Is Electrical Brain Activity a Reliable Biomarker for Opioid Analgesia in the Gut?

Objective methods for the assessment of the spinal and supraspinal effects of opioids

BACKGROUND AND PURPOSE

Opioids are potent analgesics. Opioids exert effects after interaction with opioid receptors. Opioid receptors are present in the peripheral- and central nervous system (CNS), but the analgesic effects are primarily mediated via receptors in the CNS. Objective methods for assessment of opioid effects may increase knowledge on the CNS processes responsible for analgesia. The aim of this review was to provide an overview of the most common objective methods for assessment of the spinal and supraspinal effects of opioids and discuss their advantages and limitations.

METHOD

The literature search was conducted in Pub Med (http://www.ncbi.nlm.nih.gov/pubmed) from November 2014 to June 2016, using free-text terms: "opioid", "morphine" and "oxycodone" combined with the terms "pupillometry," "magnetic resonance spectroscopy," "fMRI," "BOLD," "PET," "pharmaco-EEG", "electroencephalogram", "EEG," "evoked potentials," and "nociceptive reflex". Only original articles published in English were included.

RESULTS

For assessment of opioid effects at the supraspinal level, the following methods are evaluated: pupillometry, proton magnetic resonance spectroscopy, functional resonance magnetic imaging (fMRI), positron emission tomography (PET), spontaneous electroencephalogram (EEG) and evoked potentials (EPs). Pupillometry is a non-invasive tool used in research as well as in the clinical setting. Proton magnetic resonance spectroscopy has been used for the last decades and it is a non-invasive technique for measurement of in vivo brain metabolite concentrations. fMRI has been a widely used non-invasive method to estimate brain activity, where typically from the blood oxygen level-dependent (BOLD) signal. PET is a nuclear imaging technique based on tracing radio labeled molecules injected into the blood, where receptor distribution, density and activity in the brain can be visualized. Spontaneous EEG is typically quantified in frequency bands, power spectrum and spectral edge frequency. EPs are brain responses (assessed by EEG) to a predefined number of short phasic stimuli. EPs are quantified by their peak latencies and amplitudes, power spectrum, scalp topographies and brain source localization. For assessment of opioid effects at the spinal level, the following methods are evaluated: the nociceptive withdrawal reflex (NWR) and spinal EPs. The nociceptive withdrawal reflex can be recorded from all limbs, but it is standard to record the electromyography signal at the biceps femoris muscle after stimulation of the ipsilateral sural nerve; EPs can be recorded from the spinal cord and are typically recorded after stimulation of the median nerve at the wrist.

CONCLUSION AND IMPLICATIONS

The presented methods can all be used as objective methods for assessing the centrally mediated effects of opioids. Advantages and limitations should be considered before implementation in drug development, future experimental studies as well as in clinical settings. In conclusion, pupillometry is a sensitive measurement of opioid receptor activation in the CNS and from a practical and economical perspective it may be used as a biomarker for opioid effects in the CNS. However, if more detailed information is needed on opioid effects at different levels of the CNS, then EEG, fMRI, PET and NWR have the potential to be used. Finally, it is conceivable that information from different methods should be considered together for complementary information.

Pharmacokinetic models of morphine and its metabolites in neonates:: Systematic comparisons of models from the literature, and development of a new meta-model

