Dysmorphic neurons with ultralow intrinsic excitability are paradoxically hyperexcitable in neural network in a mouse model of focal cortical dysplasia type II.

OBJECTIVE

Focal cortical dysplasia type II (FCDII) is one of the most common causes of refractory epilepsy. Hyperactivated mechanistic target of rapamycin (mTOR) pathway resulting from genetic mutations underlies FCDII, but it remains controversial and poorly understood whether the dysmorphic neurons play a role in cortical hyperexcitability.

METHODS

Here, we performed whole-cell recordings from dysmorphic neurons and nearby normal-appearing neurons in an FCDII mouse model with a PIK3CA mutation to examine intrinsic excitability and excitatory/inhibitory (E/I) synaptic inputs. To decipher the net effects of intrinsic excitability and synaptic transmission, we delivered extracellular electric stimulation to evoke action potentials (APs) and obtained the current threshold. To explore the underlying mechanisms, immunofluorescent staining was used to analyze the neuronal morphology, and rapamycin was injected into mice to suppress the activity of mTOR pathway. γ-Aminobutyric acid type B (GABAB) receptor agonist (baclofen) was applied to modulate the excitability of dysmorphic neurons.

RESULTS

We found that dysmorphic neurons exhibited ultralow intrinsic excitability but enhanced excitatory synaptic inputs. Additionally, lower threshold intensity was required for them to evoke APs. This hyperexcitability could be attributable to mTOR-dependent increased dendritic complexity and spine density, as postnatal rapamycin application rescued these changes. Importantly, we found that activation of presynaptic GABAB receptors could specifically reduce excitatory synaptic inputs and normalize the E/I dysfunction, and thus decrease the excitability of dysmorphic neurons.

SIGNIFICANCE: Together, these findings indicate that excessive excitatory synaptic inputs prevail over reduced intrinsic excitability in dysmorphic neurons, leading to hyperexcitability in FCDII. Furthermore, they strongly suggest GABAB receptors as a potential therapeutic target for FCDII.