Behavioral pharmacology of pain

Calcium imaging for analgesic drug discovery

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Calcium imaging is an efficient way to dissect the activity of neurons in vivo.

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GCaMP indicators can be expressed in specific cell populations for in vivo imaging.

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Pain research have benefitted greatly from these features in the recent decade.

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Preclinical research is shifting towards the analysis of pain models and mechanisms.

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In vivo calcium imaging is the ideal tool for an efficient drug discovery paradigm.

Somatosensation and pain are complex phenomena involving a rangeofspecialised cell types forming different circuits within the peripheral and central nervous systems. In recent decades, advances in the investigation of these networks, as well as their function in sensation, resulted from the constant evolution of electrophysiology and imaging techniques to allow the observation of cellular activity at the population level both in vitro and in vivo. Genetically encoded indicators of neuronal activity, combined with recent advances in DNA engineering and modern microscopy, offer powerful tools to dissect and visualise the activity of specific neuronal subpopulations with high spatial and temporal resolution. In recent years various groups developed in vivo imaging techniques to image calcium transients in the dorsal root ganglia, the spinal cord and the brain of anesthetised and awake, behaving animals to address fundamental questions in both the physiology and pathophysiology of somatosensation and pain. This approach, besides giving unprecedented details on the circuitry of innocuous and painful sensation, can be a very powerful tool for pharmacological research, from the characterisation of new potential drugs to the discovery of new, druggable targets within specific neuronal subpopulations. Here we summarise recent developments in calcium imaging for pain research, discuss technical challenges and advances, and examine the potential positive impact of this technique in early preclinical phases of the analgesic drug discovery process.

Pain behaviour assessments by gait and weight bearing in surgically induced osteoarthritis and inflammatory arthritis

OBJECTIVE

. Osteoarthritis (OA) is the most common cause of joint pain. Animal models and relevant assays for measurement of pain-related behaviours are important tools for studies of mechanisms inducing and sustaining pain in OA. The aim of this study was to evaluate two different assessments of weight bearing; stationary and during locomotion, and to explore their feasibility to detect analgesic effects in vivo. Two fundamentally different mouse models of joint arthritis were investigated; surgical transection of the anterior cruciate ligament (ACLT) resulting in destabilization of the joint with subsequent structural deterioration resembling OA, and monoarthritis induced by injection of Complete Freund´s Adjuvant (CFA) into the ankle joint capsule.

DESIGN

. Mice were subjected to ACLT or CFA injection into the ankle joint. Stationary weight bearing was performed up to twenty weeks after ACLT, and for two weeks after CFA. In addition, mice with CFA-induced monoarthritis were assessed for gait and weight bearing during locomotion, and the effects of an anti-NGF antibody (MEDI578) were tested. End point histopathological analysis was performed in knee joints of ACLT mice, and in mice with ankle joint injection of CFA at eight days after injection.

RESULTS

. Both the surgical ACLT and CFA-induced monoarthritis reduced stationary weight bearing on the affected paw. The reduction in weight bearing was compensated by all other legs, but differently when stationary compared to during locomotion in the CFA-injected mice. The behavioural effects of ACLT correlated to the structural changes of the joint. In the CFA-induced monoarthritis, showing a massive infiltration of inflammatory cells at 8 days, MEDI578 significantly attenuated the pain-like behaviours.

CONCLUSIONS

. The pain-like behaviour detected is mainly due to inflammation and not to the same degree to structural changes in the joint. Behavioural effects after ACLT were too small for pharmacological evaluation of pain relief. In contrast, the inflammation after CFA injection caused a long-lasting effect on pain-like behaviours such as weight bearing and gait, which could be attenuated by administration of an anti NGF antibody.

Comparison of Burrowing and Stimuli-Evoked Pain Behaviors as End-Points in Rat Models of Inflammatory Pain and Peripheral Neuropathic Pain

Establishment and validation of ethologically-relevant, non-evoked behavioral end-points as surrogate measures of spontaneous pain in rodent pain models has been proposed as a means to improve preclinical to clinical research translation in the pain field. Here, we compared the utility of burrowing behavior with hypersensitivity to applied mechanical stimuli for pain assessment in rat models of chronic inflammatory and peripheral neuropathic pain. Briefly, groups of male Sprague-Dawley rats were habituated to the burrowing environment and trained over a 5-day period. Rats that burrowed ≤ 450 g of gravel on any 2 days of the individual training phase were excluded from the study. The remaining rats received either a unilateral intraplantar injection of Freund's complete adjuvant (FCA) or saline, or underwent unilateral chronic constriction injury (CCI) of the sciatic nerve- or sham-surgery. Baseline burrowing behavior and evoked pain behaviors were assessed prior to model induction, and twice-weekly until study completion on day 14. For FCA- and CCI-rats, but not the corresponding groups of sham-rats, evoked mechanical hypersensitivity developed in a temporal manner in the ipsilateral hindpaws. Although burrowing behavior also decreased in a temporal manner for both FCA-and CCI- rats, there was considerable inter-animal variability. By contrast, mechanical hyperalgesia and mechanical allodynia in the ipsilateral hindpaws of FCA- and CCI-rats respectively, exhibited minimal inter-animal variability. Our data collectively show that burrowing behavior is altered in rodent models of chronic inflammatory pain and peripheral neuropathic pain. However, large group sizes are needed to ensure studies are adequately powered due to considerable inter-animal variability.

Perspectives and Trends in Pharmacological Approaches to the Modulation of Pain

Pharmacological approaches to our understanding and treatment of pain have had a long history and have traditionally relied on very few drugs that either have significant side effects and abuse liability, such as the nonsteroidal anti-inflammatory drugs or the opioids, respectively, or those that have been developed for other conditions such as the tricyclic antidepressants. The pathophysiology of pain is undoubtedly complex, complicated in part by the fact that it is not a singular condition, and has a variety of etiologies and a number of associated comorbidities that make treatment interventions challenging. Moreover, there are changes in the central nervous system during the course of the development of chronic pain that, in a manner parallel to neurodegenerative disorders, likely require different pharmacological approaches in the early stages of acute pain compared to those that would be effective when pain has become chronic. This chapter reviews the current status of the field of pain research focusing on some relatively underdeveloped areas, such as pain and its associated comorbidities, and the use of transgenic animals and drug self-administration procedures in the context of analgesic assessment. This chapter also incorporates more recent developments and emerging trends in the area of epigenetics, biomarkers, and the use of induced pluripotent stem cells for pharmacological evaluation, target identification, and validation. Recent progress in the study of "organs-on-a-chip" will also be included in this overview, setting expectations for future progress that integrates these advances for deeper insights into mechanisms, novel treatments, and facilitated efforts in drug discovery.