Venlafaxine and oxycodone effects on human spinal and supraspinal pain processing: a randomized cross-over trial

Spinal Excitability in Patients with Painful Chronic Pancreatitis

Purpose

Abdominal pain is common in patients with chronic pancreatitis (CP), but management is challenging - possibly due to altered pain processing within the central nervous system rendering conventional treatments ineffective. We hypothesized that many patients with painful CP have generalized hyperalgesia correlating with central neuronal hyperexcitability.

Patients and Methods

Seventeen CP patients with pain and 20 matched healthy controls underwent experimental pain testing, including repeated pain stimuli (temporal summation), pressure algometry performed in dermatomes with same spinal innervation as the pancreatic gland (pancreatic areas) and remote dermatomes (control areas), a cold pressor test and a conditioned pain modulation paradigm. To probe central neuronal excitability, the nociceptive withdrawal reflex was elicited by electrical stimulation of the plantar skin, and electromyography was obtained from the ipsilateral anterior tibial muscle together with somatosensory evoked brain potentials.

Results

Compared to healthy controls, patients with painful CP had generalized hyperalgesia as evidenced by 45% lower pressure pain detection thresholds (P<0.05) and decreased cold pressor endurance time (120 vs 180 seconds, P<0.001). In patients, reflex thresholds were lower (14 vs 23 mA, P=0.02), and electromyographic responses were increased (16.4 vs 9.7, P=0.04) during the withdrawal reflex, reflecting predominantly spinal hyperexcitability. Evoked brain potentials did not differ between groups. A positive correlation was found between reflex thresholds and cold pressor endurance time (ρ=0.71, P=0.004).

Conclusion

We demonstrated somatic hyperalgesia in patients with painful CP associated with spinal hyperexcitability. This highlights that management should be directed at central mechanisms using, eg, gabapentinoids or serotonin-noradrenaline reuptake inhibitors.

Influence of tapentadol and oxycodone on the spinal cord and brain using electrophysiology: a randomized, placebo-controlled trial

AIMS

The aim of this study was to investigate the effects of tapentadol and oxycodone using the nociceptive withdrawal reflex and sensory evoked potentials.

METHODS

Twenty-one healthy volunteers completed a cross-over trial with oxycodone (10 mg), tapentadol (50 mg) extended-release tablets, or placebo treatment administered orally BID for 14 days. Electrical stimulations were delivered on the plantar side of the foot to evoke a nociceptive withdrawal reflex at baseline and post-interventions. Electromyography, recorded at tibialis anterior, and electroencephalography were recorded for analysis of: number of reflexes, latencies, and area under the curve of the nociceptive withdrawal reflex as well as latencies, amplitudes and dipole sources of the sensory-evoked potential.

RESULTS

Tapentadol decreased the odds ratio of eliciting nociceptive withdrawal reflex by -0.89 (P = .001, 95% confidence interval [CI] -1.46, -0.32), whereas oxycodone increased the latency of the N1 component of the sensory-evoked potential at the vertex by 12.5 ms (P = .003, 95% CI 3.35, 21.69). Dipole sources revealed that the anterior cingulate component moved caudally for all three interventions (all P < .02), and the insula components moved caudally in both the oxycodone and tapentadol arms (all P < .03).

CONCLUSION

A decrease in the number of nociceptive withdrawal reflex was observed during tapentadol treatment, possibly relating to the noradrenaline reuptake inhibition effects on the spinal cord. Both oxycodone and tapentadol affected cortical measures possible due to μ-opioid receptor agonistic effects evident in the dipole sources, with the strongest effect being mediated by oxycodone. These findings could support the dual effect analgesic mechanisms of tapentadol in humans as previously shown in preclinical studies.

