Abstracts

The hypothalamic hamartoma (HH) is a rare developmental malformation often characterized by unusual seizures associated with gelastic seizures, and refractory to medical therapy. The mechanisms of epileptogenesis operative in this subcortical lesion are unknown. Standard patch-clamp electrophysiological techniques were used to study individual cells either freshly isolated or acute slices from human HH tissue immediately following surgical resection. Over 90% of HH cells were small (6-9 mm soma) and exhibited immunoreactivity to the neuronal marker, NeuN, and to the 67 kD isoform of glutamic acid decarboxylase (GAD67), but not to GFAP (glial fibrillary acidic protein). Single cells were studied either in acutely dissociated HH cells or in 300 mm slices (visually identified with Nomarski optics under infrared illumination) for detailed studies of various ion channel conductances. Whole-cell recordings in dissociated HH cells demonstrated typical neuronal responses to depolarizing and hyperpolarization current injection. Interestingly, most dissociated HH cells exhibited robust, spontaneous [quot]pacemaker-like[quot] action potential firing, and the similar spontaneous rhythmic discharge was also observed in patch-clamp recordings from HH slices. In addition, single dissociated cells were shown to express functional TTX-sensitive Na⁺ and TEA-sensitive K⁺ channels. Under voltage clamp (V[sub]H[/sub] of [ndash]45 mV), rapid application of the excitatory neurotransmitter glutamate (1 mM), induced an inward current with a peak amplitude of 61.5 [plusmn] 4.6 pA (N=15, Mean [plusmn] SE). While, application of GABA (0.1 mM at V[sub]H[/sub] of [ndash]20 mV) induced a high-amplitude outward current of 540 [plusmn] 27.7 pA (N=20). Present study is the first to describe the electrophysiological properties of single HH cells from human epilepsy surgery. These cells exhibit many neuronal features, and many cells exhibit spontaneous pacemaker activity under our experimental conditions. While the microarchitecture and network connectivity of HH tissue remains to be defined, our findings provide an initial window into the pathophysiology of human gelastic seizures. (Supported by Barrow Scientific Foundation and the Women[apos]s Board Foundation.)