The Effect of Combined Treatment of Opioids With Methylphenidate on Nociception in Rats and Pain in Human

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.

Case report: Methylphenidate improved chronic pain in an adult patient with attention deficit hyperactivity disorder

Introduction

Chronic pain remains a health problem that is difficult to treat adequately. Its unknown cause and complex comorbidity with other illnesses, including mental disorders, amplify the severity of symptoms, which consequently decreases the quality of life of patients long term. In our clinical practice, we coincidentally found evidence that methylphenidate (MPH) effectively managed chronic pain in an adult patient with attention deficit hyperactivity disorder (ADHD). The effectiveness of MPH in the treatment of ADHD is well-established; however, its utility in treating pain remains unclear.

Case presentation

We present a rare case of a 43-year-old male patient with 15 years of chronic idiopathic pain symptoms that did not adequately respond to standard pain management, such as acetaminophen, non-opioid analgesics, and muscle relaxers. Pain also persisted after treatments with antidepressants and an epidural block. Furthermore, symptoms worsened following several sessions of modified electroconvulsive therapy. After a thorough assessment at our child and adolescent psychiatric outpatient clinic, we confirmed a diagnosis of adult ADHD with a predominantly inattentive type. Considering this newly established diagnosis, we prescribed osmotic-release oral system (OROS) methylphenidate. Within 1 month of treatment at a dose of 18 mg/day of OROS-MPH, the patient’s chronic pain unexpectedly improved dramatically, and the patient no longer experienced pain symptoms. The dosage of OROS-MPH was titrated monthly, reaching 72 mg/day as a maintenance dose, and ADHD symptoms improved after 4 months of treatment. The patient was followed up regularly for 7 years during his OROS-MPH treatment. No adverse effects were reported, including stimulant addiction. He was stable overall and functioned well in his daily activities. His pain never recurred.

Conclusion

This case report suggests that MPH may be potentially effective in treating chronic pain. Further studies are needed to confirm whether MPH improved chronic pain simultaneously with or separately from the improvement in ADHD. Moreover, elucidating the anatomical sites and molecular pharmacological mechanisms related to the action of MPH in pain modulation and perception is essential. Such sites include the descending dopaminergic pain pathway and higher cortical areas. Furthering our understanding may reinforce the justification for treating chronic pain using MPH.

Methylphenidate and Morphine Combination Therapy in a Rat Model of Chronic Pain

BACKGROUND

The incremental dose of opioids used in chronic pain management often leads to a reduced opioid analgesic effect, opioid misuse, and addiction. Central dopamine (DA) dysfunction contributes to the chronicity of pain and a decreased opioid analgesic effect. Methylphenidate (MPH/Ritalin) enhances central DA function by inhibiting DA reuptake. In this study, we used a rat model of chronic pain to examine whether combination of MPH with morphine (MOR) would improve the MOR analgesic effect under a chronic pain condition.

METHODS

Tibiotarsal joint Complete Freund's Adjuvant (CFA) injection in rats was utilized to induce chronic nociception. The analgesic effect of low-dose MPH (0.25 mg/kg), low-dose MOR (2.5 mg/kg), and their combination was examined in CFA rats. Nociceptive behavior was assessed by von Frey test. Conditioned place preference (CPP) and open field tests (OFTs) were used to examine the rewarding behavior and locomotor activity in rats, respectively.

RESULTS

Our findings are as follows: (1) in CFA rats with chronic pain, 2.5 mg/kg of MOR had less analgesic effect than 10 mg/kg of MOR at 28 days after injury (95% confidence intervals [CIs] for difference of means of von Frey threshold in gram: -11.9 [-6.5 to -17.3]); (2) in the 1-hour time window of 30-90 minutes after injection, the combination of MPH (0.25 mg/kg) with MOR (2.5 mg/kg) increased synergistically and prolonged the analgesic effect in CFA rats as compared with MPH or MOR alone (P = .01 for MPH by MOR interaction, and 95% CIs for difference of means of von Frey threshold in gram: 3.3 [1.37-6.12] for the combination versus MPH and 3.2 [1.35-5.74] for the combination versus MOR); (3) at the low dose (0.25 mg/kg), MPH did not increase locomotor activity (MOR + MPH versus MOR, P = .13) nor significantly enhanced MOR reward behavior (MOR + MPH versus MOR, P = .63) in CFA rats.

CONCLUSIONS

Our data suggest that a combination therapy using low-dose MPH and MOR may produce a MOR-sparing effect in chronic pain management.

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.