Properties of Voltage-Activated Ca²⁺ Currents in Acutely Isolated Human Hippocampal Granule Cells

Abstract

Properties of Ba2+ currents through voltage-dependent Ca2+ channels (IBa) were investigated in 61 dentate granule cells acutely isolated from the resected hippocampus of nine patients with therapy-refractory temporal lobe epilepsy (TLE). Currents with a high threshold of activation (HVA) peaked at 0 mV, and showed some time-dependent inactivation and a voltage of half-maximal steady-state inactivation (V1/2inact) of -16.4 mV. Application of saturating doses of omega-conotoxin (omega-CgTx) GVIA or nifedipine distinguished characteristic N-type (38%) and L-type (62% of HVA currents) Ca2+ currents. Combined application of both agents blocked HVA currents by > 95%. In a 10-mo-old child but not in adult patients, an omega-agatoxin IVA (omega-AgaTxIVA)-sensitive but omega-CgTx MVIIC-insensitive, noninactivating component of HVA currents (approximately 24%) was present that most probably corresponds to a P-type current. A T-type Ca2+ current could be separated from HVA components on the basis of its steady-state voltage-dependent inactivation (V1/2inact = -71.0 mV). The T-type Ca2+ current isolated by subtraction peaked at more negative potentials (-10 mV), showed a significantly more rapid time-dependent inactivation, and could be selectively blocked by low concentrations of Ni2+. It was insensitive to nifedipine and omega-CgTx GVIA. We conclude that L-, N-, and T-type currents are present in adult human dentate granule cells and an additional P-type current is present in neurons from a 10-mo-old patient. These data may provide a basis for comparison with animal models of epilepsy and for the elucidation of mechanisms of action of drugs intended for use in human disease.

Effects of Phenytoin, Carbamazepine, and Gabapentin on Calcium Channels in Hippocampal Granule Cells from Patients with Temporal Lobe Epilepsy

“…Likewise, results from binding studies provide evidence that PHT can interact with nifedipine-sensitive Ca2+ channels (36)(37)(38). Interestingly, the amount of the maximal PHT-mediated inhibition of HVA currents obtained in our study is comparable to the fraction of N-type Ca2+ current found in human hippocampal granule cells (24). Whether PHT partially antagonizes L-type Ca2+ current, as was shown by Miyazaki et al in glucagon-secreting tumor cells (39), or completely blocks N-type Ca2+ current will be subject of further investigations.…”

“…Previous studies in our laboratory demonstrated that dentate granule cells from adult human hippocampus express mainly high-voltage activated (HVA) Ca" currents including L-and N-type currents (24), whereas there has been no evidence for low-voltage-activated (LVA) Ca2+ currents. In this study, we examined the effects of PHT, CBZ, and GBP on HVA currents.…”

Calcium‐dependent inactivation of high‐threshold calcium currents in human dentate gyrus granule cells

“…While in guinea-pig DGGCs at least four different ICa components (T, L, N and P) were demonstrated (Eliot & Johnston, 1994), under our experimental conditions the predominant source of Ca¥ influx into dissociated human granule cells is a HVA ICa composed of up to three Ca¥ conductances. Our recordings also differ from those of Beck et al (1997b) in the substantially smaller peak IBa densities (60 ± 6 pA pF¢ versus the 144·3 ± 24·3 pA pF¢ in their report). This difference may stem from their use of 10 mÒ intracellular EGTA, thereby possibly lowering resting [Ca¥] and consequently basal Ca¥-dependent inactivation (see below).…”

Chronic Epileptogenic Cellular Alterations in the Limbic System After Status Epilepticus

“…Therefore, the primary utility of drugs effective in these screens would probably be against secondary generalization of seizures. However, it is clear from the studies described above and studies describing alterations in excitatory amino acid receptors (60)(61)(62)77) and other ion channels (64,(78)(79)(80) in focal tissue within the epileptic brain that many ion channels and receptors in these critical epileptogenic sites are fundamentally different from those in control tissue. Future efforts targeting these "epileptic" ion channels may have therapeutic benefit in controlling initiation of seizures rather than seizure spread.…”