To elucidate these mechanisms, we studied the spontaneous and thermally evoked firing patterns of ventrolateral periaqueductal gray (vlPAG) neurons in awake-behaving rats treated with paclitaxel to induce neuropathic pain.  In the present study, vlPAG neurons in naive rats exhibited either excitatory, inhibitory, or neutral responses to noxious thermal stimuli, as previously observed.  However, after development of behavioral hypersensitivity induced by the chemotherapeutic agent, paclitaxel, vlPAG neurons displayed increased neuronal activity and changes in thermal pain-evoked neuronal activity.  This involved elevated levels of spontaneous firing and heightened responsiveness to nonnoxious stimuli (allodynia) as well as noxious thermal stimuli (hyperalgesia) as compared with controls.  Furthermore, after paclitaxel treatment, only excitatory neuronal responses were observed for both nonnoxious and noxious thermal stimuli.  Systemic administration of gabapentin, a nonopioid analgesic, induced significant dose-dependent decreases in the elevated spontaneous and thermally evoked vlPAG neuronal firing to both nonnoxious and noxious thermal stimuli in rats exhibiting neuropathic pain, but not in naive rats.  Thus, these results show a strong correlation between behavioral hypersensitivity to thermal stimuli and increased firing of vlPAG neurons in allodynia and hyperalgesia that occur in this neuropathic pain model.
THERM