Long-term potentiation induced by single volley activation: a mechanism for bicuculline-induced enhancement of synaptic field potentials in the CA1 hippocampal region

Ketamine activated glutamatergic neurotransmission by GABAergic disinhibition in the medial prefrontal cortex

The fast-onset antidepressant actions of ketamine at subanaesthetic doses have attracted enormous interest in psychiatric disease treatment. However, the severe psychotomimetic side effects foster an urgent need to deeply understand the fast-onset antidepressant mechanism of ketamine. Ketamine, as a non-competitive NMDAR antagonist, increases the overall excitability of the mPFC, which is presumed to be essential for the antidepressant action of ketamine. However, the underlying mechanism is still elusive. Here, our results showed that low concentration of ketamine increased the activity and the excitatory/inhibitory ratio of pyramidal neurons; these changes were accompanied by diminished interneurons activity in the mPFC. Moreover, ketamine induced increases in excitatory transmission and antidepressant-like effects, which might rely on the functional intact of GABAergic system in the mPFC. These results suggest a critical role of the mPFC GABAergic system in the fast antidepressant effects of a subanaesthetic dose ketamine.

GABAA receptor antagonism ameliorates behavioral and synaptic impairments associated with MeCP2 overexpression

Methyl-CpG-binding protein 2 (MeCP2) is a ubiquitously expressed transcriptional regulator with functional importance in the central nervous system. Loss-of-function mutations in MECP2 results in the neurodevelopmental disorder, Rett syndrome, whereas increased expression levels are associated with the neurological disorder, MECP2 duplication syndrome. Previous characterization of a mouse line overexpressing Mecp2 demonstrated that this model recapitulated key behavioral features of MECP2 duplication syndrome with specific deficits in synaptic plasticity and neurotransmission. Alterations in excitation/inhibition balance have been suggested to underlie neurodevelopmental disorders with recent data suggesting that picrotoxin (PTX), a GABAA receptor antagonist, rescues certain behavioral and synaptic phenotypes in a mouse model of Down syndrome. We therefore examined whether a similar treatment regimen would impact the behavioral and synaptic phenotypes in a mouse model of MECP2 duplication syndrome. We report that chronic treatment with low doses of PTX ameliorates specific behavioral phenotypes, including motor coordination, episodic memory impairments, and synaptic plasticity deficits. These findings suggest that GABAA receptor antagonists may offer a possible therapeutic target for the treatment of MECP2 duplication syndrome.

Excitatory and inhibitory synaptic transmission is differentially influenced by two ortho-substituted polychlorinated biphenyls in the hippocampal slice preparation

Exposure to polychlorinated biphenyls impairs cognition and behavior in children. Two environmental PCBs 2,2',3,3',4,4',5-heptachlorobiphenyl (PCB170) and 2,2',3,5',6-pentachlorobiphenyl (PCB95) were examined in vitro for influences on synaptic transmission in rat hippocampal slices. Field excitatory postsynaptic potentials (fEPSPs) were recorded in the CA1 region using a multi-electrode array. Perfusion with PCB170 (10 nM) had no effect on fEPSP slope relative to baseline period, whereas (100 nM) initially enhanced then depressed fEPSP slope. Perfusion of PCB95 (10 or 100 nM) persistently enhanced fEPSP slope >200%, an effect that could be inhibited by dantrolene, a drug that attenuates ryanodine receptor signaling. Perfusion with picrotoxin (PTX) to block GABA neurotransmission resulted in a modest increase in fEPSP slope, whereas PTX+PCB170 (1-100 nM) persistently enhanced fEPSP slope in a dose dependent manner. fEPSP slope reached >250% of baseline period in the presence of PTX+100 nM PCB170, conditions that evoked marked epileptiform after-potential discharges. PCB95 and PCB170 were found to differentially influence the Ca(2+)-dependence of [(3)H]ryanodine-binding to hippocampal ryanodine receptors. Non-coplanar PCB congeners can differentially alter neurotransmission in a manner suggesting they can elicit imbalances between inhibitory and excitatory circuits within the hippocampus. Differential sensitization of ryanodine receptors by Ca(2+) appears to mediate, at least in part, hippocampal excitotoxicity by non-coplanar PCBs.

Cortical excitability is abnormal in patients with the "fixed dystonia" syndrome

A form of fixed dystonia (FD) without evidence of basal ganglia lesions or neurodegeneration has been recently characterized (Schrag et al., Brain 2004;127:2360-2372), which may overlap the clinical spectrum of either complex regional pain syndrome or psychogenic dystonia. Transcranial magnetic stimulation studies in typically mobile dystonia revealed abnormal motor cortical excitability and sensori-motor integration. We compared 12 patients with limb FD to 10 patients with primary adult-onset typically mobile dystonia and 11 age-matched healthy volunteers. Measurements at the first digital interosseus representation area on both hemispheres included: short intracortical inhibition (SICI), contralateral silent period (cSP), and short and long afferent inhibition (SAI and LAI). Repeated measure ANOVA and post-hoc t-tests were used for statistical analysis. SICI was significantly reduced in both hemispheres of patients with "typical" and FD, compared to healthy subjects. For both hemispheres, cSP duration was shorter in both fixed and "typical" dystonia patients. SAI and LAI did not significantly differ between the three groups. The abnormal cortical excitability observed in FD might represent an underlying trait predisposing to different clinical forms of dystonia.

