expression of spinal HDAC2 that depends on the injury and the cell type, suggesting a sophisticated regulation of gene expression by HDAC2.