Abstracts

Rationale: Hypothalamic hamartomas (HH) are congenital tumors of the ventral hypothalamus resulting in treatment-resistant epilepsy. HH are intrinsically epileptogenic for gelastic seizures. Electrophysiological features specific to Small and Large HH neurons have been identified, but the structure of these neurons is largely unexplored. We wished to study the microanatomical features of Small and Large HH neurons in order to gain further insights into their function within the epileptic network. Methods: Surgically-resected HH tissue from 13 patients (54% male; median age 5.3 years) was fixed, sectioned, and stained utilizing a modified Golgi-Cox method [FD NeuroTechnologies, Ellicott City, MD, USA]. Two-dimensional representations of selected cells were obtained with bright-field microscopy and a camera lucida drawing tube and analyzed for morphological features, including Sholl analysis. Neurons were assigned to the Small or Large type guided by our prior experience with single-cell microinjections subsequent to patch-clamp electrophysiological recordings. Results: Not unexpectedly, Small (n=50) and Large (n=10) HH neurons differ in the cross-sectional area of their soma (mean 115 versus 345 μm2; p<0.0001). Small neurons have universally round soma, while Large neurons are markedly pleomorphic and typically include at least one thick proximal dendrite. The two populations diverge significantly with respect their dendritic fields as shown by Sholl Analysis. Small neurons have dendrites that are thin, minimally branched and spineless (p=0.0002). 90% of Small neurons have dendrites that extend no more than 100 um from the soma. Dendritic beading is common in both types of cells. Conclusions: Small HH neurons are abundant, spontaneously-firing GABAergic neurons that occur in cellular clusters. Golgi staining of individual Small HH neurons shows simple and spatially limited dendritic fields consistent with an interneuron phenotype. Their network interactions may be largely within the local cluster. While Large HH neurons are morphologically consistent with excitatory projection-type neurons, they are much less stereotyped, suggesting that there is more diversity within the Large HH neuron subgroup than previously appreciated.