Effect of lidocaine administration at the nucleus locus coeruleus level on lateral hypothalamus-induced antinociception in the rat

Nerve Injury Triggers Time-dependent Activation of the Locus Coeruleus, Influencing Spontaneous Pain-like Behavior in Rats

BACKGROUND

Dynamic changes in neuronal activity and in noradrenergic locus coeruleus (LC) projections have been proposed during the transition from acute to chronic pain. Thus, the authors explored the cellular cFos activity of the LC and its projections in conjunction with spontaneous pain-like behavior in neuropathic rats.

METHODS

Tyrosine hydroxylase:Cre and wild-type Long-Evans rats, males and females, were subjected to chronic constriction injury (CCI) for 2 (short-term, CCI-ST) or 30 days (long-term, CCI-LT), evaluating cFos and Fluoro-Gold expression in the LC, and its projections to the spinal cord (SC) and rostral anterior cingulate cortex (rACC). These tests were carried out under basal conditions (unstimulated) and after noxious mechanical stimulation. LC activity was evaluated through chemogenetic and pharmacologic approaches, as were its projections, in association with spontaneous pain-like behaviors.

RESULTS

CCI-ST enhanced basal cFos expression in the LC and in its projection to the SC, which increased further after noxious stimulation. Similar basal activation was found in the neurons projecting to the rACC, although this was not modified by stimulation. Strong basal cFos expression was found in CCI-LT, specifically in the projection to the rACC, which was again not modified by stimulation. No cFos expression was found in the CCI-LT LCipsilateral (ipsi)/contralateral (contra)→SC. Chemogenetics showed that CCI-ST is associated with greater spontaneous pain-like behavior when the LCipsi is blocked, or by selectively blocking the LCipsi→SC projection. Activation of the LCipsi or LCipsi/contra→SC dampened pain-like behavior. Moreover, Designer Receptor Exclusively Activated by Designer Drugs (DREADDs)-mediated inactivation of the CCI-ST LCipsi→rACC or CCI-LT LCipsi/contra→rACC pathway, or intra-rACC antagonism of α-adrenoreceptors, also dampens pain-like behavior.

CONCLUSIONS

In the short term, activation of the LC after CCI attenuates spontaneous pain-like behaviors via projections to the SC while increasing nociception via projections to the rACC. In the long term, only the projections from the LC to the rACC contribute to modulate pain-like behaviors in this model.

EDITOR’S PERSPECTIVE

Intra-accumbal orexinergic system contributes to the stress-induced antinociceptive behaviors in the animal model of acute pain in rats

Stress and pain are interleaved at numerous levels - influencing each other. Stress can increase the nociception threshold in animals, long-known as stress-induced analgesia (SIA). Orexin is known as a neuropeptide that modulates pain. The effect of stress on the mesolimbic system in the modulation of pain is known. The role of the intra-accumbal orexin receptors in the modulation of acute pain by forced swim stress (FSS) is unclear. In this study, 117 adult male albino Wistar rats (270-300 g) were used. The animals were unilaterally implanted with cannulae above the NAc. The antagonist of the orexin-1 receptor (OX1r), SB334867, and antagonist of the orexin-2 receptor (OX2r), TCS OX2 29, were microinjected into the NAc in different doses (1, 3, 10, and 30 nmol/0.5 µl DMSO) before exposure to FSS for a 6-min period. The tail-flick test was carried out as an assay nociception of acute pain, and the nociceptive threshold [tail-flick latency (TFL)] was measured for 60-minute. The findings demonstrated that exposure to acute stress could remarkably increase the TFLs and antinociceptive responses. Moreover, intra-accumbal microinjection of SB334867 or TCS OX2 29 blocked the antinociceptive effect of stress in the tail-flick test. The contribution of orexin receptors was almost equally modulating SIA. The present study's findings suggest that OX1r and OX2r within the NAc modulate stress-induced antinociceptive responses. The intra-accumbal microinjection of orexin receptors antagonists declares inducing antinociceptive responses by FSS in acute pain. Proposedly, intra-accumbla orexinergic receptors have a role in the development of SIA.

