Psychostimulant drugs potentiate morphine analgesia in the formalin test

Methylphenidate attenuates signs of evoked neuropathic pain in animal model

Methylphenidate is a psychostimulant that increases dopamine and noradrenaline levels. Recent studies have shown that methylphenidate potentiates the effect of morphine and together suppress acute and chronic pain. In clinical practice, methylphenidate has been used as a treatment for ADHD and changes of pain threshold have been noted in these patients. The aim of this study was to determine the effect of methylphenidate in an animal model of peripheral neuropathic pain. Neuropathic pain was modeled by the chronic constriction of the sciatic nerve (CCI) in Wistar rats. We evaluated the effect of methylphenidate (1 mg/kg, s.c.) on evoked pain (reflex tests - plantar test, vonFrey test and operant test - thermal place preference) and on spontaneous pain (conditioned place preference). CCI induced thermal, mechanical and cold hyperalgesia/allodynia. Methyphenidate suppressed mechanical and cold hyperalgesia/allodynia, while had no effect on thermal one. Therefore, methylphenidate seems to be a new potential pharmacotherapy for the treatment of neuropathic pain.

The Role of Mesostriatal Dopamine System and Corticostriatal Glutamatergic Transmission in Chronic Pain

There is increasing recognition of the involvement of the nigrostriatal and mesolimbic dopamine systems in the modulation of chronic pain. The first part of the present article reviews the evidence indicating that dopamine exerts analgesic effects during persistent pain by stimulating the D2 receptors in the dorsal striatum and nucleus accumbens (NAc). Thereby, dopamine inhibits striatal output via the D2 receptor-expressing medium spiny neurons (D2-MSN). Dopaminergic neurotransmission in the mesostriatal pathways is hampered in chronic pain states and this alteration maintains and exacerbates pain. The second part of this article focuses on the glutamatergic inputs from the medial prefrontal cortex to the NAc, their activity changes in chronic pain, and their role in pain modulation. Finally, interactions between dopaminergic and glutamatergic inputs to the D2-MSN are considered in the context of persistent pain. Studies using novel techniques indicate that pain is regulated oppositely by two independent dopaminergic circuits linking separate parts of the ventral tegmental area and of the NAc, which also interact with distinct regions of the medial prefrontal cortex.

Comparing the Antinociceptive Effects of Methamphetamine, Buprenorphine, or Both After Chronic Treatment and Withdrawal in Male Rats

Introduction:

Methamphetamine (Meth) and Buprenorphine (BUP) modulate pain perception. However, the antinociceptive effects of their interactions, which affect through different systems, are unclear in rats. This study aimed to compare the analgesic effects of Meth, BUP, and their coadministration, as well as the effect of withdrawal from these substances on nociception in male rats.

Methods:

In this experiment, 40 male Wistar rats (weight: 250–300 g) were categorized into four groups: control, Meth, BUP, or BUP+Meth. After seven days of treatments, the antinociceptive effects were assessed using the hot plate and the tail flick tests. The differences among the groups were analyzed with ANOVA and Tukey’s post hoc tests. P values less than 0.05 were considered significant.

Results:

Meth and BUP increased the reaction times during the hot plate and tail flick tests. The combination of Meth and BUP increased reaction time more than Meth or BUP alone.

Conclusion:

The significantly high reaction times in rats treated with Meth and BUP indicate that these substances have antinociceptive effects. In addition, Meth enhanced the antinociceptive effects of BUP. These synergistic effects might occur through the dopaminergic, serotonergic, and or adrenergic systems.

Relief of Pain-Depressed Behavior in Rats by Activation of D1-Like Dopamine Receptors

