ALTERED DISCHARGES OF SPINAL WIDE DYNAMIC RANGE NEURONS AND DOWN-REGULATION OF GLUTAMATE TRANSPORTER EXPRESSION IN RATS WITH PACLITAXEL-INDUCED HYPERALGESIA J. P. CATA, H.-R. WENG, J.-H. CHEN AND P. M. DOUGHERTY* Department of Anesthesiology and Pain Research, Division of Anes- thesiology and Critical Care Medicine, University of Texas–M.D. Anderson Cancer Center, 1400 Holcombe, Unit 409, Houston, TX 77030 USA Abstract—Changes in the signaling of wide dynamic range neurons and the expression of glutamate transporters in the lumbar spinal dorsal horn of rats with Taxol-induced hyper- algesia are detailed in this report. Deep spinal lamina neu- rons have significantly increased spontaneous activity and after-discharges to noxious mechanical stimuli, increased responses to both skin heating and cooling, and increased after-discharges and abnormal windup to transcutaneous electrical stimuli. The expression of glutamate transporter proteins in the dorsal horn is decreased at the time point corresponding to the physiological changes. These results suggest a state of increased excitability develops in spinal pain-signaling neurons as a consequence of decreased glu- tamate clearance. These changes in dorsal horn neurobiol- ogy likely in turn contribute to the hyper-responsiveness to sensory stimuli seen in animals treated with Taxol and may play a role in the pain seen in cancer patients receiving Taxol. © 2005 Published by Elsevier Ltd on behalf of IBRO. Key words: chemotherapy, cancer, neuropathic pain, windup, spinal cord. Paclitaxel (Taxol) is the frontline antineoplastic agent used in the treatment of multiple solid tumor malignancies (Row- insky et al., 1993). The major dose- and treatment-limiting complication experienced by patients receiving Taxol is painful sensory motor neuropathy (Forsyth et al., 1997; Quasthoff and Hartung, 2002; Verstappen et al., 2003). Symptoms occur in a stocking and glove distribution often early in treatment with the final overall incidence and se- verity directly related to the initial and cumulative drug doses, the co-administration of other chemotherapeutic agents and the presence of other diseases like diabetes or alcoholism (Tuxen and Hansen, 1994; Rowinsky and Donehower, 1995; New et al., 1996; Connelly et al., 1996; Chaudhry et al., 2003). Taxol-induced pain improves or completely resolves in about half of the cases, but be- comes chronic in the remainder. In that treatment regi- mens are only partially effective at best, chronic Taxol chemo-neuropathy affects the quality of life and return to productivity of thousands of cancer survivors (Connelly et al., 1996). The mechanism of Taxol-induced pain is unknown, but psychophysical studies indicate that patients with chronic Taxol-induced pain have preferential dysfunction in my- elinated primary afferent fibers (Dougherty et al., 2004). Sensory encoding by neurons in the spinal dorsal horn during paclitaxel-induced hyperalgesia has not been ex- plored. There is ample evidence that multiple changes in such activity occurs in many models of experimental neu- ropathy (Palecek et al., 1992, 1993; Laird and Bennett, 1993; Pertovaara et al., 1997; Yakhnitsa et al., 1999). Increased spontaneous activity, after-discharges, and ex- panded receptive field areas in wide dynamic range (WDR) neurons were reported in rats with chronic constriction neu- ropathy (Laird and Cervero, 1989; Tabo et al., 1990; Palecek et al., 1992, 1993; Pertovaara et al., 1997; Yakhnitsa et al., 1999). Similar changes in spinal WDR neuron physiology were also observed in rats with vincristine-induced neuro- pathic pain (Weng et al., 2003). Moreover, the changes in neural function in the spinal dorsal horn accompanying chronic constriction neuropathy are paralleled by down- regulation in the expression of the glutamate-aspartate trans- porter (GLAST), the glial glutamate transporter-1 (GLT-1), and the excitatory amino acid carrier-1 (EAAC1) (Sung et al., 2003). Glutamate transporters are the key mechanism for clearance and maintenance of homeostasis in extracellular glutamate concentrations at excitatory synapses and thus control the termination of signaling at these sites (Danbolt, 2001; Gegelashvili et al., 2001). The first goal of the present study was to determine whether rats with paclitaxel-induced hyperalgesia show evidence of altered sensory encoding in the dorsal horn. In the second goal of the study, changes in the expression of glutamate transporters in rats with chemo- therapy-induced neuropathy were explored using immuno- histochemistry. EXPERIMENTAL PROCEDURES A total of 36 male Sprague–Dawley rats weighing 175–220 g were used. Seventeen of these received paclitaxel and 19 rats received vehicle. The animals were housed singly with room temperature maintained at 22 °C and with food and water provided ad libitum. All experiments were carried out in accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals and had been reviewed and approved by the University of Texas M.D. Anderson Animal Care and Use Committee. The minimal number of animals needed was used in each experiment *Corresponding author. Tel: +1-713-745-0438; fax: +1-713-745-2956. E-mail address: pdougherty@mdanderson.org (P. M. Dougherty). Abbreviations: COX, cyclooxygenase; DMSO, dimethyl sulfoxide; EAAC1, excitatory amino acid carrier-1; GLAST, glutamate-aspartate transporter; GLT-1, glial glutamate transporter-1; NMDA, N-methyl-D- aspartate; PBS, phosphate-buffered saline; WDR, wide dynamic range. Neuroscience 138 (2006) 329 –338 0306-4522/06$30.00+0.00 © 2005 Published by Elsevier Ltd on behalf of IBRO. doi:10.1016/j.neuroscience.2005.11.009 329