Evaluation of Formalin-Induced Pain Behavior and Glutamate Release in the Spinal Dorsal Horn Using In Vivo Microdialysis in Conscious Rats

Measurement of neurotransmitters in the spinal dorsal horn in conscious animals re- mains a technical challenge. Here we applied concentric microdialysis to measure formalin-induced glutamate (Glu) release in the ipsilateral dorsal horn in conscious, freely-moving rats. Hind paw formalin injection induced flinching nociceptive behaviors accompanied by increased Glu in the dorsal horn (maximum = 294%). Both flinching and Glu increase were prevented by morphine (3 mg/kg, s.c.). Accordingly, concentric microdialysis is a sensitive technique for studying neuro- chemical modulation induced by pain and analgesics in the spinal dorsal horn of awake rats. Measurement of Glu provides information on modulation of excitatory signals.