Ethanol dose-dependently attenuates NMDA-mediated thermal hyperalgesia in the rat

Experimental design and reporting standards for improving the internal validity of pre-clinical studies in the field of pain: Consensus of the IMI-Europain consortium

Background and aims

Pain is a subjective experience, and as such, pre-clinical models of human pain are highly simplified representations of clinical features. These models are nevertheless critical for the delivery of novel analgesics for human pain, providing pharmacodynamic measurements of activity and, where possible, on-target confirmation of that activity. It has, however, been suggested that at least 50% of all pre-clinical data, independent of discipline, cannot be replicated. Additionally, the paucity of “negative” data in the public domain indicates a publication bias, and significantly impacts the interpretation of failed attempts to replicate published findings. Evidence suggests that systematic biases in experimental design and conduct and insufficiencies in reporting play significant roles in poor reproducibility across pre-clinical studies. It then follows that recommendations on how to improve these factors are warranted.

Methods

Members of Europain, a pain research consortium funded by the European Innovative Medicines Initiative (IMI), developed internal recommendations on how to improve the reliability of pre-clinical studies between laboratories. This guidance is focused on two aspects: experimental design and conduct, and study reporting.

Results

Minimum requirements for experimental design and conduct were agreed upon across the dimensions of animal characteristics, sample size calculations, inclusion and exclusion criteria, random allocation to groups, allocation concealment, and blinded assessment of outcome. Building upon the Animals in Research: Reportingin vivo Experiments (ARRIVE) guidelines, reporting standards were developed for pre-clinical studies of pain. These include specific recommendations for reporting on ethical issues, experimental design and conduct, and data analysis and interpretation. Key principles such as sample size calculation, a priori definition of a primary efficacy measure, randomization, allocation concealments, and blinding are discussed. In addition, considerations of how stress and normal rodent physiology impact outcome of analgesic drug studies are considered. Flow diagrams are standard requirements in all clinical trials, and flow diagrams for preclinical trials, which describe number of animals included/excluded, and reasons for exclusion are proposed. Creation of a trial registry for pre-clinical studies focused on drug development in order to estimate possible publication bias is discussed.

Conclusions

More systematic research is needed to analyze how inadequate internal validity and/or experimental bias may impact reproducibility across pre-clinical pain studies. Addressing the potential threats to internal validity and the sources of experimental biases, as well as increasing the transparency in reporting, are likely to improve preclinical research broadly by ensuring relevant progress is made in advancing the knowledge of chronic pain pathophysiology and identifying novel analgesics.

Implications

We are now disseminating these Europain processes for discussion in the wider pain research community. Any benefit from these guidelines will be dependent on acceptance and disciplined implementation across pre-clinical laboratories, funding agencies and journal editors, but it is anticipated that these guidelines will be a first step towards improving scientific rigor across the field of pre-clinical pain research.

Psychosocial aspects of chronic widespread pain and fibromyalgia

PURPOSE

To study the impact on health status as measured by SF-36 in groups of subjects having chronic musculoskeletal pain with different degree of generalization: No chronic pain (NCP), chronic regional pain (CRP), chronic widespread pain (CWP), CWP with a stricter 'Manchester' definition (CWP-M), and clinically defined fibromyalgia (FM). The study also examines the association between psychosocial and lifestyle background variables, and these pain-groups.

METHOD

A cross-sectional study with a postal survey to 3928 subjects, constituting a representative sample of the adult general population, followed by clinical examination in a selected group of subjects with CWP. CWP and FM were diagnosed according to ACR 1990 fibromyalgia criteria. Health status was measured by SF-36 Health Survey.

RESULTS

Patients with CWP, CWP-M, and FM were found to present with more severe impairment of health status than the other two population groups. Several psychosocial factors, such as belonging to a lower socio-economic group, being an immigrant, living in a compromised housing area, having lower educational level, experiencing lower social support and having a family history of chronic pain, were associated with the populations with CWP and FM.

CONCLUSIONS

The spectrum of impact on health and association to background variables, with respect to a stricter definition of CWP, indicates that these factors are important to attend to in the understanding and management of CWP and FM.

