GABA (gamma-aminobutyric acid) is the main inhibitory transmitter in the adult brain, and it exerts its fast hyperpolarizing effect through activation of anion (predominantly Cl-)-permeant GABA(A) receptors. However, during early neuronal development, GABA(A)-receptor-mediated responses are often depolarizing, which may be a key factor in the control of several Ca2+-dependent developmental phenomena, including neuronal proliferation, migration and targeting. To date, however, the molecular mechanism underlying this shift in neuronal electrophysiological phenotype is unknown. Here we show that, in pyramidal neurons of the rat hippocampus, the ontogenetic change in GABA(A)-mediated responses from depolarizing to hyperpolarizing is coupled to a developmental induction of the expression of the neuronal (Cl-)-extruding K+/Cl- co-transporter, KCC2. Antisense oligonucleotide inhibition of KCC2 expression produces a marked positive shift in the reversal potential of GABAA responses in functionally mature hippocampal pyramidal neurons. These data support the conclusion that KCC2 is the main Cl- extruder to promote fast hyperpolarizing postsynaptic inhibition in the brain.

Biphasic GABA A -mediated postsynaptic responses can be readily evoked in CA1 pyramidal neurons of rat hippocampal slices by high-frequency stimulus (HFS) trains in the presence of ionotropic glutamate receptor antagonists. In the present experiments with sharp microelectrodes, whole-cell techniques, and K ϩ -selective microelectrodes, an HFS train (40 pulses at 100 Hz) applied in stratum radiatum close to the recording site evoked a brief hyperpolarizing IPSP (hIPSP), which turned into a prolonged (2-3 sec) depolarization (GABAmediated depolarizing postsynaptic potential; GDPSP). The I-V relationships of the postsynaptic currents (hIPSC and GDPSC) had distinct characteristics: the hIPSC and the early GDPSC showed outward rectification, whereas the late GDPSC was reduced with positive voltage steps to zero or beyond (inward rectification), but often no clear reversal was seen. That two distinct currents contribute to the generation of the GDPSP was also evident from the finding that a second HFS train at peak or late GDPSP induced a prompt GABA A -mediated hyperpolarization. The GDPSP/C was dependent on the availability of bicarbonate, but not on interstitial or intrapyramidal carbonic anhydrase activity.The HFS train evoked a rapid GABA A -mediated bicarbonatedependent increase in the extracellular

Long-term potentiation (LTP), which approximates Hebb's postulate of associative learning, typically requires depolarization-dependent glutamate receptors of the NMDA (N-methyl-Daspartate) subtype. However, in some neurons, LTP depends instead on calcium-permeable AMPA-type receptors. This is paradoxical because intracellular polyamines block such receptors during depolarization. We report that LTP at synapses on hippocampal interneurons mediating feedback inhibition is "anti-Hebbian": It is induced by presynaptic activity but prevented by postsynaptic depolarization. Anti-Hebbian LTP may occur in interneurons that are silent during periods of intense pyramidal cell firing, such as sharp waves, and lead to their altered activation during theta activity.Associative N-methyl-D-aspartate receptor (NMDAR)-dependent LTP is induced by coincident activity in afferent pathways sufficient to depolarize postsynaptic neurons (1). However, the voltage dependence of Ca 2+ -permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (CP-AMPARs) is opposite to that of NMDARs (2, 3). Because CP-AMPARs are blocked by cytoplasmic polyamines upon depolarization (4, 5), maximal Ca 2+ influx occurs when the membrane potential is relatively negative. LTP dependent on CP-AMPARs occurs in interneurons of the spinal cord and amygdala (6, 7), but its postsynaptic voltage dependence has not been explored. In hippocampal interneurons, CP-AMPARs have been implicated in long-term depression (8-10), and contribute to synaptic Ca 2+ transients, especially in the stratum oriens/alveus (11). Many interneurons in the oriens/alveus also show NMDAR-independent LTP (12). We therefore looked for associative LTP in these cells, while recording with the gramicidin perforated patch technique to preserve intracellular polyamines (13).Stimulation of pyramidal cell axon collaterals in the alveus evoked monosynaptic excitatory postsynaptic potentials (EPSPs) subthreshold for evoking action potentials. After recording a baseline, we paired high-frequency burst (HFB) stimulation (five pulses at 100 Hz, repeated 20 times) with stimulation of a second, supra-threshold, alveus pathway. "In-phase" associative pairing (phase difference ΔΦ = 0°) failed to elicit associative LTP in either pathway (n = 7; Fig. 1, A Europe PMC Funders Author ManuscriptsEurope PMC Funders Author Manuscripts two weak pathways, and then delivered HFBs to both pathways antiphase (ΔΦ = 180°). This evoked a persistent increase in EPSP initial slope in one or both pathways in all cells (n = 7; Fig. 1, C and D). LTP was elicited even when HFB stimuli were delivered to only one weak pathway (n = 7; Fig. 1, E and F). Thus, LTP at excitatory synapses on interneurons in the oriens/alveus is prevented by associative pairing, in direct contrast to NMDAR-dependent LTP (1).Can direct manipulation of the postsynaptic membrane potential similarly gate LTP induction? We delivered HFBs to one pathway coinciding with the trough (somatic voltage: −90 mV) of an imposed 4-Hz sinusoidal ...

