Contribution of dopamine receptors to periaqueductal gray-mediated antinociception

Free-operant avoidance behavior by rats after reinforcer revaluation using opioid agonists and D-amphetamine

The associative processes that support free-operant instrumental avoidance behavior are still unknown. We used a revaluation procedure to determine whether the performance of an avoidance response is sensitive to the current value of the aversive, negative reinforcer. Rats were trained on an unsignaled, free-operant lever press avoidance paradigm in which each response avoided or escaped shock and produced a 5 s feedback stimulus. The revaluation procedure consisted of noncontingent presentations of the shock in the absence of the lever either paired or unpaired with systemic morphine and in a different cohort with systemic d-amphetamine. Rats were then tested drug free during an extinction test. In both the d-amphetamine and morphine groups, pairing of the drug and shock decreased subsequent avoidance responding during the extinction test, suggesting that avoidance behavior was sensitive to the current incentive value of the aversive negative reinforcer. Experiment 2 used central infusions of D-Ala(2), NMe-Phe(4), Gly-ol(5)]-enkephalin (DAMGO), a mu-opioid receptor agonist, in the periacqueductal gray and nucleus accumbens shell to revalue the shock. Infusions of DAMGO in both regions replicated the effects seen with systemic morphine. These results are the first to demonstrate the impact of revaluation of an aversive reinforcer on avoidance behavior using pharmacological agents, thereby providing potential therapeutic targets for the treatment of avoidance behavior symptomatic of anxiety disorders.

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.

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.

Catechol-O-methyltransferase gene polymorphism and chronic human pain: a systematic review and meta-analysis

In human studies, low COMT (catechol-O-methyltransferase) activity has been associated with increased sensitivity to acute clinical preoperative or postoperative pain. We explored the association between the COMT genotype and three chronic pain conditions: migrainous headache, fibromyalgia, or chronic widespread pain and chronic musculoskeletal pain. Furthermore, we evaluated whether COMT genotype affects the efficacy of opioids in chronic pain. After a systematic literature review, we carried out meta-analyses on the three chronic pain conditions. The efficacy of opioids was evaluated using a systematic review only. The meta-analyses showed that fibromyalgia or chronic widespread pain is the only type of chronic pain that could be associated with the COMT single nucleotide polymorphism rs4680 (Val158Met). Met158, which results in the low-activity variant of COMT, is the risk allele. In chronic clinical pain, the effect of the COMT polymorphism depends on the pain condition. Low COMT activity is not associated with migrainous headache or chronic musculoskeletal pain conditions, but it may increase the risk for fibromyalgia or chronic widespread pain. Low COMT activity increases opioid receptors and enhances opioid analgesia and adverse effects in some cancer pains. Findings from animal studies that have utilized COMT inhibitors elucidate the mechanism behind these findings. In rodent pain models, COMT inhibitors are pronociceptive, except for neuropathic pain models, where nitecapone was found to be antiallodynic. The complex interplay between enhanced adrenergic and dopaminergic activity in different parts of the nociceptive system probably explains the complicated actions of low COMT activity.

Involvement of the periaqueductal gray in the effect of motor cortex stimulation

Several clinical and animal studies of different pain models reported that motor cortex stimulation (MCS) has an antinociceptive effect. In our previous study, the response of the primary somatosensory cortex (SI) to peripheral stimuli decreased after MCS. The aim of the present study was to investigate involvement of the periaqueductal gray (PAG) in this inhibitory effect of MCS. Responses of the SI to electrical stimuli applied to both forepaws of anesthetized rats were monitored to evaluate the effect of MCS. After sensory-evoked potentials (SEPs) were stable, either saline, opioid, or dopamine receptor antagonists were locally microinjected into the PAG. After drug or saline administration, MCS was applied to the forepaw area of the right motor cortex. SEPs after MCS were compared to those before MCS. In the saline group, SEPs ipsilateral to MCS decreased, but SEPs contralateral to MCS did not. The decrease in SEPs was prevented by pretreatment of the PAG with naloxone. Application of a nonspecific dopamine receptor antagonist (α-flupenthixol) to the PAG also blocked the inhibition of SEPs after MCS. Inhibition of SEPs after MCS was blocked by local application of a D1 antagonist (SCH-23390) in the PAG, but not by a D2 antagonist (eticlopride). These results suggest that the PAG participates in the inhibitory effect of MCS, and this effect of MCS may be mediated by opioid and dopamine D1 receptors within thePAG.

