Calbindin-D28k content and firing pattern of hippocampal granule cells in amygdala-kindled rats: a perforated patch-clamp study

The dentate gyrus is believed to play an important pathophysiological role during experimentally induced kindling. In this study, we investigated whether an altered content of the calcium binding protein calbindin-D28k or an increased intrinsic excitability of hippocampal granule cells contribute to the induction of the kindling phenomenon. We determined the firing pattern of granule cells in hippocampal slices using perforated patch-clamp recordings in current clamp mode. The expression of calbindin-D28k and glutamic acid decarboxylase (GAD67) by granule cells was analyzed immunohistochemically. Rats developed secondarily generalized limbic seizures within ∼11 days of twice-daily stimulation of the amygdala. As reported for other kindling paradigms, this protocol induced a clear up-regulation of GAD67 in granule cells, indicating their involvement in the induced neuronal activity. However, when comparing kindled and control rats, we could not detect any differences in intrinsic excitability: Firing frequency, after-hyperpolarisations, action potentials, input resistance and membrane potentials were nearly identical between both groups. Furthermore, we did not observe any differences in the calbindin-D28k immunoreactivity between groups. In every slice, virtually all granule cells were found to be strongly calbindin-D28k positive, and there was no apparent reduction in the general level of calbindin-D28k expression. We conclude that changes in intrinsic membrane properties or in the calbindin-D28k content of granule cells are not necessary for the development of amygdala kindling.