Involvement of GABAB Receptors in Presynaptic Inhibition of Fibers of the Descending Projections of the Spinal Cord in the Frog Rana Ridibunda

Isolated spinal cord preparations from Rana ridibunda frogs were used for studies of the effects of the GABA(B) receptor agonists (-)-baclofen (50 and 100 microM) and GABA (4-8 mM) and the specific GABA(B) receptor antagonist 2-hydroxysaclofen (100 microM) on the transmission of signals from fibers of the ventral columns monosynaptically connected with motoneurons in segments 9 and 10. These experiments showed that (-)-baclofen (50 and 100 microM) produced significant and dose-dependent suppression of excitatory postsynaptic potentials (EPSP) in motoneurons and ventral root potentials evoked by stimulation of fibers of the ipsi- and contralateral ventral columns. The inhibitory effect of (-)-baclofen (100 microM) on descending EPSP was 35-50% blocked by the GABA(B) receptor antagonist 2-hydroxysaclofen (0.2 mM). The inhibitory effect of GABA (4-8 mM) on descending EPSP was 60% blocked by the GABA(A) receptor antagonist picrotoxin (0.05 mM). (-)-Baclofen (50 microM) and GABA (4 and 6 mM) were found to have inhibitory effects on ventral root potentials evoked by stimulation of the ipsi- and contralateral ventral columns. The data obtained here, as well as data obtained by pharmacological analysis and conditioning by stimulation of the ipsi- and contralateral ventral columns, are regarded as a significant argument supporting the existence of GABA(B) receptor-mediated presynaptic inhibition of descending fibers connected monosynaptically to spinal cord motoneurons in the frog Rana ridibunda.