Columnar distribution of catecholaminergic neurons in the ventrolateral periaqueductal gray and their relationship to efferent pathways

The periaqueductal gray (PAG) is a critical brain region involved in opioid analgesia and provides efferents to descending pathways that modulate nociception. In addition, the PAG contains ascending pathways to regions involved in the regulation of reward, including the substantia nigra (SN) and the ventral tegmental area (VTA). SN and VTA contain dopaminergic neurons that are critical for the maintenance of positive reinforcement. Interestingly, the PAG is also reported to contain a population of dopaminergic neurons. In this study, the distribution of catecholaminergic neurons within the ventrolateral (vl) PAG was examined using immunocytochemical methods. In addition, the catecholaminergic PAG neurons were examined to determine whether these neurons are integrated into ascending (VTA, SN) and descending rostral ventral medulla (RVM) efferent pathways from this region. The immunocytochemical analysis determined that catecholaminergic neurons in the PAG are both dopaminergic and noradrenergic and these neurons have a distinct rostrocaudal distribution within the ventrolateral column of PAG. Dopaminergic neurons were concentrated rostrally and were significantly smaller than noradrenergic neurons. Combined immunocytochemistry and tract tracing methods revealed that catecholaminergic neurons are distinct from, but closely associated with, both ascending and descending efferent projection neurons. Finally, by electron microscopy, catecholaminergic neurons showed close dendritic appositions with other neurons in PAG, suggesting a possible nonsynaptic mechanism for regulation of PAG output by these neurons. In conclusion, our data indicate that there are two populations of catecholaminergic neurons in the vlPAG that form dendritic associations with both ascending and descending efferents suggesting a possible nonsynaptic modulation of vlPAG neurons.

Uliginosin B presents antinociceptive effect mediated by dopaminergic and opioid systems in mice

Previous studies have shown that uliginosin B inhibits dopamine reuptake in rat brain. This compound occurs in Hypericum polyanthemum and H. caprifoliatum for which was reported to have antinociceptive effect sensitive to naloxone. The aim of this study was to assess the antinociceptive effect of uliginosin B and to evaluate the involvement of opioid and dopaminergic receptors activation. Uliginosin B presented antinociceptive effect in hot-plate and abdominal writhing tests, in mice, at doses that did not impair the motor coordination (15 mg/kg, i.p.). Uliginosin B in high dose (90 mg/kg, i.p.) presented ataxic effect in the rotarod apparatus. These effects seem to be mediated by distinct receptors since the effect on the hot-plate was completely abolished by naloxone and sulpiride, but it was unaffected by SCH 23390. On the other hand, the motor impairment induced by uliginosin B was completely prevented by naloxone and partially prevented by sulpiride and SCH 23390. However, the receptors' activation appears to be indirect since uliginosin B did not bind to opioid and dopaminergic receptors. Thus, uliginosin B effects probably are due to its ability to inhibit monoamine reuptake with consequent activation of dopamine receptors and indirect stimulation of opioid system.

Functional properties of dopamine neurons and co-expression of vasoactive intestinal polypeptide in the dorsal raphe nucleus and ventro-lateral periaqueductal grey

The dorsal raphe nucleus (DRN) and ventrolateral periaqueductal grey (vlPAG) regions contain populations of dopamine neurons, often considered to be a dorsal caudal extension of the A10 group [mostly found in the ventral tegmental area (VTA)]. Recent studies suggest they are involved in promoting wakefulness and mediate some of the antinociceptive and rewarding properties of opiates. However, little is known about their electrophysiological properties. To address this, we used Pitx3-GFP and tyrosine hydroxylase (TH)-GFP mice to carry out targeted whole-cell recordings from this population in acute brain slices. We found that DRN/vlPAG dopamine neurons have characteristics similar to most VTA dopamine neurons, but distinct from dorsal raphe serotonin neurons. They fire broad action potentials at a relatively slow, regular rate, exhibit a hyperpolarization-activated inward current and delayed repolarization, and show spike-frequency adaptation in response to prolonged depolarization. In addition, they receive fast excitatory and inhibitory synaptic inputs. Moreover, we found co-expression of vasoactive intestinal polypeptide in small, periaqueductal dopamine neurons, but generally not in larger, more ventral dopamine neurons.