Involvement of A5/A7 noradrenergic neurons and B2 serotonergic neurons in nociceptive processing: a fiber photometry study

In the central nervous system, the A6 noradrenaline (NA) and the B3 serotonin (5-HT) cell groups are well-recognized players in the descending antinociceptive system, while other NA/5-HT cell groups are not well characterized. A5/A7 NA and B2 5-HT cells project to the spinal horn and form descending pathways. We recorded G-CaMP6 green fluorescence signal intensities in the A5/A7 NA and the B2 5-HT cell groups of awake mice in response to acute tail pinch stimuli, acute heat stimuli, and in the context of a non-noxious control test, using fiber photometry with a calcium imaging system. We first introduced G-CaMP6 in the A5/A7 NA or B2 5-HT neuronal soma, using transgenic mice carrying the tetracycline-controlled transactivator transgene under the control of either a dopamine β-hydroxylase or a tryptophan hydroxylase-2 promoters and by the site-specific injection of adeno-associated virus (AAV-TetO(3G)-G-CaMP6). After confirming the specific expression patterns of G-CaMP6, we recorded G-CaMP6 green fluorescence signals in these sites in awake mice in response to acute nociceptive stimuli. G-CaMP6 fluorescence intensity in the A5, A7, and B2 cell groups was rapidly increased in response to acute nociceptive stimuli and soon after, it returned to baseline fluorescence intensity. This was not observed in the non-noxious control test. The results indicate that acute nociceptive stimuli rapidly increase the activities of A5/A7 NA or B2 5-HT neurons but the non-noxious stimuli do not. The present study suggests that A5/A7 NA or B2 5-HT neurons play important roles in nociceptive processing in the central nervous system. We suggest that A5/A7/B2 neurons may be new therapeutic targets. All performed procedures were approved by the Institutional Animal Use Committee of Kagoshima University (MD17105) on February 22, 2018.

Opioid Specific Effects on Central Processing of Sensation and Pain: A Randomized, Cross-Over, Placebo-Controlled Study

Moderate to severe pain is often treated with opioids, but central mechanisms underlying opioid analgesia are poorly understood. Findings thus far have been contradictory and none could infer opioid specific effects. This placebo-controlled, randomized, 2-way cross-over, double-blinded study aimed to explore opioid specific effects on central processing of external stimuli. Twenty healthy male volunteers were included and 3 sets of assessments were done at each of the 2 visits: 1) baseline, 2) during continuous morphine or placebo intravenous infusion and 3) during simultaneous morphine + naloxone or placebo infusion. Opioid antagonist naloxone was introduced in order to investigate opioid specific effects by observing which morphine effects are reversed by this intervention. Quantitative sensory testing, spinal nociceptive withdrawal reflexes (NWR), spinal electroencephalography (EEG), cortical EEG responses to external stimuli and resting EEG were measured and analyzed. Longer lasting pain (cold-pressor test - hand in 2° water for 2 minutes, tetanic electrical), deeper structure pain (bone pressure) and strong nociceptive (NWR) stimulations were the most sensitive quantitative sensory testing measures of opioid analgesia. In line with this, the principal opioid specific central changes were seen in NWRs, EEG responses to NWRs and cold-pressor EEG. The magnitude of NWRs together with amplitudes and insular source strengths of the corresponding EEG responses were attenuated. The decreases in EEG activity were correlated to subjective unpleasantness scores. Brain activity underlying slow cold-pressor EEG (1-4Hz) was decreased, whereas the brain activity underlying faster EEG (8-12Hz) was increased. These changes were strongly correlated to subjective pain relief. This study points to evidence of opioid specific effects on perception of external stimuli and the underlying central responses. The analgesic response to opioids is likely a synergy of opioids acting at both spinal and supra-spinal levels of the central nervous system. Due to the strong correlations with pain relief, the changes in EEG signals during cold-pressor test have the potential to serve as biomarkers of opioid analgesia. PERSPECTIVE: This exploratory study presents evidence of opioid specific effects on the pain system at peripheral and central levels. The findings give insights into which measures are the most sensitive for assessing opioid-specific effects.

The molecular neurobiology and neuropathology of opioid use disorder

The number of people diagnosed with opioid use disorder has skyrocketed as a consequence of the opioid epidemic and the increased prescribing of opioid drugs for chronic pain relief. Opioid use disorder is characterized by loss of control of drug taking, continued drug use in the presence of adverse consequences, and repeated relapses to drug taking even after long periods of abstinence. Patients who suffer from opioid use disorder often present with cognitive deficits that are potentially secondary to structural brain abnormalities that vary according to the chemical composition of the abused opioid. This review details the neurobiological effects of oxycodone, morphine, heroin, methadone, and fentanyl on brain neurocircuitries by presenting the acute and chronic effects of these drugs on the human brain. In addition, we review results of neuroimaging in opioid use disorder patients and/or histological studies from brains of patients who had expired after acute intoxication following long-term use of these drugs. Moreover, we include relevant discussions of the neurobiological mechanisms involved in promoting abnormalities in the brains of opioid-exposed patients. Finally, we discuss how novel strategies could be used to provide pharmacological treatment against opioid use disorder.

