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 typi cal of fictive locomotion, accompanied by a reduction in the antidromic spi ke afterhyperpolarization (AHP). Since the AHP depended linearly on extrace llular 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 loc omotion. To confirm the specificity of this approach we also tested concent rations of N-methyl-D-aspartate (2 muM) below threshold for fictive locomot ion, 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 s ynaptic 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. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.