Temporal coding at the immature depolarizing GABAergic synapse.

In the developing hippocampus, GABA exerts depolarizing and excitatory actions and contributes to the generation of neuronal network driven giant depolarizing potentials (GDPs). Here, we studied spike time coding at immature GABAergic synapses and its impact on synchronization of the neuronal network during GDPs in the neonatal (postnatal days P2-6) rat hippocampal slices. Using extracellular recordings, we found that the delays of action potentials (APs) evoked by synaptic activation of GABA(A) receptors are long (mean, 65 ms) and variable (within a time window of 10-200 ms). During patch-clamp recordings, depolarizing GABAergic responses were mainly subthreshold and their amplification by persistent sodium conductance was required to trigger APs. AP delays at GABAergic synapses shortened and their variability reduced with an increase in intracellular chloride concentration during whole-cell recordings. Negative shift of the GABA reversal potential (E(GABA)) with low concentrations of bumetanide, or potentiation of GABA(A) receptors with diazepam reduced GDPs amplitude, desynchronized neuronal firing during GDPs and slowed down GDPs propagation. Partial blockade of GABA(A) receptors with bicuculline increased neuronal synchronization and accelerated GDPs propagation. We propose that spike timing at depolarizing GABA synapses is determined by intracellular chloride concentration. At physiological levels of intracellular chloride GABAergic depolarization does not reach the action potential threshold and amplification of GABAergic responses by non-inactivating sodium conductance is required for postsynaptic AP initiation. Slow and variable excitation at GABAergic synapse determines the level of neuronal synchrony and the rate of GDPs propagation in the developing hippocampus.

NMDA receptors pattern early activity in the developing barrel cortex in vivo.

N-methyl-D-aspartate (NMDA) type of glutamate receptors play an important role in activity-dependent plasticity in the developing cortex. However, the physiological patterns of cortical activity that activate NMDA receptors in vivo remain largely unknown. We performed full-band recordings from the barrel cortex of neonatal rats in vivo and found that the dominant pattern of the early activity, network driven spindle bursts, are associated with large amplitude NMDA receptor-dependent delta waves. The major sink of delta waves was in the dense cortical plate, which coincided with the sinks of sensory-evoked responses as well as fast spindle-burst oscillations. Pharmacological analysis revealed major contributions from NMDA and alpha-aminopropionate (AMPA) type of glutamate receptors in the generation of delta waves, whereas fast oscillations primarily involved only AMPA receptors. Our results suggest that the 2 component spindle burst is generated by rhythmic, presumably thalamocortical, synaptic input which entrains an AMPA receptor-mediated fast oscillation and who's summation generates an NMDA and AMPA receptor mediated delta wave. The massive summation of thalamocortical activity during the spindle bursts thus provides a long time window for co-incident activation of cortical neurons by the thalamocortical cells which may contribute to the formation of thalamocortical synapses in the barrel cortex during the critical period of developmental plasticity.

Postnatal changes in somatic gamma-aminobutyric acid signalling in the rat hippocampus.

During postnatal development of the rat hippocampus, gamma-aminobutyric acid (GABA) switches its action on CA3 pyramidal cells from excitatory to inhibitory. To characterize the underlying changes in the GABA reversal potential, we used somatic cell-attached recordings of GABA(A) and N-methyl-D-aspartate channels to monitor the GABA driving force and resting membrane potential, respectively. We found that the GABA driving force is strongly depolarizing during the first postnatal week. The strength of this depolarization rapidly declines with age, although GABA remains slightly depolarizing, by a few millivolts, even in adult neurons. Reduction in the depolarizing GABA driving force was due to a progressive negative shift of the reversal potential of GABA currents. Similar postnatal changes in GABA signalling were also observed using the superfused hippocampus preparation in vivo, and in the hippocampal interneurons in vitro. We also found that in adult pyramidal cells, somatic GABA reversal potential is maintained at a slightly depolarizing level by bicarbonate conductance, chloride-extrusion and chloride-loading systems. Thus, the postnatal excitatory-to-inhibitory switch in somatic GABA signalling is associated with a negative shift of the GABA reversal potential but without a hyperpolarizing switch in the polarity of GABA responses. These results also suggest that in adult CA3 pyramidal cells, somatic GABAergic inhibition takes place essentially through shunting rather than hyperpolarization. Apparent hyperpolarizing GABA responses previously reported in the soma of CA3 pyramidal cells are probably due to cell depolarization during intracellular or whole-cell recordings.