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.

Antinociceptive Activity of Trichilia catigua Hydroalcoholic Extract: New Evidence on Its Dopaminergic Effects

Trichilia catigua is a native plant of Brazil; its barks are used by some local pharmaceutical companies to prepare tonic drinks, such as Catuama. The present study was addressed to evaluate the effects of T. catigua hydroalcoholic extract in mouse nociception behavioral models, and to evaluate the possible mechanisms involved in its actions. Male Swiss mice were submitted to hot-plate, writhing and von Frey tests, after oral treatment with T. catigua extract (200 mg kg⁻¹, p.o.). The extract displayed antinociceptive effect in all three models. For characterization of the mechanisms involved in the antinociceptive action of the extract, the following pharmacological treatments were done: naloxone (2.5 mg kg⁻¹, s.c.), SR141716A (10 mg kg⁻¹, i.p.), SCH23390 (15 μg kg⁻¹, i.p.), sulpiride (50 mg kg⁻¹, i.p.), prazosin (1 mg kg⁻¹, i.p.), bicuculline (1 mg kg⁻¹, i.p.) or dl-p-chlorophenylalanine methyl ester (PCPA, 100 mg kg⁻¹, i.p.). In these experiments, the action of T. catigua extract was evaluated in the hot-plate test. The treatment with SCH23390 completely prevented the antinociceptive effect, while naloxone partially prevented it. The possible involvement of the dopaminergic system in the actions of T. catigua extract was substantiated by data showing the potentiation of apomorphine-induced hypothermia and by the prevention of haloperidol-induced catalepsy. In conclusion, the antinociceptive effects of T. catigua extract seem to be mainly associated with the activation of dopaminergic system and, to a lesser extent, through interaction with opioid pathway.

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

The present study examined the roles of dopamine and D(1)- and D(2)-like dopamine receptors in ventrolateral orbital cortex (VLO)-evoked antinociception in rats with persistent inflammatory pain. Following formalin injection into the rat unilateral hindpaw pad, the effects of dopamine receptor agonist and antagonist microinjections into the VLO on nociceptive behavior were observed. Results demonstrated that VLO microinjection of the non-selective dopamine receptor agonist apomorphine (R(-)-apomorphine hydrochloride, 1.0, 2.5 and 5.0μg) depressed later-phase nociceptive behavior induced by formalin injection; this effect was attenuated by the D(2)-like dopamine receptor antagonist S(-)-raclopride(+)-tartrate salt (raclopride, 3.0μg), but not by the D(1)-like dopamine receptor antagonist R(+)-SCH-23390 hydrochloride (SCH-23390, 1.0μg). Apomorphine-induced antinociception was mimicked by microinjection of the D(2)-like dopamine receptor agonist (-)-quinpirole hydrochloride (2.0 and 5.0μg) into the same VLO site, and this effect was antagonized by raclopride (3.0μg). In addition, microinjection of the D(1)-like dopamine receptor agonist R(+)-SKF-38393 hydrochloride (5.0μg) had no effect on formalin-induced nociceptive behavior during the later phase. However, the D(1)-like dopamine receptor antagonist SCH-23390 (2.5, 5.0 and 10μg) depressed nociceptive behavior in a dose-dependent manner. These results suggested that dopamine mediated VLO-induced antinociception via different mechanisms in the persistent inflammatory pain model; D(2)-like receptors mediated dopamine-induced antinociception, while D(1)-like dopamine receptors exhibited tonic facilitatory action on nociceptive behavior, thereby blocking D(1)-like dopamine receptors could induce antinociception.

