Post-Episode Depression of GABAergic Transmission in Spinal Neurons of the Chick Embryo

Abstract

Whole cell recordings were obtained from ventral horn neurons in spontaneously active spinal cords isolated from the chick embryo [embryonic days 10 to 11 (E10-E11)] to examine the post-episode depression of GABAergic transmission. Spontaneous activity occurred as recurrent, rhythmic episodes approximately 60 s in duration with 10- to 15-min quiescent inter-episode intervals. Current-clamp recording revealed that episodes were followed by a transient hyperpolarization (7 +/- 1.2 mV, mean +/- SE), which dissipated as a slow (0.5-1 mV/min) depolarization until the next episode. Local application of bicuculline 8 min after an episode hyperpolarized spinal neurons by 6 +/- 0.8 mV and increased their input resistance by 13%, suggesting the involvement of GABAergic transmission. Gramicidin perforated-patch recordings showed that the GABAa reversal potential was above rest potential (E(GABAa) = -29 +/- 3 mV) and allowed estimation of the physiological intracellular [Cl(-)] = 50 mM. In whole cell configuration (with physiological electrode [Cl(-)]), two distinct types of endogenous GABAergic currents (I(GABAa)) were found during the inter-episode interval. The first comprised TTX-resistant, asynchronous miniature postsynaptic currents (mPSCs), an indicator of quantal GABA release (up to 42% of total mPSCs). The second (tonic I(GABAa)) was complimentary to the slow membrane depolarization and may arise from persistent activation of extrasynaptic GABAa receptors. We estimate that approximately 10 postsynaptic channels are activated by a single quantum of GABA release during an mPSC and that about 30 extrasynaptic GABAa channels are required for generation of the tonic I(GABAa) in ventral horn neurons. We investigated the post-episode depression of I(GABAa) by local application of GABA or isoguvacine (100 microM, for 10-30 s) applied before and after an episode at holding potentials (V(hold)) -60 mV. The amplitude of the evoked I(GABA) was compared after clamping the cell during the episode at one of three different V(hold): -60 mV, below E(GABAa) resulting in Cl(-) efflux; -30 mV, close to E(GABAa) with minimal Cl(-) flux; and 0 mV, above E(GABAa) resulting in Cl(-) influx during the episode. The amplitude of the evoked I(GABA) changed according to the direction of Cl(-) flux during the episode: at -60 mV a 41% decrease, at -30 mV a 4% reduction, and at 0 mV a 19% increase. These post-episode changes were accompanied by shifts of E(GABAa) of -10, -1.2, and +7 mV, respectively. We conclude that redistribution of intracellular [Cl(-)] during spontaneous episodes is likely to be an important postsynaptic mechanism involved in the post-episode depression of GABAergic transmission in chick embryo spinal neurons.

GABA Uptake via GABA Transporter-1 Modulates GABAergic Transmission in the Immature Hippocampus

“…The decrease in GDP frequency was not studied further, but it may well reflect an increase in the time needed to restore neuronal Cl Ϫ gradients after the enhanced dissipation (and a consequent reduction in the driving force for excitatory GABA responses) that is likely to take place during the NO-711-mediated increase in GDP amplitude and duration (cf. Chub and O'Donovan, 2001).…”

GABA A transmission is a critical step in the process of triggering homeostatic increases in quantal amplitude

“…It is likely that several mechanisms are recruited to recover and maintain activity levels and that the different mechanisms may have different time courses (13). After glutamatergic blockade, activity recovers in part because of a fast loading of intracellular chloride, which strengthens the unblocked GABAergic currents (22,44). After GABAergic block, there may be fast changes in the probability of release (45) and changes in cellular excitability (46,47).…”

“…Previous studies using TTX have demonstrated that the majority of sPSCs recorded are action potentialindependent mPSCs (7,44). Therefore, experiments were performed without TTX (except where mentioned) and are referred to as mPSCs.…”

“…Therefore, experiments were performed without TTX (except where mentioned) and are referred to as mPSCs. Previous reports have suggested that motoneurons in the E10 chicken embryo primarily receive GABAergic and glutamatergic mPSCs (7,44). To establish which populations of neurotransmitters contributed to the mPSCs recorded, GABAergic and glutamatergic antagonists were acutely bath-applied.…”

Modeling Spontaneous Activity in the Developing Spinal Cord Using Activity-Dependent Variations of Intracellular Chloride

“…During development, the classical inhibitory neurotransmitters GABA and glycine can be functionally excitatory because the chloride equilibrium potential is above rest as a result of an elevated intracellular chloride concentration (Chub and O'Donovan, 2001) (for review, see Ben Ari, 2002). This leads to a predominance of excitatory synapses that renders developing networks hyperexcitable.…”