Post-conditioning experience with acute or chronic inflammatory pain reduces contextual fear conditioning in the rat

The effects of pain following disbudding on calf memory

Studies have found evidence of pain in the hours following hot-iron disbudding, but little is known about longer-lasting pain following this procedure. Work on humans and rats has shown that lasting pain can have negative effects on the formation and recall of memories. The objective of this study was to assess whether lasting pain following disbudding affects learning and memory in calves. A modified hole-board apparatus was used to assess how quickly calves were able to learn the locations of 4 bottles containing milk dispersed among 11 locations with empty bottles. At 14 d of age and after 6 d of training on this task, calves (n = 30) were randomly assigned to 3 treatments: disbudding with analgesic on the day of the procedure, disbudding with analgesic throughout the study, and sham disbudding. All calves were sedated, given a lidocaine cornual local block and a single injection of an nonsteroidal anti-inflammatory drugs. Starting on the day after their disbudding treatment, calves were tested daily using the modified hole-board apparatus. After 12 d of testing, the locations of the 4 milk-containing bottles were switched, and calves then relearned the locations of the rewarded bottles over the next 6 daily test sessions. We found general working memory (i.e., short-term memory) and reference memory (i.e., long-term memory) increased over the 12 d of testing, declined when locations were switched on d 13, and then again increased over the final 6 d of testing. We did not find an effect of treatment on any measure, perhaps because there was no lasting pain or because effects were too minor to detect using this test of spatial memory.

Cognition and Pain: A Review

Cognition is defined as the brain's ability to acquire, process, store, and retrieve information. Pain has been described as an unpleasant sensory or emotional experience, and for experiencing pain consciously, cognitive processing becomes imperative. Moreover, evaluation of pain strongly depends on cognition as it requires learning and recall of previous experiences. There could be a possible close link between neural systems involved in cognition and pain processing, and studies have reported an association between pain and cognitive impairment. In this narrative review, we explore the available evidence that has investigated cognitive changes associated with pain. We also examine the anatomical, biochemical, and molecular association of pain and neuro-cognition. Additionally, we focus on the cognitive impairment caused by analgesic medications. There is a need to improve our understanding of pathophysiology and cognitive impairment mechanisms associated with chronic pain and its treatment. This area provides a diverse opportunity for grounding future research, aiding institution of timely interventions to prevent chronic pain and associated cognitive decline, ultimately improving patient care.

Formalin-induced pain prolongs sub- to supra-second time estimation in rats

Background

Temporal estimation can be influenced by pain, which is a complex psychological and physiological phenomenon. However, the time range in which perception is most sensitive to pain remains unclear.

Methods

In the present study, we explored the effects of acute inflammatory pain on time perception in the sub- to supra-second (0.6–2.4-s) and supra-second (2–8-s) ranges in rats. Plantar formalin injection was used to induce acute inflammatory pain, and a temporal bisection task was used to measure time perception. Task test sessions were held for five consecutive days (one per day): the day before injection (baseline), immediately after injection, and the three post-injection days. The point of subjective equality (PSE, which reflects the subjective duration) and Weber fraction (which reflects temporal sensitivity) were calculated and analysed.

Results

In the 0.6–2.4-s range, the PSE was significantly lower, indicating prolonged subjective duration, in the formalin group relative to the saline group (p = 0.049) immediately after injection. Formalin-induced pain also tended to lengthened time perception in the 0.6–2.4-s range on post-injection days 2 (p = 0.06) and 3 (p = 0.054). In the 2–8-s range, formalin injection did not affect the PSE or Weber fraction.

Conclusions

The enhanced effect of pain on temporal perception in the sub- to supra-second range is observed in this study and this effect is attenuated with the prolongation of estimated time, even in rats.

2-Bromopalmitate attenuates inflammatory pain by maintaining mitochondrial fission/fusion balance and function

