A mutant thyroid hormone receptor alpha1 alters hippocampal circuitry and reduces seizure susceptibility in mice

Thyroid hormone deficiency during early developmental stages causes a multitude of functional and morphological deficits in the brain. In the present study we investigate the effects of a mutated thyroid hormone receptor TR alpha 1 and the resulting receptor-mediated hypothyroidism on the development of GABAergic neurotransmission and seizure susceptibility of neuronal networks. We show that mutant mice have a strong resistance to seizures induced by antagonizing the GABA(A) receptor complex. Likewise the hippocampal network of mutant mice shows a decreased likelihood to transform physiological into pathological rhythmic network activity such as seizure-like interictal waves. As we demonstrate the cellular basis for this behavior is formed by the excitatory nature of GABAergic neurotransmission in the mutant mice, possibly caused by altered Cl(-) homeostasis, and/or the altered patterning of calretinin-positive cells in the hippocampal hilus. This study is, to our knowledge, the first to show an effect of maternal and early postnatal hypothyroidism via TR alpha 1 on the development of GABAergic neurotransmission and susceptibility to epileptic seizures.