Morphine is commonly used for pain management in preterm neonates. The aims of this study were to compare published models of neonatal pharmacokinetics of morphine and its metabolites with a new dataset, and to combine the characteristics of the best predictive models to design a meta-model for morphine and its metabolites in preterm neonates. Moreover, the concentration-analgesia relationship for morphine in this clinical setting was also investigated. A population of 30 preterm neonates (gestational age: 23-32weeks) received a loading dose of morphine (50-100μg/kg), followed by a continuous infusion (5-10μg/kg/h) until analgesia was no longer required. Pain was assessed using the Premature Infant Pain Profile. Five published population models were compared using numerical and graphical tests of goodness-of-fit and predictive performance. Population modelling was conducted using NONMEM® and the $PRIOR subroutine to describe the time-course of plasma concentrations of morphine, morphine-3-glucuronide, and morphine-6-glucuronide, and the concentration-analgesia relationship for morphine. No published model adequately described morphine concentrations in this new dataset. Previously published population pharmacokinetic models of morphine, morphine-3-glucuronide, and morphine-6-glucuronide were combined into a meta-model. The meta-model provided an adequate description of the time-course of morphine and the concentrations of its metabolites in preterm neonates. Allometric weight scaling was applied to all clearance and volume terms. Maturation of morphine clearance was described as a function of postmenstrual age, while maturation of metabolite elimination was described as a function of postnatal age. A clear relationship between morphine concentrations and pain score was not established.

Multivariate analysis of single-sweep evoked brain potentials for pharmaco-electroencephalography

BACKGROUND AND AIMS

Current findings on altered evoked potentials (EPs) caused by morphine are based on common alterations for a group of subjects after drug administration. However, this ignores the analysis of individual responses, which may explain the clinical differences in efficacy. Therefore, we explored the individual responses to morphine in terms of the altered single-sweep characteristics in a placebo-controlled crossover study. To account for multifactorial mechanisms, several characteristics were assessed simultaneously by multivariate pattern analysis (MVPA).

METHODS

EPs were recorded from 62 channels and obtained before and after morphine and placebo administration during repeated electrical stimulations of the oesophagus in 12 healthy males. Additionally, the pain detection threshold was recorded to reflect the subjective analgesic effect in each subject. The characteristics of the sweeps were extracted by a multivariate matching pursuit algorithm with Gabor atoms implemented with a variable amplitude and constant phase across the sweeps. The single-sweep amplitudes were used as input to an MVPA algorithm to discriminate individual responses. The accuracy of the MVPA for each individual subject was used for correlation analysis of the analgesic effect.

RESULTS

The mean classification accuracy when discriminating pre- and posttreatment morphine responses was 72% (p = 0.01). The individual classification accuracy was positively correlated to the analgesic effect of morphine (p = 0.03). Furthermore, the 2 posttreatment responses were classified and validated by the classification of the 2 pretreatment responses (p = 0.001).

CONCLUSIONS

The alterations in the single-sweep EPs after morphine reflect the analgesic effect. The MVPA approach is a novel methodology for monitoring the individual efficacy of analgesics.

Single-sweep spectral analysis of contact heat evoked potentials: a novel approach to identify altered cortical processing after morphine treatment

AIMS

The cortical response to nociceptive thermal stimuli recorded as contact heat evoked potentials (CHEPs) may be altered by morphine. However, previous studies have averaged CHEPs over multiple stimuli, which are confounded by jitter between sweeps. Thus, the aim was to assess single-sweep characteristics to identify alterations induced by morphine.

METHODS

In a crossover study 15 single-sweep CHEPs were analyzed from 62 electroencephalography electrodes in 26 healthy volunteers before and after administration of morphine or placebo. Each sweep was decomposed by a continuous wavelet transform to obtain normalized spectral indices in the delta (0.5-4 Hz), theta (4-8 Hz), alpha (8-12 Hz), beta (12-32 Hz) and gamma (32-80 Hz) bands. The average distribution over all sweeps and channels was calculated for the four recordings for each volunteer, and the two recordings before treatments were assessed for reproducibility. Baseline corrected spectral indices after morphine and placebo treatments were compared to identify alterations induced by morphine.

RESULTS

Reproducibility between baseline CHEPs was demonstrated. As compared with placebo, morphine decreased the spectral indices in the delta and theta bands by 13% (P = 0.04) and 9% (P = 0.007), while the beta and gamma bands were increased by 10% (P = 0.006) and 24% (P = 0.04).

CONCLUSION

The decreases in the delta and theta band are suggested to represent a decrease in the pain specific morphology of the CHEPs, which indicates a diminished pain response after morphine administration. Hence, assessment of spectral indices in single-sweep CHEPs can be used to study cortical mechanisms induced by morphine treatment.

