Disrupted cortical conductivity in schizophrenia: TMS-EEG study.

Schizophrenia is conceptualized as a failure of cognitive integration, and altered oscillatory properties of neurocircuits are associated with its symptoms. We hypothesized that abnormal characteristics of neural networks may alter functional connectivity and distort propagation of activation in schizophrenic brains. Thus, electroencephalography (EEG) responses to transcranial magnetic stimulation (TMS) of motor cortex were compared between schizophrenia and healthy subjects. There was no difference in the initial response. However, TMS-induced waves of recurrent excitation spreading across the cortex were observed in schizophrenia, while in healthy subjects the activation faded away soon after stimulation. This widespread activation in schizophrenia was associated with increased oscillatory activities in the proximal central leads and in fronto-temporo-parietal leads bilaterally. A positive correlation was found between increased TMS-induced cortical activation in gamma frequency and positive symptoms of schizophrenia, while negative symptoms were correlated with activation in theta and delta bands. We suggest that excessive activation in response to stimulation in schizophrenia brains may lead to abnormal propagation of the signal that could potentially result in aberrant activity in areas remote from the activation origin. This mechanism may account for the positive symptoms of schizophrenia and could worsen signal to noise deficits, jeopardizing adequate information processing with ensuing cognitive deficits.

Exploring the effect of inducing long-term potentiation in the human motor cortex on motor learning.

BACKGROUND: Paired-associative stimulation (PAS) represents a neurophysiologic paradigm that involves peripheral nerve stimulation (PNS) of the median nerve, followed by the transcranial magnetic stimulation (TMS) of the contralateral motor cortex. PAS has been shown to result in long-term potentiation-like activity (PAS-LTP) if PNS precedes TMS by 25 milliseconds (PAS-25). PAS-LTP has also been shown to relate to simple motor performance. However, to date, no studies have directly investigated whether the induction of PAS-LTP is associated with enhanced motor learning. OBJECTIVE: The objective of this study was to assess the short- and long-term effect of PAS-25 on motor learning. METHODS: This was a randomized controlled pilot study in which the control condition was PAS-10, whereby PNS precedes TMS by 10 milliseconds. Motor learning was assessed using the rotary pursuit task at baseline prior to PAS-25 or PAS-10 and 45 minutes and 1 week post-PAS. RESULTS: As expected PAS-25 but not PAS-10 was associated with PAS-LTP as indexed by a significant potentiation of the motor evoked potential. Also, PAS-25 resulted in enhanced motor learning at 1 week post-PAS (F (2, 44) = 3.441, P = .041). CONCLUSIONS: This is the first PAS study showing long-term behavioral effect and suggests, albeit indirectly, that PAS-25 can trigger slowly manifesting cellular and structural changes that result in long-term improvement in motor performance. Larger studies with neurophysiologic or neuroimaging outcomes are needed to confirm such preliminary findings.

Vulnerability of glial cells to hydrogen peroxide in cultured hippocampal slices.

Massive production of free radicals (FR) has been associated with a variety of pathological conditions in the central nervous system (CNS). We have used the FR generating compound hydrogen peroxide (H2O2) in organotypic hippocampal slice cultures to model oxidative injury in the brain. Necrotic cell death was monitored for up to 48 h using propidium iodide (PI) and confocal microscopy. A 1 h exposure to H2O2 (0.5-2.5 mM) caused a dose-dependent, and region specific cell death in hippocampal slice cultures. Glial cells demonstrated a high degree of vulnerability to H2O2. During the initial 3 h post-injury period, regions of the slice where glial cell bodies predominated showed massive cell death. The majority of neurons in the pyramidal layers were spared, though at later time points they appeared damaged as well. Carboxy-dichlorofluorescein imaging revealed a corresponding early increase in ROS generation in glial cells compared to pyramidal neurons. Immunohistochemistry of PI labeled slices identified astrocytes as the cells most sensitive to H2O2 toxicity. In dissociated cell cultures of hippocampal astrocytes and neurons, astrocytes also exhibited a significantly higher sensitivity to H2O2 than neurons. Hydrogen peroxide-induced cytotoxicity in all regions of the hippocampal slice culture was significantly attenuated by pre-treatment with antioxidants (alpha-tocopherol and glutathione), and was not prevented by blockade of Ca2+ influx, or NMDA channel activation. Cyclosporin A, an inhibitor of mitochondrial permeability transition, reduced cytotoxicity in glial areas by more than 50%, while in the CA2-CA3 pyramidal layers a much smaller, but still significant, attenuation of cytotoxicity was observed. Our results suggest that mitochondria are primary targets of H2O2 toxicity, particularly in astrocytes.