Timing of the developmental switch in GABA(A) mediated signaling from excitation to inhibition in CA3 rat hippocampus using gramicidin perforated patch and extracellular recordings.

The timing of the developmental switch in the GABA(A) mediated responses from excitatory to inhibitory was studied in Wistar rat CA3 hippocampal pyramidal cells using gramicidin perforated patch-clamp and extracellular recordings. Gramicidin perforated patch recordings revealed a gradual developmental shift in the reversal potential of synaptic and isoguvacine-induced GABA(A) mediated responses from -55 +/- 4 mV at postnatal days P0-2 to -74 +/- 3 mV at P13-15 with a midpoint of disappearance of the excitatory effects of GABA at around P8. Extracellular recordings in CA3 pyramidal cell layer revealed that the effect of isoguvacine on multiple unit activity (MUA) switched from an increase to a decrease at around P10. The effect of synaptic GABA(A) mediated responses on MUA switched from an increase to a decrease at around P8. It is concluded that the developmental switch in the action of GABA via GABA(A) receptors from excitatory to inhibitory occurs in Wistar rat CA3 pyramidal cells at around P8-10, an age that coincides with the transition from immature to mature hippocampal rhythms. We propose that excitatory GABA contributes to enhanced excitability and ictogenesis in the neonatal rat hippocampus.

Membrane potential of CA3 hippocampal pyramidal cells during postnatal development.

A depolarized resting membrane potential has long been considered to be a universal feature of immature neurons. Despite the physiological importance, the underlying mechanisms of this developmental phenomenon are poorly understood. Using perforated-patch, whole cell, and cell-attached recordings, we measured the membrane potential in CA3 pyramidal cells in hippocampal slices from postnatal rats. With gramicidin perforated-patch recordings, membrane potential was -44 +/- 4 (SE) mV at postnatal days P0-P2, and it progressively shifted to -67 +/- 2 mV at P13-15. A similar developmental change of the membrane potential has been also observed with conventional whole cell recordings. However, the value of the membrane potential deduced from the reversal potential of N-methyl-d-aspartate channels in cell-attached recordings did not change with age and was -77 +/- 2 mV at P2 and -77 +/- 2 mV at P13-14. The membrane potential measured using whole cell recordings correlated with seal and input resistance, being most depolarized in neurons with high, several gigaohms, input resistance and low seal resistance. Simulations revealed that depolarized values of the membrane potential in whole cell and perforated-patch recordings could be explained by a shunt through the seal contact between the pipette and membrane. Thus the membrane potential of CA3 pyramidal cells appears to be strongly negative at birth and does not change during postnatal development.

Seizure-induced damage in the developing human: relevance of experimental models.

A considerable amount of money and effort is spent every year investigating the effects of seizure on the developing rodent brain. A critical question is the relevance of these studies to children. The goal of this chapter is to review the relationship between seizures during early development and cognitive impairment in children and rodents. While the majority of children with epilepsy have normal cognitive development, a small group of children with frequent, recurrent seizures show progressive cognitive impairment. Likewise, in rodent models recurrent seizures during early development are associated with cognitive impairment and histological changes including mossy fiber sprouting and reduced neurogenesis. Status epilepticus is associated with a lower morbidity and mortality rate in children than in adults. Status epilepticus in rodent models is associated with less cell loss and cognitive impairment than in adults. While rodent studies can offer a great deal of insight into mechanisms of seizure-induced brain damage, they also have significant limitations. No animal models have yet been developed that mimic human epileptic syndromes, such as infantile spasms, Lennox-Gastaut syndrome, or the severe myoclonic epilepsies. In addition, rodent studies supply only crude measures of learning and memory. Disturbances of language or higher cortical functions such as visual or auditory processing cannot be tested in animal models.

New concepts in neonatal seizures.

The immature brain is more prone to seizures than the older brain as a result of an imbalance between excitatory and inhibitory input. The depolarizing, rather than hyperpolarizing effect of GABA(A) during the first week of life in the rodent, and the delay in postsynaptic GABA(B) inhibition coupled with the over-expression of glutamatergic synapses contribute to this increased propensity toward seizures. It is now clear that seizures can be injurious to the immature brain, although the pattern of seizure-induced injury is age-related. While the immature brain is resistant to acute seizure-induced cell loss, there are functional abnormalities following seizures with impairment of visual-spatial memory and reduced seizure threshold. Neonatal seizures are also associated with a number of activity-dependent changes in brain development including altered synaptogenesis and reduction in neurogenesis. These results argue that neonatal seizures should no longer be considered as benign events.