Orbitostriatal encoding of reward delayed gratification and impulsivity in chronic pain.

Central robust network functional rearrangement is a characteristic of several neurological conditions, including chronic pain. Preclinical and clinical studies have shown the importance of pain-induced dysfunction in both orbitofrontal cortex (OFC) and nucleus accumbens (NAc) brain regions for the emergence of cognitive deficits. Outcome information processing recruits the orbitostriatal circuitry, a pivotal pathway regarding context-dependent reward value encoding. The current literature reveals the existence of structural and functional changes in the orbitostriatal crosstalk in chronic pain conditions, which have emerged as a possible underlying cause for reward and time discrimination impairments observed in individuals affected by such disturbances. However, more comprehensive investigations are needed to elucidate the underlying disturbances that underpin disease development. In this review article, we aim to provide a comprehensive view of the orbitostriatal mechanisms underlying time-reward dependent behaviors, and integrate previous findings on local and network malplasticity under the framework of the chronic pain sphere.

Dynamics of Lateral Habenula-Ventral Tegmental Area Microcircuit on Pain-Related Cognitive Dysfunctions.

Chronic pain is a health problem that affects the ability to work and perform other activities, and it generally worsens over time. Understanding the complex pain interaction with brain circuits could help predict which patients are at risk of developing central dysfunctions. Increasing evidence from preclinical and clinical studies suggests that aberrant activity of the lateral habenula (LHb) is associated with depressive symptoms characterized by excessive negative focus, leading to high-level cognitive dysfunctions. The primary output region of the LHb is the ventral tegmental area (VTA), through a bidirectional connection. Recently, there has been growing interest in the complex interactions between the LHb and VTA, particularly regarding their crucial roles in behavior regulation and their potential involvement in the pathological impact of chronic pain on cognitive functions. In this review, we briefly discuss the structural and functional roles of the LHb-VTA microcircuit and their impact on cognition and mood disorders in order to support future studies addressing brain plasticity during chronic pain conditions.

Altered Brain Expression of DNA Methylation and Hydroxymethylation Epigenetic Enzymes in a Rat Model of Neuropathic Pain.

The role of epigenetics in chronic pain at the supraspinal level is yet to be fully characterized. DNA histone methylation is crucially regulated by de novo methyltransferases (DNMT1-3) and ten-eleven translocation dioxygenases (TET1-3). Evidence has shown that methylation markers are altered in different CNS regions related to nociception, namely the dorsal root ganglia, the spinal cord, and different brain areas. Decreased global methylation was found in the DRG, the prefrontal cortex, and the amygdala, which was associated with decreased DNMT1/3a expression. In contrast, increased methylation levels and mRNA levels of TET1 and TET3 were linked to augmented pain hypersensitivity and allodynia in inflammatory and neuropathic pain models. Since epigenetic mechanisms may be responsible for the regulation and coordination of various transcriptional modifications described in chronic pain states, with this study, we aimed to evaluate the functional role of TET1-3 and DNMT1/3a genes in neuropathic pain in several brain areas. In a spared nerve injury rat model of neuropathic pain, 21 days after surgery, we found increased TET1 expression in the medial prefrontal cortex and decreased expression in the caudate-putamen and the amygdala; TET2 was upregulated in the medial thalamus; TET3 mRNA levels were reduced in the medial prefrontal cortex and the caudate-putamen; and DNMT1 was downregulated in the caudate-putamen and the medial thalamus. No statistically significant changes in expression were observed with DNMT3a. Our results suggest a complex functional role for these genes in different brain areas in the context of neuropathic pain. The notion of DNA methylation and hydroxymethylation being cell-type specific and not tissue specific, as well as the possibility of chronologically differential gene expression after the establishment of neuropathic or inflammatory pain models, ought to be addressed in future studies.

Role of Glutamatergic Projections from Lateral Habenula to Ventral Tegmental Area in Inflammatory Pain-Related Spatial Working Memory Deficits.

