We studied the regulation of spontaneous activity in the embryonic (day 10-11) chick spinal cord. After bath application of either an excitatory amino acid (AP-5 or CNQX) and a nicotinic cholinergic (DHbetaE or mecamylamine) antagonist, or glycine and GABA receptor (bicuculline, 2-hydroxysaclofen, and strychnine) antagonists, spontaneous activity was blocked for a period (30-90 min) but then reappeared in the presence of the drugs. The efficacy of the antagonists was assessed by their continued ability to block spinal reflex pathways during the reappearance of spontaneous activity. Spontaneous activity ceased over the 4-5 hour monitoring period when both sets of antagonists were applied together. After application of glycine and GABA receptor antagonists, the frequency of occurrence of spontaneous episodes slowed and became highly variable. By contrast, during glutamatergic and nicotinic cholinergic blockade, the frequency of occurrence of spontaneous episodes initially slowed and then recovered to stabilize near the predrug level of activity. Whole-cell recordings made from ventral spinal neurons revealed that this recovery was accompanied by an increase in the amplitude of spontaneously occurring synaptic events. We also measured changes in the apparent equilibrium potential of the rhythmic, synaptic drive of ventral spinal neurons using voltage or discontinuous current clamp. After excitatory blockade, the apparent equilibrium potential of the rhythmic synaptic drive shifted approximately 10 mV more negative to approximately -30 mV. In the presence of bicuculline, the apparent equilibrium potential of the synaptic drive shifted toward the glutamate equilibrium potential. Considered with other evidence, these findings suggest that spontaneous rhythmic output is a general property of developing spinal networks, and that GABA and glycinergic networks alter their function to compensate for the blockade of excitatory transmission.

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.

Gonzalez-Islas C, Chub N, Wenner P. NKCC1 and AE3 appear to accumulate chloride in embryonic motoneurons. J Neurophysiol 101: 507-518, 2009. First published November 26, 2008 doi:10.1152/jn.90986.2008. During early development, ␥-aminobutyric acid (GABA) depolarizes and excites neurons, contrary to its typical function in the mature nervous system. As a result, developing networks are hyperexcitable and experience a spontaneous network activity that is important for several aspects of development. GABA is depolarizing because chloride is accumulated beyond its passive distribution in these developing cells. Identifying all of the transporters that accumulate chloride in immature neurons has been elusive and it is unknown whether chloride levels are different at synaptic and extrasynaptic locations. We have therefore assessed intracellular chloride levels specifically at synaptic locations in embryonic motoneurons by measuring the GABAergic reversal potential (E GABA ) for GABA A miniature postsynaptic currents. When whole cell patch solutions contained 17-52 mM chloride, we found that synaptic E GABA was around Ϫ30 mV. Because of the low HCO 3 Ϫ permeability of the GABA A receptor, this value of E GABA corresponds to approximately 50 mM intracellular chloride. It is likely that synaptic chloride is maintained at levels higher than the patch solution by chloride accumulators. We show that the Na, is clearly involved in the accumulation of chloride in motoneurons because blocking this transporter hyperpolarized E GABA and reduced nerve potentials evoked by local application of a GABA A agonist. However, chloride accumulation following NKCC1 block was still clearly present. We find physiological evidence of chloride accumulation that is dependent on HCO 3 Ϫ and sensitive to an anion exchanger blocker. These results suggest that the anion exchanger, AE3, is also likely to contribute to chloride accumulation in embryonic motoneurons. I N T R O D U C T I O NA great deal of interest has recently been focused on the observation that neurons are depolarized by ␥-aminobutyric acid (GABA) in early development in several different parts of the nervous system (Ben-Ari et al. 2007). In this developmental period, the reversal potential for GABA A receptor currents (referred to as E GABA ) is more depolarized than the resting membrane potential because chloride, the main carrier of the GABA A current, is accumulated in immature neurons. The depolarizing nature of GABA is important in driving the spontaneous network activity (SNA) that is observed in virtually all developing circuits The modulation of intracellular chloride appears to be critical for the normal expression of SNA (Chub and O'Donovan 1998, 2001; Chub et al. 2006; Fedirchuk et al. 1999; Marchetti et al. 2005). During an episode of SNA, intracellular chloride is depleted as it passes out of the cell through activated GABA A receptor channels, thus weakening the driving force for GABAergic synapses, leaving the cord relatively less excitable. Then in the quiescent...