Clinically significant pain often includes a decrease in both behavior and mesolimbic dopamine signaling. Indirect and/or direct dopamine receptor agonists may alleviate pain-related behavioral depression. To test this hypothesis, the present study compared effects of indirect and direct dopamine agonists in a preclinical assay of pain-depressed operant responding. Male Sprague-Dawley rats with chronic indwelling microelectrodes in the medial forebrain bundle were trained in an intracranial self-stimulation (ICSS) procedure to press a lever for pulses of electrical brain stimulation. Intraperitoneal injection of dilute lactic acid served as an acute noxious stimulus to depress ICSS. Intraperitoneal lactic acid-induced depression of ICSS was dose-dependently blocked by the dopamine transporter inhibitor methylphenidate and the D1-selective agonist SKF82958, but not by the D2/3-selective agonists quinpirole, pramipexole, or sumanirole. The antinociceptive effects of methylphenidate and SKF82958 were blocked by the D1-selective antagonist SCH39166. Acid-induced stimulation of a stretching response was evaluated in separate groups of rats, but all agonists decreased acid-stimulated stretching, and antagonism experiments were inconclusive due to direct effects of the antagonists when administered alone. Taken together, these results suggest that D1-receptor stimulation is both sufficient to block acid-induced depression of ICSS and necessary for methylphenidate antinociception in this procedure. Conversely, D2/3-receptor stimulation is not sufficient to relieve pain-depressed behavior. These results support the hypothesis that pain-related depression of dopamine D1 receptor signaling contributes to pain-related depression of behavior in rats. Additionally, these results support further consideration of indirect dopamine agonists and direct D1 receptor agonists as candidate treatments for pain-related behavioral depression.

Chronic administration of modafinil induces hyperalgesia in mice: reversal by L-NG-nitro-arginine methyl ester and 7-nitroindazole

Modafinil [2-((diphenylmethyl) sulfinyl) acetamide] is a central nervous system stimulant. It has received considerable attention as a potential psychotropic agent in several psychiatric disorders. The current study was carried out to investigate the effect of modafinil after acute administration on animal models of pain in mice. Also, this study evaluated the effect of L-NG-nitroarginine methyl ester (L-NAME), 7-nitroindazole (7-NI) and naloxone following chronic administration of modafinil. Modafinil was administered in the doses of 50, 100 or 200 mg/kg once in acute study and it showed significantly increased tail-flick latency (tfl) and paw-licking latency. In formalin test modafinil (100 mg/kg) significantly reduced licking/biting time in both early and late phases in comparison to control. In chronic study, modafinil 100 mg/kg administered for 10 days, produced a progressive decrease in the reaction time (i.e., tfl/paw-licking latency) in comparison to day 1 values which started building up from day 4 and fully established at day 6, indicating hyperalgesic response. Prior administration of 7-NI (on day 7) and L-NAME (on day 10) prevented the hyperalgesic response while naloxone on day 10 did not have a significant effect on modafinil-induced hyperalgesia. These results demonstrate that modafinil has a potential role in pain as it exhibited antinociceptive effect after acute administration in a dose-dependent manner and on chronic administration it caused hyperalgesia. This hyperalgesia is reversed by nitric oxide synthase inhibitors, suggesting the possibility of involvement of nitric oxide pathway. Further studies are required to evaluate the role of modafinil in clinical pain.

Common biological pathways underlying the psychoneurological symptom cluster in cancer patients

BACKGROUND

A symptom cluster is a group of symptoms that occur together and are interrelated. The clinical implication of symptom cluster research is to use the clustering patterns of symptoms to understand the mechanisms for these symptoms and develop management strategies targeted at multiple symptoms.

OBJECTIVE

The purposes of this review were to summarize the evidence for a psychoneurological symptom cluster in cancer patients, to provide information regarding the underlying biological mechanisms for each of the psychoneurological symptoms within the cluster, and to propose possible common biological pathways that may underlie this cluster.

METHODS

A systematic review of the literature was conducted.

RESULTS

Empirical evidence exists to support a cluster of psychoneurological symptoms (ie, depressive symptoms, cognitive disturbance, fatigue, sleep disturbance, pain). At a molecular level, common biological pathways (ie, proinflammatory cytokines, hypothalamic-pituitary-adrenal axis, and monoamine neurotransmission system) may underlie the development of symptoms within this cluster. Activation of proinflammatory cytokines is proposed as a first stage of mechanistic pathway. However, other biological factors, such as lowered estrogen or hemoglobin levels, may influence psychoneurological cluster.

CONCLUSION

Additional studies are needed to confirm the roles of cytokines as well as other biological factors in the development of the psychoneurological cluster and to determine the biomarkers to identify the subgroups of cancer patients who are at greatest risk for this cluster.