The Effect of Oral Morphine on Pain-Related Brain Activation - An Experimental Functional Magnetic Resonance Imaging Study

Knowledge about cerebral mechanisms underlying pain perception and effect of analgesic drugs is important for developing methods for diagnosis and treatment of pain. The aim was to explore altered brain activation before and after morphine treatment using functional magnetic resonance imaging recorded during experimental painful heat stimulation. Functional magnetic resonance imaging data were recorded and analysed in 20 healthy volunteers (13 men and 7 women, 24.9 ± 2.6 years) in a randomized, double-blind, placebo-controlled, cross-over study. Painful stimulations were applied to the right forearm using a contact heat evoked potential stimulator (CHEPS) before and after treatment with 30 mg oral morphine and placebo. CHEPS stimulations before treatment induced activation in the anterior cingulate cortex, secondary somatosensory cortex/insula, thalamus and cerebellum (n = 16, p < 0.05). In response to morphine treatment, the spatial extent of these pain-specific areas decreased (n = 20). Reduced pain-induced activation was seen in the right insula, anterior cingulate cortex and inferior parietal cortex after morphine treatment compared to before treatment (n = 16, p < 0.05), and sensory ratings of pain perception were significantly reduced after morphine treatment (p = 0.02). No effect on pain-induced brain activation was seen after placebo treatment compared to before treatment (n = 12, p > 0.05). In conclusion, heat stimulation activated areas in the 'pain matrix' and a clinically relevant dose of orally administered morphine revealed significant changes in brain areas where opioidergic pathways are predominant. The method may be useful to investigate the mechanisms of analgesics.

Electroencephalography and analgesics

To assess centrally mediated analgesic mechanisms in clinical trials with pain patients, objective standardized methods such as electroencephalography (EEG) has many advantages. The aim of this review is to provide the reader with an overview of present findings in analgesics assessed with spontaneous EEG and evoked brain potentials (EPs) in humans. Furthermore, EEG methodologies will be discussed with respect to translation from animals to humans and future perspectives in predicting analgesic efficacy. We searched PubMed with MeSH terms 'analgesics', 'electroencephalography' and 'evoked potentials' for relevant articles. Combined with a search in their reference lists 15 articles on spontaneous EEG and 55 papers on EPs were identified. Overall, opioids produced increased activity in the delta band in the spontaneous EEG, but increases in higher frequency bands were also seen. The EP amplitudes decreased in the majority of studies. Anticonvulsants used as analgesics showed inconsistent results. The N-methyl-D-aspartate receptor antagonist ketamine showed an increase in the theta band in spontaneous EEG and decreases in EP amplitudes. Tricyclic antidepressants increased the activity in the delta, theta and beta bands in the spontaneous EEG while EPs were inconsistently affected. Weak analgesics were mainly investigated with EPs and a decrease in amplitudes was generally observed. This review reveals that both spontaneous EEG and EPs are widely used as biomarkers for analgesic drug effects. Methodological differences are common and a more uniform approach will further enhance the value of such biomarkers for drug development and prediction of treatment response in individual patients.

A novel approach to pharmaco-EEG for investigating analgesics: assessment of spectral indices in single-sweep evoked brain potentials

AIMS

To compare results from analysis of averaged and single-sweep evoked brain potentials (EPs) by visual inspection and spectral analysis in order to identify an objective measure for the analgesic effect of buprenorphine and fentanyl.

METHODS

Twenty-two healthy males were included in a randomized study to assess the changes in EPs after 110 sweeps of painful electrical stimulation to the median nerve following treatment with buprenorphine, fentanyl or placebo patches. Bone pressure, cutaneous heat and electrical pain ratings were assessed. EPs and pain assessments were obtained before drug administration, 24, 48, 72 and 144 h after beginning of treatment. Features from EPs were extracted by three different approaches: (i) visual inspection of amplitude and latency of the main peaks in the average EPs, (ii) spectral distribution of the average EPs and (iii) spectral distribution of the EPs from single-sweeps.

