Enhanced serotonin and mesolimbic dopamine transmissions in a rat model of neuropathic pain

Astroglial MicroRNA-219-5p in the Ventral Tegmental Area Regulates Nociception in Rats

BACKGROUND

The authors previously reported that noncoding microRNA miR-219-5p is down-regulated in the spinal cord in a nociceptive state. The ventral tegmental area also plays critical roles in modulating nociception, although the underlying mechanism remains unknown. The authors hypothesized that miR-219-5p in the ventral tegmental area also may modulate nociception.

METHODS

The authors studied the bidirectional regulatory role of ventral tegmental area miR-219-5p in a rat complete Freund's adjuvant model of inflammatory nociception by measuring paw withdrawal latencies. Using molecular biology technologies, the authors measured the effects of astroglial coiled-coil and C2 domain containing 1A/nuclear factor κB cascade and dopamine neuron activity on the down-regulation of ventral tegmental area miR-219-5p-induced nociceptive responses.

RESULTS

MiR-219-5p expression in the ventral tegmental area was reduced in rats with thermal hyperalgesia. Viral overexpression of ventral tegmental area miR-219-5p attenuated complete Freund's adjuvant-induced nociception from 7 days after complete Freund's adjuvant injection (paw withdrawal latencies: 6.09 ± 0.83 s vs. 3.96 ± 0.76 s; n = 6/group). Down-regulation of ventral tegmental area miR-219-5p in naïve rats was sufficient to induce thermal hyperalgesia from 7 days after lentivirus injection (paw withdrawal latencies: 7.09 ± 1.54 s vs. 11.75 ± 2.15 s; n = 8/group), which was accompanied by increased glial fibrillary acidic protein (fold change: 2.81 ± 0.38; n = 3/group) and reversed by intraventral tegmental area injection of the astroglial inhibitor fluorocitrate. The nociceptive responses induced by astroglial miR-219-5p down-regulation were inhibited by interfering with astroglial coiled-coil and C2 domain containing 1A/nuclear factor-κB signaling. Finally, pharmacologic inhibition of ventral tegmental area dopamine neurons alleviated this hyperalgesia.

CONCLUSIONS

Down-regulation of astroglial miR-219-5p in ventral tegmental area induced nociceptive responses are mediated by astroglial coiled-coil and C2 domain containing 1A/nuclear factor-κB signaling and elevated dopamine neuron activity.

The Link between Depression and Chronic Pain: Neural Mechanisms in the Brain

Chronic pain, as a stress state, is one of the critical factors for determining depression, and their coexistence tends to further aggravate the severity of both disorders. Unfortunately, their association remains unclear, which creates a bottleneck problem for managing chronic pain-induced depression. In recent years, studies have found considerable overlaps between pain- and depression-induced neuroplasticity changes and neurobiological mechanism changes. Such overlaps are vital to facilitating the occurrence and development of chronic pain and chronic pain-induced depression. In this review, we summarized the role of neuroplasticity in the occurrence and development of the two disorders in question and explored individualized application strategies of analgesic drugs and antidepressants that have different pharmacological effects in the treatment of chronic pain-induced depression. Therefore, this review may provide new insights into the understanding of association between chronic pain and depression.

Antidepressive and antinociceptive effects of ethanolic extract and fruticuline A from Salvia lachnostachys Benth leaves on rodents

Objectives

This study investigated the antidepressant and antinociceptive effects of ethanolic extract (SLEE) and pure fruticuline A obtained from Salvia lachnostachys leaves on rats and mice.

Methods

In this study, SLEE (100 mg/kg, p.o. route) was evaluated for its effects on spared nerve injury (SNI) in rats. The animals were submitted to mechanical sensitivity, forced swim (FST) and cold sensitivity tests 10 and 15 days after surgery. SLEE (100 mg/kg, p.o.) and fruticuline A (3 mg/kg, p.o.) were also evaluated with respect to nociceptive behavior induced by formalin. In addition, clonidine-induced depressive-like behavior was also analyzed.

Results

The oral administration of SLEE for up to 15 days and the subcutaneous injection of 10 mg/kg of ketamine (positive control) significantly inhibited SNI-induced mechanical hyperalgesia and decreased immobility in the FST. On the 15th day of oral treatment, SLEE prevented the SNI-induced increase in cold sensitivity. In the formalin test, SLEE and fruticuline A significantly reduced the frequency of paw licking during the first and second phases and decreased the formation of edema. In locomotor analysis (open field test without clonidine treatment), SLEE and fruticuline A did not alter the response. SLEE and fruticuline A significantly attenuated clonidine-induced suppression of spontaneous locomotor activity (squares invaded and licking) and emotionality (grooming and freezing) compared with controls, similar to the naive group.

Conclusion

SLEE exhibits antihyperalgesic, antidepressant, and antinociceptive effects, and fruticuline A appears to be at least partly responsible for the effects of SLEE. Together, these results demonstrate the antidepressive effects of SLEE and fruticuline A and indicate that both derivatives obtained from S. lachnostachys act against spontaneous neuropathic pain.

