The role of dopamine receptors in ventrolateral orbital cortex-evoked antinociception in a rat formalin test model

Activation of 5-HT5A receptor in the ventrolateral orbital cortex produces antinociceptive effects in rat models of neuropathic and inflammatory pain

The ventrolateral orbital cortex (VLO) is identified as an integral component of the endogenous analgesic system comprising a spinal cord - thalamic nucleus submedius - VLO - periaqueductal gray (PAG) - spinal cord loop. The present study investigates the effects of 5-HT5A receptor activation in the VLO on allodynia induced by spared nerve injury and formalin-evoked flinching behavior and spinal c-Fos expression in male SD rats, and further examines whether GABAergic modulation is involved in the effects evoked by VLO 5-HT5A receptor activation. We found an upregulation of 5-HT5A receptor expression in the VLO during neuropathic and inflammatory pain states. Microinjection of the non-selective 5-HT5A receptor agonist 5-CT into the VLO dose dependently alleviated allodynia, and flinching behavior and spinal c-Fos expression, which were blocked by the selective 5-HT5A receptor antagonist SB-699551. Moreover, application of the GABAA receptor antagonist bicuculline in the VLO augmented the analgesic effects induced by 5-CT in neuropathic and inflammatory pain states, whereas the GABAA receptor agonist muscimol attenuated these analgesic effects. Additionally, the 5-HT5A receptors were found to be colocalized with GABAergic neurons in the VLO. These results provide new evidence for the involvement of central 5-HT5A receptors in the VLO in modulation of neuropathic and inflammatory pain and support the hypothesis that activation of 5-HT5A receptors may inhibit the inhibitory effect of GABAergic interneurons on output neurons projecting to the PAG (GABAergic disinhibitory mechanisms), consequently activating the brainstem descending inhibitory system that depresses nociceptive transmission at the spinal cord level.

Optogenetic Approach in Trigeminal Neuralgia and Potential Concerns: Preclinical Insights

The integration of optogenetics in the trigeminal pain circuitry broadens and reinforces existing pain investigations. Similar to research on spinal neuropathic pain, the exploration of the underlying determinants of orofacial pain is expanding. Optogenetics facilitates more direct, specific, and subtle investigations of the neuronal circuits involved in orofacial pain. One of the most significant concerns of both dentistry and medicine is trigeminal neuralgia (TN) management due to its substantial impact on a patient's quality of life. Our objective is to gather insights from preclinical studies conducted in TN employing an optogenetic paradigm, thereby extending the prospects for in-depth neurobiological research. This review highlights optogenetic research in trigeminal pain circuitry involving TN. We outline the central and peripheral regions associated with pain-that have been investigated using optogenetics in the trigeminal pain circuitry. The study further reports its scope and limitations as well as its potential for future applications from bench to bedside.

Itch-specific neurons in the ventrolateral orbital cortex selectively modulate the itch processing

Itch is a cutaneous sensation that is critical in driving scratching behavior. The long-standing question of whether there are specific neurons for itch modulation inside the brain remains unanswered. Here, we report a subpopulation of itch-specific neurons in the ventrolateral orbital cortex (VLO) that is distinct from the pain-related neurons. Using a Tet-Off cellular labeling system, we showed that local inhibition or activation of these itch-specific neurons in the VLO significantly suppressed or enhanced itch-induced scratching, respectively, whereas the intervention did not significantly affect pain. Conversely, suppression or activation of pain-specific neurons in the VLO significantly affected pain but not itch. Moreover, fiber photometry and immunofluorescence verified that these itch- and pain-specific neurons are distinct in their functional activity and histological location. In addition, the downstream targets of itch- and pain-specific neurons were different. Together, the present study uncovers an important subpopulation of neurons in the VLO that specifically modulates itch processing.

Muscle-brain communication in pain: The key role of myokines

Pain is the most common reason for a physician visit, which accounts for a considerable proportion of the global burden of disease and greatly affects patients' quality of life. Therefore, there is an urgent need to identify new therapeutic targets involved in pain. Exercise-induced hypoalgesia (EIH) is a well known phenomenon observed worldwide. However, the available evidence demonstrates that the mechanisms of EIH remain unclear. One of the most accepted hypotheses has been the activation of several endogenous systems in the brain. Recently, the concept that the muscle acts as a secretory organ has attracted increasing attention. Proteins secreted by the muscle are called myokines, playing a critical role in communicating with other organs, such as the brain. This review will focus on several myokines and discuss their roles in EIH.