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Brain abnormalities caused by opioid intoxication.

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Intoxication of opioids leads to defects in brain neurocircuitries.

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Insight into the molecular mechanisms associated with craving in heroin addicts.

Objective Pain Assessment: a Key for the Management of Chronic Pain

The individual and social burdens associated with chronic pain have been escalating globally. Accurate pain measurement facilitates early diagnosis, disease progression monitoring and therapeutic efficacy evaluation, thus is a key for the management of chronic pain. Although the "golden standards" of pain measurement are self-reported scales in clinical practice, the reliability of these subjective methods could be easily affected by patients' physiological and psychological status, as well as the assessors' predispositions. Therefore, objective pain assessment has attracted substantial attention recently. Previous studies of functional magnetic resonance imaging (fMRI) revealed that certain cortices and subcortical areas are commonly activated in subjects suffering from pain. Dynamic pain connectome analysis also found various alterations of neural network connectivity that are correlated with the severity of clinical pain symptoms. Electroencephalograph (EEG) demonstrated suppressed spontaneous oscillations during pain experience. Spectral power and coherence analysis of EEG also identified signatures of different types of chronic pain. Furthermore, fMRI and EEG can visualize objective brain activities modulated by analgesics in a mechanism-based way, thus bridge the gaps between animal studies and clinical trials. Using fMRI and EEG, researchers are able to predict therapeutic efficacy and identify personalized optimal first-line regimens. In the future, the emergence of magnetic resonance spectroscopy and cell labelling in MRI would encourage the investigation on metabolic and cellular pain biomarkers. The incorporation of machine learning algorithms with neuroimaging or behavior analysis could further enhance the specificity and accuracy of objective pain assessments.

Differential effects of oxycodone and venlafaxine on resting state functional connectivity-A randomized placebo-controlled magnetic resonance imaging study

AIM

Different mechanisms may be involved in the antinociceptive effects of oxycodone (opioid) and venlafaxine (serotonin-norepinephrine reuptake inhibitor), and the aim of this study was to investigate the effect of these drugs on brain functional connectivity.

METHODS

Resting state functional magnetic resonance imaging was acquired in 20 healthy volunteers before and after a 5-day treatment with oxycodone, venlafaxine, or placebo in a randomized, double-blind, crossover study. Functional connectivity analyses were performed between four predefined seeds (dorsal anterior cingulate cortex, rostral anterior cingulate cortex, posterior insula, and prefrontal cortex), and the whole brain.

RESULTS

The overall interpretation was that there were differences between the effects of oxycodone and venlafaxine on functional connectivity. Oxycodone mainly showed decreased functional connectivity between limbic structures and to supralimbic areas (all P < 0.05). Venlafaxine also showed decreased functional connectivity between limbic structures and to supralimbic areas, but increased functional connectivity to structures in the midbrain and brain stem was also found (all P < 0.05).

CONCLUSIONS

Oxycodone and venlafaxine showed differential effects on resting-state functional connectivity as compared to placebo. This supports that the two drugs exert different mechanisms, and that the drugs in combination may exert additive effects and could potentially improve pain therapy.

Depression and chronic pain in the elderly: links and management challenges

Aging is an inevitable process and represents the accumulation of bodily alterations over time. Depression and chronic pain are highly prevalent in elderly populations. It is estimated that 13% of the elderly population will suffer simultaneously from the two conditions. Accumulating evidence suggests than neuroinflammation plays a critical role in the pathogenesis of both depression and chronic pain. Apart from the common pathophysiological mechanisms, however, the two entities have several clinical links. Their management is challenging for the pain physician; however, both pharmacologic and nonpharmacologic approaches are available and can be used when the two conditions are comorbid in the elderly patients.