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Activation of p38 mitogen-activated protein kinase (p38) in spinal microglia is implicated in spinal nociceptive processing. Minocycline, a tetracycline derivative, displays selective inhibition of microglial activation, a function that is distinct from its antibiotic activity. In the present study we examined antinociceptive effects of intrathecal (IT) administration of minocycline in experimental models of inflammation-evoked hyperalgesia in addition to the effect of minocycline on stimulation-induced activation of p38 in spinal microglia. Intrathecal minocycline produced a dose-dependent reduction of formalin-evoked second-phase flinching behaviour in rats, and prevented thermal hyperalgesia induced by carrageenan injection into the paw. In contrast, systemic delivery (intraperitoneally) of minocycline inhibited the first but not the second phase of formalin-induced flinching, and it had no effect on carrageenan-induced hyperalgesia. Centrally mediated hyperalgesia induced by IT delivery of N-methyl-d-aspartate was completely blocked by IT minocycline. An increase in phosphorylation (activation) of p38 (P-p38) was observed in the dorsal spinal cord after carrageenan paw injection, assessed by both Western blotting and immunohistochemistry. The increased P-p38 immunoreactivity was seen primarily in microglia but also in a small population of neurons. Minocycline, at the IT dose that blocked carrageenan-induced hyperalgesia, also attenuated the increased P-p38 in microglia. In addition, minocycline suppressed lipopolysaccharide-evoked P-p38 in cultured spinal microglial cells. Taken together, these findings show that minocycline given IT produces a potent and consistent antinociception in models of tissue injury and inflammation-evoked pain, and they provide strong support for the idea that this effect is mediated by direct inhibition of spinal microglia and subsequent activation of p38 in these cells.

Behavioral models indicate that persistent small afferent input, as generated by tissue injury, results in a hyperalgesia at the site of injury and a tactile allodynia in areas adjacent to the injury site. Local tissue injury and inflammation yields well-defined escape behaviors in animals and pain reports in humans. Examination of the histochemistry and electrophysiology of spinal systems has revealed considerable detail regarding the elements of systems that are activated by these stimuli. Nevertheless, the functional contribution of different spinal systems in pain processing ultimately must be defined in terms of the systems in which such end points can be measured, e.g., the behavior of the intact organism. We will consider below how certain spinal systems contribute to the observed behavioral states. Behavioral Effects of Cutaneous Stimuli After InjuryAn acute, unconditioned, thermal, or mechanical stimulus sufficient to activate polymodel nociceptive afferents (C fibers) depolarizes populations of dorsal horn wide dynamic range (WDR) neurons that project supraspinally. This output in turn evokes a supraspinally organized escape behavior. The hot plate test (thermal stimulus to the paw) or the local injection of an irritant such as formalin or capsaicin where the unconditioned stimulus evokes a somatotopically directed behavior (e.g., withdrawal or licking) are behavioral paradigms believed to reflect this underlying mechanism (1). The more intense the stimulus, the more robust will be the afferent volley and the more vigorous or shorter latencied is the escape behavior (2).An acute stimulus of intensity and duration that leads to tissue injury also produces an acute discharge. In addition, the injury leads to the local release of active factors that evoke and sustain persistent activity in the sensory afferents innervating the injured or inflamed tissue (3). Thus, in contrast to the acute response, injury leads to persistent activity in populations of small afferents and also may activate afferent populations that are excited only in the presence of local factors generated by the injury (e.g., silent ''nociceptors'') (4). Electrophysiological studies have shown that the persistent activation of spinal WDR neurons by small, but not large, afferents, will lead: (i) a progressive enhancement of the WDR response to each subsequent input, and (ii) an increase in the dimensions of the peripheral receptive field to which the spinal neuron will respond (5). This electrophysiological observation parallels behavioral changes in which the animal displays an enhanced response to a given stimulus or a reduction in the intensity of the stimulus required to evoke an escape response. Thus, the injection of an irritant (formalin) into one hind paw evokes a high frequency barrage during the first 10-20 min followed by a modest ongoing discharge over the next hour (6). Coincident with the initial afferent barrage, WDR neurons display an initial burst of activity followed by a period of quiescence and then a progress...