The lateral habenula (LHb) and the ventral tegmental area (VTA), which form interconnected circuits, have important roles in the crucial control of sensory and cognitive motifs. Signaling in the LHb-VTA pathway can be exacerbated during pain conditions by a hyperactivity of LHb glutamatergic neurons to inhibit local VTA DAergic cells. However, it is still unclear whether and how this circuit is endogenously engaged in pain-related cognitive dysfunctions. To answer this question, we modulated this pathway by expressing halorhodopsin in LHb neurons of adult male rats, and then selectively inhibited the axonal projections from these neurons to the VTA during a working memory (WM) task. Behavioral performance was assessed after the onset of an inflammatory pain model. We evaluated the impact of the inflammatory pain in the VTA synapses by performing immunohistochemical characterization of specific markers for GABAergic (GAD65/67) and dopaminergic neurons (dopamine transporter (DAT), dopamine D2 receptor (D2r) and tyrosine hydroxylase (TH)). Our results revealed that inhibition of LHb terminals in the VTA during the WM delay-period elicits a partial recovery of the performance of pain animals (in higher complexity challenges); this performance was not accompanied by a reduction of nociceptive responses. Finally, we found evidence that the pain-affected animals exhibit VTA structural changes, namely with an upregulation of GAD65/67, and a downregulation of DAT and D2r. These results demonstrate a role of LHb neurons and highlight their responsibility in the stability of the local VTA network, which regulates signaling in frontal areas necessary to support WM processes.

Altered prefrontal-striatal theta-band oscillatory dynamics underlie working memory deficits in neuropathic pain rats.

BACKGROUND: Prelimbic medial prefrontal cortex (PL-mPFC) and nucleus accumbens core region (NAcc) play an important role in supporting several executive cognitive mechanisms, such as spatial working memory (WM). Recently, this circuit has been also associated with both sensory and affective components of pain. However, it is still unclear whether this circuit is endogenously engaged in neuropathic pain-related cognitive dysfunctions. METHODS: To answer this question, we induced the expression of halorhodopsin in local PL-mPFC neurons projecting to NAcc, and then selectively inhibited the terminals of these neurons in the NAcc while recording neural activity during the performance of a delayed non-match to sample (DNMS) spatial WM task. Within-subject behavioural performance and PL-mPFC to NAcc circuit neural activity was assessed after the onset of a persistent rodent neuropathic pain model-spared nerve injury (SNI). RESULTS: Our results revealed that the induction of the neuropathy reduced WM performance, and altered the interplay between PL-mPFC and NAcc neurons namely in increasing the functional connectivity from NAcc to PL-mPFC, particularly in the theta-band spontaneous oscillations; in addition, these behavioural and functional perturbations were partially reversed by selective optogenetic inhibition of PL-mPFC neuron terminals into the NAcc during the DNMS task delay-period, without significant antinociceptive effects. CONCLUSIONS: Altogether, these results strongly suggest that the PL-mPFC excitatory output into the NAcc plays an important role in the deregulation of WM under pain conditions. SIGNIFICANCE: Selective optogenetic inhibition of prefrontal-striatal microcircuit reverses pain-related working memory deficits but has no significant impact on pain responses. Neuropathic pain underlies an increase of functional connectivity between the nucleus accumbens core area and the prelimbic medial prefrontal cortex mediated by theta-band activity.

Bidirectional optogenetic modulation of prefrontal-hippocampal connectivity in pain-related working memory deficits.

Dysfunction of the prefrontal-hippocampal circuit has been identified as a leading cause to pain-related working-memory (WM) deficits. However, the underlying mechanisms remain poorly determined. To address this issue, we implanted multichannel arrays of electrodes in the prelimbic cortex (PL-mPFC), and in the dorsal hippocampal CA1 field (dCA1) to record the neural activity during the performance of a delayed non-match to sample (DNMS) task. The prefrontal-hippocampal connectivity was selectively modulated by bidirectional optogenetic inhibition or stimulation of local PL-mPFC glutamatergic calcium/calmodulin-dependent protein kinase-II alpha (CaMKIIα) expressing neurons during the DNMS task delay-period. The within-subject behavioral performance was assessed using a persistent neuropathic pain model - spared nerve injury (SNI). Our results showed that the induction of the neuropathic pain condition affects the interplay between PL-mPFC and dCA1 regions in a frequency-dependent manner, and that occurs particularly across theta oscillations while rats performed the task. In SNI-treated rats, this disruption was reversed by the selective optogenetic inhibition of PL-mPFC CaMKIIα-expressing neurons during the last portion of the delay-period, but without any significant effect on pain responses. Finally, we found that prefrontal-hippocampal theta connectivity is strictly associated with higher performance levels. Together, our findings suggest that PL-mPFC CaMKIIα-expressing neurons could be modulated by painful conditions and their activity may be critical for prefrontal-hippocampal connectivity during WM processing.

Selective optogenetic inhibition of medial prefrontal glutamatergic neurons reverses working memory deficits induced by neuropathic pain.