RESULTS

Visual inspection revealed no difference between active treatments and placebo (all P > 0.05). Spectral distribution of the averaged potentials showed a decrease in the beta (12-32 Hz) band for fentanyl (P = 0.036), which however did not correlate with pain ratings. Spectral distribution in the single-sweep EPs revealed significant increases in the theta, alpha and beta bands for buprenorphine (all P < 0.05) as well as theta band increase for fentanyl (P = 0.05). For buprenorphine, beta band activity correlated with bone pressure and cutaneous heat pain (both P = 0.04, r = 0.90).

CONCLUSION

In conclusion single-sweep spectral band analysis increases the information on the response of the brain to opioids and may be used to identify the response to analgesics.

Peripheral and central nervous contribution to gastrointestinal symptoms in diabetic patients with autonomic neuropathy

BACKGROUND & AIMS

Long-term diabetes mellitus (DM) has been associated with neuronal changes in the enteric, peripheral and/or central nervous system. Moreover, abnormal visceral sensation and gastrointestinal (GI) symptoms are seen in up to 75% of patients. To explore the role of diabetic autonomic neuropathy (DAN) in patients with long-standing DM, we investigated psychophysical responses and neuronal activity recorded as evoked brain potentials and dipolar source modelling.

METHODS

Fifteen healthy volunteers and 14 type-1 DM patients with DAN were assessed with a symptom score index characterizing upper GI abnormalities. Multichannel (62) electroencephalography was recorded during painful electrical stimulation of the lower oesophagus. Brain activity to painful stimulations was modelled using Brain Electrical Source Analysis (besa).

RESULTS

Diabetic patients had higher stimulus intensities to evoke painful sensation (p ≤ 0.001), longer latencies of N2 and P2 components (both p ≤ 0.001), and lower amplitudes of P1-N2 and N2-P2 complexes (p ≤ 0.001; p = 0.02). Inverse modelling of brain sources showed deeper bilateral insular dipolar source localization (p = 0.002). Symptom score index was negatively correlated with the depth of insular activity (p = 0.004) and positively correlated with insular dipole strength (p = 0.03).

CONCLUSION

DM patients show peripheral and central neuroplastic changes. Moreover, the role of abnormal insular processing may explain the appearance and persistence of GI symptoms related to DAN. This enhanced understanding of DAN may have future clinical and therapeutical implications.

Human experimental pain models for assessing the therapeutic efficacy of analgesic drugs

Pain models in animals have shown low predictivity for analgesic efficacy in humans, and clinical studies are often very confounded, blurring the evaluation. Human experimental pain models may therefore help to evaluate mechanisms and effect of analgesics and bridge findings from basic studies to the clinic. The present review outlines the concept and limitations of human experimental pain models and addresses analgesic efficacy in healthy volunteers and patients. Experimental models to evoke pain and hyperalgesia are available for most tissues. In healthy volunteers, the effect of acetaminophen is difficult to detect unless neurophysiological methods are used, whereas the effect of nonsteroidal anti-inflammatory drugs could be detected in most models. Anticonvulsants and antidepressants are sensitive in several models, particularly in models inducing hyperalgesia. For opioids, tonic pain with high intensity is attenuated more than short-lasting pain and nonpainful sensations. Fewer studies were performed in patients. In general, the sensitivity to analgesics is better in patients than in healthy volunteers, but the lower number of studies may bias the results. Experimental models have variable reliability, and validity shall be interpreted with caution. Models including deep, tonic pain and hyperalgesia are better to predict the effects of analgesics. Assessment with neurophysiologic methods and imaging is valuable as a supplement to psychophysical methods and can increase sensitivity. The models need to be designed with careful consideration of pharmacological mechanisms and pharmacokinetics of analgesics. Knowledge obtained from this review can help design experimental pain studies for new compounds entering phase I and II clinical trials.