Cortical Modulation of Nociception

Nociception is the neuronal process of encoding noxious stimuli and could be modulated at peripheral, spinal, brainstem, and cortical levels. At cortical levels, several areas including the anterior cingulate cortex (ACC), prefrontal cortex (PFC), ventrolateral orbital cortex (VLO), insular cortex (IC), motor cortex (MC), and somatosensory cortices are involved in nociception modulation through two main mechanisms: (i) a descending modulatory effect at spinal level by direct corticospinal projections or mostly by activation of brainstem structures (i.e. periaqueductal grey matter (PAG), locus coeruleus (LC), the nucleus of raphe (RM) and rostroventral medulla (RVM)); and by (ii) cortico-cortical or cortico-subcortical interactions. This review summarizes evidence related to the participation of the aforementioned cortical areas in nociception modulation and different neurotransmitters or neuromodulators that have been studied in each area. Besides, we point out the importance of considering intracortical neuronal populations and receptors expression, as well as, nociception-induced cortical changes, both functional and connectional, to better understand this modulatory effect. Finally, we discuss the possible mechanisms that could potentiate the use of cortical stimulation as a promising procedure in pain alleviation.

Activation of ventrolateral orbital cortex improves mouse neuropathic pain-induced anxiodepression

Depression and anxiety are frequently observed in patients suffering from neuropathic pain. The underlying mechanisms remained unclear. The ventrolateral orbital cortex (VLO) has attracted considerable interest in its role in antidepressive effect in rodents. In the present study, we further investigated the role of the VLO in the anxiodepressive consequences of neuropathic pain in a chronic constriction injury of infraorbital nerve-induced trigeminal neuralgia (TN) mouse model. Elevated plus maze, open field, forced swimming, tail suspension, and sucrose preference tests were used to evaluate anxiodepressive-like behaviors. The results show that chemogenetic activation of bilateral VLO neurons, especially CaMK2A+ pyramidal neurons, blocked the TN-induced anxiodepressive-like behaviors. Chemogenetic and optogenetic activation of VGLUT2+ or inhibition of VGAT+ VLO neurons was sufficient to produce an antianxiodepressive effect in TN mice. Pharmacological activation of D1-like receptors (D1Rs) but not D2Rs in the VLO significantly alleviated TN-induced depressive-like behaviors. Electrophysiological recordings revealed a decreased excitability of VLO excitatory neurons following neuropathic pain. Furthermore, activation of submedius thalamic nucleus-VLO (Sm-VLO) projection mimicked the antianxiodepressive effect of VLO excitation. Conversely, activation of VLO-periaqueductal gray matter (PAG) projection had no effect on TN-induced anxiodepressive behaviors. This study provides a potentially novel mechanism-based therapeutic strategy for the anxiodepressive consequences of neuropathic pain.

Bilateral Parkinson's disease model rats exhibit hyperalgesia to subcutaneous formalin administration into the vibrissa pad

We bilaterally injected 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle of rats and developed bilateral Parkinson’s disease (PD) model rats in order to experimentally investigate the neural mechanisms underlying the alteration of nociception in the orofacial region of patients with PD. We explored the effects of dopamine depletion on nociception by investigating behavioral responses (face rubbing) triggered by subcutaneous administration of formalin into the vibrissa pad. We also assessed the number of c-Fos–immunoreactive (c-Fos-IR) cells in the superficial layers of the trigeminal spinal subnucleus caudalis (Vc). Subcutaneous formalin administration evoked a two-phase increase in face rubbing. We observed the first increase 0–5 min after formalin administration (first phase) and the second increase 10–60 min after administration (second phase). The number of face rubbing behaviors of 6OHDA–injected rats did not significantly change compared with saline–injected rats in both phases. Significant increase of c-Fos-IR cells in the Vc was found in 6-OHDA–injected rats after formalin administration compared with those in saline–injected rats after formalin administration. We also assessed expression of c-Fos-IR cells in the paraventricular nucleus (PVN), and significant decrease of c-Fos-IR cells in the PVN of 6-OHDA–injected rats was found. Taken together, these findings suggest that bilateral dopaminergic denervation evoked by 6-OHDA administration causes hyperalgesia in the trigeminal region and the PVN may be involved in the hyperalgesia.

