Alina Stegemann

A painful episode can lead to a life-long increase in an individual's experience of pain. Fearful anticipation of imminent pain could play a role in this phenomenon, but the neurobiological underpinnings are unclear because fear can both suppress and enhance pain. Here, we show in mice that long-term associative fear memory stored in neuronal engrams in the prefrontal cortex determines whether a painful episode shapes pain experience later in life. Furthermore, under conditions of inflammatory and neuropathic pain, prefrontal fear engrams expand to encompass neurons representing nociception and tactile sensation, leading to pronounced changes in prefrontal connectivity to fear-relevant brain areas. Conversely, silencing prefrontal fear engrams reverses chronically established hyperalgesia and allodynia. These results reveal that a discrete subset of prefrontal cortex neurons can account for the debilitating comorbidity of fear and chronic pain and show that attenuating the fear memory of pain can alleviate chronic pain itself.

Analysis of neural encoding and plasticity processes frequently relies on studying spatial patterns of activity-induced immediate early genes’ expression, such as c-fos. Quantitatively analyzing the numbers of cells expressing the Fos protein or c-fos mRNA is a major challenge owing to large human bias, subjectivity and variability in baseline and activity-induced expression. Here, we describe a novel open-source ImageJ/Fiji tool, called ‘Quanty-cFOS’, with an easy-to-use, streamlined pipeline for the automated or semi-automated counting of cells positive for the Fos protein and/or c-fos mRNA on images derived from tissue sections. The algorithms compute the intensity cutoff for positive cells on a user-specified number of images and apply this on all the images to process. This allows for the overcoming of variations in the data and the deriving of cell counts registered to specific brain areas in a highly time-efficient and reliable manner. We validated the tool using data from brain sections in response to somatosensory stimuli in a user-interactive manner. Here, we demonstrate the application of the tool in a step-by-step manner, with video tutorials, making it easy for novice users to implement. Quanty-cFOS facilitates a rapid, accurate and unbiased spatial mapping of neural activity and can also be easily extended to count other types of labelled cells.

Abstract A painful episode can lead to life-long increase in an individual’s experience of pain; however, mechanisms for such long-lasting modulation are not well understood. Fearful anticipation of imminent pain has been suggested to play a role1, 2, but the neurobiological underpinnings are unclear, since fear can paradoxically both suppress and enhance pain2,3. Here, we show in mice that in contrast to acute or innate fear, chronic fear memory encoded and stored in neuronal engrams in the prefrontal cortex is both necessary and sufficient to define whether a painful episode profoundly shapes the experience of ongoing pain at a later point in life. Furthermore, in conditions of inflammatory and neuropathic pain, prefrontal engrams for fear memory expand to encompass neurons representing nociception and tactile sensation, leading to pronounced changes in prefrontal connectivity to key brain areas. In contrast, silencing prefrontal fear memory engrams reverses chronically established hyperalgesia and allodynia. These results reveal that a discrete subset of prefrontal cortical neurons can account for the debilitating comorbidity of long-term fear and chronic pain and show that attenuating the fear memory of pain can alleviate chronic pain itself.

Background: Post-stroke pain (PSP) affects nearly half of stroke survivors, severely compromising quality of life. The causes of PSP remain underexplored, although there is likely a complex interplay of lesion effects, psychological factors and mobility that play a role in the development of PSP. The aim of the study was to investigate clinical characteristics associated with PSP, as well as structural and functional correlates of PSP using lesion symptom (LSM) and network mapping (LNM). Methods: We analyzed data from the INSPiRE-TMS cohort, encompassing 1,022 minor ischemic stroke patients. Pain severity and psychological factors (EQ5D-3L questionnaire) were assessed annually for up to 3 years. In a sub-group of 391 patients with available imaging data, LSM and LNM analyses were conducted to identify neural correlates of PSP. Results: Overall, 47% of patients reported pain 1-year post-stroke. Multivariable regression analyses identified baseline anxiety as being associated with PSP assessed at 1-year post-stroke (OR 2.90, 95% CI 1.17-7.17, p=0.021). LSM did not identify any voxels associated with new severe pain. LNM identified a network involving anterior cingulate cortex, thalamus and insular cortex. Adjusting for anxiety highlighted distinct network contributions, suggesting interactive effects of psychological states on pain perception. Automated comparison to large metanalytical findings using Neurosynth associated the terms 'pain' and 'nociception' most strongly to the identified network. Conclusion: PSP is closely associated with psychological factors such as anxiety. LNM of PSP revealed disruptions in a pain-related neural network consistent with prior pain research. These results warrant external validation and could guide future network-targeted neuromodulation therapies. Registration: URL: https://www.clinicaltrials.gov ; Unique identifier: NCT01586702 .