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We have previously shown that the atypical methylxanthine, propentofylline, reduces mechanical allodynia after peripheral nerve transection in a rodent model of neuropathy. In the present study, we sought to determine whether propentofylline-induced glial modulation alters spinal glutamate transporters, glutamate transporter-1 (GLT-1) and glutamate-aspartate transporter (GLAST) in vivo, which may contribute to reduced behavioral hypersensitivity after nerve injury. In order to specifically examine the expression of the spinal glutamate transporters, a novel line of double transgenic GLT-1-enhanced green fluorescent protein (eGFP)/GLAST-Discosoma Red (DsRed) promoter mice was used. Adult mice received propentofylline (10 mg/kg) or saline via i.p. injection starting 1 h prior to L5-spinal nerve transection and then daily for 12 days. Mice receiving saline exhibited punctate expression of both eGFP (GLT-1 promoter activation) and DsRed (GLAST promoter activation) in the dorsal horn of the spinal cord, which was decreased ipsilateral to nerve injury on day 12. Propentofylline administration reinstated promoter activation on the injured side as evidenced by an equal number of eGFP (GLT-1) and DsRed (GLAST) puncta in both dorsal horns. As demonstrated in previous studies, propentofylline induced a concomitant reversal of L5 spinal nerve transection-induced expression of glial fibrillary acidic protein (GFAP). The ability of propentofylline to alter glial glutamate transporters highlights the importance of controlling aberrant glial activation in neuropathic pain and suggests one possible mechanism for the anti-allodynic action of this drug.

Increasing evidence points to a role for spinal neuroimmune dysregulation (glial cell activation and cytokine expression) in the pathogenesis of chronic pain. Suppression of astrocytic and microglial activation with the methylxanthine derivative, propentofylline, pre-emptively attenuates the development of nerve injury-induced allodynia. Currently, we investigated the ability of systemic propentofylline to reverse existing, long-term allodynia after nerve injury--a clinically relevant paradigm. Rats received L5 spinal nerve transection or sham surgery and the development of mechanical allodynia was assessed daily for 2 weeks, at which time injured rats exhibited robust responses to non-noxious von Frey filaments. On days 14-27, rats received either saline or 101 mg/kg propentofylline by intraperitoneal (i.p.) injection. On day 28 or 42 (after a 14-day drug washout period), lumbar spinal cord sections were processed for assessment of astrocytic glial fibrillary acidic protein (GFAP) and microglial OX-42 (antibody against CR3/CD11b). Propentofylline treatment to nerve injured rats resulted in significant reversal of allodynia that lasted throughout the 14-day washout period. Spinal microglial activation was observed at days 28 and 42 post-injury at the protein level, in the absence of mRNA level changes. Less robust increases in GFAP immunoreactivity were observed at days 28 and 42 post-transection. Interestingly, propentofylline treatment suppressed microglial activation at both time points in this paradigm. Taken together, our results highlight the clinical potential of the glial modulating agent, propentofylline, for the treatment of neuropathic pain as well as a role for microglia in the long-term maintenance of allodynia.

Reactive astrocytes display decreased glutamate transporters, such as GLT-1, and as a result synaptic glutamate clearance is impaired. In addition, these activated astrocytes are immunocompetent and release algesic mediators that can sensitize neurons in the spinal cord. Currently, we evaluated the effect of propentofylline (PPF), an experimental antiallodynic agent, on the phenotype and glutamate transporter expression of astrocytes. Primary astrocyte cultures, which represent an activated phenotype with a polygonal morphology and low GLT-1 expression, were treated for 3 or 7 days with 10, 100, or 1,000 microM PPF or dibutyryl-cAMP (db-cAMP), a known inducer of GLT-1 expression. PPF dose-dependently induced astrocytes to display a mature phenotype, with elongated processes and a stellate shape, as well as increased GLT-1 and GLAST immunoreactivity, similar to that seen with db-cAMP. Real time RT-PCR and Western blot analysis clearly demonstrated that PPF caused a potent dose-dependent induction of GLT-1 and GLAST mRNA and protein in these astrocytes. Importantly, the observed increase in glutamate transporters was found to have a functional effect, with significantly enhanced glutamate uptake in astrocytes treated with 100 or 1,000 microM PPF that was sensitive to dihydrokainate inhibition, suggesting it is GLT-1 mediated. Finally, the effect of PPF on lipopolysaccharide-induced chemokine release was investigated. Interestingly, PPF was able to dampen both MCP-1 (CCL2) and MIP-2 (CXCL2) release from astrocytes while db-cAMP significantly enhanced this chemokine expression. These findings suggest that PPF is capable of differentiating astrocytes to a homeostatic, mature phenotype, competent for glutamate clearance and distinct from that induced by db-cAMP.