Effect of xenon on brain injury, neurological outcome, and survival in patients after aneurysmal subarachnoid hemorrhage-study protocol for a randomized clinical trial

BACKGROUND

Aneurysmal subarachnoid hemorrhage (aSAH) is a neurological emergency, affecting a younger population than individuals experiencing an ischemic stroke; aSAH is associated with a high risk of mortality and permanent disability. The noble gas xenon has been shown to possess neuroprotective properties as demonstrated in numerous preclinical animal studies. In addition, a recent study demonstrated that xenon could attenuate a white matter injury after out-of-hospital cardiac arrest.

METHODS

The study is a prospective, multicenter phase II clinical drug trial. The study design is a single-blind, prospective superiority randomized two-armed parallel follow-up study. The primary objective of the study is to explore the potential neuroprotective effects of inhaled xenon, when administered within 6 h after the onset of symptoms of aSAH. The primary endpoint is the extent of the global white matter injury assessed with magnetic resonance diffusion tensor imaging of the brain.

DISCUSSION

Despite improvements in medical technology and advancements in medical science, aSAH mortality and disability rates have remained nearly unchanged for the past 10 years. Therefore, new neuroprotective strategies to attenuate the early and delayed brain injuries after aSAH are needed to reduce morbidity and mortality.

TRIAL REGISTRATION

ClinicalTrials.gov NCT04696523. Registered on 6 January 2021. EudraCT, EudraCT Number: 2019-001542-17. Registered on 8 July 2020.

Risk for opioid misuse in chronic pain patients is associated with endogenous opioid system dysregulation

µ-Opioid receptors (MOR) are a major target of endogenous and exogenous opioids, including opioid pain medications. The µ-opioid neurotransmitter system is heavily implicated in the pathophysiology of chronic pain and opioid use disorder and, as such, central measures of µ-opioid system functioning are increasingly being considered as putative biomarkers for risk to misuse opioids. To explore the relationship between MOR system function and risk for opioid misuse, 28 subjects with chronic nonspecific back pain completed a clinically validated measure of opioid misuse risk, the Pain Medication Questionnaire (PMQ), and were subsequently separated into high (PMQ > 21) and low (PMQ ≤ 21) opioid misuse risk groups. Chronic pain patients along with 15 control participants underwent two separate [¹¹C]-carfentanil positron emission tomography scans to explore MOR functional measures: one at baseline and one during a sustained pain-stress challenge, with the difference between the two providing an indirect measure of stress-induced endogenous opioid release. We found that chronic pain participants at high risk for opioid misuse displayed higher baseline MOR availability within the right amygdala relative to those at low risk. By contrast, patients at low risk for opioid misuse showed less pain-induced activation of MOR-mediated, endogenous opioid neurotransmission in the nucleus accumbens. This study links human in vivo MOR system functional measures to the development of addictive disorders and provides novel evidence that MORs and µ-opioid system responsivity may underlie risk to misuse opioids among chronic pain patients.

Brain β-Amyloid and Atrophy in Individuals at Increased Risk of Cognitive Decline

BACKGROUND AND PURPOSE

The relationship between brain β-amyloid and regional atrophy is still incompletely understood in elderly individuals at risk of dementia. Here, we studied the associations between brain β-amyloid load and regional GM and WM volumes in older adults who were clinically evaluated as being at increased risk of cognitive decline based on cardiovascular risk factors.

MATERIALS AND METHODS

Forty subjects (63-81 years of age) were recruited as part of a larger study, the Finnish Geriatric Intervention Study to Prevent Cognitive Impairment and Disability. Neuroimaging consisted of PET using ¹¹C Pittsburgh compound-B and T1-weighted 3D MR imaging for the measurement of brain β-amyloid and GM and WM volumes, respectively. All subjects underwent clinical, genetic, and neuropsychological evaluations for the assessment of cognitive function and the identification of cardiovascular risk factors.

RESULTS

Sixteen subjects were visually evaluated as showing cortical β-amyloid (positive for β-amyloid). In the voxel-by-voxel analyses, no significant differences were found in GM and WM volumes between the samples positive and negative for β-amyloid. However, in the sample positive for β-amyloid, increases in ¹¹C Pittsburgh compound-B uptake were associated with reductions in GM volume in the left prefrontal (P = .02) and right temporal lobes (P = .04).

CONCLUSIONS

Our results show a significant association between increases in brain β-amyloid and reductions in regional GM volume in individuals at increased risk of cognitive decline. This evidence is consistent with a model in which increases in β-amyloid incite neurodegeneration in memory systems before cognitive impairment manifests.

