Abstracts

Rationale:
People with temporal lobe epilepsy (TLE) are at higher risk of developing reproductive endocrine comorbidities than the general population. Gonadotropin-releasing hormone (GnRH) neurons in the hypothalamus are the final output in the central control of fertility and thus play an important role in the regulation of reproductive function. Previous work demonstrated increased GnRH neuron intrinsic excitability in both male and female GnRH-Cre:Ai9 mice in the intrahippocampal kainic acid (IHKA) mouse model of TLE.¹ Voltage-gated potassium (K⁺) channels modulate the pattern of intrinsic excitability in many neurons. Here, we examined whether K⁺ currents in GnRH neurons are altered in the IHKA mouse model, using a second mouse strain with fluorescently labeled GnRH neurons.

Methods:
Adult male GnRH-GFP transgenic mice were housed in a standard environment with a 14/10 h light/dark cycle (1900 h lights off). Once the pups reached P56, they were injected with either KA (50 nl of 20 mM) or control saline into the right dorsal CA1 region of the hippocampus. We used video screening of seizures immediately after KA injection for 3-5 h and histopathological assessment of hippocampal sclerosis (cresyl violet staining) and gliosis (GFAP staining) to confirm IHKA injection. At two months post-injection, we prepared 300-μm coronal brain slices and used whole-cell current-clamp or voltage-clamp electrophysiology to record action potential firing or voltage-gated K⁺ currents, respectively, in GnRH neurons expressing GFP between 1100-1500 h. The voltage- and time-dependence of activation and inactivation of fast-inactivating A-type (I_A) and slowly inactivating K-type (I_K) currents were quantified.

Results:
We confirmed the increased intrinsic excitability phenotype in GnRH neurons from IHKA male GnRH-GFP mice (n = 7 cells, 5 mice per group, p = 0.011). However, no changes in voltage-dependence of inactivation and activation of I_A and I_K currents in GnRH neurons were identified. There was also no difference in the time-dependence of recovery from inactivation of I_A. However, the time course of inactivation of I_A was slower in cells from IHKA mice (n = 6 cells, 4 mice per group, p = 0.003).

Conclusions:
Increased intrinsic excitability in GnRH neurons from IHKA male GnRH-GFP mice suggests that this phenotype is preserved across mouse strains. GnRH neurons from both IHKA and control mice exhibit I_A and I_K currents. I_A in GnRH neurons inactivates at a slower rate when compared to controls. However, this pattern contradicts the increased intrinsic excitability phenotype. This study is significant because the findings suggest that changes in I_A and I_K currents do not underlie increased GnRH neuron excitability in IHKA male mice, and thus would not be effective targets for therapeutic intervention. However, GnRH neuron functional properties display prominent sex differences, and future experiments will examine changes in I_A and I_K currents in IHKA females.

References: 1. Li et al., eNeuro 2018

Funding: R01 NS105825 (C.A.C.-H.)