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Electric fields due to synaptic currents sharpen excitatory transmission

S Sylantyev, TM Jensen, DM Kullmann, DA Rusakov
Collaboration: M Y Xiao (Göteborg), AI Ivanov (Marseille)

The shape of excitatory synaptic signals, which underlie information processing in the brain, depends on the neurotransmitter profile in the synaptic cleft. Electric interactions between synaptic currents and charged neurotransmitter molecules potentially contribute to this profile.

We have shown that interactions between ion currents and glutamate molecules in the cleft can explain an increased dwell time of neurotransmitter in the cleft (and therefore a prolongation of transmission) upon postsynaptic depolarisation. Voltage-dependent temporal tuning of excitatory synaptic responses may thus contribute to signal integration in neural circuits.

The optimal height of the synaptic cleft

LP Savtchenko, DA Rusakov

Signal integration in the brain is determined by the concentration profile of neurotransmitter in the synaptic cleft. According to a traditional view, narrower clefts should correspond to higher intra-cleft concentrations of neurotransmitter and therefore to the enhanced activation of synaptic receptors.

We have argued that narrowing the cleft also increases electrical resistance of the intra-cleft medium and therefore reduces local receptor currents. Detailed theoretical analyses and Monte-Carlo simulations suggest that these contrasting phenomena result in a relatively narrow range of cleft heights at which the synaptic receptor current reaches its maximum. This matches closely experimental estimates. We have shown therefore that a simple principle may underlie the synaptic cleft architecture to maximize synaptic strength.

Nanodiffusion in the brain extracellular space

K Zheng, DA Rusakov
Collaboration: K Suhling (KCL)

Signal transfer in the brain relies on rapid diffusion of transmitter molecules in the extracellular space. Although extracellular diffusivity is thus of a fundamental importance for neural communication, it has hitherto been evaluated only in the bulk of the tissue.

We have developed time-resolved fluorescence anisotropy imaging microscopy to measure extracellular diffusivity in the neuropil on a scale that is much smaller than extracellular gaps. We have applied this approach to ex-vivo brain tissue (acute hippocampal slices) using a small fluorescence probe. The results provide previously unattainable insights into the fundamental physical properties of the extracellular medium in the brain.

Synapse-specific coupling of kainate autoreceptors and Ca2+ stores in hippocampal mossy fibers

R Scott, P Ragunathan, DM Kullmann, DA Rusakov
Collaboration: T Lalic, M Capogna (Oxford)

Signal transmission between hippocampal mossy fibres and their multiple cell targets exhibits remarkable target-dependent and use-dependent modification, including changes associated with epilepsy. The underlying mechanisms involving presynaptic kainate receptors remain intensely debated.

We have found that a single discharge in a single axon can activate presynaptic kainate-type glutamate receptors, triggering action potential-dependent Ca2+ release from Ca2+ stores. This phenomenon occurs at some specialized synapses made by hippocampal mossy fibres but not at others. Thus, kainate autoreceptor - Ca2+ store coupling acts as a synapse-specific, short-range mechanism.

Ca2+ channels in evoked neurotransmitter release at individual synapses and neurological disease

Y Ermolyuk, C Henneberger, F Alder, R Surges, DA Rusakov, DM Kullmann, K Volynski

Synaptic release of neurotransmitters is triggered by Ca2+ entry via multiple types of Ca2+ channels with different biophysical and pharmacological properties. How they contribute to the extensive heterogeneity in activity-andneuromodulator-dependent plasticity of neurotransmitter release exhibited by distinct synapses is poorly understood.

We have devised new methods to relate presynaptic Ca2+ dynamics to vesicular release based on fluorescence microscopy and electrophysiology. These methods are revealing how different presynaptic Ca2+ channels trigger the release of synaptic vesicles and influence plasticity. We are also applying them to study how inherited mutations of Ca2+ channels cause migraine, ataxia and epilepsy. The anticipated outcome of these experiments should provide novel and important insights not only into the roles of different Ca2+ channels in neurotransmitter release, but also into the molecular mechanisms of several Ca2+ channelopathies.

Synaptic mechanism of action of levetiracetam

R Surges, MC Walker, K Volynski

Levetiracetam is a novel antiepileptic drug with a broad antiepileptic potential, negligible metabolic effects and a low propensity for pharmacokinetic drug interactions. Although it is widely used in the treatment of focal and generalised epilepsy, its cellular mechanism of action is elusive.

We are studying how levetiracetam modulates the release of neurotransmitters and presynaptic Ca2+ dynamics at the level of individual excitatory and inhibitory hippocampal synapses, both under control conditions and in experimental models of increased network activity. This may lead to the development of even more potent antiepileptic drugs, as well as shedding light on mechanisms of epilepsy.

Roles of glutamate and nicotinic acetylcholine receptors in the induction of distinct forms of plasticity in hippocampal interneurons

I Oren, C Le Duigou, DM Kullmann
Collaboration: KP Lamsa, P Somogyi (Oxford)

Hippocampal interneurons exhibit at least two forms of use-dependent long-term potentiation (LTP) of excitatory synaptic transmission. Many puzzles surround these phenomena, including how they map onto the different types of interneurons that occur in the brain. Moreover, multiple types of glutamate receptors can potentially trigger different forms of short- and long-term plasticity.