Dopaminergic and serotonergic mechanisms in the modulation of pain: In vivo studies in human brain

Here we review the literature assessing the roles of the brain dopaminergic and serotonergic systems in the modulation of pain as revealed by in vivo human studies using positron emission tomography. In healthy subjects, dopamine D₂/D₃ receptor availability particularly in the striatum and serotonin 5-HT_1A and 5-HT_2A receptor availabilities in the cortex predict the subject's response to tonic experimental pain. High availability of dopamine D₂/D₃ or serotonin 5-HT_2A receptors is associated with high pain intensity, whereas high availability of 5-HT_1A receptors associates with low pain intensity. Chronic neuropathic pain is associated with high striatal dopamine D₂/D₃ receptor availability, for which low endogenous dopamine tone is a plausible explanation, although a compensatory increase in striatal dopamine D₂/D₃ receptor density may also contribute. In contrast, chronic musculoskeletal pain is associated with low baseline availability of striatal dopamine D₂/D₃ receptors. In healthy subjects, brain serotonin 5-HT_1A as well as dopamine D₂/D₃ receptor availabilities associate with the subject's response criterion rather than the capacity to discriminate painful thermal stimuli suggesting that these neurotransmitter systems act mainly on non-sensory rather than sensory factors of thermally induced pain experience. Additionally, 5-HT_1A receptor availability predicts the subject's discriminative ability but not response criterion for non-painful tactile test stimuli, while no such correlation is observed with dopamine D₂/D₃ receptors. These findings suggest that dopamine acting on striatal dopamine D₂/D₃ receptors and serotonin acting on cortical 5-HT_1A and 5-HT_2A receptors contribute to top-down pain regulation in humans.

Building up analgesia in humans via the endogenous μ-opioid system by combining placebo and active tDCS: a preliminary report

Transcranial Direct Current Stimulation (tDCS) is a method of non-invasive brain stimulation that has been frequently used in experimental and clinical pain studies. However, the molecular mechanisms underlying tDCS-mediated pain control, and most important its placebo component, are not completely established. In this pilot study, we investigated in vivo the involvement of the endogenous μ-opioid system in the global tDCS-analgesia experience. Nine healthy volunteers went through positron emission tomography (PET) scans with [11C]carfentanil, a selective μ-opioid receptor (MOR) radiotracer, to measure the central MOR activity during tDCS in vivo (non-displaceable binding potential, BPND)--one of the main analgesic mechanisms in the brain. Placebo and real anodal primary motor cortex (M1/2mA) tDCS were delivered sequentially for 20 minutes each during the PET scan. The initial placebo tDCS phase induced a decrease in MOR BPND in the periaqueductal gray matter (PAG), precuneus, and thalamus, indicating activation of endogenous μ-opioid neurotransmission, even before the active tDCS. The subsequent real tDCS also induced MOR activation in the PAG and precuneus, which were positively correlated to the changes observed with placebo tDCS. Nonetheless, real tDCS had an additional MOR activation in the left prefrontal cortex. Although significant changes in the MOR BPND occurred with both placebo and real tDCS, significant analgesic effects, measured by improvements in the heat and cold pain thresholds, were only observed after real tDCS, not the placebo tDCS. This study gives preliminary evidence that the analgesic effects reported with M1-tDCS, can be in part related to the recruitment of the same endogenous MOR mechanisms induced by placebo, and that such effects can be purposely optimized by real tDCS.

3D-neuronavigation in vivo through a patient's brain during a spontaneous migraine headache