Neuroinflammation-a co-occurring phenomenon linking chronic pain and opioid dependence

Chronic pain is a disease that encompasses both sensory and emotional elements. Opioids are highly effective analgesics because they target both of these elements, by inhibiting pain pathways and alleviating negative affect (including depression) by engaging reward or hedonic pathways. Unfortunately, chronic opioid use is limited by the development of unwanted side effects, such as tolerance, hyperalgesia, and abuse liability. Thus, the challenge of providing effective pain treatment while minimizing these unwanted side effects is an ongoing issue with significant clinical and societal impact. In this review, we posit that neuroinflammation within the central nervous system is a shared phenomenon between chronic pain and opioids that contributes to pain sensitization and negative affect. The implications for pain progression, addiction liability, and alternative treatment strategies are discussed.

How Accurate Appraisal of Behavioral Costs and Benefits Guides Adaptive Pain Coping

Coping with pain is a complex phenomenon encompassing a variety of behavioral responses and a large network of underlying neural circuits. Whether pain coping is adaptive or maladaptive depends on the type of pain (e.g., escapable or inescapable), personal factors (e.g., individual experiences with coping strategies in the past), and situational circumstances. Keeping these factors in mind, costs and benefits of different strategies have to be appraised and will guide behavioral decisions in the face of pain. In this review we present pain coping as an unconscious decision-making process during which accurately evaluated costs and benefits lead to adaptive pain coping behavior. We emphasize the importance of passive coping as an adaptive strategy when dealing with ongoing pain and thus go beyond the common view of passivity as a default state of helplessness. In combination with passive pain coping, we highlight the role of the reward system in reestablishing affective homeostasis and discuss existing evidence on a behavioral and neural level. We further present neural circuits involved in the decision-making process of pain coping when circumstances are ambiguous and, therefore, costs and benefits are difficult to anticipate. Finally, we address the wider implications of this topic by discussing its relevance for chronic pain patients.

Can the Lateral Habenula Crack the Serotonin Code?

The lateral habenula (LHb) and the serotonergic system both contribute to motivational states by encoding rewarding and aversive signals. Converging evidence suggests that perturbation of these systems is critical for the pathophysiology of mood disorders. Anatomical and functional studies indicate that the serotonergic system and the LHb are interconnected in a forward-feedback loop. However, how serotonin release modifies the synaptic and cellular properties of LHb neurons and whether this has any behavioral repercussions remain poorly investigated. In this review article, we discuss insights gained from rodents and humans regarding the implications of the serotonin system and the LHb in aversion encoding and related disorders. We then describe the type, properties and pharmacology of serotonergic receptors expressed throughout the LHb. Finally, we discuss physiological data reporting how serotonergic signaling modifies synaptic transmission and neuronal activity within the LHb. Altogether, we combine a mechanistic- and circuit-level knowledge to provide an overview on how the LHb integrates serotonergic signals, a process potentially contributing to LHb-dependent encoding of valenced external stimuli.

Are the emergence of affective disturbances in neuropathic pain states contingent on supraspinal neuroinflammation?

Neuro-immune interactions contribute to the pathogenesis of neuropathic pain due to peripheral nerve injury. A large body of preclinical evidence supports the idea that the immune system acts to modulate the sensory symptoms of neuropathy at both peripheral and central nervous system sites. The potential involvement of neuro-immune interactions in the highly debilitating affective disturbances of neuropathic pain, such as depression, anhedonia, impaired cognition and reduced motivation has received little attention. This is surprising given the widely accepted view that sickness behaviour, depression, cognitive impairment and other neuropsychiatric conditions can arise from inflammatory mechanisms. Moreover, there is a set of well-described immune-to-brain transmission mechanisms that explain how peripheral inflammation can lead to supraspinal neuroinflammation. In the last 5years increasing evidence has emerged that peripheral nerve injury induces supraspinal changes in cytokine or chemokine expression and alters glial cell activity. In this systematic review, based on strong preclinical evidence, we advance the argument that the emergence of affective disturbances in neuropathic pain states are contingent on pro-inflammatory mediators in the interconnected hippocampal-medial prefrontal circuitry that subserve affective behaviours. We explore how dysregulation of inflammatory mediators in these networks may result in affective disturbances through a wide variety of neuromodulatory mechanisms. There are also promising results from clinical trials showing that anti-inflammatory agents have efficacy in the treatment of a variety of neuropsychiatric conditions including depression and appear suited to sub-groups of patients with elevated pro-inflammatory profiles. Thus, although further research is required, aggressively targeting supraspinal pro-inflammatory mediators at critical time-points in appropriate clinical populations is likely to be a novel avenue to treat debilitating affective disturbances in neuropathic conditions.

Modulation of pain, nociception, and analgesia by the brain reward center

The midbrain dopamine center comprises a key network for reward, salience, motivation, and mood. Evidence from various clinical and preclinical settings points to the midbrain dopamine circuit as an important modulator of pain perception and pain-induced anxiety and depression. This review summarizes recent findings that shed light to the neuroanatomical, electrophysiological and molecular adaptations that chronic pain conditions promote in the mesolimbic dopamine system. Chronic pain states induce changes in neuronal plasticity and functional connectivity in several parts of the brain reward center, including nucleus accumbens, the ventral tegmental area and the prefrontal cortex. Here, we discuss recent findings on the mechanisms involved in the perception of chronic pain, in pain-induced anxiety and depression, as well as in pain-killer addiction vulnerability. Several new studies also show that the mesolimbic dopamine circuit potently modulates responsiveness to opioids and antidepressants used for the treatment of chronic pain. We discuss recent data supporting a role of the brain reward pathway in treatment efficacy and we summarize novel findings on intracellular adaptations in the brain reward circuit under chronic pain states.