Predictors of Incident Chronic Widespread Pain Among Veterans Following the First Gulf War

OBJECTIVE

We sought to determine the predictors of incident chronic widespread pain (CWP), specifically, the effect of preexisting symptoms, stress, and psychosocial factors in the subsequent development of CWP among veterans from the first Gulf War (GW).

METHODS

We conducted a structured telephone survey (baseline) of military personnel originally from Iowa who were either eligible for or deployed to Operation Desert Shield/Desert Storm, approximately 5 years postconflict. We conducted a follow-up, clinical, in-person study of those who met a priori-defined outcomes of symptoms of cognitive dysfunction, depression, or CWP, and also a sample of those who did not meet any of the outcomes of interest.

RESULTS

A total of 370 of 602 evaluated GW veterans were free of CWP 5 years postconflict. At follow-up, 69 (19%) of these had developed CWP. A positive family history of medically unexplained persistent symptoms [odds ratio (OR)=4.8 (2.3, 13.2)] was strongly associated with CWP. At baseline, individuals who reported preexisting symptoms of bronchitis [OR=4.9 (1.9, 12.3)] and cognitive dysfunction [OR=2.1 (1.1, 4.2)] were more likely to develop CWP. Alcohol use [OR=0.2 (0.1, 0.7)] was protective against CWP. Rather than combat-related exposure per se, the perception of stress at the time of the GW [OR=1.6 (1.1, 2.3)] correlated with CWP.

DISCUSSION

Among the GW veterans evaluated longitudinally in this study, family history, predeployment symptoms, and the level of perceived stress during the GW were associated with subsequent development of CWP.

Experiments with nitric oxide synthase inhibitors in spinal nerve ligated rats provide no evidence of a role for nitric oxide in neuropathic mechanical allodynia

We have investigated the effect of treatment with N(omega)-nitro-l-arginine methylester (l-NAME), a non-selective nitric oxide synthase inhibitor (NOS), both before and after the induction of mechanical allodynia by tight ligation of the left L5 and L6 spinal nerves in rats (SNL rats). The degree of mechanical allodynia was measured by tactile threshold for paw flinching with von Frey filaments. Intraperitoneal (i.p.) administration of l-NAME (3-30 mg/kg) 1 week after the spinal nerve ligation produced a dose-dependent reduction of the behavioral signs of mechanical allodynia, but the effect was not reversed by pretreatment with l-arginine (300 mg/kg). N(omega)-Nitro-l-arginine (l-NNA, i.p., 30 mg/kg), aminoguanidine (AG, i.p., 30 mg/kg) and a potent neuronal NOS inhibitor (LY457963, i.p., 30 mg/kg) did not reduce mechanical sensitivity in the SNL rats. Furthermore, using an ex vivo NOS activity assay, l-NAME partially inhibited the spinal NOS activity, whereas LY457963 almost completely inhibited the spinal NOS activity. Prior administration of l-NAME (i.p., 30 mg/kg) or of MK-801 (0.5 mg/kg), an NMDA antagonist, 30 min before the spinal nerve ligation significantly prevented the development of mechanical allodynia after spinal nerve ligation for an extended period of time. High doses of l-arginine (100 mg/kg or 300 mg/kg, i.p.), however, did not reverse the preemptive effect of l-NAME. These results suggest that neither the anti-allodynic nor the preemptive effects of l-NAME are mediated by NOS inhibition.

Characteristics of temporal summation of second pain sensations elicited by brief contact of glabrous skin by a preheated thermode

