Extensive studies in rodents suggest that serotonin (5-HT) modulates nociceptive responses through the stimulation of several receptor types. However, it remains to demonstrate that these receptors participate in the control of nociception under physiological conditions. Pain behaviors of mutants which do not express 5-HT1A, 5-HT1B, 5-HT2A or 5-HT3A receptors, or lacking the 5-HT transporter, compared to paired wild-type mice of the same genetic background, were examined using validated tests based on different sensory modalities. Mechanical (von Frey filaments, tail pressure, tail clip tests), thermal (radiant heat, 46 degrees C water bath, hot-plate test) and formalin-induced nociception were determined in 2- to 3-month-old males. 5-HT1A knock-out mice differed from wild-types by higher thermal sensitivity (hot-plate test only), and 5-HT1B knock-out mice by higher thermal and formalin sensitivity. Both 5-HT2A and 5-HT3A knock-out mice differed from wild-types by a dramatic decrease in the formalin-induced nociceptive responses for phase II (16-45 min after injection/inflammatory phase). In contrast, neither mechanical, thermal nor formalin-induced nociception differed between mutants lacking the 5-HT transporter and paired wild-type mice. Although differences in spontaneous locomotor activity in 5-HT1B-/- (increase) and 5-HT3A-/- (decrease) knock-out mice versus paired wild-types might have confounded differences in nociception, acute 5-HT receptor blockade by selective antagonists was found to replicate in wild-type mice the effects on pain behavior, but not on locomotor activity, of the respective gene knock-out in mutants. These results support the conclusion that the complex control of pain mechanisms by 5-HT, acting at multiple receptors, is physiologically relevant in mice.

1 Peripheral lesion to the trigeminal nerve may induce severe pain states. Several lines of evidence have suggested that the antimigraine e ect of the triptans with 5-HT 1B/1D receptor agonist properties may result from inhibition of nociceptive transmission in the spinal nucleus of the trigeminal nerve by these drugs. On this basis, we have assessed the potential antinociceptive e ects of sumatriptan and zolmitriptan, compared to dihydroergotamine (DHE), in a rat model of trigeminal neuropathic pain. 2 Chronic constriction injury was produced by two loose ligatures of the infraorbital nerve on the right side. Responsiveness to von Frey ®lament stimulation of the vibrissal pad was used to evaluate allodynia. 3 Two weeks after ligatures, rats with a chronic constriction of the right infraorbital nerve displayed bilateral mechanical hyper-responsiveness to von Frey ®lament stimulation of the vibrissal pad with a mean threshold of 0.38+0.04 g on the injured side and of 0.43+0.04 g on the contralateral (left) side (versus 512.5 g on both sides in the same rats prior to nerve constriction injury). 4 Sumatriptan at a clinically relevant dose (100 mg kg 71 , s.c.) led to a signi®cant reduction of the mechanical allodynia-like behaviour on both the injured and the contralateral sides (peak-e ects 6.3+1.1 g and 4.4+0.7 g, respectively). A more pronounced e ect was obtained with zolmitriptan (100 mg kg 71 , s.c.) (peak-e ects: 7.4+0.9 g and 3.2+1.3 g) whereas DHE (50 ± 100 mg kg 71 , i.v.) was less active (peak-e ect *1.5 g). 5 Subcutaneous pretreatment with the 5-HT 1B/1D receptor antagonist, GR 127935 (3 mg kg 71 ), prevented the anti-allodynia-like e ects of triptans and DHE. Pretreatment with the 5-HT 1A receptor antagonist, WAY 100635 (2 mg kg 71 , s.c.), did not alter the e ect of triptans but signi®cantly enhanced that of DHE (peak e ect 4.3+0.5 g). 6 In a rat model of peripheral neuropathic pain, which consisted of a unilateral loose constriction of the sciatic nerve, neither sumatriptan (50 ± 300 mg kg 71 ) nor zolmitriptan (50 ± 300 mg kg 71 ) modi®ed the thresholds for paw withdrawal and vocalization in response to noxious mechanical stimulation. 7 These results support the rationale for exploring the clinical e cacy of brain penetrant 5-HT 1B/1D receptor agonists as analgesics to reduce certain types of trigeminal neuropathic pain in humans.

E-52862 is a selective σ1R antagonist currently undergoing phase II clinical trials for neuropathic pain and represents a potential first-in-class analgesic. Here, we investigated the effect of single and repeated administration of E-52862 on different pain-related behaviours in several neuropathic pain models in rats: mechanical allodynia in cephalic (trigeminal) neuropathic pain following chronic constriction injury of the infraorbital nerve (IoN), mechanical hyperalgesia in streptozotocin (STZ)-induced diabetic polyneuropathy, and cold allodynia in oxaliplatin (OX)-induced polyneuropathy. Mechanical hypersensitivity induced after IoN surgery or STZ administration was reduced by acute treatment with E-52862 and morphine, but not by pregabalin. In the OX model, single administration of E-52862 reversed the hypersensitivity to cold stimuli similarly to 100 mg/kg of gabapentin. Interestingly, repeated E-52862 administration twice daily over 7 days did not induce pharmacodynamic tolerance but an increased antinociceptive effect in all three models. Additionally, as shown in the STZ and OX models, repeated daily treatment with E-52862 attenuated baseline pain behaviours, which supports a sustained modifying effect on underlying pain-generating mechanisms. These preclinical findings support a role for σ1R in neuropathic pain and extend the potential for the use of selective σ1R antagonists (e.g., E-52862) to the chronic treatment of cephalic and extra-cephalic neuropathic pain.

Drug combination for the treatment of pain is common clinical practice. Co-crystal of Tramadol-Celecoxib (CTC) consists of two active pharmaceutical ingredients (APIs), namely the atypical opioid tramadol and the preferential cyclooxygenase-2 inhibitor celecoxib, at a 1:1 molecular ratio. In this study, a non-formulated 'raw' form of CTC administered in suspension (referred to as ctc) was compared with both tramadol and celecoxib alone in a rat plantar incision postoperative pain model. For comparison, the strong opioids morphine and oxycodone, and a tramadol plus acetaminophen combination at a molecular ratio of 1:17 were also tested. Isobolographic analyses showed that ctc exerted synergistic mechanical antiallodynic (experimental ED = 2.0 ± 0.5 mg/kg, i.p.; theoretical ED = 3.8 ± 0.4 mg/kg, i.p.) and thermal (experimental ED = 2.3 ± 0.5 mg/kg, i.p.; theoretical ED = 9.8 ± 0.8 mg/kg, i.p.) antihyperalgesic effects in the postoperative pain model. In contrast, the tramadol and acetaminophen combination showed antagonistic effects on both mechanical allodynia and thermal hyperalgesia. No synergies between tramadol and celecoxib on locomotor activity, motor coordination, ulceration potential and gastrointestinal transit were observed after the administration of ctc. Overall, rat efficacy and safety data revealed that ctc provided synergistic analgesic effects compared with each API alone, without enhancing adverse effects. Moreover, ctc showed similar efficacy but improved safety ratio (80, measured as gastrointestinal transit vs postoperative pain ED ratios) compared with the strong opioids morphine (2.5) and oxycodone (5.8). The overall in vivo profile of ctc supports the further investigation of CTC in the clinical management of moderate-to-severe acute pain as an alternative to strong opioids.