Monoaminergic Antidepressants in the Relief of Pain: Potential Therapeutic Utility of Triple Reuptake Inhibitors (TRIs)

Over 75% of depressed patients suffer from painful symptoms predicting a greater severity and a less favorable outcome of depression. Imaging, anatomical and functional studies have demonstrated the existence of common brain structures, neuronal pathways and neurotransmitters in depression and pain. In particular, the ascending serotonergic and noradrenergic pathways originating from the raphe nuclei and the locus coeruleus; respectively, send projections to the limbic system. Such pathways control many of the psychological functions that are disturbed in depression and in the perception of pain. On the other hand, the descending pathways, from monoaminergic nuclei to the spinal cord, are specifically implicated in the inhibition of nociception providing rationale for the use of serotonin (5-HT) and/or norepinephrine (NE) reuptake inhibitors (SSRIs, NRIs, SNRIs), in the relief of pain. Compelling evidence suggests that dopamine (DA) is also involved in the pathophysiology and treatment of depression. Indeed, recent insights have demonstrated a central role for DA in analgesia through an action at both the spinal and suprasinal levels including brain regions such as the periaqueductal grey (PAG), the thalamus, the basal ganglia and the limbic system. In this context, dopaminergic antidepressants (i.e., containing dopaminergic activity), such as bupropion, nomifensine and more recently triple reuptake inhibitors (TRIs), might represent new promising therapeutic tools in the treatment of painful symptoms with depression. Nevertheless, whether the addition of the dopaminergic component produces more robust effects than single- or dual-acting agents, has yet to be demonstrated. This article reviews the main pathways regulating pain transmission in relation with the monoaminergic systems. It then focuses on the current knowledge regarding the in vivo pharmacological properties and mechanism of action of monoaminergic antidepressants including SSRIs, NRIs, SNRIs and TRIs. Finally, a synthesis of the preclinical studies supporting the efficacy of these antidepressants in analgesia is also addressed in order to highlight the relative contribution of 5-HT, NE and DA to nociception.

Endogenous opiates and behavior: 2009

This paper is the 32nd consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2009 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

The role of dopamine receptors in ventrolateral orbital cortex-evoked anti-nociception in a rat model of neuropathic pain

The present study examined the role of dopamine and D(1)-and D(2)-like dopamine receptors in ventrolateral orbital cortex (VLO)-evoked anti-hypersensitivity in a rat model of neuropathic pain, as well as the possible underlying mechanisms. Results showed that microinjection of apomorphine [(R(-)-apomorphine hydrochloride)], a non-selective dopamine receptor agonist, into the VLO attenuated spared nerve injury (SNI)-induced mechanical allodynia in a dose-dependent manner. This effect was completely blocked by the D(2)-like dopamine receptor antagonist S(-)-raclopride(+)-tartrate salt (1.5 microg), but was enhanced by the D(1)-like dopamine receptor antagonist SCH23390 (R(+)-SCH-23390 hydrochloride, 5.0 microg). The attenuating effect of apomorphine on mechanical allodynia was mimicked by application of the D(2)-like dopamine receptor agonist quinpirole [((-)-quinpirole hydrochloride, 0.5, 1.0, and 2.0 microg)]. In addition, microinjection of larger doses (10 and 20 microg) of SCH23390 into the VLO significantly attenuated allodynia. Furthermore, microinjections of GABA(A) receptor antagonists, bicuculline [(+)-bicuculline,(S), 9(R)] and picrotoxin (200 and 300 ng for both drugs), into the VLO attenuated mechanical allodynia. A small dose of bicuculline or picrotoxin (100 ng) resulted in increased quinpirole (0.5 microg)-induced anti-allodynia. In contrast, GABA(A) receptor agonists, muscimol hydrochloride (250 ng) or THIP [(2,5,6,7-retrahydroisoxazolo(5,4-c)pyridine-3-ol hydrochloride, 1.0 microg)], blocked quinpirole (2.0 microg)-induced attenuation. These results suggest that the dopaminergic system is involved in mediating VLO-induced anti-hypersensitivity, activation of D(2)-like dopamine receptors, and inhibition of D(1)-like receptors resulting in anti-hypersensitivity. In addition, the mechanisms of GABAergic disinhibition might be involved in D(2)-like receptor mediating effects in neuropathic pain.