We have focused on one subtype of interneurons that mediate feedback inhibition in the hippocampus, to ask whether Ca2+ influx via kainate receptors is able to trigger the induction of LTP. We have, moreover, related the effects of acute activation of glutamate and alpha7 nicotinic receptor activation by exogenous agonists to the effects of synaptic activation of these receptors. The results are helping to shed light on how reversible and long-term changes in inhibition can be induced.

Mechanisms and plasticity of hippocampal population activity

T Akam, DM Kullmann

The hippocampal formation is able to sustain several distinct patterns of network activity characterized by population oscillations at different characteristic frequencies. These oscillations involve different populations of interneurons, and are thought to be important for encoding information.

We have established an in vitro model of gamma oscillations, and are asking how afferent signals are able to reset the intrinsic rhythm, depending on precisely when they are delivered. This is a pre-requisite to understand how different areas of the brain can oscillate either independently or together (with a characteristic phase relationship).

Modulation of neurotransmitter release by presynaptic GABAA receptors

R Scott, DA Rusakov, DM Kullmann
Collaboration: A Ruiz (School of Pharmacy)

We have previously identified presynaptic GABAA receptors in mossy fibres in the hippocampus. This is a highly unexpected novel form of modulation of transmission, which has since been indentified in several other sites in the brain.

We have started to identify the GABAA receptor types that mediate presynaptic modulation of mossy fibre signaling, as well as the consequences of the phenomenon for orthodromic transmission, with direct recordings from presynaptic boutons and with multiphoton fluorescence imaging.

Channelopathies

J Heeroma, S Rajakulendran, S Tomlinson, DM Kullmann
Collaboration: E Fletcher, C Farmer, E Matthews, S Schorge, MG Hanna (Molecular Neuroscience), H Bostock (Sobell Dept)

We have previously reported on several ion channel mutations that are associated with epilepsy as well as episodic ataxia. The consequences of these mutations for neuronal excitability and for neurotransmitter release remain poorly understood.

In order to understand the effects of the mutations on neuron function, we have expressed them with lentiviral vectors in neuronal cultures. This has allowed us to examine how they alter action potential threshold, rate and duration, as well as neurotransmitter release probability. In parallel, we continue to study the effects of alternative splicing of sodium channels, as well as muscle channelopathies, which may share mechanisms in common with CNS channelopathies. We have also begun a study that asks whether the consequences of channel mutations can be detected as changes in peripheral nerve excitability.

Treatment and pathology of status epilepticus

MC Walker, S Sisodiya, Pi-Shan Chang, M Thom
Collaboration: K Chandler (Royal Veterinary College)

Status epilepticus is a medical emergency with high morbidity and mortality. It is associated with neuronal damage leading to neurological deficits, and chronic epilepsy. 30-40% of patients are resistant to treatment and require anaesthesia in an intensive care unit. We are looking into the treatment of drug-resistant status epilepticus. We are also investigating the mechanisms of the associated damage to nerve cells in the brain and how to prevent this.

In collaboration with Dr Kate Chandler we are investigating the histopathological consequences of status epilepticus, mechanisms of preventing neuronal damage and the development of epilepsy.

Mechanisms underlying epileptogenesis and regulating network excitability

V Hovhannisyan, I Pavlov, S Schorge, MC Walker
Collaboration: K Hashemi (Open Source Instruments, Brandeis University), A Pitkanen (Kuopio), G Sperk (Innsbruck), M Shah (School of Pharmacy), R Scott, M Lythgoe (¹û¶³Ó°Ôº ICH)

Many people develop epilepsy after a specific brain injury, such as a stroke, brain tumour, head injury or status epilepticus. We are investigating the changes in the brain that lead to epilepsy (epileptogenesis). We are specifically interested to know how communication between neurons changes following status epilepticus and how this makes the brain predisposed to spontaneous seizures.

Over the last year, we have completed the final stages of the development of a wireless animal telemetry unit at the Institute of Neurology. This will allow us to continuously monitor seizures in animals for weeks, so facilitating the careful mapping of the development of epilepsy following a brain insult. In a series of studies, we are investigating further the mechanisms underlying the development of epilepsy. In particular, we are studying changes in GABAergic inhibition that occur during the development of epilepsy both in status epilepticus and following head injury. We have also investigated the role of changes in the cationic current Ih in the entorhinal, changes in adenosine modulation of transmission, and the role of inflammation in epileptogenesis.

GABAergic inhibition in controlling brain excitability

I Pavlov, DM Kullmann, MC Walker
Collaboration: A Semyanov (Tokyo)

GABA is the major inhibitory neurotransmitter in the brain. Much remains to be understood in the functions of GABAergic inhibition in controlling the excitability of individual neurons.

We have continued to study tonic inhibition, in which extracellular GABA acts on extrasynaptic GABAA receptors on pyramidal neurons. We have found that this tonic inhibition has unexpected biophysical properties that have unique effects on neuronal excitability and on the temporal fidelity of neuronal transmission.