Opioid -and ␦-receptors are present on the central terminals of primary afferents, where they are thought to inhibit neurotransmitter release. This mechanism may mediate analgesia produced by spinal opiates; however, when they used neurokinin 1 receptor (NK1R) internalization as an indicator of substance P release, Trafton et al. (1999) noted that this evoked internalization was altered only modestly by morphine delivered intrathecally at spinal cord segment S1-S2. We reexamined this issue by studying the effect of opiates on NK1R internalization in spinal cord slices and in vivo. ]-enkephalin (DPDPE) (1 M). In vivo, hindpaw compression induced NK1R internalization in ipsilateral laminas I-II. This evoked internalization was significantly reduced by morphine (60 nmol), DAMGO (1 nmol), and DPDPE (100 nmol), but not by the agonist trans-(1S,2S)-3,4-dichloro-N-mathyl-N-[2-(1-pyrrolidinyl)cyclohexyl]-benzeneacetamide hydrochloride (200 nmol), deliveredat spinal cord segment L2 using intrathecal catheters. These doses of the and ␦ agonists were equi-analgesic as measured by a thermal escape test. Lower doses neither produced analgesia nor inhibited NK1R internalization. In contrast, morphine delivered by percutaneous injections at S1-S2 had only a modest effect on thermal escape, even at higher doses. Morphine decreased NK1R internalization after systemic delivery, but at a dose greater than that necessary to produce equivalent analgesia. All effects were reversed by naloxone. These results indicate that lumbar opiates inhibit noxious stimuli-induced neurotransmitter release from primary afferents at doses that are confirmed behaviorally as analgesic.

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Tachykinin- and calcitonin gene-related peptide (CGRP) immunoreactivities were localized by immunohistochemistry in the same nerves of the kidney, renal pelvis and ureter as well as in spinal ganglion cells of both the guinea-pig and man. The tachykinin and CGRP-immunoreactive nerves in the ureter were present within the smooth muscle layers, around blood vessels, close to and within the lining epithelium. The levels of neurokinin A-, substance P- and CGRP-like immunoreactivity per tissue weight, as determined by radioimmunoassay, were about 30-100-fold higher in the guinea-pig than in the human ureter, which was in good agreement with the relative density of immunoreactive nerve fibres, as seen by immunohistochemistry. Capsaicin treatment caused an almost total disappearance of both neurokinin A-, substance P- and CGRP-immunoreactive nerve fibres in the guinea-pig ureter and a 90% depletion of neurokinin A, substance P- and CGRP-like immunoreactivity, further supporting a sensory origin of these nerves. Reversed-phase high performance liquid chromatography of water extracts of the human ureter revealed the presence of neurokinin A- and eledoisin-like material using antiserum K12, which does not cross-react with substance P. Most of the CGRP-like immunoreactivity in human ureter extracts co-eluted with synthetic human CGRP. Capsaicin both caused inhibition of spontaneous motility of the human ureter in vitro and initiated contractions in some preparations. Neurokinin A and neuropeptide K potently initiated phasic contractions of the ureter, while substance P had only minor contractile effects. CGRP inhibited both spontaneous and neurokinin A-induced ureteric contractions. In conclusion, peptides with potent opposite motility effects are present in the same, presumably sensory nerves of the ureter in both the guinea-pig and man. It will be of importance to determine whether local release of neuropeptides can account for ureteric motility changes accompanying sensory nerve activation upon ureteral obstruction, by e.g. renal calculi.

Antagonist studies show that spinal p38 mitogen-activated protein kinase plays a crucial role in spinal sensitization. However, there are two p38 isoforms found in spinal cord and the relative contribution of these two to hyperalgesia is not known. Here we demonstrate that the isoforms are distinctly expressed in spinal dorsal horn: p38a in neurons and p38b in microglia. In lieu of isoform selective inhibitors, we examined the functional role of these two individual isoforms in nociception by using intrathecal isoform-specific antisense oligonucleotides to selectively block the expression of the respective isoform. In these rats, down-regulation of spinal p38b, but not p38a, prevented nocifensive flinching evoked by intraplantar injection of formalin and hyperalgesia induced by activation of spinal neurokinin-1 receptors through intrathecal injection of substance P. Both intraplantar formalin and intrathecal substance P produced an increase in spinal p38 phosphorylation and this phosphorylation (activation) was prevented when spinal p38b, but not p38a, was down-regulated. Thus, spinal p38b, probably in microglia, plays a significant role in spinal nociceptive processing and represents a potential target for pain therapy.

Based on previous work, we hypothesized that activation of spinal NMDA-receptor initiates activation of the p38 mitogen-activated protein kinase (p38 MAPK) pathway, leading to spinal release of prostaglandins and hyperalgesia. Accordingly, we examined the effect of intrathecal SD-282, a selective p38 MAPK inhibitor, on NMDA-induced release of prostaglandin E(2) (PGE(2)) and thermal hyperalgesia. Inhibition of spinal p38 MAPK attenuated both NMDA-evoked release of PGE(2) and thermal hyperalgesia. NMDA injection led to increased phospho-p38 MAPK immunoreactivity in superficial (I-II) dorsal laminae. Co-labeling studies revealed co-localization of activated p38 MAPK predominantly with microglia but also with a small subpopulation of neurons. Taken together these data suggest a role for p38 MAPK in NMDA-induced PGE(2) release and hyperalgesia, and that microglia is involved in spinal nociceptive processing.