Dopamine D1 and D2 receptors mediate analgesic and hypnotic effects of l-tetrahydropalmatine in a mouse neuropathic pain model

RATIONALE

Levo-tetrahydropalmatine (l-THP), an active ingredient of Corydalis yanhusuo, has been reported to be a partial agonist for dopamine D₁ receptors (D₁R) and an antagonist for D₂R. Although it has been safely used clinically in China for decades as an analgesic with sedative/hypnotic properties, there are few studies that address the mechanisms by which l-THP exerts its beneficial effects in chronic pain-induced sleep disturbance.

OBJECTIVES

To investigate the effects and mechanisms of l-THP on sleep disturbance in a neuropathic pain-like condition.

METHODS

A mouse model of chronic neuropathic pain induced by partial sciatic nerve ligation (PSNL) was employed. The antinociceptive and hypnotic effects of l-THP were evaluated by measurement of mechanical allodynia, thermal hyperalgesia, and electroencephalogram (EEG) recordings in PSNL mice. Pharmacological approaches and c-Fos expression were used to clarify the mechanisms of l-THP.

RESULTS

Intraperitoneal injection of l-THP at 5 and 10 mg/kg not only significantly increased the mechanical threshold by 134.4% and 174.8%, and prolonged the thermal latency by 49.4% and 69.2%, but also increased non-rapid eye movement sleep by 17.5% and 29.6%, and decreased sleep fragmentation in PSNL mice, compared with the vehicle control. Moreover, the antinociceptive effect of l-THP was prevented by D₁R antagonist SCH23390 or D₂R agonist quinpirole; meanwhile, the hypnotic effect of l-THP was blocked by quinpirole rather than by SCH23390. Immunohistochemistry demonstrated that l-THP inhibited c-Fos overexpression induced by PSNL in the cingulate cortex and the periaqueductal gray.

CONCLUSIONS

These findings indicated that l-THP exerted analgesic effects by agonism D₁R and antagonism D₂R, and the antagonism of D₂R mediated the hypnotic effect of l-THP in PSNL mice.

The pronociceptive role of 5-HT6 receptors in ventrolateral orbital cortex in a rat formalin test model

Recent studies have shown the 5-HT₆ receptors are expressed in regions which are important in pain processing such as the cortex, amygdala, thalamus, PAG, spinal cord and dorsal root ganglia (DRG), suggesting a putative role of 5-HT₆ receptors in pain modulation. The ventrolateral orbital cortex (VLO) is part of an endogenous analgesic system, consisting of the spinal cord - thalamic nucleus submedius (Sm) - VLO - periaqueductal gray (PAG) - spinal cord loop. The present study assessed the possible role of 5-HT₆ receptors in the VLO in formalin-induced inflammatory pain model. Firstly we found that microinjection of selective 5-HT₆ receptor agonists EMD-386088 (5 μg in 0.5 μl) and WAY-208466 (8 μg in 0.5 μl) both augmented 5% formalin-induced nociceptive behavior. Microinjection of selective 5-HT₆ receptor antagonist SB-258585 (1,2 and 4 μg in 0.5 μl) significantly reduced formalin-induced flinching. Besides, the pronociceptive effects of EMD-386088 and WAY-208466 were dramatically reduced by SB-258585, implicating 5-HT₆ receptor mechanisms in mediating these responses. In addition, the pronociceptive effect of EMD-386088 was also prevented by the adenylate cyclase (AC) inhibitor SQ-22536 (2 nmol in 0.5 μl) and the protein kinase A (PKA) inhibitor H89 (10 nmol in 0.5 μl), respectively. We further confirmed the above results with quantification of spinal c-fos expression. Taken together, our results suggested that 5-HT₆ receptors play a pronociceptive role in the VLO in the rat formalin test due to its activation of AC - PKA pathway. Therefore, cerebral cortical 5-HT₆ receptors could be a new target to develop analgesic drugs.