IMPLICATIONS FOR PRACTICE

This information can be used by researchers and clinicians to guide the selection of symptom management strategies that are ideally targeted to the biological mechanisms that underlie this symptom cluster.

Behavioral and Antinociceptive Effects of Different Psychostimulant Drugs in Prenatally Methamphetamine-Exposed Rats

Prenatal exposure to methamphetamine (METH) increases nociceptive sensitivity in adult rats. As the strong analgesics have high abuse potential and drugs of abuse are known to have analgesic properties, the aim was to study analgesic effect of different psychostimulants in control and prenatally METH-exposed rats. Latencies of withdrawal reflexes of hind limbs and the tail on thermal nociceptive stimuli were repeatedly measured in 15-min intervals after the application of 5 mg/kg s.c. of amphetamine (AMPH), methamphetamine (METH), cocaine (COC), 3,4-methylenedioxymethamphetamine (MDMA) or morphine (MOR). In all groups, AMPH induced on hind limbs stronger analgesia than METH and MDMA whereas COC and MOR were practically without any effect. On the tail, effect of AMPH did not differ from that of MOR. All psychostimulants increased defecation in comparison with MOR and in all groups the number of defecation boluses positively correlated with analgesia of the hind limbs. We did not confirm that prenatal exposure to METH makes adult rats more sensitive either to same drug or to other psychostimulants. The different analgesic potencies of psychostimulants and MOR at different body sites indicate the possible existence of a somatotopic organization of pain inhibition, which is controlled by different mechanisms.

Methylphenidate: established and expanding roles in symptom management

Methylphenidate is a psychostimulant originally used for the treatment of attention-deficit disorder. Methylphenidate inhibits neuronal neurotransmitter transporters involved in the uptake of dopamine and norepinephrine at the level of the synapse. Inhibition of these transmitter transporters leads to increased concentrations of dopamine and norepinephrine in the synapse, which results in increasing alertness. The stimulant effect of methylphenidate has been used for the treatment of major depression, poststroke depression, cognitive enhancement in patients with brain tumors, neurodegenerative disorders, HIV disease, fatigue, and as a treatment for delirium and sedation associated with opioid use. Other areas where methylphenidate has been evaluated include gait disorders in the elderly individuals and the treatment of apathy in dementia. Analgesic effects have been demonstrated in preclinical models but true analgesic effects remain to be proven in humans. This article reviews the current use of methylphenidate for symptom management with a critical look at the evidence base for its efficacy in the conditions described.

Enhancing effects of morphine on methamphetamine-induced reinforcing behavior and its association with dopamine release and metabolism in mice

Polydrug abuse has become a significant problem worldwide, and the combined use of methamphetamine (MA) and morphine (M) is now highly prevalent among addicts. In the present study, we investigated the neurobehavioral effects of repeated treatment regimens of these drugs (i.p. administration of 0.75 mg/kg/day MA, 5 mg/kg/day M, and their combination for five consecutive days followed by once weekly for five consecutive weeks) in mice. In addition, we used an in vivo microdialysis technique to study the changes in extracellular concentrations of dopamine (DA) and its metabolites in the mouse striatum after challenge administration of these drugs. The results showed that systemic M increased MA-induced conditioned place preference (CPP), as revealed by higher CPP values which were also maintained for a longer duration compared with those induced by an identical dose of MA or M alone. Subsequent to challenge with combined MA and M, mice exhibited an increase in stereotyped behavior, which appeared to be associated with an elevation of extracellular concentration of DA in the striatum. Our findings suggest that M not only produces synergistic effects on MA-induced CPP, but also interacts with MA to induce stereotyped behavioral sensitization which is mediated by an increase in DA outflow in the striatum. These findings provide insight into the behavioral and neurochemical basis responsible for the combined abuse liability of MA and M.

Modafinil treatment of opioid-induced sedation

STUDY OBJECTIVE

The purpose of this study was to assess the efficacy of modafinil in combating opioid-induced sedation.

DESIGN

A 1-year retrospective chart review of all patients receiving modafinil, a wake-promoting agent, to treat opioid-induced sedation. Opioid-induced sedation was measured using Epworth Sleepiness Scale (ESS).

SETTING

Outpatient, private practice.

