Spontaneous and locomotor-related GABAergic input onto primary afferents in the neonatal rat

The in vitro brain stem-spinal cord preparation of neonatal rats (0-5 days old) was used to examine the contribution of GABA(A) (gamma-aminobutyric ac id) receptors to the spontaneous and locomotor-related antidromic firing in the dorsal roots of neonatal rats. Spontaneous bursts of antidromic discha rges were generated by the underlying afferent terminal depolarizations rea ching spiking threshold. The number of antidromic action potentials increas ed significantly in saline solution with Cl- concentration reduced to 50% o f control. Bath application of the GABA(A) receptor antagonist bicuculline, at low concentrations (1-2 mu M), or picrotoxin blocked the antidromic dis charges in the dorsal roots almost completely. The increase in Cl- conducta nce was therefore mediated by an activation of GABA(A) receptors. Increasin g the concentration of bicuculline to 10-20 mu M never blocked these discha rges further. On the contrary, in half of the preparations, the number of a ntidromic action potentials was higher in the presence of high concentratio ns of bicuculline (10-20 mu M) than in the presence of picrotoxin or low co ncentrations of bicuculline. This suggests that bicuculline, at high concen trations, may have other effects, in addition to blocking GABA(A) receptors , Dorsal root firing was observed during fictive locomotion induced by bath application of excitatory amino acids and serotonin. A rhythmical pattern was often demonstrated. Bicuculline at low concentrations caused a decrease of the antidromic discharge whereas, at high concentrations, bursts of dis charges appeared. A double-bath with a barrier built at the L3 level was th en used to separate the mechanisms which generate locomotion from those med iating primary afferent depolarizations. Excitatory amino acids and seroton in were perfused in the rostral pool only. Decreasing the concentration of chloride in the caudal bath caused a sharp increase in the number of antidr omic action potentials recorded from the L5 dorsal root. These discharges, which were modulated in phase with the locomotor rhythm, were blocked by bi cuculline. These data demonstrate the existence of a locomotor-related GABA ergic input onto primary afferent terminals in the neonatal rat.