GABAergic inhibition suppresses paroxysmal network activity in the neonatal rodent hippocampus and neocortex

Excitatory actions of endogenously released GABA contribute to initiation of ictal epileptiform activity in the developing hippocampus

In the developing rat hippocampus, ictal epileptiform activity can be elicited easily in vitro during the first three postnatal weeks. Changes in neuronal ion transport during this time cause the effects of GABA(A) receptor (GABA(A)-R) activation to shift gradually from strongly depolarizing to hyperpolarizing. It is not known whether the depolarizing effects of GABA and the propensity for ictal activity are causally linked. A key question is whether the GABA-mediated depolarization is excitatory, which we defined operationally as being sufficient to trigger action potentials. We assessed the effect of endogenous GABA on ictal activity and neuronal firing rate in hippocampal slices from postnatal day 1 (P1) to P30. In extracellular recordings, there was a strong correlation between the postnatal age at which GABA(A)-R antagonists decreased action potential frequency (P23) and the age at which ictal activity could be induced by elevated potassium (P23). In addition, there was a strong correlation between the fraction of slices in which ictal activity was induced by elevated potassium concentrations and the fractional decrease in action potential firing when GABA(A)-Rs were blocked in the presence of ionotropic glutamate receptor antagonists. Finally, ictal activity induced by elevated potassium was blocked by the GABA(A)-R antagonists bicuculline and SR-95531 (gabazine) and increased in frequency and duration by GABA(A)-R agonists isoguvacine and muscimol. Thus, the propensity of the developing hippocampus for ictal activity is highly correlated with the effect of GABA on action potential probability and reversed by GABA(A) antagonists, indicating that GABA-mediated excitation is causally linked to ictal activity in this developmental window.

The role of chloride-dependent inhibition and the activity of fast-spiking neurons during cortical spike-wave electrographic seizures

The conventional view is that the cortical paroxysmal depolarizing shift is a giant excitatory postsynaptic potential enhanced by various intrinsic neuronal currents. Other results point out, however, that synaptic inhibition remains functional in many forms of paroxysmal activities and that intense activation of GABAergic interneurons may accentuate the excitation of target pyramidal cells. To determine the role played by cortical inhibitory neurons in paroxysmal discharges, we used single and dual intracellular recordings from electrophysiologically identified neocortical neurons during spontaneously occurring and electrically induced spike-wave electrographic seizures in vivo. Conventional fast-spiking neurons (presumably local inhibitory interneurons) fired at a very high frequency during paroxysmal depolarizing shifts, which corresponded to the electroencephalogram 'spike' components of spike-wave complexes. The firing of fast-spiking neurons preceded the discharges of neighboring regular-spiking neurons. During electrographic seizures, the reversal potential of the GABA (type A)-mediated potentials in regular-spiking neurons was shifted to positive values by 20-30 mV. Data also show that the prolonged hyperpolarizations during the electroencephalogram 'wave' components of spike-wave electrographic seizures do not contain Cl(-)-dependent inhibitory potentials. Moreover, Cl(-)-dependent mechanisms were reduced or absent during the fast runs that are associated with spike-wave complexes in some paroxysms. We conclude that the strong activity of cortical inhibitory neurons during paroxysmal depolarizing shifts induces Cl(-)-dependent depolarizing postsynaptic potentials in target pyramidal neurons, which facilitate the development of electrographic seizures.

Molecular biology and ontogeny of gamma-aminobutyric acid (GABA) receptors in the mammalian central nervous system

gamma-Aminobutyric acid (GABA) is the predominant inhibitory neurotransmitter in the mammalian central nervous system. After release from nerve terminals, GABA binds to at least two classes of postsynaptic receptors (ie, GABAA and GABAB), which are nearly ubiquitous in the brain. GABAA receptors are postsynaptic heteropentameric complexes that display unique physiologic and pharmacologic properties based on subunit composition. Activation of GABAA receptors in mature neurons results in membrane hyperpolarization, which is mediated principally by inward chloride flux, whereas in early stages of brain development, GABAA receptor activation causes depolarization of the postsynaptic membrane. GABA, receptors reside both presynaptically and postsynaptically, exist as heterodimers and are coupled to voltage-dependent ion channels through interactions with heterotrimeric G proteins. This review summarizes the molecular biology and ontogeny of GABAA and GABAB receptors, highlighting some of their putative roles during normal brain development as well as in disease states such as epilepsy.

Is there more to GABA than synaptic inhibition?

In the mature brain, GABA (gamma-aminobutyric acid) functions primarily as an inhibitory neurotransmitter. But it can also act as a trophic factor during nervous system development to influence events such as proliferation, migration, differentiation, synapse maturation and cell death. GABA mediates these processes by the activation of traditional ionotropic and metabotropic receptors, and probably by both synaptic and non-synaptic mechanisms. However, the functional properties of GABA receptor signalling in the immature brain are significantly different from, and in some ways opposite to, those found in the adult brain. The unique features of the early-appearing GABA signalling systems might help to explain how GABA acts as a developmental signal.