A growing body of research, generated primarily from MRI-based studies, shows that migraine appears to occur, and possibly endure, due to the alteration of specific neural processes in the central nervous system. However, information is lacking on the molecular impact of these changes, especially on the endogenous opioid system during migraine headaches, and neuronavigation through these changes has never been done. This study aimed to investigate, using a novel 3D immersive and interactive neuronavigation (3D-IIN) approach, the endogenous µ-opioid transmission in the brain during a migraine headache attack in vivo. This is arguably one of the most central neuromechanisms associated with pain regulation, affecting multiple elements of the pain experience and analgesia. A 36 year-old female, who has been suffering with migraine for 10 years, was scanned in the typical headache (ictal) and nonheadache (interictal) migraine phases using Positron Emission Tomography (PET) with the selective radiotracer [(11)C]carfentanil, which allowed us to measure µ-opioid receptor availability in the brain (non-displaceable binding potential - µOR BPND). The short-life radiotracer was produced by a cyclotron and chemical synthesis apparatus on campus located in close proximity to the imaging facility. Both PET scans, interictal and ictal, were scheduled during separate mid-late follicular phases of the patient's menstrual cycle. During the ictal PET session her spontaneous headache attack reached severe intensity levels; progressing to nausea and vomiting at the end of the scan session. There were reductions in µOR BPND in the pain-modulatory regions of the endogenous µ-opioid system during the ictal phase, including the cingulate cortex, nucleus accumbens (NAcc), thalamus (Thal), and periaqueductal gray matter (PAG); indicating that µORs were already occupied by endogenous opioids released in response to the ongoing pain. To our knowledge, this is the first time that changes in µOR BPND during a migraine headache attack have been neuronavigated using a novel 3D approach. This method allows for interactive research and educational exploration of a migraine attack in an actual patient's neuroimaging dataset.

μ-Opioid activation in the midbrain during migraine allodynia - brief report II

We investigated in vivo the allodynic response of the central μ-opioid system during spontaneous migraine headaches, following a sustained pain threshold challenge on the trigeminal ophthalmic region. Six migraineurs were scanned during the ictal and interictal phases using positron emission tomography (PET) with the selective μ-opioid receptor (μOR) radiotracer [11C]carfentanil. Females were scanned during the mid-late follicular phase of two separate cycles. Patients showed ictal trigeminal allodynia during the thermal challenge that was concurrent and positively correlated with μOR activation in the midbrain, extending from red nucleus to ventrolateral periaqueductal gray matter. These findings demonstrate for the first time in vivo the high μOR activation in the migraineurs' brains in response to their allodynic experience.

Association of μ-Opioid Activation in the Prefrontal Cortex with Spontaneous Migraine Attacks - Brief Report I

We evaluated in vivo the μ-opioid system during spontaneous episodic migraine headaches. Seven patients were scanned at different phases of their migraine using positron emission tomography with the selective μ-opioid receptor (μOR) radiotracer [11C]carfentanil. In the ictal phase, there was μOR activation in the medial prefrontal cortex, which was strongly associated with the μOR availability level during the interictal phase. Furthermore, μ-opioid binding changes showed moderate negative correlation with the combined extension and severity of the attacks. These results indicate for the first time that there is high μOR activation in the migraineurs' brains during headache attacks in response to their pain.

Striatal μ-opioid receptor availability predicts cold pressor pain threshold in healthy human subjects

Previous PET studies in healthy humans have shown that brain μ-opioid receptor activation during experimental pain is associated with reductions in the sensory and affective ratings of the individual pain experience. The aim of this study was to find out whether brain μ-opioid receptor binding at the resting state, in absence of painful stimulation, can be a long-term predictor of experimental pain sensitivity. We measured μ-opioid receptor binding potential (BP(ND)) with μ-opioid receptor selective radiotracer [(11)C]carfentanil and positron emission tomography (PET) in 12 healthy male subjects. Later, we recruited these subjects to participate in a separate psychophysical testing session to measure cold pressor pain threshold, cold pressor pain tolerance and tactile sensitivity with von Frey monofilaments. We used both voxel-by-voxel and region-of-interest image analyses to examine the potential associations between μ-opioid receptor BP(ND) and psychophysical measures. The results show that striatal μ-opioid receptor BP(ND) predicts cold pressor pain threshold, but not cold pressor pain tolerance or tactile sensitivity. This finding suggests that striatal μ-opioid receptor density is involved in setting individual pain threshold.

Correlation of human cold pressor pain responses with 5-HT(1A) receptor binding in the brain

We determined whether serotonin 5-HT(1A) receptor availability in the brain is associated with cold pressor pain (CPP) or sympathetic reflex responses. Psychophysical testing was performed in eleven healthy males who had participated in a positron emission tomography study with [carbonyl-(11)C]WAY-100635 ligand for the assessment of 5-HT(1A) receptor binding potential (BP). Psychophysical testing consisted of determining CPP threshold, tolerance, intensity, unpleasantness and CPP threshold modulation by conditioning CPP. Autonomic control was assessed by determining the cutaneous vasoconstriction responses in the finger induced by CPP and Valsalva maneuver. CPP intensity was inversely correlated with 5-HT(1A) BP in multiple cortical and subcortical areas, including the prefrontal and cingulate cortices, insula, amygdala and the dorsal raphe. CPP unpleasantness was not significantly correlated with 5-HT(1A) BP in any of the regions of interest. Increase of CPP threshold by conditioning CPP was directly correlated with 5-HT(1A) BP in the amygdala and medial prefrontal cortex. Vasoconstriction induced by Valsalva but not CPP was directly correlated with 5-HT(1A) BP in the ventral part of the anterior cingulate cortex and the anterior insula. The results suggest that 5-HT(1A) receptors in the brain influence pain and Valsalva-induced sympathetic vasoconstriction reflex. In general, subjects with high availability of 5-HT(1A) receptors have low CPP intensity accompanied by a high capacity for central suppression of pain or a sympathetic vasoconstriction response by a Valsalva maneuver.