Dopamine D3 receptor knockout mice exhibit abnormal nociception in a sex-different manner

Pain is a complex and subjective experience. Previous studies have shown that mice lacking the dopamine D3 receptor (D3RKO) exhibit hypoalgesia, indicating a role of the D3 receptor in modulation of nociception. Given that there are sex differences in pain perception, there may be differences in responses to nociceptive stimuli between male and female D3RKO mice. In the current study, we examined the role of the D3 receptor in modulating nociception in male and female D3RKO mice. Acute thermal pain was modeled by hot-plate test. This test was performed at different temperatures including 52°C, 55°C, and 58°C. The von Frey hair test was applied to evaluate mechanical pain. And persistent pain produced by peripheral tissue injury and inflammation was modeled by formalin test. In the hot-plate test, compared with wild-type (WT) mice, D3RKO mice generally exhibited longer latencies at each of the three temperatures. Specially, male D3RKO mice showed hypoalgesia compared with male WT mice when the temperature was 55°C, while for the female mice, there was a statistical difference between genotypes when the test condition was 52°C. In the von Frey hair test, both male and female D3RKO mice exhibited hypoalgesia. In the formalin test, the male D3RKO mice displayed a similar nociceptive behavior as their sex-matched WT littermates, whereas significantly depressed late-phase formalin-induced nociceptive behaviors were observed in the female mutants. These findings indicated that the D3 receptor affects nociceptive behaviors in a sex-specific manner and that its absence induces more analgesic behavior in the female knockout mice. © 2016 Wiley Periodicals, Inc.

The Antinociceptive Properties of the Corydalis yanhusuo Extract

Corydalis yanhusuo. W.T. extracts (YHS) are widely used for the treatment of pain and inflammation. There are a few studies that assessed the effects of YHS in pain assays; however, none of these studies has systematically compared its activities in the different pain animal modes namely: acute, inflammatory and chronic pain. Furthermore, little is known about the mechanism of YHS activity in these assays. The aim of this study was to systematically evaluate the antinociceptive properties of YHS by testing it in four standardized pain assays and to investigate its mechanism. YHS antinociceptive properties were analyzed in the tail flick, the formalin paw licking, the von Frey filament and the hot box assays after spinal nerve ligation, which monitors acute nociceptive, persistent inflammatory and chronic neuropathic pain, respectively. YHS pharmacological profile was determined by screening it against a battery of G-protein coupled receptors and its mechanism of action was studied using knock-out mice. Our study shows that YHS, at a non-sedative dose, increases the tail flick latency in the tail flick assay without resulting in development of tolerance. YHS also decreases paw licking time in the formalin assay. Further, YHS increases paw withdraw threshold and latency in the von Frey filament and the hot box assays, respectively. In vitro, YHS exhibits prominent dopamine receptor antagonistic properties. In dopamine D2 receptor knockout mice, its antinociceptive effects are attenuated in acute and neuropathic pain but not inflammatory pain assays. Our results therefore indicate that YHS effectively attenuates acute, inflammatory and neuropathic pain, without causing tolerance. The effects on acute and neuropathic pain, but not inflammatory pain, are at least partially mediated through dopamine D2 receptor antagonism. Since YHS is a dietary supplement commercially available in the United States, our data suggest that it might be a candidate for alternative pain treatment.

Neuropathic Pain due to Small Fiber Neuropathy in Aging: Current Management and Future Prospects

Over the last 10 years, the diagnosis small fiber neuropathy (SFN) has gained recognition worldwide. Patients often suffer from severe neuropathic pain that may be difficult to treat. A substantial subset of patients with SFN is aged 65 years or older, and these patients often exhibit comorbidities and usage of multiple drugs, making neuropathic pain treatment more challenging. In this review, we highlight relevant pathophysiological aspects and discuss currently used therapeutic strategies for neuropathic pain. Possible pitfalls in neuropathic pain treatment in the elderly will be underlined.