Alterations of the neocortical GABAergic system in the pilocarpine model of temporal lobe epilepsy: neuronal damage and immunocytochemical changes in chronic epileptic rats

A wealth of previous studies reported pathological alterations in extrahippocampal regions in mesial temporal lobe epilepsy. Previous experimental findings have also demonstrated that the entorhinal cortex and the neocortex are damaged in different animal models of acute limbic seizures. The present study was aimed at verifying possible alterations in neocortical areas, and, in particular, structural changes of GABAergic interneurons in the sensorimotor cortex, in pilocarpine-induced chronic epilepsy in the rat. Series of sections were Nissl stained and processed for immunocytochemistry using antibodies that recognize nonphosphorylated neurofilament (SMI311), glial fibrillary acidic protein (GFAP), the calcium-binding protein parvalbumin (PV) which is expressed by a subset of cortical GABAergic neurons, the GABA transporter (GAT1), and isoform 65 of glutamic acid decarboxylase (GAD65), the GABA synthetic enzyme. Epileptic rats showed decreased cortical thickness, and diffuse gliosis was observed with GFAP antibody. Neurofilament alterations were also detected in sections processed using SMI311 antiserum. In addition, a diffuse decrease of PV, GAD65, and GAT1 immunoreactivity was observed in the sensorimotor cortex. Altered expression of PV, GAD65, and GAT1 pointed out specific neocortical disturbances in GABAergic inhibition, which could play a crucial role in seizure generation and expression. Thus, the present findings indicate that damage of GABAergic interneurons could be strictly associated with neocortical hyperexcitability in temporal lobe epilepsy.

Physiological effects of sustained blockade of excitatory synaptic transmission on spontaneously active developing neuronal networks--an inquiry into the reciprocal linkage between intrinsic biorhythms and neuroplasticity in early ontogeny

Spontaneous bioelectric activity (SBA) taking the form of extracellularly recorded spike trains (SBA) has been quantitatively analyzed in organotypic neonatal rat visual cortex explants at different ages in vitro, and the effects investigated of both short- and long-term pharmacological suppression of glutamatergic synaptic transmission. In the presence of APV, a selective NMDA receptor blocker, 1-2- (but not 3-)week-old cultures recovered their previous SBA levels in a matter of hours, although in imitation of the acute effect of the GABAergic inhibitor picrotoxin (PTX), bursts of action potentials were abnormally short and intense. Cultures treated either overnight or chronically for 1-3 weeks with APV, the AMPA/kainate receptor blocker DNQX, or a combination of the two were found to display very different abnormalities in their firing patterns. NMDA receptor blockade for 3 weeks produced the most severe deviations from control SBA, consisting of greatly prolonged and intensified burst firing with a strong tendency to be broken up into trains of shorter spike clusters. This pattern was most closely approximated by acute GABAergic disinhibition in cultures of the same age, but this latter treatment also differed in several respects from the chronic-APV effect. In 2-week-old explants, in contrast, it was the APV+DNQX treated group which showed the most exaggerated spike bursts. Functional maturation of neocortical networks, therefore, may specifically require NMDA receptor activation (not merely a high level of neuronal firing) which initially is driven by endogenous rather than afferent evoked bioelectric activity. Putative cellular mechanisms are discussed in the context of a thorough review of the extensive but scattered literature relating activity-dependent brain development to spontaneous neuronal firing patterns.

Early development of neuronal activity in the primate hippocampus in utero

Morphological studies suggest that the primate hippocampus develops extensively before birth, but little is known about its functional development. Patch-clamp recordings of hippocampal neurons and reconstruction of biocytin-filled pyramidal cells were performed in slices of macaque cynomolgus fetuses delivered by cesarean section. We found that during the second half of gestation, axons and dendrites of pyramidal cells grow intensively by hundreds of micrometers per day to attain a high level of maturity near term. Synaptic currents appear around midgestation and are correlated with the level of morphological differentiation of pyramidal cells: the first synapses are GABAergic, and their emergence correlates with the growth of apical dendrite into stratum radiatum. A later occurrence of glutamatergic synaptic currents correlates with a further differentiation of the axodendritic tree and the appearance of spines. Relying on the number of dendritic spines, we estimated that hundreds of new glutamatergic synapses are established every day on a pyramidal neuron during the last third of gestation. Most of the synaptic activity is synchronized in spontaneous slow ( approximately 0.1 Hz) network oscillations reminiscent of the giant depolarizing potentials in neonatal rodents. Epileptiform discharges can be evoked by the GABA(A) receptor antagonist bicuculline by the last third of gestation, and postsynaptic GABA(B) receptors contribute to the termination of epileptiform discharges. Comparing the results obtained in primates and rodents, we conclude that the template of early hippocampal network development is conserved across the mammalian evolution but that it is shifted toward fetal life in primate.