Involvement of descending pain control system regulated by orexin receptor signaling in the induction of central post-stroke pain in mice

Central post-stroke pain (CPSP) is a type of neuropathic pain for which the mechanism and relevant drug pathways remain unknown. Recently, it was reported that intracerebroventricular (ICV) administration of orexin-A suppresses pain and ischemia. In this study, we tested the role of orexin-A in CPSP induction in mice. Male ddY mice were subjected to 30 min of bilateral carotid artery occlusion (BCAO). CPSP was assessed by von Frey test. Colocalization of orexin 1 receptor (OX1R) with various neuron markers were determined by double-immunofluorescence. The hindpaw withdrawal responses to mechanical stimuli were significantly increased 3 days post-BCAO compared with those of sham groups. ICV injection of orexin-A dose-dependently suppressed BCAO-induced mechanical allodynia. These effects were inhibited by pre-treatment with SB334867 (an OX1R antagonist; ICV injection), yohimbine (a noradrenaline α₂ receptor antagonist; intrathecal (IT) injection), and WAY100635 (a serotonin 5-HT_1A receptor antagonist; IT injection), but not TCS OX2 29 (an OX2R antagonist; ICV injection). OX1R colocalized with TH (a noradrenergic neuron marker) and TPH (a serotonergic neuron marker) in the locus ceruleus (LC) and the rostral ventromedial medulla (RVM), respectively. The number of c-Fos positive cells in the LC and the RVM of BCAO mice was increased at 90 min after ICV injection of orexin-A compared to saline group. These results indicate that orexin-A/OX1R signaling plays an important role through activation of the descending pain control system in the induction of CPSP in mice.

Cerebral Ischemia Changed the Effect of Metabosensitive Muscle Afferents on Somatic Reflex Without Affecting Thalamic Activity

The purpose of the present study was to examine the contribution of group III and IV metabosensitive afferents at spinal and supraspinal levels in rats subjected to middle cerebral artery occlusion (MCAO) with reperfusion during the acute phase. Animals were randomized in Control (n = 23), SHAM (n = 18), MCAO-D1 (n = 10), and MCAO-D7 (n = 20) groups. Rats performed the Electrical Von Frey and the Adhesive removal tests before the surgery and at day 1 (D1), D3, and D7 after MCAO. Animals were subjected to electrophysiological recordings including the responses of group III/IV metabosensitive afferents to combinations of chemical activators and the triceps brachii somatic reflex activity at D1 or D7. The response of ventral posterolateral (VPL) thalamic nuclei was also recorded after group III/IV afferent activation. Histological measurements were performed to assess the infarct size and to confirm the location of the recording electrodes into the VPL. Behavioral results indicated that MCAO induced disorders of both mechanical sensibility and motor coordination of paretic forepaw during 7 days. Moreover, injured animals exhibited an absence of somatic reflex inhibition from the group III/IV afferents at D1, without affecting the response of both these afferents and the VPL. Finally, the regulation of the central motor drive by group III/IV afferents was modified at spinal level during the acute phase of cerebral ischemia and it might contribute to the observed behavioral disturbances.

No evidence of potentiation of buprenorphine by milnacipran in healthy subjects using a nociceptive test battery

BACKGROUND

Serotonin-norepinephrine reuptake inhibitors inhibit the reuptake of serotonin and noradrenalin and are used in the treatment of neuropathic pain. Animal studies suggest that milnacipran co-administered with opioids may potentiate the analgesic effect of μ-opioid receptor agonists. This study hypothesized that co-administration of milnacipran and buprenorphine would have a synergistic effect in evoked pain models in healthy subjects.

METHODS

This was a randomized double-blinded, placebo-controlled, four-way cross-over, multiple dose clinical trial to investigate the analgesic effects of buprenorphine (placebo, 0.5, 1 and 3 μg/kg) in combination with milnacipran (placebo, 25 and 50 mg) in healthy subjects.

RESULTS

11 healthy men were enrolled in the study. Buprenorphine alone showed a dose-response relationship indicative of anti-nociception in the pain tests. Following milnacipran administration, no changes were seen in the pharmacodynamic measurements for pain, psychomotor function, body stability or eye movements. For the electrical tests, cold pressor test and pressure pain test, buprenorphine alone was superior when compared with buprenorphine plus milnacipran. No differences in pharmacodynamic variables, besides an increase in pupil/iris ratio, were observed after repeated administration of milnacipran 50 mg. Single and multiple doses of 25 or 50 mg milnacipran did not further potentiate the anti-nociceptive effects of buprenorphine.

CONCLUSIONS

Buprenorphine showed dose-dependent effects consistent with its pharmacological profile. Milnacipran alone did not affect any of the pain variables. The combination of both buprenorphine and milnacipran did not potentiate or show a synergistic effect on the pain models used in this study.

SIGNIFICANCE

Buprenorphine is known to be a potent opioid agonist. Animal studies suggest that milnacipran co-administered with opioids may potentiate the analgesic effect of μ-opioid receptor agonists. Here, we found that buprenorphine showed a dose-dependent analgesic effect, but that no potentiation or synergy on a battery of evoked pain tasks could be observed after co-administration of both milnacipran and buprenorphine.