Central sensitization

Abstracts

B05 - Central Sensitization (638) The endogenous NMDA antagonist and NOS inhibitor agmatine inhibits spinal long term potentiation (LTP) G. Wilcox, A. Fiska, F. Haugan, F. Svendsen, L. Rygh, A. Tjolsen, K. Hole; University of Bergen, Bergen, Norway Plasticity in spinal nociceptive circuits may play an important role in clinical pain, particularly when pain becomes long lasting or chronic. Previously we have shown that long-term potentiation (LTP) can be induced in single wide dynamic range (WDR) neurons in the dorsal horn after high-frequency stimulation of the sciatic nerve in intact urethaneanaesthetized rats. We have also shown in behavioral studies that intrathecally administered agmatine, an endogenous neurotransmitter that reduces NMDA receptor currents and competitively inhibits nitric oxide synthase (NOS), can interrupt the development of long-term hyperalgesia. This unusual action suggested that agmatine interfered with a process of long-term plasticity in spinal cord. Because both NMDA receptors and NOS are thought to be involved in LTP, we sought to study agmatine‘s effect on the induction and persistence of spinal LTP. Extracellular single unit recordings of dorsal horn WDR cells were made, and spikes evoked by single test stimuli were counted every 5 min. LTP was induced by a conditioning stimulation procedure (20 trains of 2 s duration, 10 s intervals, 100 Hz, 0.5 ms pulses, 6 times C-fibre threshold). Fifty ␮L of 40 mM agmatine was applied topically to the exposed spinal cord 5 min before or 1 hour after the conditioning stimulus. The responses to test stimulation were recorded for 3 h after the conditioning stimulus. Pretreatment, but not post-treatment, with 40 mM agmatine inhibited LTP induction. Agmatine in higher concentrations (60 mM and 120 mM) also reduced LTP in a dose-dependent fashion. The present data indicate that agmatine inhibits LTP induction better than established LTP. The data are promising with regard to agmatine or similar substances as possible therapies for chronic pain.

(639) Ca2-permeable AMPA/KA receptor antagonists produce a dose-dependent block on secondary allodynia evoked by thermal injury T. Jones, L. Sorkin; University of California-San Diego, La Jolla, CA Intrathecal pre-injury treatment (pre-T) with NMDA receptor (NMDAr) antagonists blocks development of central sensitization in a number of pain models. In contrast, secondary (2°) mechanical allodynia evoked by thermal injury (52.5°C for 45 s) applied to the hind paw heel of the rat is not blocked by intrathecal pre-T with NMDAr antagonists; it is, however, blocked by intrathecal non-NMDA, AMPA/KAr antagonists or specific calcium-permeable (Ca2⫹-perm) AMPA/KA receptor antagonists. These findings suggest a role for these receptors in the development of central sensitization. The present study used the same injury model and assessed the effects of the AMPA/KA receptor antagonist, CNQX, and specific Ca2⫹-perm-AMPA/KA receptor antagonists, philanthotoxin (PHTx) and joro spider toxin (JST) when given as post-injury treatments (post-Ts). Intrathecal pre-T with saline resulted in 2° mechanical allodynia (p⬍0.0001) when measured with calibrated von Frey filaments. This allodynia was blocked by intrathecal pre-T with 5␮g PHTx, 3␮g JST or 10␮g CNQX (p⬃0.057, 0.495 and 0.443, respectively; p⬎0.05 means no allodynia). Intrathecal post-T with saline 5 or 30 minutes after injury also resulted in 2° mechanical allodynia (p⬍0.0001). Allodynia at both time points was blocked with intrathecal 10␮g CNQX (p⬃0.231 and 0.870). Intrathecal 5-minute post-Ts with PHTx or JST revealed a dose-dependency where only increased doses of PHTx (10␮g, p⬃0.191) and JST (5␮g, p⬃0.822) blocked 2° mechanical allodynia. Intrathecal PHTx and JST were no longer effective when given as 30-minute post-Ts at doses that did not produce motor deficits when assessed by activity on a rotarod. In conclusion, Ca2⫹-perm-AMPA/KArs have a role in central sensitization development and resultant pain responses similar to that known for NMDArs. Secondary mechanical allodynia in this model was not maintained by Ca2⫹-perm-AMPA/KArs, but instead required activation of Ca2⫹-impermeable AMPA/KArs. This supports a role for non-NMDAr antagonists as potential clinical treatment options.