Inflammatory pain activates astrocytes and increases inflammatory cytokine release in the spinal cord. Mitochondrial fusion and fission rely on the functions of dynamin-related protein 1 (Drp1) and optic atrophy 1 (OPA1), which are essential for the synaptic transmission and plasticity. In the present study, we aimed to explore the effects of 2-bromopalmitate (2-BP), an inhibitor of protein palmitoylation, on the modulation of pain behavior. Rats were intraplantar injected with complete Freund's adjuvant (CFA) to establish an inflammatory pain model. In the spinal cord of rats with CFA-induced inflammatory pain, the expression of astrocyte-specific glial fibrillary acidic protein (GFAP) and contents of proinflammatory cytokines IL-1β and TNF-α were increased. Mitochondrial Drp1 was increased, while OPA1 was decreased. Consequently, CFA induced reactive oxygen species (ROS) production and Bcl-2-associated X protein (BAX) expression. The intrathecal administration of 2-BP significantly reversed the pain behaviors of the inflammatory pain in rats. Moreover, 2-BP also reduced the Drp1 expression, elevated the OPA1 expression, and further reduced the GFAP, IL-1β, and TNF-α expression and ROS production. Furthermore, in vitro study proved a similar effect of 2-BP on the regulation of Drp1 and OPA1 expression. 2-BP also increased the mitochondrial membrane potential and decreased the levels of BAX, ROS, and proinflammatory cytokines. These results indicate that 2-BP may attenuate the inflammatory pain of CFA-treated rats via regulating mitochondrial fission/fusion balance and function.

Persistent pain intensifies recall of consolidated fear memories

Ensembles of principal neurons in the basolateral amygdala (BLA) generate the initial engrams for fear memories, while projections from the BLA to the medial prefrontal cortex (mPFC) are essential for the encoding, transfer and storage of remote fear memories. We tested the effects of chronic pain on remote fear memories in mice.

Male mice underwent classic fear conditioning by pairing a single tone (conditional stimulus, CS) with a single electric foot shock (unconditional stimulus, US). Sciatic nerve constriction was used to induce neuropathic pain at various time points before or after the fear conditioning. The mice with sciatic nerve cuffs implanted 48 h after the fear conditioning showed an increased freezing response to CS when compared to mice without cuffs or when compared to mice in which the nerve cuffing was performed 48 h before the fear conditioning. The enhancing effect of pain on consolidated fear memory was further tested and mice in which the nerve cuffing was performed 14 days after the fear conditioning also showed an increased fear response when tested 56 days later.

We used immunostaining to detect morphological changes in the BLA that could suggest a mechanism for the observed increase in fear response. We found an increased number of calbindin/parvalbumin positive neurons in the BLA and increased perisomatic density of GAD65 on projection neurons that connect BLA to mPFC in mice with nerve cuffs. Despite the strong increase of c-Fos expression in BLA and mPFC that was induced by fear recall, neither the BLA to mPFC nor the mPFC to BLA projection neurons were activated in mice with nerve cuffs. Furthermore, non-injured mice had an increased fear response when BLA to mPFC projections were inhibited by a chemogenetic method.

In conclusion, this study provides evidence that persistent pain has a significant impact on consolidated fear memories. Very likely the underlying mechanism for this phenomenon is increased inhibitory input onto the BLA to mPFC projection neurons, possibly from neurons with induced parvalbumin expression. Conceivably, the increased fear response to consolidated fear memory is a harbinger for the later development of anxiety and depression symptoms associated with chronic pain.

Neonatal pain and stress disrupts later-life pavlovian fear conditioning and sensory function in rats: Evidence for a two-hit model

Early life trauma has been linked to increased risks for anxiety, depression, and chronic pain. We used rodent models of acute and inflammatory neonatal pain to explore effects on fear conditioning and somatosensory function. Hindpaw needle pricks or handling on postnatal days (PNDs) 1-7 caused lasting impacts on affective and somatosensory function when assessed at later ages, PNDs 24 (postweaning), 45 (adolescence), or 66 (adulthood). First, auditory, but not contextual, freezing was mildly disrupted regardless of age. Second, a profound postfear conditioning tactile hypersensitivity was observed in neonatally stressed, postweaning rats. In the absence of fear conditioning, the mechanical hypersensitivity was not observed, consistent with a two-hit model of psychopathology. Injections of 2% α-carrageenan did not have the same lasting impact but was slightly protective against observed effects of neonatal vehicle injections. Basal and elicited corticosterone levels postweaning were not altered by neonatal pain or handling. These data demonstrate that neonatal adversity can have lasting impacts on affective and somatosensory function that differs regardless of age.

Microinjection of 2-arachidonoyl glycerol into the rat ventral hippocampus differentially modulates contextually induced fear, depending on a persistent pain state