IL-10 and IL-1β mediate neuropathic-pain like behavior in the ventrolateral orbital cortex

Previous evidence has shown that the glial cells can be activated by peripheral nerve injury and release both pro-inflammatory and anti-inflammatory cytokines, which play crucial roles in the establishment and maintenance of neuropathic pain. The present study examined the roles of anti-inflammatory cytokine IL-10 and pro-inflammatory IL-1β on allodynia induced by spared nerve injury (SNI) in the ventrolateral orbital cortex (VLO) in the rat. The mechanical paw withdrawal threshold (PWT) was measured using von-Frey filaments. Microinjection of IL-10 (0.1, 0.5, 1 μg/0.5 μl) into the VLO, contralateral to the site of nerve injury attenuated allodynia; PWT increased in a dose-dependent manner. Similar to IL-10, administration of rabbit anti-rat IL-1β antibody (0.1, 1.0 and 10 ng/0.5 μl) into the same VLO site also alleviated allodynia with a dose-dependent fashion. Moreover, western blotting results showed expression levels of IL-10 and IL-1β significantly up-regulated in the contralateral VLO of SNI rats as compared with that of sham-operated rats. These results suggest that anti-inflammatory cytokine IL-10 and pro-inflammatory cytokine IL-1β mediate neuropathic-pain like behavior at the cerebral cortex level; IL-10 released from activated glial cells in the VLO can potentially attenuate allodynia while IL-1β released from activated glial cells in the VLO can potentially maintain or facilitate allodynia. These results provide new insights and site for therapy at the cerebral cortex level in neuropathic pain condition.

Interaction between the dopaminergic and opioidergic systems in dorsal hippocampus in modulation of formalin-induced orofacial pain in rats

The hippocampus is a region of the brain that serves several functions. The dopaminergic system acts through D1- and D2-like receptors to interfere in pain modulation and the opioid receptors play major roles in analgesic processes and there are obvious overlaps between these two systems. The present study investigated the interaction between the opioidergic and dopaminergic systems in the dorsal hippocampus (CA1) region for formalin-induced orofacial pain. Two guide cannulae were stereotaxically implanted in the CA1 region and morphine (0.5, 1, 2 and 4 μg/0.5 μl saline) and naloxone (0.3, 1 and 3 μg/0.5 μl saline) were used as the opioid receptor agonist and antagonist, respectively. SKF-38393 (1 μg/0.5 μl saline) was used as a D1-like receptor agonist, quinpirole (2 μg/0.5 μl saline) as a D2-like receptor agonist, SCH-23390 (0.5 μg/0.5 μl saline) as a D1-like receptor antagonist and sulpiride (3 μg/0.5 μl DMSO) as a D2-like receptor antagonist. To induce orofacial pain, 50 μl of 1% formalin was subcutaneously injected into the left side of the upper lip. Our results showed that different doses of morphine significantly reduced orofacial pain in both phases induced by formalin. Naloxone (1 and 3 μg) reversed morphine induced analgesia in CA1. SKF-38393 and quinpirole with naloxone (1 μg) significantly decreased formalin-induced orofacial pain in both phases. SCH-23390 had no effect on the antinociceptive response of morphine in both phases of orofacial pain. Sulpiride reversed the antinociceptive effects of morphine only in the first phase, but this result was not significant. Our findings suggest that there is cross-talk between the opioidergic and dopaminergic systems. Opioidergic neurons also exerted antinociceptive effects by modulation of the dopaminergic system in the CA1 region of the brain.

Advances in cortical modulation of pain

Pain is an intricate phenomenon composed of not only sensory-discriminative aspects but also of emotional, cognitive, motivational, and affective components. There has been ample evidence for the existence of an extensive cortical network associated with pain processing over the last few decades. This network includes the anterior cingulate cortex, forebrain, insular cortex, ventrolateral orbital cortex, somatosensory cortex, occipital cortex, retrosplenial cortex, motor cortex, and prefrontal cortex. Diverse neurotransmitters participate in the cortical circuits associated with pain processing, including glutamate, gamma-aminobutyric acid, dopamine, and opioids. This work examines recent rodent studies about cortical modulation of pain, mainly at a molecular level.