Evidence for impaired long-term potentiation in schizophrenia and its relationship to motor skill learning.

Several lines of evidence suggest that schizophrenia (SCZ) is associated with disrupted plasticity in the cortex. However, there is little direct neurophysiological evidence of aberrant long-term potentiation (LTP)-like plasticity in SCZ and little human evidence to establish a link between LTP to learning and memory. LTP was evaluated using a neurophysiological paradigm referred to as paired associative stimulation (PAS). PAS involves pairing of median nerve electric stimulation with transcranial magnetic stimulation (TMS) over the contralateral motor cortex (for abductor pollicis brevis muscle activation) delivered at 25-ms interstimulus interval. This pairing was delivered at a frequency of 0.1 Hz for 30 min. LTP was reflected by the change in motor evoked potentials (MEPs) before and after PAS. In addition, motor skill learning was assessed using the rotary pursuit task. Compared with healthy subjects, patients with SCZ demonstrated significant MEP facilitation deficits following PAS and impaired rotary-pursuit motor learning. Across all subjects there was a significant association between LTP and motor skill learning. These data provide evidence for disrupted LTP in SCZ, whereas the association between LTP with motor skill learning suggests that the deficits in learning and memory in SCZ may be mediated through disordered LTP.

Succinic semialdehyde dehydrogenase deficiency: GABAB receptor-mediated function.

The succinic semialdehyde dehydrogenase (SSADH) null mouse (SSADH(-/-)) represents a viable animal model for human SSADH deficiency and is characterized by markedly elevated levels of both gamma-hydroxybutyric acid (GHB) and gamma-aminobutyric acid (GABA) in brain, blood, and urine. In physiological concentrations, GHB acts at the GHB receptor (GHBR), but in high concentrations such as those observed in the brains of children with SSADH deficiency, GHB is thought to be a direct agonist at the GABABR receptor (GABABR). We tested the hypothesis that both GHBR and GABABR-mediated function are perturbed in SSADH deficiency. Therefore, we examined the high affinity binding site for GHB as well as the expression and function of the GABABR in mutant mice made deficient in SSADH (SSADH(-/-)). There was a significant decrease in binding of the specific GABABR antagonist, [3H]CGP-54626A at postnatal day (PN)7 and PN14 in SSADH(-/-) when compared to wild type control animals (SSADH(+/+)), particularly in hippocampus. GABABR-mediated synaptic potentials were decreased in SSADH(-/-). Immunoblot analysis of GABABR1a, R1b, and R2 in SSADH(-/-) indicated a trend towards a region-specific and time-dependent decrease of GABABR subunit protein expression. There was no difference between SSADH(-/-) and wild type in binding of either [3H]GHB or a specific GHBR antagonist to the GHBR. These data suggest that the elevated levels of GABA and GHB that occur in SSADH(-/-) lead to a use-dependent decrease in GABABR-mediated function and raise the possibility that this GHB- and GABA-induced perturbation of GABABR could play a role in the pathogenesis of the seizures and mental retardation observed in SSADH deficiency.

Status epilepticus in mice deficient for succinate semialdehyde dehydrogenase: GABAA receptor-mediated mechanisms.

The epilepsy that occurs in SSADH deficiency has a seizure phenotype similar to that occurring in the SSADH(-/-) mouse. We examined the expression and function of the GABA(A) receptor (GABA(A)R) in SSADH-deficient mice. A selective decrease in binding of [(35)S]tert-butylbicyclophosphorothionate was observed in SSADH(-/-) mice at postnatal day 7 that was progressive until the third postnatal week of life when, at the nadir of the decreased [(35)S]tert-butylbicyclophosphorothionate binding, generalized convulsive seizures emerged that rapidly evolved into status epilepticus. We also observed a substantial downregulation of the beta(2) subunit of GABA(A)R, a reduction in GABA(A)-mediated inhibitory postsynaptic potentials, and augmented postsynaptic population spikes recorded from hippocampal slices. The SSADH(-/-) mouse model represents a powerful investigative tool for understanding the pathophysiology of the seizures associated with human SSADH deficiency. These data raise the possibility that progressive dysfunction of the GABA(A)R may be involved in the development of seizures in SSDAH-deficient mice. Elucidation of the precise fundamental mechanisms of the perturbation of the GABA(A)R-mediated function in SSADH(-/-) mice could lead to the development of novel treatment modalities designed to reduce the neurological morbidity in children with SSADH deficiency.