Nerve injury induces transient locus coeruleus activation over time: role of the locus coeruleus-dorsal reticular nucleus pathway

ABSTRACT

The transition from acute to chronic pain results in maladaptive brain remodeling, as characterized by sensorial hypersensitivity and the ensuing appearance of emotional disorders. Using the chronic constriction injury of the sciatic nerve as a model of neuropathic pain in male Sprague-Dawley rats, we identified time-dependent plasticity of locus coeruleus (LC) neurons related to the site of injury, ipsilateral (LCipsi) or contralateral (LCcontra) to the lesion, hypothesizing that the LC→dorsal reticular nucleus (DRt) pathway is involved in the pathological nociception associated with chronic pain. LCipsi inactivation with lidocaine increased cold allodynia 2 days after nerve injury but not later. However, similar blockade of LCcontra reduced cold allodynia 7 and 30 days after inducing neuropathy but not earlier. Furthermore, lidocaine blockade of the LCipsi or LCcontra reversed pain-induced depression 30 days after neuropathy. Long-term pain enhances phosphorylated cAMP-response element binding protein expression in the DRtcontra but not in the DRtipsi. Moreover, inactivation of the LCcontra→DRtcontra pathway using dual viral-mediated gene transfer of designer receptor exclusively activated by designer drugs produced consistent analgesia in evoked and spontaneous pain 30 days postinjury. This analgesia was similar to that produced by spinal activation of α2-adrenoreceptors. Furthermore, chemogenetic inactivation of the LCcontra→DRtcontra pathway induced depressive-like behaviour in naïve animals, but it did not modify long-term pain-induced depression. Overall, nerve damage activates the LCipsi, which temporally dampens the neuropathic phenotype. However, the ensuing activation of a LCcontra→DRtcontra facilitatory pain projection contributes to chronic pain, whereas global bilateral LC activation contributes to associated depressive-like phenotype.

The role of dentate gyrus dopaminergic receptors in the lateral hypothalamic-induced antinociception during persistent inflammatory pain in male rats

The lateral hypothalamus (LH) is one of the key brain areas involved in pain modulation. Also, the dentate gyrus (DG) of the hippocampus expresses various receptors, including dopaminergic receptors. Dopaminergic receptors play a key role in pain transmission and modulation within the brain. The present study aimed to investigate the involvement of DG dopaminergic receptors in the LH-induced antinociception during the presence of inflammatory pain. Male Wistar rats were used in this study. Cannulae were unilaterally implanted in their skull for microinjections into the LH and DG. The LH was chemically stimulated by carbachol injection (250 nM/0.5 μl saline). In separate groups, different doses (0.25, 1, and 4 μg/0.5 μl vehicle) of the D1- and D2-like dopamine receptor antagonists (SCH23390 and Sulpiride, respectively) were microinjected into the DG, 5 min prior to intra-LH injection of carbachol. Five min after the second injection, formalin test as a persistent inflammatory pain model in animals was done in all rats. The results revealed that carbachol could induce antinociception following formalin injection into rat's hind paw. The 4 μg dose of both antagonists significantly reduced the LH stimulation-induced antinociception in both phases of formalin pain responses. Although the 1 μg dose of sulpiride significantly reduced antinociception during both phases, 1 μg SCH23390 could only reduce this antinociception during the late phase. These findings demonstrate the involvement of DG dopaminergic receptors in the LH-induced antinociception. The results also suggest that the effectiveness of DG dopaminergic receptors is more pronounced during the late phase of formalin-induced pain responses.

Projection-specific dopamine neurons in the ventral tegmental area participated in morphine-induced hyperalgesia and anti-nociceptive tolerance in male mice

BACKGROUND

Long-term morphine use is associated with serious side effects, such as morphine-induced hyperalgesia and analgesic tolerance. Previous investigations have documented the association between dopamine (DA) neurons in the ventral tegmental area (VTA) and pain. However, whether VTA DA neurons are implicated in morphine-induced hyperalgesia and analgesic tolerance remains elusive.

METHODS

Initially, we observed behavioural effects of lidocaine administration into VTA or ablation of VTA DA neurons on morphine-induced hyperalgesia and anti-nociceptive tolerance. Subsequently, c-Fos expression in nucleus accumbens (NAc) shell-projecting and medial prefrontal cortex (mPFC)-projecting VTA DA neurons after chronic morphine treatment was respectively investigated. Afterwards, the effects of chemogenetic manipulation of NAc shell-projecting or mPFC-projecting DA neurons on morphine-induced hyperalgesia and anti-nociceptive tolerance were observed. Additionally, effects of chemogenetic manipulation of VTA GABA neurons on c-Fos expression in VTA DA neurons were investigated.

RESULTS

Lidocaine injection into VTA relieved established hyperalgesia and anti-nociceptive tolerance whereas ablation of VTA DA neurons prevented the development of morphine-induced hyperalgesia and anti-nociceptive tolerance. Chronic morphine treatment increased c-Fos expression in NAc shell-projecting DA neurons, rather than in mPFC-projecting DA neurons. Chemogenetic manipulation of NAc shell-projecting DA neurons had influence on morphine-induced hyperalgesia and tolerance. However, chemogenetic manipulation of mPFC-projecting DA neurons had no significant effects on morphine-induced hyperalgesia and anti-nociceptive tolerance. Chemogenetic manipulation of VTA GABA neurons affected the c-Fos expression in VTA DA neurons.

