Gap Junctions Contribute to Ictal/Interictal Genesis in Human Hypothalamic Hamartomas

• Neuronal-type functional gap junctions are present in HH tissue.
• Expression of neuronal-type gap junctions is more abundant in HH tissue relative to normal hypothalamus (mammillary body).
• In HH slices, pharmacological block of neuronal-type gap junctions significantly reduces seizure-like discharges.This paper evaluates the role that gap junctions contribute to seizure activity utilized human hypothalamic hamartoma (HH) tissue after surgical resection. We find that 1) gap junctions are present in HH tissue, 2) gap junctions are more abundant in HH tissue relative to normal hypothalamus (mammillary body), and 3) pharmacological blockade of gap junctions in freshly-resected HH tissue slices can decrease the seizure-like discharges. These results provide evidence that gap junctions participate in the generation of seizures from HH tissue and suggest further research into the possibility that gap junction blocking medications may improve seizures in patients with HH.

Human hypothalamic hamartoma (HH) is a rare subcortical lesion associated with treatment-resistant epilepsy. Cellular mechanisms responsible for epileptogenesis are unknown. We hypothesized that neuronal gap junctions contribute to epileptogenesis through synchronous activity within the neuron networks in HH tissue. We studied surgically resected HH tissue with Western-blot analysis, immunohistochemistry, electron microscopy, biocytin microinjection of recorded HH neurons, and microelectrode patch clamp recordings with and without pharmacological blockade of gap junctions. Normal human hypothalamus tissue was used as a control. Western blots showed increased expression of both connexin-36 (Cx36) and connexin-43 (Cx43) in HH tissue compared with normal human mammillary body tissue. Immunohistochemistry demonstrated that Cx36 and Cx43 are expressed in HH tissue, but Cx36 was mainly expressed within neuron clusters while Cx43 was mainly expressed outside of neuron clusters. Gap-junction profiles were observed between small HH neurons with electron microscopy. Biocytin injection into single recorded small HH neurons showed labeling of adjacent neurons, which was not observed in the presence of a neuronal gap-junction blocker, mefloquine. Microelectrode field recordings from freshly resected HH slices demonstrated spontaneous ictal/interictal-like discharges in most slices. Bath-application of gap-junction blockers significantly reduced ictal/interictal-like discharges in a concentration-dependent manner, while not affecting the action-potential firing of small gamma-aminobutyric acid (GABA) neurons observed with whole-cell patch-clamp recordings from the same patient's HH tissue. These results suggest that neuronal gap junctions between small GABAergic HH neurons participate in the genesis of epileptic-like discharges. Blockade of gap junctions may be a new therapeutic strategy for controlling seizure activity in HH patients.