Stability of local medial prefrontal cortex (mPFC) network activity is believed to be critical for sustaining cognitive processes such as working memory (WM) and decision making. Dysfunction of the mPFC has been identified as a leading cause to WM deficits in several chronic pain conditions; however, the underlying mechanisms remain largely undetermined. Here, to address this issue, we implanted multichannel arrays of electrodes in the prelimbic region of the mPFC and recorded the neuronal activity during a food-reinforced delayed nonmatch to sample (DNMS) task of spatial WM. In addition, we used an optogenetic technique to selectively suppress the activity of excitatory pyramidal neurons that are considered the neuronal substrate for memory retention during the delay period of the behavioral task. Within-subject behavioral performance and pattern of neuronal activity were assessed after the onset of persistent pain using the spared nerve injury model of peripheral neuropathy. Our results show that the nerve lesion caused a disruption in WM and prelimbic spike activity and that this disruption was reversed by the selective inhibition of prelimbic glutamatergic pyramidal neurons during the delay period of the WM task. In spared nerve injury animals, photoinhibition of excitatory neurons improved the performance level and restored neural activity to a similar profile observed in the control animals. In addition, we found that selective inhibition of excitatory neurons does not produce antinociceptive effects. Together, our findings suggest that disruption of balance in local prelimbic networks may be crucial for the neurological and cognitive deficits observed during painful syndromes.

Animal models of congenital hypoalgesia: Untapped potential for assessing pain-related plasticity.

Over the last 20 years a large number of transgenic mouse models have been produced showing different degrees of congenital hypoalgesia; some of these models mimic known human conditions while others seemingly have no human counterpart. However, very little significant contributions to our understanding of pain neurobiology were obtained from this multitude of animal models; in most cases the study of these animals was limited to the characterization of its pain perception without addressing the long term consequences of their hypoalgesic condition. In this review we discuss the untapped potential that these animal models of congenital hypoalgesia hold for future studies addressing brain plasticity during permanent conditions of reduced pain perception, and that may result in important insights on the interplay between pain, emotion, and cognition. Revisiting hypoalgesia using modern techniques of functional neurophysiology in awake animals may complement the recent literature of functional clinical and preclinical studies that improve our understanding of the central malplasticity caused by pain.

Increased fronto-hippocampal connectivity in the Prrxl1 knockout mouse model of congenital hypoalgesia.

Despite the large number of studies addressing how prolonged painful stimulation affects brain functioning, there are only a handful of studies aimed at uncovering if persistent conditions of reduced pain perception would also result in brain plasticity. Permanent hypoalgesia induced by neonatal injection of capsaicin or carrageenan has already been shown to affect learning and memory and to induce alterations in brain gene expression. In this study, we used the Prrxl1 model of congenital mild hypoalgesia to conduct a detailed study of the neurophysiological and behavioral consequences of reduced pain experience. Prrxl1 knockout animals are characterized by selective depletion of small diameter primary afferents and abnormal development of the superficial dorsal laminae of the spinal cord, resulting in diminished pain perception but normal tactile and motor behaviour. Behavioral testing of Prrxl1 mice revealed that these animals have reduced anxiety levels, enhanced memory performance, and improved fear extinction. Neurophysiological recordings from awake behaving Prrxl1 mice show enhanced altered fronto-hippocampal connectivity in the theta- and gamma-bands. Importantly, although inflammatory pain by Complete Freund Adjuvant injection caused a decrease in fronto-hippocampal connectivity in the wild-type animals, Prrxl1 mice maintained the baseline levels. The onset of inflammatory pain also reverted the differences in forebrain expression of stress- and monoamine-related genes in Prrxl1 mice. Altogether our results suggest that congenital hypoalgesia may have an effect on brain plasticity that is the inverse of what is usually observed in animal models of chronic pain.

Effect of Motor Impairment on Analgesic Efficacy of Dopamine D2/3 Receptors in a Rat Model of Neuropathy.

Testing the clinical efficacy of drugs that also have important side effects on locomotion needs to be properly designed in order to avoid erroneous identification of positive effects when the evaluation depends on motor-related tests. One such example is the evaluation of analgesic role of drugs that act on dopaminergic receptors, since the pain perception tests used in animal models are based on motor responses that can also be compromised by the same substances. The apparent analgesic effect obtained by modulation of the dopaminergic system is still a highly disputed topic. There is a lack of acceptance of this effect in both preclinical and clinical settings, despite several studies showing that D2/3 agonists induce antinociception. Some authors raised the hypothesis that this antinociceptive effect is enhanced by dopamine-related changes in voluntary initiation of movement. However, the extent to which D2/3 modulation changes locomotion at analgesic effective doses is still an unresolved question. In the present work, we performed a detailed dose-dependent analysis of the changes that D2/3 systemic modulation have on voluntary locomotor activity and response to four separate tests of both thermal and mechanical pain sensitivity in adult rats. Using systemic administration of the dopamine D2/3 receptor agonist quinpirole, and of the D2/3 antagonist raclopride, we found that modulation of D2/3 receptors impairs locomotion and exploratory activity in a dose-dependent manner across the entire range of tested dosages. None of the drugs were able to consistently diminish either thermal or mechanical pain perception when administered at lower concentrations; on the other hand, the larger concentrations of raclopride (0.5-1.0 mg/kg) strongly abolished pain responses, and also caused severe motor impairment. Our results show that administration of both agonists and antagonists of dopaminergic D2/3 receptors affects sensorimotor behaviors, with the effect over locomotion and exploratory activity being stronger than the observed effect over pain responses.

