Low doses of morphine, the most commonly used opioid analgesic, have been shown to significantly reduce the affective but not the sensory intensive dimension of pain. This suggests differential dose-response relationships of opioid analgesia on the sensory and affective components of pain. We investigated the effects of different alfentanil plasma concentration levels (0, 19.6+/-2.7, 47.2+/-7.6, and 76.6+/-11.3 ng/ml) on pain-related brain activation achieved by short pulses of gaseous CO(2) delivered to the nasal mucosa, using functional magnetic resonance imaging (fMRI) on a 3.0 T MRI scanner in 16 non-carriers and 9 homozygous carriers of the mu-opioid receptor gene variant OPRM1 118A>G. Increasing opioid concentrations had differential effects in brain regions processing the sensory and affective dimensions of pain. In brain regions associated with the processing of the sensory intensity of pain (primary and secondary somatosensory cortices, posterior insular cortex), activation decreased linearly in relation to alfentanil concentrations, which was significantly less pronounced in OPRM1 118G carriers. In contrast, in brain regions known to process the affective dimension of pain (parahippocampal gyrus, amygdala, anterior insula), pain-related activation disappeared at the lowest alfentanil dose, without genotype differences.

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1 The pain relieving properties of imipramine (100 mg orally), tramadol (150 mg orally), and anpirtoline (60 mg orally) were compared in 16 healthy subjects in a cross-over, double-blind, randomized, and placebo-controlled study. Anpirtoline exhibits analgesia which is possibly mediated via serotoninergic pathways, whereas tramadol exerts its effects at opioid receptors. The pain-relieving effect of the tricyclic antidepressant imipramine may involve both serotoninergic and opioid mechanisms. 2 Chemo-somatosensory event-related potentials (CSSERP) were recorded after painful stimulation of the nasal mucosa with carbon dioxide. Subjects rated the perceived intensity of the stimuli by means of a visual analogue scale. In addition, acoustically evoked responses were recorded, the spontaneous EEG was analyzed in the frequency domain, the subjects' vigilance was assessed in a tracking task, and side effects of the drugs were monitored. 3 Anpirtoline and tramadol produced a decrease of both CSSERP amplitudes and subjective estimates of pain, the effects of the former compound being greater. In contrast, after administration of imipramine no change of CSSERP amplitudes could be detected, whereas the subjective estimate of pain intensity decreased significantly. This was accompanied by a significant decrease of arousal indicating that pain relief produced by acute administration of imipramine was primarily related to its sedation action.4 The analgesic properties of anpirtoline were demonstrated in man. Tramadol was characterized as a weak opioid analgesic. In contrast, imipramine appeared to produce its pain-relieving effects predominantly by non-specific actions. It is hypothesized that different analgesics may change ERP sources in a drug-specific manner.

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