Objective Although postoperative cognitive dysfunction (POCD) often complicates recovery from major surgery, the pathogenic mechanisms remain unknown. We explored whether systemic inflammation, in response to surgical trauma, triggers hippocampal inflammation and subsequent memory impairment, in a mouse model of orthopedic surgery. Methods C57BL/6J, knock out (lacking interleukin [IL]-1 receptor, IL-1R−/−) and wild type mice underwent surgery of the tibia under general anesthesia. Separate cohorts of animals were tested for memory function with fear conditioning tests, or euthanized at different times to assess levels of systemic and hippocampal cytokines and microglial activation; the effects of interventions, designed to interrupt inflammation (specifically and nonspecifically), were also assessed. Results Surgery caused hippocampal-dependent memory impairment that was associated with increased plasma cytokines, as well as reactive microgliosis and IL-1β transcription and expression in the hippocampus. Nonspecific attenuation of innate immunity with minocycline prevented surgery-induced changes. Functional inhibition of IL-1β, both in mice pretreated with IL-1 receptor antagonist and in IL-1R−/− mice, mitigated the neuroinflammatory effects of surgery and memory dysfunction. Interpretation A peripheral surgery-induced innate immune response triggers an IL-1β-mediated inflammatory process in the hippocampus that underlies memory impairment. This may represent a viable target to interrupt the pathogenesis of postoperative cognitive dysfunction.

Brain-derived neurotrophic factor (BDNF) is synthesized by small neuron cell bodies in the dorsal root ganglia (DRG) and is anterogradely transported to primary afferent terminals in the dorsal horn where it is involved in the modulation of painful stimuli. Here we show that BDNF is released in the rat isolated dorsal horn after chemical stimulation by capsaicin or electrical stimulation of dorsal roots. Capsaicin superfusion (1-100 M) induced a dose-dependent release of BDNF, measured using ELISA. The highest dose of capsaicin also induced a depletion of BDNF protein in the dorsal horn. BDNF release was also seen after electrical stimulation of the dorsal roots at C-fiber strength. This release was encoded by specific patterns of afferent fiber stimulation. Neither continuous low-frequency (480 pulses, 1 Hz) nor tetanic high-frequency (300 pulses in 3 trains, 100 Hz) stimulation evoked release of BDNF, although substance P (SP) release was observed under both of these conditions. However, BDNF was released after short bursts of high-frequency stimulation (300 pulses in 75 trains, 100 Hz) along with SP and glutamate. The NMDA antagonist D-AP-5 inhibited electrically evoked BDNF release. BDNF release was also measured after systemic or intrathecal NGF treatment. This upregulated BDNF content in the DRG and increased the capsaicin-evoked release of BDNF. Similarly, the amount of BDNF released by burst stimulation was increased after NGF treatment. This activity-dependent release continued to be encoded solely by this stimulation pattern. These experiments demonstrate that BDNF release in the dorsal horn is encoded by specific patterns of afferent fiber stimulation and is mediated by NMDA receptor activation.

Persistent herpes zoster-associated pain is a significant clinical problem and an area of largely unmet therapeutic need. Progress in elucidating the underlying pathophysiology of zoster-associated pain and related co-morbidity behaviour, in addition to appropriately targeted drug development has been hindered by the lack of an appropriate animal model. This study further characterises a recently developed rat model of zoster-associated hypersensitivity and investigates (a) response to different viral strains; (b) relationship between viral inoculum concentration ('dose') and mechanical hypersensitivity ('response'); (c) attenuation of virus-associated mechanical hypersensitivity by clinically useful analgesic drugs; and (d) measurement of pain co-morbidity (anxiety-like behaviour) and pharmacological intervention in the open field paradigm (in parallel with models of traumatic peripheral nerve injury). VZV was propagated on fibroblast cells before subcutaneous injection into the glabrous footpad of the left hind limb of adult male Wistar rats. Control animals received injection of uninfected fibroblast cells. Hind-limb reflex withdrawal thresholds to mechanical, noxious thermal and cooling stimuli were recorded at specified intervals post-infection. Infection with all viral strains was associated with a dose-dependent mechanical hypersensitivity but not a thermal or cool hypersensitivity. Systemic treatment with intraperitoneal (i.p.) morphine (2.5mg/kg), amitriptyline (10mg/kg), gabapentin (30mg/kg), (S)-(+)-ibuprofen (20mg/kg) and the cannnabinoid WIN55,212-2 (2mg/kg) but not the antiviral, acyclovir (50mg/kg), was associated with a reversal of mechanical paw withdrawal thresholds. In the open field paradigm, virus-infected and nerve-injured animals demonstrated an anxiety-like pattern of ambulation (reduced entry into the central area of the open arena) which was positively correlated with mechanical hypersensitivity. This may reflect painrelated comorbidity. Further, anxiety-like behaviour was attenuated by acute i.p. administration of gabapentin (30mg/kg) in nerve-injured, but not virus-infected animals. This model will prove useful Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. NIH Public Access Author ManuscriptNeuroscience. Author manuscript; available in PMC 2008 May 22. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript in elucidating the pathophysiology of zoster-associated pain and provide a tool for pre-clinical screening of analgesic drugs. Keywordszoster-associated pain; postherpetic neuralgia; neuropathy; analgesia; open f...

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Damage to peripheral nerves is associated with changes in excitability and/or phenotype of primary afferent neurons as well as increased neuronal excitability (central sensitization) and reduced inhibitory tone in the dorsal horn. For instance, in dorsal root ganglia (DRG) brain derived neurotrophic factor (BDNF) is down-regulated in small cells whilst de novo expressed in large diameter cells. In the dorsal horn, GABA content is decreased. In this study, in a dorsal horn, 'with dorsal roots attached' preparation obtained from spinal nerve lesioned Wistar rats, stimulation of ipsilateral dorsal roots at either A fibre or A + C fibre strength did not evoke release of BDNF. In separate experiments, activity-induced release of GABA in the isolated dorsal horn of neuropathic rats was significantly reduced compared to release in sham operated rats. GABA release could be significantly restored following topical application of BDNF through the dorsal horn preparation. Finally, neuropathic rats developed thermal and mechanical hypersensitivity and thermal hyperalgesia was reduced by intrathecal injection of BDNF. We concluded that BDNF-induced release of GABA could be a mechanism to explain the antinociceptive action of intrathecal BDNF in neuropathic animals. Furthermore, reduced availability of sensory neuron-derived BDNF might contribute to the reduced GABAergic tone in the dorsal horn of neuropathic rats.