Activation of dopaminergic D2/D3 receptors modulates dorsoventral connectivity in the hippocampus and reverses the impairment of working memory after nerve injury.

Dopamine plays an important role in several forms of synaptic plasticity in the hippocampus, a crucial brain structure for working memory (WM) functioning. In this study, we evaluated whether the working-memory impairment characteristic of animal models of chronic pain is dependent on hippocampal dopaminergic signaling. To address this issue, we implanted multichannel arrays of electrodes in the dorsal and ventral hippocampal CA1 region of rats and recorded the neuronal activity during a food-reinforced spatial WM task of trajectory alternation. Within-subject behavioral performance and patterns of dorsoventral neuronal activity were assessed before and after the onset of persistent neuropathic pain using the Spared Nerve Injury (SNI) model of neuropathic pain. Our results show that the peripheral nerve lesion caused a disruption in WM and in hippocampus spike activity and that this disruption was reversed by the systemic administration of the dopamine D2/D3 receptor agonist quinpirole (0.05 mg/kg). In SNI animals, the administration of quinpirole restored both the performance-related and the task-related spike activity to the normal range characteristic of naive animals, whereas quinpirole in sham animals caused the opposite effect. Quinpirole also reversed the abnormally low levels of hippocampus dorsoventral connectivity and phase coherence. Together with our finding of changes in gene expression of dopamine receptors and modulators after the onset of the nerve injury model, these results suggest that disruption of the dopaminergic balance in the hippocampus may be crucial for the clinical neurological and cognitive deficits observed in patients with painful syndromes.

Prefrontal cortex and mediodorsal thalamus reduced connectivity is associated with spatial working memory impairment in rats with inflammatory pain.

The medial prefrontal cortex (mPFC) and the mediodorsal thalamus (MD) form interconnected neural circuits that are important for spatial cognition and memory, but it is not known whether the functional connectivity between these areas is affected by the onset of an animal model of inflammatory pain. To address this issue, we implanted 2 multichannel arrays of electrodes in the mPFC and MD of adult rats and recorded local field potential activity during a food-reinforced spatial working memory task. Recordings were performed for 3weeks, before and after the establishment of the pain model. Our results show that inflammatory pain caused an impairment of spatial working memory performance that is associated with changes in the activity of the mPFC-MD circuit; an analysis of partial directed coherence between the areas revealed a global decrease in the connectivity of the circuit. This decrease was observed over a wide frequency range in both the frontothalamic and thalamofrontal directions of the circuit, but was more evident from MD to mPFC. In addition, spectral analysis revealed significant oscillations of power across frequency bands, namely with a strong theta component that oscillated after the onset of the painful condition. Finally, our data revealed that chronic pain induces an increase in theta/gamma phase coherence and a higher level of mPFC-MD coherence, which is partially conserved across frequency bands. The present results demonstrate that functional disturbances in mPFC-MD connectivity are a relevant cause of deficits in pain-related working memory.

Impaired spatial memory performance in a rat model of neuropathic pain is associated with reduced hippocampus-prefrontal cortex connectivity.

Chronic pain patients commonly complain of working memory deficits, but the mechanisms and brain areas underlying this cognitive impairment remain elusive. The neuronal populations of the mPFC and dorsal CA1 (dCA1) are well known to form an interconnected neural circuit that is crucial for correct performance in spatial memory-dependent tasks. In this study, we investigated whether the functional connectivity between these two areas is affected by the onset of an animal model of peripheral neuropathic pain. To address this issue, we implanted two multichannel arrays of electrodes in the mPFC and dCA1 of rats and recorded the neuronal activity during a food-reinforced spatial working memory task in a reward-based alternate trajectory maze. Recordings were performed for 3 weeks, before and after the establishment of the spared nerve injury model of neuropathy. Our results show that the nerve lesion caused an impairment of working memory performance that is temporally associated with changes in the mPFC populational firing activity patterns when the animals navigated between decision points-when memory retention was most needed. Moreover, the activity of both recorded neuronal populations after the nerve injury increased their phase locking with respect to hippocampal theta rhythm. Finally, our data revealed that chronic pain reduces the overall amount of information flowing in the fronto-hippocampal circuit and induces the emergence of different oscillation patterns that are well correlated with the correct/incorrect performance of the animal on a trial-by-trial basis. The present results demonstrate that functional disturbances in the fronto-hippocampal connectivity are a relevant cause for pain-related working memory deficits.