Association of striatal dopamine D2/D3 receptor binding potential with pain but not tactile sensitivity or placebo analgesia

Striatal dopamine D2/D3 receptors have been suggested to play a role in pain sensitivity and placebo effect. We studied whether the association of dopamine D2/D3 receptor binding potential (BP) with sensory thresholds is specific to the modality of pain, and whether striatal dopamine D2/D3 receptor BP predicts the magnitude of placebo analgesia. Pain and tactile thresholds, and placebo analgesia were assessed in eight healthy human male subjects who had previously participated in a dopamine D2/D3 receptor positron emission tomography study with [11C]raclopride. The results show that the cutaneous heat pain threshold was inversely correlated with dopamine D2/D3 receptor BP in the right putamen, but responses to tactile stimulation did not correlate with striatal dopamine D2/D3 receptor BP. Placebo-induced elevation of the heat pain threshold did not correlate with striatal dopamine D2/D3 receptor BP. These results suggest that the influence of striatal dopamine D2/D3 receptors on sensory thresholds is selective for the modality of pain. Moreover, striatal dopamine D2/D3 receptor BP appears not to predict individual's analgesic response to placebo.

Striatal dopamine D2 receptors in modulation of pain in humans: a review

We review evidence indicating that the striatum and striatal dopamine D2 receptors are involved in the regulation of pain in humans. Painful stimulation produces an increase in regional cerebral blood flow in the human striatum. Pain is a common symptom in patients with nigrostriatal dopaminergic hypofunction. Positron emission tomography findings show that a low dopamine D2 receptor availability in the striatum of healthy subjects (indicating either a low density of dopamine D2 receptors or a high synaptic concentration of dopamine) is associated with a high cold pain threshold and a low capacity to recruit central pain inhibition by conditioning stimulation. Patients with chronic orofacial pain have higher dopamine D2 receptor availability than their age-matched controls. We propose that the striatal dopamine D2 receptor may be an important target for the diagnosis and treatment of chronic pain.

Striatal dopamine D2/D3 receptor availability correlates with individual response characteristics to pain

We studied in healthy humans the contribution of cerebral dopamine D2/D3 receptors to individual differences in response characteristics to painful stimulation. Positron emission tomography was used to measure the dopamine D2/D3 binding potential (D2/D3 BP) with [(11)C]raclopride in the striatum (n = 8) and with [(11)C]FLB 457 in the extrastriatal regions (n = 11). Sensitivity to cutaneous heat pain was assessed by a traditional threshold method and by an analysis based on the signal detection theory which allows the separation of an individual subject's discriminative capacity from the response criterion, i.e. the area under the receiver operating characteristic curve provides a measure of the sensory discriminability (sensory factor) and the response criterion gives an estimate of the subject's response bias or attitude (nonsensory factor). The pain threshold and response criterion were inversely correlated with the D2/D3 BP in the right putamen, whereas the discriminative capacity was not significantly correlated with the D2/D3 BP in any brain region. The correlation of the D2/D3 BP in the putamen with the pain threshold and the subject's response criterion may rather be explained by a dopaminergic effect on nonsensory factors determining the subject's attitude towards pain than by a dopaminergic effect on the subject's discriminative capacity. Alternatively, striatal dopamine D2/D3 receptors could control a modulatory pathway producing a parallel shift in the stimulus-response function for sensory signals, mimicking a change in the subject's response criterion.

