The chemokine CC motif ligand 1 (CCL1) participates in immune cell recruitment and, as for other chemokines, is also involved in nociceptive processing. In contrast with previous reports indicating its participation in allodynia and cold hypernociception when spinally administered, its ability to evoke heat thermal analgesia, mediated by circulating leukocytes and endocannabinoids, after systemic administration has recently been reported. Aiming to explore the possible role played by CCL1 on spinal nociception, we study here the effect of its intrathecal administration on thermal nociception in mice. The intrathecal administration of CCL1 (0.3-30 ng) produced dose-dependent analgesia as measured by the unilateral hot plate test. This analgesia evoked by CCL1 peaked 1 h after injection, was prevented by the CCR8 antagonist R243 and was accompanied by a reduction of c-Fos protein expression in dorsal horn spinal neurons. Blood leukocyte depletion did not modify CCL1-evoked analgesic responses that, in contrast, were abolished by the microglial inhibitor minocycline, but not the astroglial inhibitor aminoadipate, suggesting the involvement of spinal microglial cells. Furthermore, analgesia remained unmodified by the coadministration of cannabinoid type 1 or 2 receptors antagonists (AM251 or SR144285). However, it was reversed by naloxone but not by cyprodime or naltrindole (selective antagonists of mu- or delta- opioid receptors). The inhibitory effect induced by the selective kappa-opioid receptor antagonist, nor-binaltorphimine, and by an anti-dynorphin A 1-17 antibody indicates that analgesia evoked by spinal CCL1 is mediated by endogenous dynorphins acting on kappa-opioid receptors. Endogenous dynorphin and microglia behave as key players in heat thermal analgesia evoked by spinal CCL1 in mice.
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