CONCLUSIONS

These findings revealed the involvement of NAc shell-projecting VTA DA neurons in morphine-induced hyperalgesia and anti-nociceptive tolerance, and may shed new light on the clinical management of morphine-induced hyperalgesia and analgesic tolerance.

PERSPECTIVE

This study demonstrated that NAc shell-projecting DA neurons rather than mPFC-projecting DA neurons in the VTA were implicated in morphine-induced hyperalgesia and anti-nociceptive tolerance. Our findings may pave the way for the discovery of novel therapies for morphine-induced hyperalgesia and analgesic tolerance.

The Involvement of Central Noradrenergic Pathway in the Analgesic Effect of Bee Venom Acupuncture on Vincristine-Induced Peripheral Neuropathy in Rats

Vincristine is a vinca alkaloid anti-mitotic drug with a broad spectrum of effects on solid and hematologic cancers. The major dose-limiting factor of this anti-cancer regimen is painful peripheral neuropathy. However, no gold-standard analgesic option has been used clinically. In this study, we investigated the effects and mechanism of bee venom acupuncture (BVA) to alleviate peripheral neuropathic pain induced by repeated intraperitoneal infusions of vincristine (1 mg/kg/day, days 1–5 and 8–12) in rats. Subcutaneous injection with bee venom (BV, 1.0 mg/kg) at the ST36 acupoint ameliorated cold and mechanical hypersensitivity (i.e., aberrant withdrawal responses in acetone drop and von Frey hair tests, respectively). In vivo extracellular recording demonstrated that BVA inhibited cutaneous cold (acetone) and mechanical (brush, press, and pinch) stimuli-elicited abnormal hyperexcitation of the spinal wide dynamic range (WDR) neurons in vincristine-treated rats. In addition, the microinjection of lidocaine into the ipsilateral locus coeruleus or the antagonism of the spinal α₂-adrenergic receptors clearly reversed the effects of BVA on cold and mechanical hypersensitivity, indicating a vital role of the descending noradrenergic modulation in analgesia. These findings suggest that BVA could be a potential therapeutic option for vincristine-induced peripheral neuropathy.

Role of orexin-1 and -2 receptors within the nucleus accumbens in the acquisition of sensitization to morphine in rats

It has been reported that orexins A and B are involved in the mediation of drug reward. In addition, the nucleus accumbens (NAc) has an important role in the development of morphine-conditioned place preference (CPP) and morphine sensitization. In the present study, we aimed to evaluate the role of orexin receptors within the NAc in morphine sensitization using CPP paradigm. Adult male Wistar rats were used and were bilaterally implanted by two cannulae in the NAc. The animals received intra-accumbal administration of OX1 or OX2 receptor antagonists, SB-334867 (0.1, 1, and 10 nM/side) or TCS OX2 29 (2, 10, and 20 nM/side), 10 min before morphine injection during the sensitization period, during which the animals received repeated administration of morphine (5 mg/kg; s.c.) once daily for three days followed by 5 morphine injection-free days. Then the CPP paradigm was conducted for the evaluation of morphine rewarding properties by injecting a sub-threshold dose of morphine (0.5 mg/kg; s.c.). The results showed that bilateral administration of OX1 receptor antagonist into the NAc reduced acquisition of morphine sensitization in a dose-dependent manner, but OX2 receptor antagonist produced similar effect only at its highest dose, indicating that OX1 and OX2 receptors within the NAc are involved in the acquisition of morphine sensitization.

Noradrenergic Locus Coeruleus pathways in pain modulation

The noradrenergic system is crucial for several activities in the body, including the modulation of pain. As the major producer of noradrenaline (NA) in the central nervous system (CNS), the Locus Coeruleus (LC) is a nucleus that has been studied in several pain conditions, mostly due to its strategic location. Indeed, apart from a well-known descending LC-spinal pathway that is important for pain control, an ascending pathway passing through this nucleus may be responsible for the noradrenergic inputs to higher centers of the pain processing, such as the limbic system and frontal cortices. Thus, the noradrenergic system appears to modulate different components of the pain experience and accordingly, its manipulation has distinct behavioral outcomes. The main goal of this review is to bring together the data available regarding the noradrenergic system in relation to pain, particularly focusing on the ascending and descending LC projections in different conditions. How such findings influence our understanding of these conditions is also discussed.

