Evidence for increased extracellular K(+) as an important mechanism for dorsal root induced alternating rhythmic activity in the neonatal rat spinal cord in vitro.

Intracellular recording from rat lumbar motoneurones showed that patterned electrical stimulation of one dorsal root induced alternating patterns typical of fictive locomotion, accompanied by a reduction in the antidromic spike afterhyperpolarization (AHP). Since the AHP depended linearly on extracellular K(+) ([K(+)](o)), from the AHP fall we calculated that [K(+)](o) rose to 7.9+/-0.4 mM, a value within the 7-10 mM range known to elicit fictive locomotion. To confirm the specificity of this approach we also tested concentrations of N-methyl-D-aspartate (2 microM) below threshold for fictive locomotion, and we observed that, despite strong excitation, the estimated [K(+)](o) was 6 mM, i.e. outside the range for fictive locomotion. We suggest that synaptic activity induced by a certain pattern of electrical stimulation of afferent fibres evoked persistent elevation in [K(+)](o) which triggered the locomotor network to generate rhythmic patterns.