Talks and posters
Bryan's award winning presentation!
Congratulations Bryan on your award winning presentation!
2nd place in the Student Brain Symposium 2011
Neurophysiological mechanisms in a mouse model of early onset epileptic encephalopathy
Bryan Leaw, Christopher Reid and Steven Petrou
Howard Florey Institute, Parkville, 3010
Dravet syndrome is a severe epileptic encephalopathy, which lies at the most severe end of the Genetic Epilepsy with Febrile Seizure Plus (GEFS+) spectrum. SCN1A mutations are the most common cause of GEFS+ and account for >70% of patients with Dravet syndrome. Recently however, a patient with Dravet syndrome was reported to be homozygous for a mutation in SCN1B, encoding the β1 subunit of sodium channels. To further elucidate the pathogenesis of SCN1B based Dravet syndrome, we investigated a mouse model homozygous for a common GEFS+ SCN1B mutation, C121W. Mice homozygous for this C121W mutation exhibited the seizure susceptibility and abnormal gait of human Dravet patients, and mirrored human patient responses to specific anti-epileptic drugs (AED). This strongly supported the C121W mouse as a model of Dravet syndrome, therefore we performed neuronal analysis with the presence of the C121W mutation. We examined homozygous subicular neurons, which have been suggested to have a role in thermal seizure genesis. These neurons displayed hallmarks of increased excitability, consisting broader APs and a left-shifted input-output relationship. Interestingly, these neurons displayed intrinsically higher membrane input resistance, which could account for this hyperexcitability. We then tested a novel AED, retigabine, which activates potassium channels and thereby would potentially lower membrane resistance back to wild-type levels. Work done in our laboratory then showed that upon application of retigabine, susceptibility to thermal seizures in these mice was strongly suppressed. This highlights the potential of our mouse model in identifying potential AEDs that could target specific human epileptic syndromes.
 Bryan's Sobr presentation.pdf | [ ] | 989 Kb |
Kim's award winning poster!
Congratulations Kim on your award winning poster!
1st place in the Student Brain Symposium 2011
1st place in the 8th Asian & Oceanian Epilepsy Congress 2010
Low blood glucose precipitates spike-and-wave discharge activities in a mouse model of epilepsy
Tae Hwan Kim1.3, Christopher A. Reid1,3, Samuel F. Berkovic2 and Steve Petrou1,3
1. Florey Neuroscience Institute The University of Melbourne, Parkville, Australia
2. Department of Medicine, Austin Health, The University of Melbourne, Heidelberg West, Australia
3. Centre for Neuroscience, The University of Melbourne, Parkville, Australia
Absence seizures are defined as unprovoked behavioural arrest associated with generalized spike and wave discharge (SWD) on EEGs. Absence epilepsies have a largely genetic aetiology but it is well known that environmental effects such as decreased vigilance and voluntary hyperventilation may induce seizures. However, we still don’t have a complete understanding of all the environmental precipitants. Low glucose in general has not been considered an environmental factor but the manifestation of absence epilepsy in patients with brain glucose transporter deficiencies has raised this possibility. Here we investigate if lowering blood glucose can precipitate SWD activity in a animal model of absence epilepsy.
This study highlights a highly reproducible, immediate, reversible and dose dependent impact of low blood glucose on SWD expression in the animal model of absence epilepsy. We also showed that overnight fasting can reduce blood glucose levels sufficiently to precipitate SWD activity. Our findings suggest that low blood glucose needs to be considered as a potential environmental risk factor in absence epilepsy motivating further clinical studies into this phenomenon.