PATIENTS

Eleven adult patients, six female and five male, being treated with opioids for chronic, nonmalignant pain.

RESULTS

A significant decrease was observed between pretreatment and posttreatment ESS measurements during modafinil treatment.

CONCLUSION

The results suggest an improvement in opioid-induced sedation in patients treated for nonmalignant pain.

Opioids: a review

Recent discoveries in opioid pharmacology help explain the enormous variability in clinical responses to these powerful analgesics. Although there is only one m opioid receptor gene, splice variants of that gene's expression result in a panoply of different functioning receptors. Other sources of variable response include polymorphisms in the m opioid receptor regulatory region, and pharmacokinetic differences because of cytochrome P-450 mono-oxygenase heterogeneity. Analgesic tolerance is likely the key phenomenon limiting the benefit of opioids. A plethora of intracellular pathways affects this. Among them are the N-methyl-D-aspartate receptor, protein kinase C gamma activity, nitric oxide synthase, and GM1 ganglioside content of the neuronal membrane. Clinical studies undercut the routine use of meperidine in most settings. Other studies have shown better ways to diminish opioid side effects.

Palliative uses of methylphenidate in patients with cancer: a review

PURPOSE

Cancer is, unfortunately, often a terminal disease. The goal of therapy for many patients with cancer is palliation of symptoms common at the end of life, including pain, depression, and cognitive dysfunction. Methylphenidate is a psychostimulant most commonly used in the treatment of attention deficit hyperactivity disorder. In this report, we review the use of methylphenidate in the palliative care of patients with cancer.

METHODS

This review was written on the basis of a computerized literature search of Medline. We considered all English language publications from 1966 to present using the following key words: methylphenidate, palliative care, and cancer. Forty-nine articles were identified as being relevant for this review.

RESULTS

On the basis of this review, we came to the conclusion that methylphenidate is used to ameliorate opioid-induced somnolence, to augment the analgesic effects of opioids, to treat depression, and to improve cognitive function in patients with cancer.

CONCLUSION

The medical literature supports the palliative use of methylphenidate in the care of patients with cancer. Further placebo-controlled trials are needed to elucidate the precise role that methylphenidate will have in providing symptom relief to dying patients.

Characterization of the analgesic properties of nomifensine in rats

The analgesic properties of the catecholamine uptake inhibitor nomifensine were investigated in the tail immersion, hot plate and formalin tests. Systemic administration of nomifensine produced analgesia only in the formalin test. The analgesia was dose-dependent (0.625-5 mg/kg), and the highest dose completely abolished nociceptive behaviors induced by 2% formalin. The analgesia was not affected by the opioid antagonist naltrexone (2.5-40 microg s.c.) but was dose-dependently reversed by the D2 antagonist eticlopride (181.3-270 microg/kg i.p.). Neither naltrexone nor eticlopride affected formalin pain scores. Nomifensine analgesia appears to be dopamine-mediated but independent of opioid mechanisms.

NMDA receptor antagonist disrupts acute and chronic effects of methylphenidate

Methylphenidate (MPD) is a drug widely used for treating attention-deficit/hyperactivity disorder in children. Because of its extensive consumption and because it has pharmacological stimulant properties similar to amphetamine and cocaine, MPD has the potential of abuse. N-methyl-D-aspartate (NMDA) receptors are suggested to be involved in CNS effects of stimulants, and antagonists of the NMDA receptor can potentially alter the stimulants' effects. Dizocilpine (MK-801), a non-competitive antagonist of the NMDA receptor, has been reported to prevent sensitization elicited by repeated administration of amphetamine and cocaine. The objective of the present study was to use the tail-flick latency assay, rectal temperature, and body weight gain to assess effects of repetitive treatment of MPD and whether MK-801 treatment would alter these effects in Sprague-Dawley rats. It was found that: (Ia) Acute administration of MPD or MK-801 did not alter the tail-flick latency, (Ib) Repeated administration of MPD decreased tail-flick latency, while repeated administration of MK-801 had no significant effect on tail-flick latency, (Ic) MK-801 given prior to or with MPD reversed the chronic effect on tail-flick latency produced by MPD; (IIa) When both drugs were independently given, MPD elicited a decrease in rectal temperature, while MK-801 alone produced an increase in temperature, (IIb) When given together, MK-801 had a transient effect in blocking the sensitization to MPD but failed to reverse the sensitization of MPD once it had developed; and (III) Both MK-801 and MPD caused an unstable pattern of body weight gain. Hence, the results of this study in rats suggest that MK-801 can modulate non-motor effects of MPD.

