Heterotopic neurons with altered inhibitory synaptic function in an animal model of malformation-associated epilepsy.
Children with brain malformations often exhibit an intractable form of epilepsy. Although alterations in cellular physiology and abnormal histology associated with brain malformations has been studied extensively, synaptic function in malformed brain regions remains poorly understood. We used an animal model, rats exposed to methylazoxymethanol (MAM) in utero, featuring loss of lamination and distinct nodular heterotopia to examine inhibitory synaptic function in the malformed brain. Previous in vitro and in vivo studies demonstrated an enhanced susceptibility to seizure activity and neuronal hyperexcitability in these animals. Here we demonstrate that inhibitory synaptic function is enhanced in rats exposed to MAM in utero. Using in vitro hippocampal slices and whole-cell voltage-clamp recordings from visualized neurons, we observed a dramatic prolongation of GABAergic IPSCs onto heterotopic neurons. Spontaneous IPSC decay time constants were increased by 195% and evoked IPSC decay time constants by 220% compared with age-matched control CA1 pyramidal cells; no change in IPSC amplitude or rise time was observed. GABA transport inhibitors (tiagabine and NO-711) prolonged evoked IPSC decay kinetics of control CA1 pyramidal cells (or normotopic cells) but had no effect on heterotopic neurons. Immunohistochemical staining for GABA transporters (GAT-1 and GAT-3) revealed a low level of expression in heterotopic cell regions, suggesting a reduced ability for GABA reuptake at these synapses. Together, our data demonstrate that GABA-mediated synaptic function at heterotopic synapses is altered and suggests that inhibitory systems are enhanced in the malformed brain.
Epilepsy Research Laboratory, Department of Neurological Surgery and The Graduate Program in Neuroscience, University of California, San Francisco, San Francisco, California 94143, USA.
Maria Elisa
ME
Calcagnotto
Mercedes F
MF
Paredes
Scott C
SC
Baraban
D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism; D002828:Choristoma / Q000139:chemically induced / Q000150:complications / Q000473:pathology / Q000503:physiopathology*; D004195:Disease Models, Animal; D004827:Epilepsy / Q000139:chemically induced / Q000150:complications / Q000473:pathology / Q000503:physiopathology*; D005260:Female; D050485:GABA Plasma Membrane Transport Proteins; D006624:Hippocampus / Q000187:drug effects / Q000473:pathology / Q000503:physiopathology; D066298:In Vitro Techniques; D008297:Male; D008565:Membrane Proteins / Q000378:metabolism; D026901:Membrane Transport Proteins / Q000378:metabolism; D008746:Methylazoxymethanol Acetate / Q000031:analogs & derivatives*; D009421:Nervous System Malformations / Q000139:chemically induced / Q000150:complications / Q000503:physiopathology*; D009433:Neural Inhibition / Q000187:drug effects / Q000502:physiology; D009474:Neurons / Q000187:drug effects / Q000473:pathology / Q000502:physiology; D027361:Organic Anion Transporters; D018408:Patch-Clamp Techniques; D011247:Pregnancy; D011297:Prenatal Exposure Delayed Effects; D051381:Rats; D017207:Rats, Sprague-Dawley; D013569:Synapses / Q000187:drug effects / Q000473:pathology / Q000502:physiology; D005680:gamma-Aminobutyric Acid / Q000378:metabolism / Q000494:pharmacology
D002352:Carrier Proteins; D050485:GABA Plasma Membrane Transport Proteins; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D027361:Organic Anion Transporters; C493415:Slc6a1 protein, rat; D005680:gamma-Aminobutyric Acid; D008746:Methylazoxymethanol Acetate; C030667:methylazoxymethanol