19 (640) Purinergic receptor mechanisms are involved in central sensitization in trigeminal subnucleus caudalis (Medullary Dorsal Horn) C. Chiang, S. Zhang, J. Hu, J. Dostrovsky, B. Sessle; Faculty of Dentistry, University of Toronto, Toronto, ON We have demonstrated that application of the inflammatory irritant and small-fiber excitant mustard oil (MO) to the tooth pulp produces prolonged (⬎40 min) neuroplastic changes reflecting central sensitization in brainstem nociceptive neurons of trigeminal subnucleus caudalis (Vc); this Vc central sensitization is blocked by the P2X receptor antagonist PPADS superfused (i.t.) onto Vc, which suggests that purinergic receptor mechanisms are involved in the central sensitization. The aim of this study was to determine if apyrase (a nucleotide-degrading enzyme) superfusion also attenuates central sensitization in Vc. In urethane/␣-chloralose-anesthesitized adult rats, extracellular recordings were made of nociceptive-specific (NS) neurons in deep Vc laminae. The mechanoreceptive field (RF), mechanical activation threshold and responses to graded pinch or pressure of each neuron were assessed as baseline values. Then apyrase was superfused (30 unit/ml, 0.8 ml/hr) onto Vc throughout an 80-min observation period; saline superfusion served as control. Twenty min after apyrase superfusion started, MO was applied to the molar pulp and six similar assessments at 10 min intervals were then made. After saline pretreatment, MO application produced prolonged significant increases in RF size and responses to graded pinch or pressure and a significant decrease in threshold of the NS neurons (P⬍0..001, repeated measures ANOVA, n⫽6). After apyrase pretreatment, analogous MO application did not produce any significant changes in NS neurons (P⬎0.2, n⫽7). Endogenous ATP acting through purinergic receptor mechanisms may be involved in the initiation of central sensitization in Vc nociceptive neurons. Supported by NIH DE04786.

(641) CGRP receptor activation produces PKA and PKC dependent mechanical hyperalgesia and central sensitization R. Sun, Y. Tu, N. Lawand, Q. Lin, W. Willis; Department of Anatomy and Neurosciences, University of Texas Medical Branch, Galveston, TX Calcitonin gene-related peptide (CGRP), acting through CGRP1 receptors, produced behavioral signs of hyperalgesia in rats and sensitization of wide dynamic range (WDR) neuron in the spinal cord dorsal horn (Sun, et al., 2003). Although, involvement of CGRP receptors in central sensitization has been confirmed, the second messenger systems activated by CGRP receptor stimulation and that are involved in pain transmission are not clear. This study tested whether the hyperalgesic and wide dynamic range (WDR) neuron sensitizing effects of CGRP receptor activation is medicated by PKA or PKC signaling. Intrathecal injection of CGRP in rats produced mechanical hyperalgesia, as shown by paw withdrawal threshold tests. CGRP-induced hyperalgesia was attenuated significantly by the CGRP1 receptor antagonist, CGRP8-37. The effect was attenuated significantly by a protein kinase A (PKA) inhibitor (H89) or a protein kinase C (PKC) inhibitor (chelerythrine chloride). Electrophysiological experiments demonstrated that superfusion of the spinal cord with CGRP induced the sensitization of spinal dorsal horn neurons. The effect could be blocked by CGRP8-37. Either a PKA or a PKC inhibitor (H89 or chelerythrine) also attenuated this effect of CGRP. These results are consistent with the hypothesis that CGRP produces hyperalgesia by a direct action on CGRP receptors in the spinal cord dorsal horn and that this effect of CGRP is dependent on PKA and PKC signaling. In conclusion, CGRP produces mechanical hyperalgesia and sensitizes WDR neurons in dorsal horn in the absence of nerve injury via an action at CGRP1 receptors. These effects of CGRP are mediated by both PKA and PKC second-messenger pathways. Supported by NIH grants NS09743 and NS11255.