Probing pain aversion in rats with the "Heat Escape Threshold" paradigm.

The aversive aspect of pain constitutes a major burden faced by pain patients. This has been recognized by the pain research community, leading to the development of novel methods focusing on affective-motivational behaviour in pain model animals. The most common tests used to assess pain aversion in animals require cognitive processes, such as associative learning, complicating the interpretation of results. To overcome this issue, studies in recent years have utilized unconditioned escape as a measure of aversion. However, the vast majority of these studies quantify jumping - a common escape behaviour in mice, but not in adult rats, thus limiting its use. Here, we present the "Heat Escape Threshold" (HET) paradigm for assessing heat aversion in rats. We demonstrate that this method can robustly and reproducibly detect the localized effects of an inflammatory pain model (intraplantar carrageenan) in male and female Sprague-Dawley rats. In males, a temperature that evoked unconditioned escape following carrageenan treatment also induced real-time place avoidance (RTPA). Systemic morphine more potently alleviated carrageenan-induced heat aversion (as measured by the HET and RTPA methods), as compared to reflexive responses to heat (as measured by the Hargreaves test), supporting previous findings. Next, we examined how blocking of excitatory transmission to the lateral parabrachial nucleus (LPBN), a key node in the ascending pain system, affects pain behaviour. Using the HET and Hargreaves tests, we show that intra-LPBN application of glutamate antagonists reverses the effects of carrageenan on both affective and reflexive pain behaviour, respectively. Finally, we employed the HET paradigm in a generalized opioid-withdrawal pain model. Withdrawal from a brief systemic administration of remifentanil resulted in a long-lasting and robust increase in heat aversion, but no change in reflexive responses to heat. Taken together, these data demonstrate the utility of the HET paradigm as a novel tool in preclinical pain research.

Erenumab and galcanezumab in chronic migraine prevention: effects after treatment termination.

BACKGROUND: Monoclonal antibodies (mAbs) targeting the CGRP pathway are safe and efficacious therapies for the prevention of migraine. In this study we assessed the effects of discontinuation of preventive erenumab and galcanezumab treatment in patients with chronic migraine. METHODS: This retrospective pooled analysis included completers of the open-label extension study phase for the preventive treatment of chronic migraine with galcanezumab (NCT02614261; 9 months) and erenumab (NCT02174861; 12 months) in a single headache center. We compare migraine data until week 12 after open-label treatment completion, when patients did not have any pharmacological preventive medication, to study baseline values of the double-blind trial period, and to the last 4 weeks of the open-label extension. The assessment included changes in monthly migraine days, headache hours, days with severe headache and acute headache medication use. RESULTS: Data from 16 patients after galcanezumab (n = 9) and erenumab (n = 7) open-label treatment completion were analyzed. The mean number of monthly migraine days was 18.38 ± 3.74 at baseline, and 12.19 ± 4.53 in the last 4 weeks of the open-label extension (p < 0.001). Monthly migraine days remained significantly reduced compared to baseline during the entire 12-week observation period after open-label termination (p = 0.002), with a reduction of 5.38 ± 4.92 in weeks 1-4 (p = 0.001), 4.75 ± 4.15 in weeks 5-8 (p = 0.001), and 3.93 ± 5.45 in weeks 9-12 (p = 0.014). There was no significant difference in monthly migraine days between the 12 weeks after open-label termination and the last 4 weeks of the open-label phase (p = 0.228). All other analyses revealed numerical improvement through week 12 in comparison to baseline. CONCLUSIONS: In this small, self-selected cohort, the results indicate a therapeutic effect of monoclonal antibodies targeting the CRGP pathway in chronic migraine prevention after treatment termination up to 12 weeks.

Differential activation of spinal and parabrachial glial cells in a neuropathic pain model.

The clinical burden faced by chronic pain patients is compounded by affective comorbidities, such as depression and anxiety disorders. Emerging evidence suggests that reactive glial cells in the spinal cord dorsal horn play a key role in the chronification of pain, while supraspinal glia are important for psychological aspects of chronic pain. The lateral parabrachial nucleus (LPBN) in the brainstem is a key node in the ascending pain system, and is crucial for the emotional dimension of pain. Yet, whether astrocytes and microglia in the LPBN are activated during chronic pain is unknown. Here, we evaluated the occurrence of glial activation in the LPBN of male Sprague-Dawley rats 1, 4, and 7 weeks after inducing a chronic constriction injury (CCI) of the sciatic nerve, a prevalent neuropathic pain model. CCI animals developed mechanical and thermal hypersensitivity that persisted for at least 4 weeks, and was mostly reversed after 7 weeks. Using immunohistochemical staining and confocal imaging, we found that CCI caused a strong increase in the expression of the astrocytic marker GFAP and the microglial marker Iba1 in the ipsilateral spinal dorsal horn, with peak expression observed 1 week post-injury. Moreover, morphology analysis revealed changes in microglial phenotype, indicative of microglia activation. In contrast, CCI did not induce any detectable changes in either astrocytes or microglia in the LPBN, at any time point. Thus, our results indicate that while neuropathic pain induces a robust glial reaction in the spinal dorsal horn, it fails to activate glial cells in the LPBN.

Opioids Induce Bidirectional Synaptic Plasticity in a Brainstem Pain Center in the Rat.

Opioids are powerful analgesics commonly used in pain management. However, opioids can induce complex neuroadaptations, including synaptic plasticity, that ultimately drive severe side effects, such as pain hypersensitivity and strong aversion during prolonged administration or upon drug withdrawal, even following a single, brief administration. The lateral parabrachial nucleus (LPBN) in the brainstem plays a key role in pain and emotional processing; yet, the effects of opioids on synaptic plasticity in this area remain unexplored. Using patch-clamp recordings in acute brainstem slices from male and female Sprague Dawley rats, we demonstrate a concentration-dependent, bimodal effect of opioids on excitatory synaptic transmission in the LPBN. While a lower concentration of DAMGO (0.5 µM) induced a long-term depression of synaptic strength (low-DAMGO LTD), abrupt termination of a higher concentration (10 µM) induced a long-term potentiation (high-DAMGO LTP) in a subpopulation of cells. LTD involved a metabotropic glutamate receptor (mGluR)-dependent mechanism; in contrast, LTP required astrocytes and N-methyl-D-aspartate receptor (NMDAR) activation. Selective optogenetic activation of spinal and periaqueductal gray matter (PAG) inputs to the LPBN revealed that, while LTD was expressed at all parabrachial synapses tested, LTP was restricted to spino-parabrachial synapses. Thus, we uncovered previously unknown forms of opioid-induced long-term plasticity in the parabrachial nucleus that potentially modulate some adverse effects of opioids. PERSPECTIVE: We found a previously unrecognized site of opioid-induced plasticity in the lateral parabrachial nucleus, a key region for pain and emotional processing. Unraveling opioid-induced adaptations in parabrachial function might facilitate the identification of new therapeutic measures for addressing adverse effects of opioid discontinuation such as hyperalgesia and aversion.