Rapid memory formation relies, at least in part, on long-term potentiation (LTP) of excitatory synapses. Inhibitory interneurons of the hippocampus, which are essential for information processing, have recently been found to exhibit not one, but two forms of LTP. One form resembles LTP that occurs in pyramidal neurons, which depends on N-methyl-D-aspartate receptors and is triggered by coincident pre- and postsynaptic activity. The other depends on Ca2+ influx through glutamate receptors that preferentially open when the postsynaptic neuron is at rest. Here we review these contrasting forms of LTP and describe how they are mirrored by two forms of long-term depression. We further discuss how the remarkable plasticity of glutamatergic synapses on interneurons greatly enhances the computational capacity of the cortical microcircuit.

Cortical information processing requires a delicate balance of excitatory and inhibitory signaling. How is this balance preserved during hippocampal memory encoding, which involves NMDA receptor-dependent long term potentiation (LTP)? This form of LTP occurs at synapses between pyramidal neurons but has not been detected in feed-forward inhibitory interneurons. We show that paired pre- and postsynaptic activity evokes pathway-specific LTP in half of rat stratum radiatum interneurons if cytoplasmic integrity is preserved. LTP occurs in aspiny feed-forward interneurons and propagates to pyramidal neurons as an enhancement of disynaptic inhibition. We also show that when LTP is restricted to synapses on pyramidal neurons, the temporal fidelity of synaptic integration and action potential generation in pyramidal cells is compromised. However, when LTP also occurs at synapses on feed-forward interneurons, temporal fidelity is preserved. We propose that Hebbian LTP at synapses driving disynaptic inhibition is necessary to maintain information processing without degradation during memory encoding.

Epileptic seizures are characterized by periods of hypersynchronous, hyperexcitability within brain networks. Most seizures involve two stages: an initial tonic phase, followed by a longer clonic phase that is characterized by rhythmic bouts of synchronized network activity called afterdischarges (ADs). Here we investigate the cellular and network mechanisms underlying hippocampal ADs in an effort to understand how they maintain seizure activity. Using in vitro hippocampal slice models from rats and mice, we performed electrophysiological recordings from CA3 pyramidal neurons to monitor network activity and changes in GABAergic signaling during epileptiform activity. First, we show that the highest synchrony occurs during clonic ADs, consistent with the idea that specific circuit dynamics underlie this phase of the epileptiform activity. We then show that ADs require intact GABAergic synaptic transmission, which becomes excitatory as a result of a transient collapse in the chloride (Cl Ϫ ) reversal potential. The depolarizing effects of GABA are strongest at the soma of pyramidal neurons, which implicates somatic-targeting interneurons in AD activity. To test this, we used optogenetic techniques to selectively control the activity of somatic-targeting parvalbumin-expressing (PV ϩ ) interneurons. Channelrhodopsin-2-mediated activation of PV ϩ interneurons during the clonic phase generated excitatory GABAergic responses in pyramidal neurons, which were sufficient to elicit and entrain synchronous AD activity across the network. Finally, archaerhodopsin-mediated selective silencing of PV ϩ interneurons reduced the occurrence of ADs during the clonic phase. Therefore, we propose that activity-dependent Cl Ϫ accumulation subverts the actions of PV ϩ interneurons to perpetuate rather than terminate pathological network hyperexcitability during the clonic phase of seizures.

The intact hippocampal formation (IHF) of neonatal or young rats can be kept alive for an extended period in a fully submerged chamber with excellent morphological preservation. Field or patch-clamp recordings, intracellular Ca2+ measurements, and 3-D reconstruction of biocytin-filled neurons can be performed routinely. The generation and propagation of network-driven activities can be studied within the IHF or between connected intact structures such as the septum and the hippocampus or two hippocampi, and the use of a dual chamber enables the application of drugs separately to each structure. This preparation will be useful to study intact neuronal networks in the developing hippocampus in vitro.

Some interneurons of the hippocampus exhibit NMDA receptor-independent long-term potentiation (LTP) that is induced by presynaptic glutamate release when the postsynaptic membrane potential is hyperpolarized. This "anti-Hebbian" form of LTP is prevented by postsynaptic depolarization or by blocking AMPA and kainate receptors. Although both AMPA and kainate receptors are expressed in hippocampal interneurons, their relative roles in anti-Hebbian LTP are not known. Because interneuron diversity potentially conceals simple rules underlying different forms of plasticity, we focus on glutamatergic synapses onto a subset of interneurons with dendrites in stratum oriens and a main ascending axon that projects to stratum lacunosum moleculare, the oriens-lacunosum moleculare (O-LM) cells. We show that anti-Hebbian LTP in O-LM interneurons has consistent induction and expression properties, and is prevented by selective inhibition of AMPA receptors. The majority of the ionotropic glutamatergic synaptic current in these cells is mediated by inwardly rectifying Ca 2ϩ -permeable AMPA receptors. Although GluR5-containing kainate receptors contribute to synaptic currents at high stimulus frequency, they are not required for LTP induction. Glutamatergic synapses on O-LM cells thus behave in a homogeneous manner and exhibit LTP dependent on Ca 2ϩ -permeable AMPA receptors.