Enhanced learning and memory and altered GABAergic synaptic transmission in mice lacking the alpha 5 subunit of the GABAA receptor

The alpha5 subunit of the GABA(A) receptor is localized mainly to the hippocampus of the mammalian brain. The significance of this rather distinct localization and the function of alpha5-containing GABA(A) receptors has been explored by targeted disruption of the alpha5 gene in mice. The alpha5 -/- mice showed a significantly improved performance in a water maze model of spatial learning, whereas the performance in non-hippocampal-dependent learning and in anxiety tasks were unaltered in comparison with wild-type controls. In the CA1 region of hippocampal brain slices from alpha5 -/- mice, the amplitude of the IPSCs was decreased, and paired-pulse facilitation of field EPSP (fEPSP) amplitudes was enhanced. These data suggest that alpha5-containing GABA(A) receptors play a key role in cognitive processes by controlling a component of synaptic transmission in the CA1 region of the hippocampus.

Temporal overlap of excitatory and inhibitory afferent input in guinea-pig CA1 pyramidal cells

1. The temporal interaction of evoked synaptic excitation and GABAA-mediated inhibition was examined in CA1 pyramidal cells. Single and paired intracellular recordings were carried out in pyramidal cell dendrites and somata, and interneurons of the guinea-pig hippocampal slice. Current-clamp, sharp electrode and whole-cell voltage-clamp recordings were made. 2. Kinetics of dendritic and somatic inhibitory responses were similar. Notably, kinetics of dendritic unitary IPSPs were as fast as kinetics of somatic unitary IPSPs. 3. GABAA-mediated influences were present throughout the orthodromic pyramidal cell EPSP/EPSC. Comparison of the kinetics of pharmacologically isolated monosynaptic IPSPs, IPSCs and inhibitory conductances (g GABAA), showed fastest kinetics for g GABAA. Close temporal overlap was observed between monosynaptic g GABAA and the rising phase of the evoked EPSP/EPSC. The onset of g GABAA coincided with or preceded onset of the EPSP/EPSC. 4. Onsets of feedforward IPSPs coincided with the rising phase of the pyramidal cell EPSP in > 80 % of paired recordings. Fastest feedforward inhibitory responses exerted near complete overlap with evoked excitation. 5. Onsets of recurrent IPSPs did not occur during the rising phase of the evoked EPSP, but > 3.0 ms after the peak of the pyramidal cell EPSP. 6. Orthodromically evoked interneuron spikes were observed at stimulation intensities that were below the threshold for eliciting EPSPs in concomitantly recorded pyramidal cells. The activation of feedforward inhibitory responses by weakest excitatory input, and the large temporal overlap between feedforward inhibition and evoked excitation, suggest that in situ any excitatory input in CA1 is effectively controlled by fast synaptic inhibition.

Effects of GABA(A) inhibition on the expression of long-term potentiation in CA1 pyramidal cells are dependent on tetanization parameters

Long-term potentiation (LTP) of excitatory synaptic responses of principal neurons in the hippocampus is accompanied by changes in GABAergic inhibition mediated by interneurons. The impact of inhibition on LTP of excitatory postsynaptic responses in CA1 pyramidal cells was assessed by monitoring changes in field potentials evoked by Schaffer collateral stimulation in hippocampal slices in vitro. First, to determine the effect of inhibition on population EPSPs, slices were exposed to the GABA(A) receptor antagonist bicuculline (10 microM). Both the slope and amplitude of field EPSPs (fEPSPs) were significantly enhanced by bicuculline indicating that inhibition modulates excitatory postsynaptic responses of pyramidal cells. To assess if stimulation-dependent changes in inhibition influence LTP of excitatory responses of pyramidal cells, LTP was examined in the presence and absence of bicuculline (20 microM) following either 100 Hz tetanization, or theta-patterned stimulation (short bursts delivered at 5 Hz). In normal medium, 100 Hz stimulation produced marked short-term potentiation that decayed 5-10 min post-tetanus and both stimulation paradigms produced similar LTP at 30 min post-tetanus. In comparison, LTP of the fEPSP slope and amplitude was significantly enhanced after theta-patterned stimulation, but not after 100 Hz stimulation, in bicuculline. The greater potentiation of field responses following theta-patterned stimulation in the presence of bicuculline indicates that a larger potentiation of excitatory responses was unmasked during suppression of inhibitory inputs. These results suggest that a long-lasting enhancement of inhibition in pyramidal cells was also induced following theta-patterned stimulation in normal ACSF. Since suppression of inhibition did not uncover a significantly larger potentiation following 100 Hz tetanization, the influence of inhibition on LTP of excitatory responses appears to be stimulation-dependent. In conclusion, theta-patterned stimulation appears to be more effective at inducing plasticity within inhibitory circuits, and this plasticity may partially offset concurrent increases in the excitability of the CA1 network.