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 GABAA (gamma-aminobutyric acid) receptors to the spontaneous and locomotor-related antidromic firing in the dorsal roots of neonatal rats. Spontaneous bursts of antidromic discharges were generated by the underlying afferent terminal depolarizations reaching spiking threshold. The number of antidromic action potentials increased significantly in saline solution with Cl- concentration reduced to 50% of control. Bath application of the GABAA receptor antagonist bicuculline, at low concentrations (1-2 microM), or picrotoxin blocked the antidromic discharges in the dorsal roots almost completely. The increase in Cl- conductance was therefore mediated by an activation of GABAA receptors. Increasing the concentration of bicuculline to 10-20 microM never blocked these discharges further. On the contrary, in half of the preparations, the number of antidromic action potentials was higher in the presence of high concentrations of bicuculline (10-20 microM) than in the presence of picrotoxin or low concentrations of bicuculline. This suggests that bicuculline, at high concentrations, may have other effects, in addition to blocking GABAA 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 discharges appeared. A double-bath with a barrier built at the L3 level was then used to separate the mechanisms which generate locomotion from those mediating primary afferent depolarizations. Excitatory amino acids and serotonin were perfused in the rostral pool only. Decreasing the concentration of chloride in the caudal bath caused a sharp increase in the number of antidromic action potentials recorded from the L5 dorsal root. These discharges, which were modulated in phase with the locomotor rhythm, were blocked by bicuculline. These data demonstrate the existence of a locomotor-related GABAergic input onto primary afferent terminals in the neonatal rat.

Antidromic discharges of dorsal root afferents in the neonatal rat.

Presynaptic inhibition of primary afferents can be evoked from at least three sources in the adult animal: 1) by stimulation of several supraspinal structures; 2) by spinal reflex action from sensory inputs; or 3) by the activity of spinal locomotor networks. The depolarisation in the intraspinal afferent terminals which is due, at least partly, to the activation of GABA(A) receptors may be large enough to reach firing threshold and evoke action potentials that are antidromically conducted into peripheral nerves. Little is known about the development of presynaptic inhibition and its supraspinal control during ontogeny. This article, reviewing recent experiments performed on the in vitro brainstem/spinal cord preparation of the neonatal rat, demonstrates that a similar organisation is present, to some extent, in the new-born rat. A spontaneous activity consisting of antidromic discharges can be recorded from lumbar dorsal roots. The discharges are generated by the underlying afferent terminal depolarizations reaching firing threshold. The number of antidromic action potentials increases significantly in saline solution with chloride concentration reduced to 50% of control. Bath application of the GABA(A) receptor antagonist, bicuculline (5-10 microM) blocks the antidromic discharges almost completely. Dorsal root discharges are therefore triggered by chloride-dependent GABA(A) receptor-mediated mechanisms; 1) activation of descending pathways by stimulation delivered to the ventral funiculus (VF) of the spinal cord at the C1 level; 2) activation of sensory inputs by stimulation of a neighbouring dorsal root; or 3) pharmacological activation of the central pattern generators for locomotion evokes antidromic discharges in dorsal roots. VF stimulation also inhibited the response to dorsal root stimulation. The time course of this inhibition overlapped with that of the dorsal root discharge suggesting that part of the inhibition of the monosynaptic reflex may be exerted at a presynaptic level. The existence of GABA(A) receptor-independent mechanisms and the roles of the antidromic discharges in the neonatal rat are discussed.