Anti-porin antibodies prevent excitotoxic and ischemic damage to brain tissue.

The mitochondrial permeability transition (MPT) is a converging event for different molecular routes leading to cellular death after excitotoxic/oxidative stress, and is considered to represent the opening of a pore in the mitochondrial membrane. There is evidence that the outer mitochondrial membrane protein porin is involved in the MPT and apoptosis. We present here a proof-of-principle study to address the hypothesis that anti-porin antibodies can prevent excitotoxic/ischemia-induced cell death. We generated anti-porin antibodies and show that the F(ab)(2) fragments penetrate living cells, reduce Ca(2+)-induced mitochondrial swelling as other MPT blockers do, and decrease neuronal death in dissociated and organotypic brain slice cultures exposed to excitotoxic and ischemic episodes. These observations present direct evidence that anti-porin antibody fragments prevent cell damage in brain tissue, that porin is a crucial protein involved in mitochondrial and cell dysfunction, and that it is conceivable that antibodies can be used as therapeutic agents.

Gap junctions and neuronal injury: protectants or executioners?

The authors review concepts and recent experimental observations that relate gap junctional communication to the pathophysiology of neuronal injury, specifically ischemic or traumatic damage. The role played by this type of direct intercellular communication during the progression of the injuries can be conceived to be either detrimental or beneficial, depending on the arguments employed. The data indicate that, far from being a simple matter of judgment, the contribution of gap junctions to cell injury is a complicated phenomenon that depends on the specific insult and network in which it operates.

Ischemia-induced brain damage depends on specific gap-junctional coupling.

Ischemic brain injury results in neuronal loss and associated neurologic deficits. Although there is some evidence that intercellular communication via gap junctions can spread oxidative cell injury, the possible role of gap-junctional communication in ischemia-induced cell death is the object of debate. Because gap junctions directly connect the cytoplasms of coupled cells, they offer a way to propagate stress signals from cell to cell. The authors investigated the contribution of gap-junctional communication to cell death using an in vitro ischemia model, which was reproduced by submersion of organotypic hippocampal slices into glucose-free deoxygenated medium. The gap-junctional blocker carbenoxolone significantly decreased the spread of cell death, as measured by propidium iodide staining, over a 48-hour period after the ischemic episode. Carbenoxolone ameliorated the hypoxia-induced impairment of the intrinsic neuronal electrophysiologic characteristics, as measured by whole-cell patch clamp recordings. To determine whether specific connexins were involved in the spread of postischemic cell death, the authors partially reduced the synthesis of specific connexins using antisense oligodeoxynucleotides. Simultaneous knockdown of two connexins localized mostly in neurons, connexins 32 and 26, resulted in significant neuroprotection 48 hours after the hypoxic-hypoglycemic episode. Similarly, partial reduction of the predominant glial connexin 43 significantly decreased cell death. These results indicate that gap-junctional communication contributes to the propagation of hypoxic injury and that specific gap junctions could be a novel target to reduce brain damage.

Specific gap junctions enhance the neuronal vulnerability to brain traumatic injury.

Traumatic brain injury results in neuronal loss and associated neurological deficits. Although most research on the factors leading to trauma-induced damage focuses on synaptic or ionic mechanisms, the possible role of direct intercellular communication via gap junctions has remained unexplored. Gap junctions connect directly the cytoplasms of coupled cells; hence, they offer a way to propagate stress signals from cell to cell. We investigated the contribution of gap junctional communication (GJC) to cell death using an in vitro trauma model. The impact injury, induced by a weight dropped on the distal CA1 area of organotypic hippocampal slices, results in glutamate-dependent cell loss. The gap junctional blockers carbenoxolone and octanol decreased significantly post-traumatic cell death, measured by propidium iodide staining over a 72 hr period after the impact. Dye coupling in the pyramidal layers was enhanced immediately after the injury and decreased over the following 24 hr. To determine whether specific connexins were involved in the spread of trauma-induced cell death, we used organotypic slices from connexin43 (Cx43) knock-out mice, as well as acute knock-outs by incubation with antisense oligodeoxynucleotides. Simultaneous knockdown of two neuronal connexins resulted in significant neuroprotection. Slices from the null-mutant Cx43 mice, as well as the acute Cx43 knockdown, also showed decreased cell death after the impact. The gap junctional blockers alleviated the trauma-induced impairment of synaptic function as measured by electrophysiological field potential recordings. These results indicate that GJC enhances the cellular vulnerability to traumatic injury. Hence, specific gap junctions could be a novel target to reduce injury and secondary damage to the brain and maximize recovery from trauma.