The effect of dopamine on pain-related neurons in the parafascicular nucleus of rats

Dopamine (DA) regulates pain perception in the central nervous system (CNS). However, the mechanism of the action of DA in pain-related neurons of the parafascicular nucleus (Pf) is not clear. The present study aimed to determine the effect of DA and its receptor antagonist, droperidol on the pain-evoked responses of the pain-excited neurons (PEN) and pain-inhibited neurons (PIN) in the Pf of rats and to analyze the mechanisms underlying this effect. The trains of electric impulses applied to the sciatic nerve were used as noxious stimulation. The discharges of PEN and PIN in the Pf were recorded by using a glass microelectrode. The results showed that, in the Pf, intra-Pf microinjection of DA (5 microg/0.5 microl) increased the frequency of noxious stimulation-induced discharges of the PEN and decreased the frequency of those of the PIN, while the intra-Pf administration of droperidol (0.15 microg/0.5 microl) produced an opposite effect. On the basis of the above-mentioned findings, we could conclude that DA and its receptors in the Pf are involved in the modulation of the nociceptive response by regulating the discharges of PEN and PIN.

Preclinical and early clinical investigations related to monoaminergic pain modulation

The balance between descending controls, both excitatory and inhibitory, can be altered in various pain states. There is good evidence for a prominent alpha(2)-adrenoceptor-mediated inhibitory system and 5-HT(3) (and likely also 5-HT(2)) serotonin receptor-mediated excitatory controls originating from brainstem and midbrain areas. The ability of cortical controls to influence spinal function allows for top-down processing through these monoamines. The links between pain and the comorbidities of sleep problems, anxiety, and depression may be due to the dual roles of noradrenaline and of 5-HT in these functions and also in pain. These controls appear, in the cases of peripheral neuropathy, spinal injury, and cancer-induced bone pain to be driven by altered peripheral and spinal neuronal processes; in opioid-induced hyperalgesia, however, the same changes occur without any pathophysiological peripheral process. Thus, in generalized pain states in which fatigue, mood changes, and diffuse pain occur, such as fibromyalgia and irritable bowel syndrome, one could suggest an abnormal engagement of descending facilitations with or without reduced inhibitions but with central origins. This would be an endogenous central malfunction of top-down processing, with the altered monoamine systems underlying the observed symptoms. A number of analgesic drugs can either interact with or have their actions modulated by these descending systems, reinforcing their importance in the establishment of pain but also in its control.

Early methylphenidate exposure enhances morphine antinociception and tolerance in adult rats

Methylphenidate (MPH) is often used to reduce the symptoms of Attention-Deficit/Hyperactivity Disorder. Early MPH treatment in rats has been shown to enhance adult morphine-induced antinociception. Although this enhanced antinociception could improve pain treatment, it could also lead to enhanced tolerance to morphine. This hypothesis was tested by examining the effects of MPH administration during the pre-weanling period on morphine-induced antinociception and tolerance in adulthood. Male and female Sprague-Dawley rats received daily IP injections of saline or MPH (2 or 5 mg/kg) for 10 consecutive days beginning on post-natal day (PD) 11. At 60 days of age, morphine (0, 1.8, 3.2, 5.6, 10.0, and 18 mg/kg) antinociception was assessed. Beginning one day later, rats received two daily injections of either saline or morphine (5 mg/kg) for two consecutive days to induce tolerance. On PD 63 cumulative doses of morphine were administered as before to assess the development of tolerance. Rats pretreated with MPH showed enhanced acute morphine antinociception compared to saline pretreated controls. In addition, tolerance to morphine was greater in rats pretreated with MPH early in life. The magnitude of this decrease in morphine potency was dependent on the dose of MPH, such that animals that received 5 mg/kg of MPH from PD 11 to 20 showed the greatest tolerance. These findings demonstrate that MPH exposure during the pre-weanling period has long-lasting effects that include enhanced morphine antinociception and tolerance.