Dopamine transporter genotype dependent effects of apomorphine on cold pain tolerance in healthy volunteers

The aims of this study were to assess the effects of the dopamine agonist apomorphine on experimental pain models in healthy subjects and to explore the possible association between these effects and a common polymorphism within the dopamine transporter gene. Healthy volunteers (n = 105) participated in this randomized double-blind, placebo-controlled, cross-over trial. Heat pain threshold and intensity, cold pain threshold, and the response to tonic cold pain (latency, intensity, and tolerance) were evaluated before and for up to 120 min after the administration of 1.5 mg apomorphine/placebo. A polymorphism (3′-UTR 40-bp VNTR) within the dopamine transporter gene (SLC6A3) was investigated. Apomorphine had an effect only on tolerance to cold pain, which consisted of an initial decrease and a subsequent increase in tolerance. An association was found between the enhancing effect of apomorphine on pain tolerance (120 min after its administration) and the DAT-1 polymorphism. Subjects with two copies of the 10-allele demonstrated significantly greater tolerance prolongation than the 9-allele homozygote carriers and the heterozygote carriers (p = 0.007 and p = 0.003 in comparison to the placebo, respectively). In conclusion, apomorphine administration produced a decrease followed by a genetically associated increase in cold pain tolerance.

Microinjection of valproic acid into the ventrolateral orbital cortex enhances stress-related memory formation

There is collecting evidence suggesting that the process of chromatin remodeling such as changes in histone acetylation contribute to the formation of stress-related memory. Recently, the ventrolateral orbital cortex (VLO), a major subdivision of orbitofrontal cortex (OFC), was shown to be involved in antidepressant-like actions through epigenetic mechanisms. Here, we further investigated the effects of the histone deacetylase inhibitor (HDACi) valproic acid (VPA) on stress-related memory formation and the underlying molecular mechanisms by using the traditional two-day forced swimming test (FST). The results showed that VPA significantly increased the immobility time on day 2 when infused into the VLO before the initial forced swim stress on day 1. The learned immobility response to the stress was associated with increased phosphorylation of extracellular signal-regulated kinase (ERK) in VLO and hippocampus on the first day. The levels of phosphorylated ERK (phospho-ERK) in VLO and hippocampus were significantly decreased when retested 24 h later. The pretreatment with intra-VLO VPA infusion further reduced the activation of ERK on day 2 and day 7 compared with the saline controls. Moreover, the VPA infusion pretreatment also induced a significantly decreased BDNF level in the VLO on day 2, whereas no change was detected in the hippocampus. These findings suggest that VPA enhance the memories of emotionally stressful events and the ERK activity is implicated in stimulating adaptive and mnemonic processes in case the event would recur.

Proteomics identification of annexin A2 as a key mediator in the metastasis and proangiogenesis of endometrial cells in human adenomyosis

Adenomyosis is a common estrogen-dependent disorder of females characterized by a downward extension of the endometrium into the uterine myometrium and neovascularization in ectopic lesions. It accounts for chronic pelvic pain, dysmenorrhea, menorrhagia, and infertility in 8.8-61.5% women worldwide. However, the molecular mechanisms for adenomyosis development remain poorly elucidated. Here, we utilized a two-dimensional polyacrylamide gel electrophoresis/MS-based proteomics analysis to compare and identify differentially expressed proteins in matched ectopic and eutopic endometrium of adenomyosis patients. A total of 93 significantly altered proteins were identified by tandem MS analysis. Further cluster analysis revealed a group of estrogen-responsive proteins as dysregulated in adenomyosis, among which annexin A2, a member of annexin family proteins, was found up-regulated most significantly in the ectopic endometrium of adenomyosis compared with its eutopic counterpart. Overexpression of ANXA2 was validated in ectopic lesions of human adenomyosis and was found to be tightly correlated with markers of epithelial to mesenchymal transition and dysmenorrhea severity of adenomyosis patients. Functional analysis demonstrated that estrogen could remarkably up-regulate ANXA2 and induce epithelial to mesenchymal transition in an in vitro adenomyosis model. Enforced expression of ANXA2 could mediate phenotypic mesenchymal-like cellular changes, with structural and functional alterations in a β-catenin/T-cell factor (Tcf) signaling-associated manner, which could be reversed by inhibition of ANXA2 expression. We also proved that enforced expression of ANXA2 enhanced the proangiogenic capacity of adenomyotic endometrial cells through HIF-1α/VEGF-A pathway. In vivo, we demonstrated that ANXA2 inhibition abrogated endometrial tissue growth, metastasis, and angiogenesis in an adenomyosis nude mice model and significantly alleviated hyperalgesia. Taken together, our data unraveled a dual role for ANXA2 in the pathogenesis of human adenomyosis through conferring endometrial cells both metastatic potential and proangiogenic capacity, which could serve as a potential therapeutic target for the treatment of adenomyosis patients.

