Electrophysiological properties and subunit composition of GABA_a receptors in patients with gelastic seizures and hypothalamic hamartoma

Abnormalities in GABA_A receptor structure and/or function have been associated with various forms of epilepsy in both humans and animals. Whether this is true for patients with gelastic seizures and hypothalamic hamartoma (HH) is unknown. In this study, we characterized the pharmacological properties and native subunit composition of GABA_A receptors on acutely dissociated single neurons from surgically resected HH tissues using patch-clamp, immunocytochemical, and RT-PCR techniques. We found that 1) GABA induced an inward current (I_GABA) at a holding potential of -60 mV; 2) I_GABA was mimicked by the GABA_A receptor agonist muscimol and blocked by the GABA_A receptor antagonist bicuculline, suggesting that I_GABA was mediated principally through the GABA _A receptor; 3) the EC₅₀ and Hill coefficient derived from the I_GABA concentration-response curve were 6.8 μM and 1.9, respectively; 4) the current-voltage curve was linear at a reversal potential close to zero; and 5) I_GABA exhibited low sensitivity to zinc and diazepam but higher sensitivity to pentobarbital and pregnanolone. Additionally, using Xenopus oocytes microtransplanted with normal human hypothalamic tissue, we confirmed that the functional properties of GABA_A receptors were similar to those seen in small isolated HH neurons. Finally, the expression profile of GABA_A receptor subunits obtained from normal control human hypothalamic tissue was identical to that from surgically resected human HH tissue. Taken together, our data indicate that GABA_A receptors on small HH neurons exhibit normal pharmacosensitivity and subunit composition. These findings bear relevance to a broader understanding of inhibitory neurotransmission in human HH tissue.