Contextual control of conditioned pain tolerance and endogenous analgesic systems

A neural circuit for sex-dependent conditioned pain hypersensitivity in mice

The neural mechanisms underlying sex-specific pain, in which males and females exhibit distinct responses to pain, remain poorly understood. Here we show that in a mouse model of male-specific pain hypersensitivity response to pain conditioning environments (contextual pain hypersensitivity model), elevated free-testosterone leads to hyperactivity of glutamatergic neurons in the medial preoptic area (GlumPOA) through activation of androgen receptor signaling, which in turn induces contextual pain hypersensitivity in male mice. Although not observed in naïve female mice, this pain phenotype could be induced in females via chronic administration of testosterone propionate. In addition, GlumPOA neurons send excitatory inputs to GABAergic neurons in the ventrolateral periaqueductal gray (GABAvlPAG) that are required for contextual pain hypersensitivity. Our study thus demonstrates that testosterone/androgen receptor signaling enhances GlumPOA → GABAvlPAG pathway activity, which drives a male-specific contextual pain hypersensitivity, providing insight into the basis of sexually dimorphic pain response.

Direct comparisons of neural activity during placebo analgesia and nocebo hyperalgesia between humans and rats

Placebo analgesia and nocebo hyperalgesia can profoundly alter pain perception, offering critical implications for pain management. While animal models are increasingly used to explore the underlying mechanisms of these phenomena, it remains unclear whether animals experience placebo and nocebo effects in a manner comparable to humans or whether the associated neurobiological pathways are conserved across species. In this study, we introduce a novel framework for comparing brain activity between humans and rodents during placebo analgesia and nocebo hyperalgesia. Using c-Fos immunohistochemistry in rats and fMRI in humans, we examined neural activity in 70 pain-related brain regions, identifying both conserved and species-specific connectivity changes. Functional connectivity analysis, refined by pruning connections based on known anatomical pathways, revealed significant overlap in key regions, including the amygdala, anterior cingulate cortex, and nucleus accumbens, highlighting conserved circuits driving placebo and nocebo responses. This cross-species methodology offers a powerful new approach for investigating the neurobiology of pain modulation, bridging the gap between animal models and human studies. Identifying these common connections validates the use of animal models and enables preclinical researchers to focus on circuits that are conserved across species, ensuring greater translational relevance when developing new and effective treatments for pain conditions.

Behaviorally conditioned effects of psychoactive drugs in experimental animals: What we have learned from nearly a century of research and what remains to be learned

Continuous treatment with drugs is a crucial requirement for managing various clinical conditions, including chronic pain and neuropsychiatric disorders such as depression or schizophrenia. Associative learning processes, i.e. Pavlovian conditioning, can play an important role for the effects of drugs and could open new avenues for optimizing patient treatment. In this narrative literature review, we summarize available data in experimental animals regarding the behaviorally conditioned effects of psychostimulants such as d-amphetamine and cocaine, the dopamine receptor agonist apomorphine, the dopamine receptor antagonist haloperidol, morphine and antidepressant drugs. In each section, the drug under discussion is briefly introduced, followed by a detailed examination of conditioning features, including doses and dosing regimens, characteristics of the conditioning process such as test environments or specific conditioned stimuli, testing and conditioned response characteristics, possible extinction or reconditioning or reversal training, neural mechanisms, and finally, the potential clinical relevance of the research area related to the drug. We focus on key outcomes, delve into methodical issues, identify gaps in current knowledge, and suggest future research directions.

A developmental framework for understanding the influence of sex and gender on health: Pediatric pain as an exemplar

Sex differences are a robust finding in many areas of adult health, including cardiovascular disease, psychiatric disorders, and chronic pain. However, many sex differences are not consistently observed until after the onset of puberty. This has led to the hypothesis that hormones are primary contributors to sex differences in health outcomes, largely ignoring the relative contributions of early developmental influences, emerging psychosocial factors, gender, and the interaction between these variables. In this paper, we argue that a comprehensive understanding of sex and gender contributions to health outcomes should start as early as conception and take an iterative biopsychosocial-developmental perspective that considers intersecting social positions. We present a conceptual framework, informed by a review of the literature in basic, clinical, and social science that captures how critical developmental stages for both sex and gender can affect children's health and longer-term outcomes. The literature on pediatric chronic pain is used as a worked example of how the framework can be applied to understanding different chronic conditions.

IUPHAR review: Navigating the role of preclinical models in pain research

Chronic pain is a complex and challenging medical condition that affects millions of people worldwide. Understanding the underlying mechanisms of chronic pain is a key goal of preclinical pain research so that more effective treatment strategies can be developed. In this review, we explore nociception, pain, and the multifaceted factors that lead to chronic pain by focusing on preclinical models. We provide a detailed look into inflammatory and neuropathic pain models and discuss the most used animal models for studying the mechanisms behind these conditions. Additionally, we emphasize the vital role of these preclinical models in developing new pain-relief drugs, focusing on biologics and the therapeutic potential of NMDA and cannabinoid receptor antagonists. We also discuss the challenges of TRPV1 modulation for pain treatment, the clinical failures of neurokinin (NK)- 1 receptor antagonists, and the partial success story of Ziconotide to provide valuable lessons for preclinical pain models. Finally, we highlight the overall success and limitations of current treatments for chronic pain while providing critical insights into the development of more effective therapies to alleviate the burden of chronic pain.

Learning pain in context: Response-conditioned placebo analgesia and nocebo hyperalgesia in male rats with chronic neuropathic pain

BACKGROUND

Animal models of placebo analgesia and nocebo hyperalgesia have great potential to assist in the development of novel treatments for chronic pain that exploit or inhibit these phenomena. This study sought to elicit both conditioned placebo analgesia and conditioned nocebo hyperalgesia in rats with chronic neuropathic pain using non-pharmacological, contextual conditioning approaches, similar to those most often used in humans.

METHODS

Sciatic nerve-injured male Sprague-Dawley rats (n = 80), and sham controls (n = 16), underwent a conditioning procedure in which three different thermal stimulus intensities (4 °C, 20 °C or 30 °C) were paired with contextual cues. Injured hind paw withdrawal behaviours were used to determine pain sensitivity, and either conditioned analgesia or conditioned hyperalgesia was evoked by re-exposing the rats to the same context with either an increased or decreased thermal stimulus, respectively.

RESULTS

Stronger conditioned analgesia and conditioned hyperalgesia were seen when rats were conditioned in a more complex environment, highlighting the importance of context in these processes. Rats that did not undergo conditioning procedures showed fewer hind paw withdrawals, indicating a learned component to these pain behaviours.

CONCLUSIONS

Our data call attention to context and learning as two critical factors in the development of placebo and nocebo effects in male rodents with a neuropathic injury. Additionally, the response-conditioning model we present in this study affords better comparisons between human and animal studies, in particular for those seeking to identify commonalities in the neurobiological mechanisms of placebo and nocebo responses.