The endogenous cannabinoid (endocannabinoid) system plays a key role in the modulation of aversive and nociceptive behaviour. The components of the endocannabinoid system are expressed throughout the hippocampus, a brain region implicated in both conditioned fear and pain. In light of evidence that pain can impact on the expression of fear-related behaviour, and vice versa, we hypothesised that exogenous administration of the endocannabinoid 2-arachidonoyl glycerol (2-AG) into the ventral hippocampus (vHip) would differentially regulate fear responding in the absence vs. the presence of formalin-evoked nociceptive tone. Fear-conditioned rats showed significantly increased freezing and a reduction in formalin-evoked nociceptive behaviour upon re-exposure to a context previously paired with footshock. Bilateral microinjection of 2-AG into the vHip significantly reduced contextually induced freezing in non-formalin-treated rats, and reduced formalin-evoked nociceptive behaviour in non-fear-conditioned rats. In contrast, 2-AG microinjection had no effect on fear responding in formalin-treated rats, and no effect on nociceptive behaviour in fear-conditioned rats. The inhibitory effect of 2-AG on fear-related behaviour, but not pain-related behaviour, was blocked by co-administration of the cannabinoid receptor 1 (CB1) antagonist/inverse agonist rimonabant. Tissue levels of the endocannabinoids N-arachidonoylethanolamide (anandamide, AEA) and 2-AG were similar in the vHip of fear-conditioned rats receiving formalin injection and the vHip of fear-conditioned rats receiving saline injection. However, the levels of AEA and 2-AG were significantly lower in the contralateral ventrolateral periaqueductal grey of formalin-treated fear-conditioned rats than in that of their saline-treated counterparts. These data suggest that 2-AG-CB1 receptor signalling in the vHip has an anti-aversive effect, and that this effect is abolished in the presence of a persistent pain state.

Chronic pain: the role of learning and brain plasticity

Based on theoretical considerations and recent observations, we argue that continued suffering of chronic pain is critically dependent on the state of motivational and emotional mesolimbic-prefrontal circuitry of the brain. The plastic changes that occur within this circuitry in relation to nociceptive inputs dictate the transition to chronic pain, rendering the pain less somatic and more affective in nature. This theoretical construct is a strong departure from the traditional scientific view of pain, which has focused on encoding and representation of nociceptive signals. We argue that the definition of chronic pain can be recast, within the associative learning and valuation concept, as an inability to extinguish the associated memory trace, implying that supraspinal/cortical manipulations may be a more fruitful venue for adequately modulating suffering and related behavior for chronic pain. We briefly review the evidence generated to date for the proposed model and emphasize that the details of underlying mechanisms remain to be expounded.

Suppression of voluntary wheel running in rats is dependent on the site of inflammation: evidence for voluntary running as a measure of hind paw-evoked pain

Decreased voluntary wheel running has recently been proposed as a preclinical pain measure for inflammatory pain, but whether this reflects pain evoked by use of the affected limbs is unknown. To assess the role of inflammation site as a determinant of this measure, complete Freund's adjuvant (CFA), formalin, or equivolume vehicle was subcutaneously injected into the plantar surface of the hind paws (bilateral) or L1 dorsum dermatome (leaving paws unaffected) of male Sprague Dawley rats. CFA-induced hind paw mechanical allodynia (P < .001) did not correlate with reduced voluntary wheel running. Intraplantar formalin did not attenuate voluntary running, despite eliciting robust licking/writhing/flinching behavior and hind paw mechanical allodynia (P < .001). Subcutaneous L1 dorsum dermatome formalin, but not CFA, induced licking/writhing/flinching behavior (P < .001), but neither induced hind paw mechanical allodynia or attenuated voluntary running. That voluntary running is decreased by hind paw CFA, but not by L1 dorsum CFA, implies that the behavior is a measure of CFA-induced pain evoked by use of the affected limbs rather than supraspinal pain processing that is independent of inflammation site. Furthermore, the results suggest that interpretation of voluntary wheel running data cannot simply be explained by correlation with mechanical allodynia.

PERSPECTIVE

Whether decreased voluntary running is dependent on inflammation site is unknown. We show that intraplantar, but not L1 dorsum, CFA suppressed voluntary running and formalin-induced licking/writhing/flinching behavior but had no effect on voluntary running. These data suggest that suppressed voluntary running by CFA likely reflects pain evoked by use of the affected limbs.

Current challenges in translational pain research

The current gap between basic science research and the development of new analgesics presents a serious challenge for the future of pain medicine. This challenge is particularly difficult in the search for better treatment for comorbid chronic pain conditions because: (i) animal 'pain' models do not simulate multidimensional clinical pain conditions; (ii) animal behavioral testing does not assess subjective pain experience; (iii) preclinical data provide little assurance regarding the direction of new analgesic development; and (iv) clinical trials routinely use over-sanitized study populations and fail to capture the multidisciplinary consequences of comorbid chronic pain. Therefore, a paradigm shift in translational pain research is necessary to transform the current strategy from focusing on molecular switches of nociception to studying pain as a system-based integral response that includes psychosocial comorbidities. Several key issues of translational pain research are discussed in this review.