| Poster (ILAE Congress).pdf | [ ] | 328 Kb |
Evan's poster at Neuroeng 2008
The consequences of epilepsy causing ion channel mutations in the dentate gyrus
Evan Alexander Thomas, Chris Alan Reid, Steven Petrou
Howard Florey Institute, Parkville, 3010
In recent years hundreds of epilepsy mutations in voltage gated ion channel diseases have been identified. In these cases we know exactly what the cause of the disease so this gives an unprecedented opportunity to understand how brains become epileptic. Part of this process will involve computer modelling of seizure prone networks. An important class of ion channels are the voltage gated sodium channels that are responsible for action potential initiation and propagation. Mutations in these channels affect how they responds to voltage, for examples shifting steady voltage dependence of activation or inactivation or alter gating rates. Some changes are predicted to increase neural excitability while will decrease excitability. Usually several such changes occur simultaneously. In a previous study (Thomas et al, Neuroscience 2007 147:1034-46), we found that shifts in the voltage dependence of activation had the most profound influence on excitability. Left shifts, which increase open probability, dramatically increased firing rate and lowered firing threshold. The simple neuron models were less sensitive to shifts in the voltage dependence of inactivation, and less sensitive again to changes in gating rates. We wished to extend our previous study in two ways. Firstly, we wanted to test in more realistic single neuron models. The simple neuron models that we used previously did not have realistic morphology and had only the minimum number of conductances required to generate action potentials. Secondly, although seizure susceptibility must start with a cellular phenotype, seizures are manifestly a network phenomena. We therefore performed a sensitivity analysis in the dentate gyrus. This structure has been implicated in temporal lobe epilepsy. In order to assess a neuron’s input/output behaviour we stimulated neurons with current injections and measured the resulting firing frequency. In some cases, this produced counterintuitive results. For example, left shifting the steady state voltage dependence of activation might be predicted to increase sodium channel availability and hence increase excitability. However, in response to long current injections the firing rate of neurons with the mutant channels was lower than wild type neurons. This is because increasing sodium channel availability increased action potential amplitude, which in turn, increased the action potential duty cycle increasing the firing frequency. In order to assess network function we stimulated with a constant frequency, random input from the perforant path and measured firing rate averaged across the network. In the control case the network showed moderate accommodation in response to long duration inputs. Left shifting the voltage dependence of activation reduced the accommodation and a right shift increased accommodation. By contrast, altering activation rates and inactivation had little effect. In all cases the network returned to rest on cessation of input and no other stables states were found. In conclusion, the dentate gyrus is unlikely to be the focus of seizures caused by ion channel mutations without additional structural changes. However, mutations will effect how much activity flows into deeper hippocampal structures and this together with changes in these structures may explain hippocampal based seizures.
Evan Neuroeng 2008 poster (Steve's talk at Neuroeng 2008
Using modelling to indentify the sodium channel subunit involved in neuropathic pain
Evan Thomas1, Herjend Teny2, Chin Kiong Quek2, Fan Yahua2, Wang Ying2, Steven Petrou1
1Howard Florey Institute, Parkville, 3010 2Department of Electrical Engineering, University of Melbourne, Parkville, 3010
Spinal cord injury, amputation and genetic disease can cause the sensation of pain in the absence of noxious stimuli. This pain can be extremely severe, impacting the quality of life of sufferers and placing a financial burden on society. These syndromes are difficult to treat because there are no effective drugs and the drugs that are available have severe side effects. In normal transduction of noxious stimuli, action potentials are generated in specialized structures in the skin and subcutaneous tissue. These action potentials travel antidromically along the axons of primary sensory neurons in the dorsal root ganglia (DRG neurons) and are then transmitted into higher structures in the spinal cord and then onto the brain. In neuropathic pain ectopic action potentials are generated in the cell body. Three electrophysiological changes are observed in DRG neurons. Firstly, neurons change from phasic to tonic firing. Normally, these neurons will fire only a single action potential in response to a depolarizing current injection. In the disease state, these neurons will fire a burst of action potentials when stimulated. Secondly, these neurons develop a 3mV subthreshold membrane oscillation (SMO). Thirdly, these neurons display a 5mV depolarizing after potential (DAP). The SMO means that the neuron is closer to firing threshold and more likely to fire an action potential in response to noise or other otherwise subthreshold events. The DAP has the ability to drive the membrane above threshold again. Because the neuron now has the ability to fire multiple spikes the DAP can trigger another spike. This leads to regenerative spike firing, which is the basis of the inappropriate pain. A number of lines of evidence point to the upregulation of one or more NaV1.7, NaV1.8 or NaV1.9 sodium channels as being responsible for neuropathic pain. The goal of this study was to use models of DRG neurons with realistic kinetics and voltage dependence for the three sodium channels. Increasing the conductance of NaV1.7 and NaV1.8 could convert neurons from phasic to tonic firing, however the amount of extra conductance required was not physiologically credible. No amount of additional NaV1.7 and NaV1.8 conductance was able to generate a DAP. However, the model was extremely sensitive to NaV1.9. A small amount of this conductance was able to produce both tonic firing and a DAP. We also found that SMO can be caused by a persistent (non-inactivating) channel with a voltage dependence of activation equal to that of NaV1.9. We conclude that upregulation NaV1.9 can produce the electrophysiological changes that underlie neuropathic pain. This allows us to design directed experiments to test this hypothesis.
Steve Talk NeuroEng 2008 (