Acute Experimental Pain Responses in Methadone- and Buprenorphine/Naloxone-Maintained Patients Administered Additional Opioid or Gabapentin: A Double-Blind Crossover Pilot Study.

OBJECTIVE: The study objective was to identify the analgesic efficacy of three different pharmacological strategies in patients receiving methadone or buprenorphine as opioid agonist treatment (OAT). The three pharmacological approaches, a) increasing maintenance methadone/buprenorphine dose by 30%, b) adding oxycodone, or c) adding a single dose of gabapentin, were compared with a control condition of the participant's usual OAT dose. DESIGN: A randomized, controlled, double-blinded, double-dummy, within-subject crossover study. SUBJECTS: Nine participants on stable doses of methadone and eight participants on stable doses of buprenorphine were recruited. SETTING: An outpatient opioid treatment clinic in inner city Sydney, Australia. METHODS: The cold pressor tolerance test was used to examine experimental pain threshold and tolerance. Ratings of subjective drug effects and safety measures (physiological and cognitive) were assessed. RESULTS: There was no difference in the primary outcome measures of pain thresholds or tolerance between the conditions examined. Interindividual variability was evident. Differences in some subjective measures were identified, including lower pain recall, lower "bad effects," and higher global satisfaction in the additional methadone condition. In the buprenorphine arm, increased drug liking and "bad effects" were detected with oxycodone administration, while increased subjective intoxication was identified with gabapentin. CONCLUSIONS: There was no evidence of an objective improvement in analgesia with any condition compared with control. Further research is required to optimize pain management strategies in this population.

Do gabapentin or pregabalin directly modulate the µ receptor?

BACKGROUND: Pregabalin and gabapentin improve neuropathic pain symptoms but there are emerging concerns regarding their misuse. This is more pronounced among patients with substance use disorder, particularly involving opioids. Co-ingestion of gabapentinoids with opioids is increasingly identified in opioid related deaths, however, the molecular mechanism behind this is still unclear. We have sought to determine whether pregabalin or gabapentin directly modulates acute µ receptor signaling, or µ receptor activation by morphine. METHODS: The effects of pregabalin and gabapentin were assessed in HEK 293 cells stably transfected with the human µ receptor. Their effect on morphine induced hyperpolarization, cAMP production and ERK phosphorylation were studied using fluorescent-based membrane potential assay, bioluminescence based CAMYEL assay and ELISA assay, respectively. Pregabalin/gabapentin effects on morphine-induced hyperpolarization were also investigated in AtT20 cells. RESULTS: Pregabalin or gabapentin (1 µM, 100 µM each) did not activate the µ receptor or affect K channel activation or ERK phosphorylation produced by morphine. Neither drug affected the desensitization of K channel activation produced by prolonged (30 min) application of morphine. Gabapentin (1 µM, 100 µM) and pregabalin (1 µM) did not affect inhibition of forskolin-stimulated cAMP production by morphine. However, pregabalin (100 µM) potentiated forskolin mediated cAMP production, although morphine still inhibited cAMP levels with a similar potency to control. DISCUSSION: Pregabalin or gabapentin did not activate or modulate µ receptor signaling in three different assays. Our data do not support the hypothesis that gabapentin or pregabalin augment opioid effects through direct or allosteric modulation of the µ receptor. Pregabalin at a high concentration increases cAMP production independent of morphine. The mechanism of enhanced opioid-related harms from co-ingestion of pregabalin or gabapentin with opioids needs further investigation.