Prolonged nociceptive responses to hind paw formalin injection in rats with a spinal cord injury

Unilateral lesioning of the spinal dorsal horn with the excitotoxin quisqualic acid (QUIS) leads to robust degeneration of dorsal horn grey matter, and robust pain-related symptoms, such as cutaneous hypersensitivity, persist long after injury. A possible mechanism that underlies the pain-related symptoms is the disruption of dorsal horn inhibitory neuron function, leading to decreased inhibition of nociceptive neurons. Five percent formalin was injected into the hind paw of rats with either a QUIS lesion or sham lesion. Both QUIS- and sham-lesioned rats displayed bi-phasic hind paw flinches following formalin injection, but a prolonged response was observed in QUIS-lesioned rats. The expression of the immediate-early gene product Fos in the dorsal horn ipsilateral to formalin injection was similar between QUIS- and sham-lesioned rats. In QUIS-lesioned rats, however, there was a marked absence of dorsal horn neurons, particularly GABAergic neurons, compared to sham-lesioned rats. The prolonged nociceptive response observed with a unilateral QUIS lesion may be due to generalized changes in dorsal horn neuron function including a loss of inhibitory neuron function.