Acute Chemical Pain Research Articles

Lysophosphatidic acid activates nociceptors and causes pain or itch depending on the application mode in human skin.

Lysophosphatidic acid (LPA) is involved in the pathophysiology of cholestatic pruritus and neuropathic pain. Slowly conducting peripheral afferent C-nerve fibers are crucial in the sensations of itch and pain. In animal studies, specialized neurons ("pruriceptors") have been described, expressing specific receptors, eg, from the Mas-related G-protein-coupled receptor family. Human nerve fibers involved in pain signaling ("nociceptors") can elicit itch if activated by focalized stimuli such as cowhage spicules. In this study, we scrutinized the effects of LPA in humans by 2 different application modes on the level of psychophysics and single nerve fiber recordings (microneurography). In healthy human subjects, intracutaneous LPA microinjections elicited burning pain, whereas LPA application through inactivated cowhage spicules evoked a moderate itch sensation. Lysophosphatidic acid microinjections induced heat hyperalgesia and hypersensitivity to higher electrical stimulus frequencies. Pharmacological blockade of transient receptor potential channel A1 or transient receptor potential channel vanilloid 1 reduced heat hyperalgesia, but not acute chemical pain. Microneurography revealed an application mode-dependent differential activation of mechanosensitive (CM) and mechanoinsensitive C (CMi) fibers. Lysophosphatidic acid microinjections activated a greater proportion of CMi fibers and more strongly than CM fibers; spicule application of LPA activated CM and CMi fibers to a similar extent but excited CM fibers more and CMi fibers less intensely than microinjections. In conclusion, we show for the first time in humans that LPA can cause pain as well as itch dependent on the mode of application and activates afferent human C fibers. Itch may arise from focal activation of few nerve fibers with distinct spatial contrast to unexcited surrounding afferents and a specific combination of activated fiber subclasses might contribute.

Synthesis and Pain Perception of New Analogues of Phencyclidine in NMRI Male Mice

Phencyclidine (PCP, I) and many of its derivatives have demonstrated many pharmacological effects. They interact with a number of neurotransmitter systems within the central nervous system. For example, Phencyclidine is a noncompetitive antagonist of the N-methyl-d-aspartate (NMDA) subtype of the glutamate receptor, and it causes the release and inhibits the reuptake of monoaminergic neurotransmitters, including dopamine, serotonin and norepinephrine. In this study, new thienyl (TCP, II), as well as benzothiophen (BTCP, III) derivatives (IV-VII) were synthesized. The acute and chronic pain activities of these drugs were studied using the tail immersion and formalin tests on mice and the results were compared with PCP, TCP and control groups at dosage of 10 mg/kg. The results indicated that the drug VII produced more analgesic effects on acute chemical pain in formalin test compared with other drugs. In addition, this analgesic effect was remarkably seen for drugs II, VI and VII in chronic pain in the mentioned test in comparison with other drugs. Also, the results showed that acute thermal pain could be diminished by drugs VI, II and I compared with other drugs in tail immersion test. It can be concluded that more analgesic effects of new BTCP analogues (VI and VII) may be concerned with antinociception activities of benzothiophene group and also with binding to cocaine site on the dopamine transporter receptor which seems to be more potent than PCP receptor in decreasing pain.

Synthesis and analgesic effects of 1-[1-(2-methylphenyl)(cyclohexyl)]-3-piperidinol as a new derivative of phencyclidine in mice

Phencyclidine (1-(1-phenylcyclohexyl) piperidine, CAS 956-90-1, PCP, I) and its derivatives have shown many analgesic effects. In this research, a new derivative of PCP (1-[1-(2-methylphenyl) (cyclohexyl)l3-piperidinol, PD, II) was synthesized and its analgesic (acute and chronic pains) effects were examined on rats using tail immersion (as a model of acute thermal pain) and formalin (as a model of acute and chronic chemical pain) tests and the results are compared to PCP and control groups. The results indicated that II produces higher analgesic effects in the tail immersion test compared to the PCP and control groups, with a marked and significant increase in tail immersion latency for the doses 1, 5 and 10 mg/kg. The formalin test showed that PD (II) is not effective in acute chemical pain (phase I, 0-5 min after injection) in all doses but chronic pain (initial-phase II, 15-40 min after injection) is significantly attenuated by this compound compared to PCP and saline (control) in dosesof 5 and 10 mg/kg. It is concluded that II is effective in acute thermal (in all doses) and chronic (doses of 5 and 10 mg/kg) pains; however, it is not effective in acute chemical pain compared to PCP and control.

Synthesis and analgesic effects of new pyrrole derivatives of phencyclidine in mice

Phencyclidine (1-(1-phenylcyclohexyl)piperidine, CAS 956-90-1, PCP, I) and many of its analogues have shown some pharmacological effects. In this study, new pyrrole derivatives of I (1-(1-phenylcyclohexyl)pyrrole, II and 1-[1-(4-methylphenyl)(cyclohexyl)]pyrrole, III) and their intermediates were synthesized and the acute and chronic pains were examined on mice using tail immersion (as a model of acute thermal pain) and formalin (as a model of acute and chronic chemical pain) tests and the results were compared with the PCP and control groups. The results indicated that III generated higher analgesic effects in the tail immersion test compared to the PCP and control (dimethyl sulfoxide, DMSO) groups, demonstrating a marked and significant increase in tail immersion latency, but this effect was not observed for II in the dose of 1 mg/kg. The formalin test showed that III was effective in acute chemical pain (phase I, 0-5 min after injection), but was not effective for II at the same dosage compared to the PCP and control groups. Also chronic pain will be significantly attenuated by III but II was not effective as compared to the other groups. It is concluded that substitution of the aromatic pyrrole ring instead of piperidine in the PCP molecule will not be effective alone in tail immersion and formalin tests but the addition of a methyl group (with high electron donating and dipole moments) on the phenyl group plus substitution of the aromatic pyrrole ring can be effective in acute and chronic pain compared to the PCP and control groups.

New morpholine analogues of phencyclidine: Chemical synthesis and pain perception in rats

Phencyclidine (PCP, I) and most its derivatives have demonstrated some pharmacological effects. Accordingly, in this study, the new methoxy (III) and hydroxy-methyl (IV) morpholine PCP derivatives were synthesized. The acute and chronic pain activities of these drugs (III, IV) were investigated by tail immersion and formalin tests on rats and the results were compared with those in PCP, PCM (PCP-morpholine, II), and methyl-PCM (V). Findings indicated that III (6mg/kg, i.p.) generates more analgesic effects in tail immersion test in comparison with I and II in 20, 40, 45 and 55min post-injection. These effects were observed in 10, 20, 40, 45 and 50min after the application of IV (at the same dosage). This analgesic effect was markedly seen in 20, 40, 45 and 50min after compound IV's application in comparison with the drugs (I–V). In formalin test analysis, the acute chemical pain (Phase I) could not be affected by any drugs (I–V) while chronic formalin pain would be diminished by these new synthesized drugs (III and IV), especially in late Phase II, compared to I and II at the dosage of 6mg/kg. It is, therefore, concluded that these new synthesized PCP derivates including methoxy-PCM (III) and hydroxy-methyl-PCM (IV) could substantially and respectively diminish acute thermal and chronic chemical pains.