Abstracts

Rationale: Hypothalamic hamartoma (HH) is a developmental malformation occurring in the ventral hypothalamus and is often associated with a range of neurological and endocrine disorders. The epileptic syndrome in HH patients is often characterized by gelastic (laughing) seizures beginning in early infancy, and they are usually refractory to standard anti-epileptic therapies. The epileptogenic mechanisms of gelastic seizures are largely unknown and perhaps include abnormalities in central excitatory and/or inhibitory systems or disrupted excitation/inhibition balance. In this study, we evaluated ionotropic glutamate receptor expression and function using freshly-resected HH tissues. Methods: Ionotropic glutamate receptor subunit expression was assessed by RT-PCR techniques using HH and normal (control) human hypothalamic tissues. The function and pharmacology of these receptors were characterized using patch-clamp recordings from singly dissociated neurons from HH tissue or from HH slices. Results: Most ionotropic glutamate receptor subunits were expressed in both HH and control samples except NR2C and NR3B, which were exclusively expressed in HH tissues. There was no difference in mRNA expression of all other subunits when HH and control tissues were compared. The mRNA expression profile suggests human HH neurons express a variety of ionotropic glutamate receptor subunits that may assemble together to form diverse, functional receptors on HH neurons. Under patch-clamp voltage-clamp conditions, 1 mM glutamate induced small inward-current responses in all recorded HH neurons, but GABA (0.1 mM) induced much larger currents in these same neurons. Interestingly, under Mg2+-free plus 3 μM glycine conditions, NMDA induced relatively large current responses compared to AMPA- and KA-induced currents. These results suggest ionotropic glutamate receptors are both expressed and functional on HH neurons. Under current-clamp conditions, 1 mM glutamate significantly increased firing frequency of dissociated HH neurons. In HH slices, 1 mM glutamate strongly activated recorded neurons. These results suggest that although the amplitude of current responses induced by glutamate is small, ionotropic glutamate receptors appear to play an important role in regulation of HH neuronal excitation. Conclusions: In human HH, there is heterogeneity with respect to ionotropic glutamate receptor function, pharmacology and modulation, which may play a role in epileptogenesis of gelastic seizures. This work was supported by grants of NIH (NS-056104) and Barrow neuroscience Foundation.