Temporal summation of sensory intensity was investigated in normal subjects using novel methods of thermal stimulation. A Peltier thermode was heated and then applied in a series of brief (700 ms) contacts to different sites on the glabrous skin of either hand. Repetitive contacts on the thenar or hypothenar eminence, at interstimulus intervals (ISIs) of 3 s, progressively increased the perceived intensity of a thermal sensation that followed each contact at an onset latency > 2 s. Temporal summation of these delayed (late) sensations was proportional to thermode temperature over a range of 45-53 degrees C, progressing from a nonpainful level (warmth) to painful sensations that could be rated as very strong after 10 contacts. Short-latency pain sensations rarely were evoked by such stimuli and never attained levels substantially above pain threshold for the sequences and temperatures presented. Temporal summation produced by brief contacts was greater in rate and amount than increases in sensory intensity resulting from repetitive ramping to the same temperature by a thermode in constant contact with the skin. Variation of the interval between contacts revealed a dependence of sensory intensity on interstimulus interval that is similar to physiological demonstrations of windup, where increasing frequencies of spike train activity are evoked from spinal neurons by repetitive activation of unmyelinated nociceptors. However, substantial summation at repetition rates of > or = 0.33 Hz was observed for temperatures that produced only late sensations of warmth when presented at frequencies < 0.16 Hz. Measurements of subepidermal skin temperature from anesthetized monkeys revealed different time courses for storage and dissipation of heat by the skin than for temporal summation and decay of sensory intensity for the human subjects. For example, negligible heat loss occurred during a 6-s interval between two trials of 10 contacts at 0.33 Hz, but ratings of sensory magnitude decreased from very strong levels of pain to sensations of warmth during the same interval. Evidence that temporal summation of sensory intensity during series of brief contacts relies on central integration, rather than a sensitization of peripheral receptors, was obtained using two approaches. In the first, a moderate degree of temporal summation was observed during alternating stimulation of adjacent but nonoverlapping skin sites at 0.33 Hz. Second, temporal summation was significantly attenuated by prior administration of dextromethorphan, a N-methyl-D-aspartate receptor antagonist.

Ethanol-induced state-dependent learning is mediated by 5-hydroxytryptamine3 receptors but not by N-methyl-D-aspartate receptor complex

Involvement of N-methyl-D-aspartate (NMDA) receptor complex and 5-hydroxytryptamine3 (5-HT3) receptors in state-dependent learning (SDL) induced by ethanol (EtOH) was investigated in the step-through passive avoidance task in rats. Pre-training injections of EtOH or MK-801 reduced step-through latency in the test session conducted 24 h after the training session. Pre-test as well as pre-training injections of EtOH failed to reduce the latency, while pre-training and pre-test injections of MK-801 reduced the latency. These results show that EtOH but not MK-801 produces SDL. SDL induced by EtOH was blocked by ICS205-930 injected before either the training or test session. However, ICS205-930 failed to block SDL induced by diazepam and muscimol. These results suggest that NMDA receptor complex may not be involved in SDL, and that 5-HT3 receptors may contribute to SDL induced by EtOH but not by diazepam and muscimol.

The role of nitric oxide in the development and maintenance of the hyperalgesia produced by intraplantar injection of carrageenan in the rat

Activation of the N-methyl-D-aspartate (NMDA) receptor has been reported to be involved in the mechanisms that underlie thermal hyperalgesia produced by the intraplantar injection of carrageenan. As NMDA-mediated thermal hyperalgesia produced in models of acute and persistent pain have been reported to involve production of nitric oxide, we examined the role of nitric oxide in both the development and maintenance of the thermal hyperalgesia produced by the intraplantar injection of carrageenan. In addition, we examined the role of nitric oxide in the maintenance of the mechanical hyperalgesia produced by intraplantar injection of carrageenan. Prior to the intraplantar injection of carrageenan (2 mg in 100 microliters) there was no significant difference in thermal withdrawal latencies or mechanical withdrawal thresholds between the left and right hindpaws. Three hours after injection of carrageenan into the left hindpaw, rats showed evidence of a significantly faster thermal withdrawal latency and lower mechanical withdrawal threshold of the left hindpaw compared to the right hindpaw. In addition, the left hindpaw was significantly increased in size (diameter) compared with the right hindpaw. In these same rats, the intrathecal administration of saline, NG-nitro-L-arginine methyl ester (L-NAME; 2-200 nmol) or the inactive enantiomer, NG-nitro-D-arginine methyl ester (D-NAME; 200 nmol) did not produce any significant change in thermal nociceptive withdrawal latencies in the non-injected paw. However, administration of L-NAME (2-20 nmol), but not saline or D-NAME produced a dose dependent and reversible block of the thermal hyperalgesia for a period of up to 3 h.(ABSTRACT TRUNCATED AT 250 WORDS)