Spatial integration of cold pressor pain sensation in humans

Spatial integration of cold pressor pain (CPP) in the hand was studied in healthy human subjects by measuring the latency to the ice water-induced first pain sensation with and without conditioning CPP. CPP alone showed a marked spatial summation effect. When conditioning and test CPP were applied at the same time, conditioning CPP suppressed test CPP both in an adjacent and a distant site. When test CPP was applied after the conditioning CPP (i.e. pain induced by conditioning CPP was considerably stronger than that evoked by test CPP) conditioning CPP suppressed the test CPP only in a distant site but enhanced it in an adjacent site. A decrease in the test stimulus area increased the suppressive effect by conditioning CPP. Thus, CPP shows spatial summation or inhibition depending on experimental parameters.

Kainate down-regulates a subset of GABAA receptor subunits expressed in cultured mouse cerebellar granule cells

The effect of kainate, an agonist selective for ionotropic AMPA/kainate type of glutamate receptors, on GABAA receptor subunit expression in cultured mouse cerebellar granule cells was studied using quantitative RT-PCR, ligand binding and electrophysiology. Chronic kainate treatment, without producing excitotoxicity, resulted in preferential, dose- and time-dependent down-regulation of alpha1, alpha6 and beta2 subunit mRNA expression, the expression of beta3, gamma2 and delta subunit mRNAs being less affected. The down-regulation was reversed by DNQX, an AMPA/kainate-selective glutamate receptor antagonist. A 14-day kainate treatment resulted in 46% decrease of total [3H]Ro 15-4513 binding to the benzodiazepine sites. Diazepam-insensitive [3H]Ro 15-4513 binding was decreased by 89% in accordance with very low amount of alpha6 subunit mRNA present. Diazepam-sensitive [3H]Ro 154513 binding was decreased only by 40%, contrasting >90% decrease in alpha1 subunit mRNA expression. However, this was consistent with lower potentiation of GABA-evoked currents in kainate-treated than control cells by the alpha1-selective benzodiazepine site ligand zolpidem, suggesting compensatory expression of alpha5 (and/or alpha2 or alpha3) subunits producing diazepam-sensitive but zolpidem-insensitive receptor subtypes. In conclusion, chronic kainate treatment of cerebellar granule cells selectively down-regulates oil, alpha6 and beta2 subunits resulting in altered GABAA receptor pharmacology.

Dopamine D2 receptor binding in the human brain is associated with the response to painful stimulation and pain modulatory capacity

The pain modulatory role of dopamine D2 receptors of the human forebrain was studied by determining the association between dopamine D2 receptor binding potential and the response to experimental pain. Nineteen healthy male volunteers participated in a dopamine D2 receptor positron emission tomography study. The extrastriatal regions of interest studied with [11C]FLB 457 as radioligand (n = 11) were the anterior cingulum, the medial and lateral thalamus, the medial and lateral frontal cortex, and the medial and lateral temporal cortex. The striatal regions of interest studied with [11C]raclopride (n = 8) were the caudate nucleus and the putamen. The latency to the ice water-induced cold pain threshold and tolerance were determined in a separate psychophysical test session. Moreover, the cutaneous heat pain threshold and its elevation by concurrent cold pain in the contralateral hand were determined in each subject. Cold pain threshold was inversely correlated with D2 binding potential in the right putamen and the cold pain tolerance was inversely correlated with D2 binding potential in the right medial temporal cortex. The magnitude of heat pain threshold elevation induced by concurrent cold pain was directly correlated with D2 binding potential in the left putamen. Other correlations of D2 binding potentials in varying brain regions with sensory responses were not significant. A psychophysical control study (n = 10) showed that cold pain responses were identical in the right and left hand. The results indicate that dopamine D2 receptor binding potential in the human forebrain, particularly in the striatum, may be an important parameter in determining the individual cold pain response and the potential for central pain modulation. Accordingly, an individual with only few available D2 receptors in the forebrain is likely to have a high tonic level of pain suppression, combined with a low capacity to recruit more (dopaminergic) central pain inhibition by noxious conditioning stimulation.

Spatial discrimination of one versus two test stimuli in the human skin: dissociation of mechanisms depending on the task and the modality of stimulation

We determined whether the ability to localize a single noxious cold versus innocuous tactile stimulus is associated with the ability to discriminate between two stimuli in the forearm skin of healthy human subjects. When single stimuli were applied, the localization of a noxious cold stimulus was at least as good as that of a tactile stimulus. With both of these modalities, the localization of a single stimulus was more accurate in the radial-ulnar than proximo-distal direction. In contrast to localization of a single stimulus, the discrimination of two cutaneous test stimuli from each other was an order of magnitude worse for noxious cold than touch. This dissociation indicates that localization of single cutaneous stimuli and discrimination of two cutaneous stimuli from each other are based on different mechanisms.