Abstracts

RATIONALE: Genetically epilepsy-prone rats (GEPR) are naturally susceptible to seizures. Seizures can be induced by many different stimuli, but audiogenic seizures (AGS) are easily elicited, and the neuronal network subserving AGS is well-understood. AGS are initiated in inferior colliculus (IC) and spread from there to other brain areas. In previous studies we found that both the fast and the slow calcium-dependent afterhyperpolarizations following action potentials are reduced in GEPR, which leads to a marked increase in action potential firing. In the present study, we tested the hypothesis that voltage-sensitive calcium currents are reduced in GEPR.
METHODS: GEPR (GEPR-9 strain) and Sprague-Dawley (SD) rats aged 6 to 8 weeks were compared. GEPR had either 0 seizures, or 3 AGS given once daily. Dissociated IC neurons were cultured in serum-free growth medium and studied after 4 to 7 days [italic]in vitro[/italic]. Voltage-sensitive calcium currents were studied after blockade of fast sodium and potassium currents using tetrodotoxin, tetraethylammonium, 4-aminopyridine, and intracellular cesium. Barium (10 millimolar) was used as the charge carrier instead of calcium (0 millimolar). High-voltage-activated (HVA) currents were elicited by 200 ms step depolarizations from a holding potential of -60 mV, a protocol that elicits high-voltage-activated (HVA) currents. Peak current density was measured by dividing peak current by cell capacitance, and conductance was calculated from current-voltage curves.
RESULTS: We found that peak HVA current density was markedly reduced in GEPR with 0 seizures (-1.86 [plusminus] 0.39 pA/pF, N = 21) and 3 seizures (-1.81 [plusminus] 0.40 pA/pF), compared to SD neurons (-4.67 [plusminus] 0.97 pA/pF, N = 23). The 60% / 61% reduction of current density in GEPR with 0 and 3 seizures was statistically significant (ANOVA F(2,61) = 0.03, with post-hoc student-Neuman Keuls tests indicating P [lt] 0.05 for each group). There was no difference in GEPR with 0 seizures compared to GEPR with 3 seizures. Plots of conductance/peak conductance versus voltage showed no difference in the voltage-sensitivity of calcium currents in the three groups.
CONCLUSIONS: Voltage-sensitive calcium currents are markedly reduced in IC neurons of this animal model of epilepsy. These data indicate that reduced calcium currents are the likely cause of the previously-described deficits in calcium-dependent afterhyperpolarizations and increased action potential firing. Reduced calcium currents are likely to be a major contributor to epilepsy in this animal model.
At the end of this activity the participant will be able to understand the possible importance of reduced calcium channel currents in epilepsy.
[Supported by: NINDS R29 NS34564, SIU Central Research Committee]