l-Acetylcarnitine Induces Analgesia by Selectively Up-Regulating mGlu2 Metabotropic Glutamate Receptors

Abstract

L-Acetylcarnitine (LAC, 100 mg/kg, s.c.), a drug commonly used for the treatment of painful neuropathies, substantially reduced mechanical allodynia in rats subjected to monolateral chronic constriction injury (CCI) of the sciatic nerve and also attenuated acute thermal pain in intact rats. In both cases, induction of analgesia required repeated injections of LAC, suggesting that the drug induces plastic changes within the nociceptive pathway. In both CCI-and sham-operated rats, a 24-day treatment with LAC increased the expression of metabotropic glutamate (mGlu) receptors 2 and 3 in the lumbar segment of the spinal cord, without changing the expression of mGlu1a or -5 receptors. A similar up-regulation of mGlu2/3 receptors was detected in the dorsal horns and dorsal root ganglia of intact rats treated with LAC for 5-7 days, a time sufficient for the induction of thermal analgesia. Immunohistochemical analysis showed that LAC treatment enhanced mGlu2/3 immunoreactivity in the inner part of lamina II and in laminae III and IV of the spinal cord. An increased mGlu2/3 receptor expression was also observed in the cerebral cortex but not in the hippocampus or cerebellum of LAC-treated animals.Reverse transcription-polymerase chain reaction combined with Northern blot analysis showed that repeated LAC injections selectively induced mGlu2 mRNA in the dorsal horns and cerebral cortex (but not in the hippocampus). mGlu3 mRNA levels did not change in any brain region of LAC-treated animals. To examine whether the selective up-regulation of mGlu2 receptors had any role in LAC-induced analgesia, we have used the novel compound LY 341495, which is a potent and systemically active mGlu2/3 receptor antagonist. LAC-induced analgesia was largely reduced 45 to 75 min after a single injection of LY 341495 (1 mg/kg, i.p.) in both CCI rats tested for mechanical allodynia and intact rats tested for thermal pain. We conclude that LAC produces analgesia against chronic pain produced not only by peripheral nerve injury but also by acute pain in intact animals and that LACinduced analgesia is associated with and causally related to a selective up-regulation of mGlu2 receptors. This offers the first example of a selective induction of mGlu2 receptors and discloses a novel mechanism for drug-induced analgesia.Neuropathic pain is characterized by spontaneous pain, allodynia, hyperalgesia, and pain summation and is a hallmark of neuropathies caused by traumatic injury, diabetes, and viral infections. An enhanced sensitivity of dorsal horn neurons to sensory stimulation (a phenomenon referred as "central sensitization") is commonly recognized as one of the mechanisms underlying chronic pain. In animal models of neuropathic pain, a sustained release of glutamate and peptides induces long-term potentiation at the synapses between primary afferent fibers and second-order neurons in the dorsal horns (reviewed by Sandkuhler, 2000). Induction of longterm potentiation requires the simultaneous activation of neurokinin receptors, NMDA recepto...

Endogenous activation of metabotropic glutamate receptors supports the proliferation and survival of neural progenitor cells

“…Western blot analysis of mGlu receptor proteins was carried out as described previously 44 using polyclonal antibodies against mGlu1a, mGlu4, mGlu5, mGlu6, mGlu7, mGlu8 (Upstate Technology) and mGlu2/3 (Chemicon International, Temecula, CA, USA) receptors. 3 H]inositol phosphates accumulating during the reaction was measured as described previously.…”

“…44 The following primers were used for each metabotropic glutamate receptor subtype indicated below: mGlu1: forward GCTGTA CCTACTATGCCTTC, reverse AGACCATGACACAGACTTGC; mGlu2: forward CTACAGTGATGTCTCCATCC, reverse AAAGCCTCAATGCCT GTCTC; mGlu3: forward CAAGTGACTACAGAGTGCAG, reverse CTGTC ACCAATGCTCAGCCTC; mGlu4: forward CCAACGAGGATGACATCA GG, reverse CACAGGTCACGGTGCATGG; mGlu5: forward GTCCTTCT GTTGATCCTGTC, reverse ATGCAGCATGGCCTCCACTC; mGlu6: forward GCCAGTCAGATGATTCCACC, reverse GCCTGGTACCTGGAA GATGTC; mGlu7: forward CCAGATGTGGCAGTGTGTTC, reverse CGA GTCTTGATGGCATACAC; mGlu8: forward ATCACCTTCAGCCTCATC TC, reverse TGTGACCACAGCCTTGAAGC.…”

L-Acetylcarnitine: A Proposed Therapeutic Agent for Painful Peripheral Neuropathies

“…Indirect activation of muscarinic receptors as well as the activation of the PLC-IP 3 pathway has been postulated to mediate the antinociceptive effect of L-acetylcarnitine in experimental models of acute pain [14,15]. Moreover, increased expression of metabotropic glutamate receptor 2 (mGlu2) in regions of the nervous system involved in pain transmission after chronic administration of L-acetylcarnitine has been proposed to account for the antinociceptive efficacy of the drug in acute pain and in chronic pain after nerve injury in rats [7]. In vitro studies performed in cultured DRG neurons showed that the increased expression of mGlu2 receptors occurs mainly in the large diameter neurons from which large myelinated Abeta fibers originate [8].…”

“…The antinociceptive effect of L-acetylcarnitine has been shown in different pain models in rodents [7,14,15], and several mechanisms have been proposed. Indirect activation of muscarinic receptors as well as the activation of the PLC-IP 3 pathway has been postulated to mediate the antinociceptive effect of L-acetylcarnitine in experimental models of acute pain [14,15].…”

Acetyl-l-carnitine requires phospholipase C-IP3 pathway activation to induce antinociception

“…More recently it has been reported that ALCAR induces antinociception by a central indirect cholinergic mechanism (Ghelardini et al, 2002). An up-regulation of mGlu2 receptors in ALCAR-treated animals has also been postulated (Chiechio et al, 2002). Literature data reported that the metabolic intracellular events induced by the activation of the muscarinic system are blocked not only by scopolamine, but also by the Glu2 receptor antagonist LY341495 (Johnson et al, 2000), indicating the presence of a correlation between the glutamatergic and muscarinic system, being the muscarinic system activation downstream to the mGlu2 receptors upregulation.…”