We applied randomized double pulse stimulation for assessing the effects of
GABA and a GABA(A) antagonist on compound action potentials in dorsal colu
mn axons isolated from adult rat. We stimulated the axons with double pulse
s at 0.2 Hz and randomly varied interpulse intervals between 3, 4, 5, 8, 10
, 20, 30, 50 and 80 ms. Action potentials were measured using glass micropi
pettes. The first pulse was used to condition the response activated by the
second test pulse. Concentrations of GABA of 1 mM, 100 mu M and 10 mu M di
d not affect action potential amplitudes or latencies activated by conditio
ning pulses. In the control studies, before drug administration, test pulse
s induced response amplitudes that were significantly decreased at 3-, 4- a
nd 5-ms interpulse intervals. The test action potential amplitudes were 84.
6 +/- 2.5%, 89.0 +/- 3.9% and 93.3 +/- 3.6% (mean +/- S.E.M.) of conditioni
ng pulse levels, respectively. At 3-ms interpulse intervals, test response
latencies were prolonged to 104.3 +/- 1.0%, but were unchanged at the other
interpulse intervals. The 10 mu M, 100 mu M and 1 mM concentrations of GAB
A affected test response amplitudes. Application of 100 mu M GABA reduced t
he amplitudes of test responses at 3-, 4-, 5- and 8-ms interpulse intervals
, to 59.2 +/- 3.0%, 70.0 +/- 3.0%, 80.2 +/- 1.1% and 88.6 +/- 3.6% of the c
onditioning pulse amplitudes, respectively. At both 100 mu M and 1 mM conce
ntrations, GABA significantly prolonged the latencies of test responses. Tr
eatment with 100 mu M GABA prolonged the latencies of test responses at 3-,
4- and 5-ms interpulse intervals, to 119.3 +/- 3.1%, 107.3 +/- 2.8% and 10
5.5 +/- 2.5% of conditioning pulse latencies, respectively. The addition of
100 mu M bicuculline methochloride, a GABA(A) antagonist, eliminated the e
ffects of 100 mu M GABA. The combined application of GABA and bicuculline (
both 100 mu M) did not affect amplitudes or latencies of test responses.
These results suggest that GABA(A) receptor subtypes are present on the spi
nal dorsal column axons of adult rat, and that they modulate the excitabili
ty of the axons. The randomized double pulse methods reveal that GABA incre
ases refractoriness of adult rat dorsal column axons. (C) 1999 IBRO. Publis
hed by Elsevier Science Ltd.