Differences in carbachol dose, pain condition, and sex following lateral hypothalamic stimulation

Lateral hypothalamic (LH) stimulation produces antinociception in female rats in acute, nociceptive pain. Whether this effect occurs in neuropathic pain or whether male-female sex differences exist is unknown. We examined the effect of LH stimulation in male and female rats using conditions of nociceptive and neuropathic pain. Neuropathic groups received chronic constriction injury (CCI) to induce thermal hyperalgesia, a sign of neuropathic pain. Nociceptive rats were naive for CCI, but received the same thermal stimulus following LH stimulation. To demonstrate that CCI ligation produced thermal hyperalgesia, males and females received either ligation or sham surgery for control. Both males and females demonstrated significant thermal hyperalgesia following CCI ligation (p<0.05), but male sham surgery rats also showed a significant left-right difference not present in female sham rats. In the second experiment, rats randomly assigned to CCI or nociceptive groups were given one of three doses of the cholinergic agonist carbachol (125, 250, or 500 nmol) or normal saline for control, microinjected into the left LH. Paw withdrawal from a thermal stimulus (paw withdrawal latency; PWL) was measured every 5 min for 45 min. Linear mixed models analysis showed that males and females in both pain conditions demonstrated significant antinociception, with the 500-nmol dose producing the greatest effect across groups compared with controls for the left paw (p<0.05). Female CCI rats showed equivalent responses to the three doses, while male CCI rats showed more variability for dose. However, nociceptive females responded only to the 500-nmol dose, while nociceptive males responded to all doses (p<0.05). For right PWL, only nociceptive males showed a significant carbachol dose response. These findings are suggestive that LH stimulation produces antinociception in male and female rats in both nociceptive and neuropathic pain, but dose response differences exist based on sex and pain condition.

Activation of NK₁ receptors in the locus coeruleus induces analgesia through noradrenergic-mediated descending inhibition in a rat model of neuropathic pain

BACKGROUND AND PURPOSE

The locus coeruleus (LC) is a major source of noradrenergic projections to the dorsal spinal cord, and thereby plays an important role in the modulation of nociceptive information. The LC receives inputs from substance P (SP)-containing fibres from other regions, and expresses the NK(1) tachykinin receptor, a functional receptor for SP. In the present study, we investigated the roles of SP in the LC in neuropathic pain.

EXPERIMENTAL APPROACH

Chronic constriction injury (CCI) of the left sciatic nerve was performed in rats to induce neuropathic pain. After development of neuropathic pain, SP was injected into the LC and the nocifensive behaviours were assessed. The involvement of noradrenergic descending inhibition in SP-induced analgesia was examined by i.t. administration of yohimbine, an α(2) -adrenoceptor antagonist. NK(1) receptor expression in the LC was examined by immunohistochemistry.

KEY RESULTS

In CCI rats, mechanical allodynia was alleviated by SP injection into the LC. These effects were abolished by prior injection of WIN 51708, an NK(1) receptor antagonist, into the LC or i.t. treatment with yohimbine. NK(1) receptor-like immunoreactivity was observed in noradrenergic neurons throughout the LC in intact rats, and remained unchanged after CCI.

CONCLUSION AND IMPLICATIONS

SP in the LC exerted analgesic effects on neuropathic pain through NK(1) receptor activation and resulted in facilitation of spinal noradrenergic transmission. Accordingly, manipulation of the SP/NK(1) receptor signalling pathway in the LC may be a promising strategy for effective treatment of neuropathic pain.

Orexin A in the ventral tegmental area induces conditioned place preference in a dose-dependent manner: involvement of D1/D2 receptors in the nucleus accumbens

It has been shown that orexin A in the ventral tegmental area (VTA) is necessary for development of morphine place preference. Additionally, D1 and D2 dopamine receptors in the nucleus accumbens (NAc) have critical roles in motivation and reward. However, little is known about the function of orexin in conditioned place preference (CPP) in rats and involvement of D1/D2 receptors in the NAc. In the present study, we investigated the effect of direct administration of orexin A into the VTA, and examined the role of intra-accumbal dopamine receptors in development (acquisition) of reward-related behaviors in the rats. Adult male Wistar rats were unilaterally implanted by two separate cannulae into the VTA and NAc. The CPP paradigm was used, and, conditioning score and locomotor activity were recorded by Ethovision software. The results showed that unilateral intra-VTA administration of orexin A (27, 53 and 107 ng/0.3 μl saline) during conditioning phase induced CPP in a dose-dependent manner. The most effective dose of intra-VTA orexin-A in eliciting CPP was 107 ng. However, intra-NAc administration of SCH 23390 (0.25, 1 and 4 μg/0.5 μl saline), a D1 receptor antagonist, and sulpiride (0.25, 1 and 4 μg/0.5 μl DMSO), a D2 receptor antagonist, inhibited the development of orexin-induced CPP. The inhibitory effect of D2 but not D1 receptor antagonist was exerted in a dose-dependent manner. It is supposed that the activation of VTA dopaminergic neuron by orexin impresses the D2 receptors more than D1 receptors in the NAc.