Dopamine affects the change of pain-related electrical activity induced by morphine dependence

Morphine is among the most effective analgesics. However, many evidences suggest that, besides the well-know analgesic activity, repeated opioids treatment can induce some side effects such as dependence, hyperalgesia and tolerance. The mechanism of noxious information transmission in the central nervous system after dependence is not clear. An important neurotransmitter, dopamine (DA) participates not only in the process of opioid dependence but also in pain modulation in the central nervous system. In the present study we observed changes of electrical activities of pain-excitation neurons (PENs) and pain-inhibition neurons (PINs) in the caudate nucleus (Cd) following the development of morphine dependence. We also observed the role of DA on these changes. Our results revealed that both the latency of PEN discharges and the inhibitory duration of PIN discharges decreased, and the net increased values of PEN and PIN discharges increased in the Cd of morphine dependent rats. Those demonstrated that electrical activities of both PENs and PINs increased in morphine dependent rats. DA inhibited the electrical activities of PENs and enhanced those of PINs in morphine dependent rats.

Sertraline inhibits formalin-induced nociception and cardiovascular responses

The objective of the present study was to determine the antihyperalgesic effect of sertraline, measured indirectly by the changes of sciatic afferent nerve activity, and its effects on cardiorespiratory parameters, using the model of formalin-induced inflammatory nociception in anesthetized rats. Serum serotonin (5-HT) levels were measured in order to test their correlation with the analgesic effect. Male Wistar rats (250-300 g) were divided into 4 groups (N = 8/per group): sertraline-treated group (Sert + Saline (Sal) and Sert + Formalin (Form); 3 mg·kg-1·day-1, ip, for 7 days) and saline-treated group (Sal + Sal and Sal + Form). The rats were injected with 5% (50 µL) formalin or saline into the right hind paw. Sciatic nerve activity was recorded using a silver electrode connected to a NeuroLog apparatus, and cardiopulmonary parameters (mean arterial pressure, heart rate and respiratory frequency), assessed after arterial cannulation and tracheotomy, were monitored using a Data Acquisition System. Blood samples were collected from the animals and serum 5-HT levels were determined by ELISA. Formalin injection induced the following changes: sciatic afferent nerve activity (+50.8 ± 14.7%), mean arterial pressure (+1.4 ± 3 mmHg), heart rate (+13 ± 6.8 bpm), respiratory frequency (+4.6 ± 5 cpm) and serum 5-HT increased to 1162 ± 124.6 ng/mL. Treatment with sertraline significantly reduced all these parameters (respectively: +19.8 ± 6.9%, -3.3 ± 2 mmHg, -13.1 ± 10.8 bpm, -9.8 ± 5.7 cpm) and serum 5-HT level dropped to 634 ± 69 ng/mL (P < 0.05). These results suggest that sertraline plays an analgesic role in formalin-induced nociception probably through a serotonergic mechanism.

Dopamine release is involved in antinociceptive effect of theophylline

The methylxanthines, e.g., theophylline, are widely used for the treatment of bronchial asthma. Additionally, a pain relieving effect of theophylline has been reported in patients as well as in experimental animals. The mechanism of this antinociceptive action is not clear. In this study, involvement of dopaminergic system in theophylline-induced antinociception was evaluated using tail flick test model. Swiss albino mice, (either sex, weighing 25-30 g) with base line tail flick latencies (TFL) between 2.0 and 3.5 s, were used. TFL was recorded before and at intervals of 15, 30, 45, and 60 min. after drug treatment. The experimental protocol was duly approved by the Institutional Animal Ethics Committee, All India Institute of Medical Sciences, New Delhi, India. To determine the role of dopaminergic system, the mice were pretreated with either D1 or D2 dopaminergic receptor antagonists SCH 23390 and haloperidol, respectively, prior to treatment with theophylline. Another group of animals received apomorphine along with theophylline. The dose of theophylline used, i.e., 10 mg/kg, intraperitoneally (i.p.), had shown a significant increase in TFLs. The theophylline-induced antinociception, 10 mg/kg, i.p., was reversed by pretreatment with both D1 and D2 dopaminergic receptor antagonists SCH 23390 and haloperidol as well as with apomorphine (1 mg/kg) pretreatment. The results suggest that theophylline-induced antinociception is due to release of dopamine.