Dynamics of Circadian Thalamocortical Flow of Information during a Peripheral Neuropathic Pain Condition.

It is known that the thalamocortical loop plays a crucial role in the encoding of sensory-discriminative features of painful stimuli. However, only a few studies have addressed the changes in thalamocortical dynamics that may occur after the onset of chronic pain. Our goal was to evaluate how the induction of chronic neuropathic pain affected the flow of information within the thalamocortical loop throughout the brain states of the sleep-wake cycle. To address this issue we recorded local field potentials (LFPs) - both before and after the establishment of neuropathic pain in awake freely moving adult rats chronically implanted with arrays of multielectrodes in the lateral thalamus and primary somatosensory cortex. Our results show that the neuropathic injury induced changes in the number of wake and slow-wave-sleep (SWS) state episodes, and especially in the total number of transitions between brain states. Moreover, partial directed coherence - analysis revealed that the amount of information flow between cortex and thalamus in neuropathic animals decreased significantly, indicating that the overall thalamic activity had less weight over the cortical activity. However, thalamocortical LFPs displayed higher phase-locking during awake and SWS episodes after the nerve lesion, suggesting faster transmission of relevant information along the thalamocortical loop. The observed changes are in agreement with the hypothesis of thalamic dysfunction after the onset of chronic pain, and may result from diminished inhibitory effect of the primary somatosensory cortex over the lateral thalamus.

Instability of spatial encoding by CA1 hippocampal place cells after peripheral nerve injury.

Several authors have shown that the hippocampus responds to painful stimulation and suggested that prolonged painful conditions could lead to abnormal hippocampal functioning. The aim of the present study was to evaluate whether the induction of persistent peripheral neuropathic pain would affect basic hippocampal processing such as the spatial encoding performed by CA1 place cells. These place cells fire preferentially in a certain spatial position in the environment, and this spatial mapping remains stable across multiple experimental sessions even when the animal is removed from the testing environment. To address the effect of prolonged pain on the stability of place cell encoding, we chronically implanted arrays of electrodes in the CA1 hippocampal region of adult rats and recorded the multichannel neuronal activity during a simple food-reinforced alternation task in a U-shaped runway. The activity of place cells was followed over a 3-week period before and after the establishment of an animal model of neuropathy, spared nerve injury. Our results show that the nerve injury increased the number of place fields encoded per cell and the mapping size of the place fields. In addition, there was an increase in in-field coherence while the amount of spatial information content that a single spike conveyed about the animal location decreased over time. Other measures of spatial tuning (in-field firing rate, firing peak and number of spikes) were unchanged between the experimental groups. These results demonstrate that the functioning of spatial place cells is altered during neuropathic pain conditions.

Comparison of anesthetic depth indexes based on thalamocortical local field potentials in rats.

BACKGROUND: Local field potentials may allow a more precise analysis of the brain electrical activity than the electroencephalogram. In this study, local field potentials were recorded in the thalamocortical axis of rats to (i) compare the performance of several indexes of anesthetic depth and (ii) investigate the existence of thalamocortical correlated or disrupted activity during isoflurane steady-state anesthesia. METHODS: Five rats chronically implanted with microelectrodes were used to record local field potentials in the primary somatosensory cortex and ventroposterolateral thalamic nuclei at six periods: before induction of anesthesia; in the last 5 min of randomized 20-min steady-state end-tidal 0.8, 1.1, 1.4, and 1.7% isoflurane concentrations; and after recovery. The approximate entropy, the index of consciousness, the spectral edge frequency, and the permutation entropy were estimated using epochs of 8 s. A correction factor for burst suppression was applied to the spectral edge frequency and to the permutation entropy. The correlation between the derived indexes and the end-tidal isoflurane was calculated and compared for the two studied brain regions indexes. Coherence analysis was also performed. RESULTS: The burst suppression-corrected permutation entropy showed the highest correlation with the end-tidal isoflurane concentration, and a high coherence was obtained between the two studied areas. CONCLUSIONS: The permutation entropy corrected with the classic burst suppression ratio is a promising alternative to other indexes of anesthetic depth. Furthermore, high coherence level of activity exists between the somatosensory cortical and thalamic regions, even at deep isoflurane stages.