Schizophrenia is a common psychiatric disorder of high incidence, affecting approximately 1% of the world population. The essential neurotransmitter pathology of schizophrenia remains poorly defined, despite huge advances over the past half-century in identifying neurochemical and pathological abnormalities in the disease. The dopamine/serotonin hypothesis has originally provided much of the momentum for neurochemical research in schizophrenia. In recent years, the attention has, however, shifted to the glutamate system, the major excitatory neurotransmitter in the CNS and towards a concept of functional imbalance between excitatory and inhibitory transmission at the network level in various brain regions in schizophrenia. The evidence indicating a central role for the NMDA-receptor subtype in the aetiology of schizophrenia has led to the NMDA-hypofunction model of this disease and the use of phencyclidines as a means to induce the NMDA-hypofunction state in animal models. The purpose of this review is to discuss recent findings highlighting the importance of the NMDA-hypofunction model of schizophrenia, both from a clinical perspective, as well as in opening a line of research, which enables electrophysiological studies at the cellular and network level in vitro. In particular, changes in excitation–inhibition (E/I) balance in the NMDA-hypofunction model of the disease and the resulting changes in network behaviours, particularly in gamma frequency oscillatory activity, will be discussed.

GABA B receptors (GABA B Rs) mediate slow inhibitory effects on neuronal excitability and synaptic transmission in the brain. However, the GABA B R agonist baclofen can also promote excitability and seizure generation in human patients and animals models. Here we show that baclofen has concentration-dependent effects on the hippocampal network in a mouse model of mesial temporal lobe epilepsy. Application of baclofen at a high dose (10 mg/kg i.p.) reduced the power of γ oscillations and the frequency of pathological discharges in the Cornu Ammonis area 3 (CA3) area of freely moving epileptic mice. Unexpectedly, at a lower dose (1 mg/kg), baclofen markedly increased γ activity accompanied by a higher incidence of pathological discharges. Intracellular recordings from CA3 pyramidal cells in vitro further revealed that, although at a high concentration (10 μM), baclofen invariably resulted in hyperpolarization, at low concentrations (0.5 μM), the drug had divergent effects, producing depolarization and an increase in firing frequency in epileptic but not control mice. These excitatory effects were mediated by the selective muting of inhibitory cholecystokinin-positive basket cells (CCK + BCs), through enhanced inhibition of GABA release via presynaptic GABA B Rs. We conclude that cell type-specific up-regulation of GABA B R-mediated autoinhibition in CCK + BCs promotes aberrant high frequency oscillations and hyperexcitability in hippocampal networks of chronic epileptic mice.presynaptic inhibition | mTLE model | patch clamp N euronal activity in the hippocampus shows oscillations in behavior-relevant frequency ranges including γ frequencies (30-80 Hz) (1). γ activity is prominent in the aroused brain and has been implicated in higher-level brain functions, such as sensory binding, perception (2), and storage and recall of information (3, 4). At the same time, γ frequency oscillations are also prevalent in epileptic patients and are most often observed at seizure onset during in depth EEG recordings (5). The GABAergic system plays a pivotal role in the generation of γ oscillations (6-8). However, it remains to be resolved how distinct GABAergic receptor subtypes, in particular GABA B receptors (GABA B Rs), contribute to the generation and modulation of pathological network oscillatory activity.GABA B Rs mediate slow inhibitory effects and control synaptic transmission and the excitability of neurons in cortical networks. GABA B Rs are expressed both postsynaptically in somatodendritic compartments and presynaptically in axon terminals, in excitatory principal cell and inhibitory interneurons (9-11). The effects of GABA B R activation on the network are dominated by inhibition leading to an overall dampened population activity. However, if GABAergic interneurons are effected dominantly, as observed for example, during high-frequency stimulation, GABA B R activation can produce disinhibition in principal cells (12,13). Accordingly, the role of GABA B Rs in epilepsy and seizure generation remains ambiguous. GABA B Rs ...

No abstract

Midbrain raphe nuclei provide strong serotonergic projections to the hippocampus, in which serotonin (5-HT) exerts differential effects mediated by multiple 5-HT receptor subtypes. The functional relevance of this diversity of information processing is poorly understood. Here we show that serotonin via 5-HT 1B heteroreceptors substantially reduces synaptic excitation of cholecystokinin-expressing interneurons in area CA1 of the rat hippocampus, in contrast to parvalbumin-expressing basket cells. The reduction is input specific, affecting only glutamatergic synaptic transmission originating from CA1 pyramidal cells. As a result, serotonin selectively decreases feedback inhibition via 5-HT 1B receptor activation and subsequently increases the integration time window for spike generation in CA1 pyramidal cells. Our data imply an important role for serotonergic modulation of GABAergic action in subcortical control of hippocampal output.

No abstract