Attenuated Glial K(+) Clearance Contributes to Long-Term Synaptic Potentiation Via Depolarizing GABA in Dorsal Horn Neurons of Rat Spinal Cord

It has been reported that long-term enhancement of superficial dorsal horn (DH(s)) excitatory synaptic transmission underlies central sensitization, secondary hyperalgesia, and persistent pain. We tested whether impaired clearance of K(+) and glutamate by glia in DH(s) may contribute to initiation and maintenance of the CNS pain circuit and sensorimotor abnormalities. Transient exposure of the spinal cord slice to fluorocitrate (FC) is shown to be accompanied by a protracted decrease of the DH(s) optical response to repetitive electrical stimulation of the ipsilateral dorsal root, and by a similarly protracted increase in the postsynaptic response of the DH(s) like LTP. It also is shown that LTP(FC) does not occur in the presence of APV, and becomes progressively smaller as [K(+)](o) in the perfusion solution decreased from 3.0 mM to 0.0 mM. Interestingly LTP(FC) is reduced by bath application of Bic. Whole-cell patch recordings were carried out to evaluate the effects of FC on the response of DH(s) neurons to puffer-applied GABA. The observations reveal that transient exposure to FC is reliably accompanied by a prolonged (>1 hr) depolarizing shift of the equilibrium potential for the DH(s) neuron transmembrane ionic currents evoked by GABA. Considered collectively, the findings demonstrate that LTP(FC) involves (1) elevation of [K(+)](o) in the DH(s), (2) NMDAR activation, and (3) conversion of the effect of GABA on DH(s) neurons from inhibition to excitation. It is proposed that a transient impairment of astrocyte energy production can trigger the cascade of dorsal horn mechanisms that underlies hyperalgesia and persistent pain.