Methylphenidate: its pharmacology and uses

Methylphenidate is a commonly used medication in the United States. This central nervous system stimulant has a mechanism of action distinct from that of amphetamine. The Food and Drug Administration has approved methylphenidate for the treatment of attention-deficit/hyperactivity disorder and narcolepsy. Treatment with methylphenidate has been advocated in patients with traumatic brain injury and stroke, cancer patients, and those with human immunodeficiency virus infection. Placebo-controlled trials have documented its efficacy as an adjunctive agent in the treatment of depression and pain. This article reviews the current understanding of the mechanism of action and efficacy of methylphenidate in various clinical conditions.

Electroacupuncture potentiates the antinociceptive effect of intrathecal endomorphin-1 in the rat formalin test

Endomorphin-1 is a novel endogenous mu-opioid peptide. In this study, we examined the effects of 2 Hz electroacupuncture in the rat tail flick test and the formalin test (a persistent noxious model). Moreover, we investigated if the electroacupuncture potentiated the effect of intrathecal endomorphin-1. The results demonstrated that electroacupuncture alone produced a significant antinociception in the tail flick test, but not in the formalin test, and that intrathecal endomorphin-1 dose-dependently suppressed the biphasic nociceptive behavior in the formalin test. Electroacupuncture enhanced the antinociceptive effect of intrathecal endomorphin-1 in the formalin test, resulting in a significant leftward shift in the dose-response curves for intrathecal endomorphin-1 antinociception. The enhanced effect was antagonized by intraperitoneal naltrexone. The study suggests that electroacupuncture may potentiate the intrathecal endomorphin-1 antinociception partially mediated by opioid receptors.

The role of dopamine in the nucleus accumbens in analgesia

Opioid and psychostimulant drugs have long been used for the relief of chronic pain in the clinical situation. Animal studies confirm that these drugs alleviate persistent or tonic pain. Little is known, however, about the neural systems underlying the suppression of tonic pain except that they are different from those mediating the suppression of phasic (i.e., sharp and short-lasting) pain. Although spinal and brainstem-descending pain suppression mechanisms play a role in mediating the inhibition of tonic pain, it appears that this response is additionally mediated by the activation of mechanisms lying rostral to the brainstem. Recent studies suggest that the activation of mesolimbic dopamine (DA) neurons, arising from the cell bodies of the ventral tegmental area (VTA) and projecting to the nucleus accumbens (NAcc), plays an important role in mediating the suppression of tonic pain. Other studies suggest that this pain-suppression system involving the activation of mesolimbic DA neurons is naturally triggered by exposure to stress, through the endogenous release of opioids and substance P (SP) in the midbrain.

Potentiation of opioid analgesia by psychostimulant drugs: a review

Recent research has investigated drug combinations that enhance the analgesic effectiveness of their component substances. Many studies have examined the combination of opioids and psychostimulant drugs, such as amphetamine and methylphenidate. Despite the positive results reported in the literature, this combination is rarely used in clinical practice. The purpose of this paper is to review the literature on the opioid-amphetamine combination. Experiments with animal and human subjects provide convincing evidence that d-amphetamine or methylphenidate potentiate the analgesic effects of morphine. Psychostimulant drugs have been shown in animal studies to possess intrinsic analgesic properties and to have the ability to enhance the analgesic properties of opioids when both types of drugs are given in combination. Studies with human subjects have confirmed the enhancement of opioid analgesia by amphetamines and, in addition, have demonstrated that psychostimulant drugs produce a decrease in somnolence and an increase in general cognitive abilities. The greater cognitive alertness, moreover, allows the use of larger opioid doses, which can produce a substantial increase in analgesia. These results indicate another possible method to enhance the quality of life in patients with difficult pain problems. Although the enhanced cognitive effects are well established, the effects on pain need further study to determine the mechanisms of action and the drug combinations and administration patterns that would maximize their effects.