Calcium current activated by potassium ions in voltage-clamped rat hippocampal pyramidal neurones

Synchronous and asynchronous exocytosis induced by subthreshold high K+ at Cs(+)-loaded terminals of rat hippocampal neurons

Transmitter release at Cs(+)-loaded autaptic terminals was selectively activated by the subthreshold concentration of external K+, and Ca(2+) channel types and transmitter pools involved in synchronous and asynchronous exocytosis were studied. When a neuron was depolarized to +30 mV by applying a current through a pipette containing Cs(+) for >30 s, a rapid external K+ jump to 3.75-10 mM, otherwise ineffective, produced an outward current (K10 response). K10 responses were initially graded (type-1) and then became a spike and plateau-shape with (type-2) or without a latency (type-3). On repolarization to -60 mV, a high K+ jump induced inward currents (called also K10 response) similar to those at +30 mV, whose shape changed from that of type-3, then type-2 and finally type-1 over 30 min. During a period favorable for inducing a type-3 response, a current similar to this response was generated by a voltage pulse (+ 80 or 90 mV, 20 or 30 ms) to the cell soma. Currents similar to K10 responses were rarely induced by a high K+ jump without a conditioning depolarization except for some cells, but consistently produced when 3 mM Cs(+) and 50 microM 4-aminopyridine were externally applied for tens of minutes. Picrotoxin, 6-cyano-7-nitroquinoxaline-2,3-dione with 3-[(RS)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid or Cd(2+) in, or Ca(2+) removal from, a high-K+ solution blocked all the K10 responses, while a plateau remaining after a high K+ jump was not blocked by Ca(2+) removal immediately after the K+ jump. Thus Cs(+) loading and decreased K+ concentration in autaptic terminals by a conditioning depolarizing current selectively sensitize the terminals to a subthreshold high K+ jump for depolarization to activate synchronous or asynchronous transmitter release. Nicardipine (5-10 microM) blocked type-1 and -2 responses but not type-3 responses, while omega-conotoxin (10 microM) blocked all the types of K10 response in the presence of nicardipine. Increasing the interval of high K+ jumps biphasically increased the magnitude of K10 response, preferentially in the postjump fraction reflecting purely the asynchronous activation of exocytotic machinery, and decreased the reduction of miniature postsynaptic current frequency after a K10 response. These results suggest the roles of N(P/Q)-type Ca(2+) channels in synchronous exocytosis at the terminals, L-type Ca(2+) channels in initiating a Ca(2+) action potential at the parent axon and both types in asynchronous exocytosis and also suggest the different releasable pools of transmitter for two modes of exocytosis in cultured hippocampal neurons.

Clotrimazole analogues: effective blockers of the slow afterhyperpolarization in cultured rat hippocampal pyramidal neurones

1. The pharmacology of the slow afterhyperpolarization (sAHP) was studied in cultured rat hippocampal pyramidal neurones. 2. Clotrimazole, its in vivo metabolite, 2-chlorophenyl-bisphenyl-methanol (CBM) and the novel analogues, UCL 1880 and UCL 2027, inhibited the sI(AHP) with similar IC50s (1-2 microM). 3. Clotrimazole and CBM also inhibited the high voltage-activated (HVA) Ca2+ current in pyramidal neurones with IC50s of 4.7 microM and 2.2 microM respectively. UCL 1880 was a less effective Ca2+ channel blocker, reducing the HVA Ca2+ current by 50% at 10 microM. At concentrations up to 10 microM, UCL 2027 had no effect on the Ca2+ current, indicating that its effects on the sI(AHP) were independent of Ca2+ channel block. 4. Clotrimazole also inhibited both the outward holding current (IC50=2.8 microM) present at a potential of -50 mV and the apamin-sensitive medium AHP (mAHP; IC50 approximately amp;10 microM). The other clotrimazole analogues tested had smaller effects on these two currents. The present work also shows that 100 nM UCL 1848, an inhibitor of apamin-sensitive conductances, abolishes the mAHP. 5. Currents were recorded from HEK293 cells transfected with hSK1 and rSK2. The SK currents were very sensitive to inhibition by UCL 1848 but were not significantly reduced by the sI(AHP) inhibitor, UCL 2027 (10 microM). 10 microM UCL 1880 reduced the hSK1 current by 40%. 6. UCL 2027 appears to be the first relatively selective blocker of the sAHP to be described. Furthermore, the ability of UCL 2027 to block the sAHP with minimal effect on SK1 channel activity questions the role of this channel in the sAHP.

Ca(2+) channels involved in the generation of the slow afterhyperpolarization in cultured rat hippocampal pyramidal neurons

The advantages of using isolated cells have led us to develop short-term cultures of hippocampal pyramidal cells, which retain many of the properties of cells in acute preparations and in particular the ability to generate afterhyperpolarizations after a train of action potentials. Using perforated-patch recordings, both medium and slow afterhyperpolarization currents (mI(AHP) and sI(AHP), respectively) could be obtained from pyramidal cells that were cultured for 8-15 days. The sI(AHP) demonstrated the kinetics and pharmacologic characteristics reported for pyramidal cells in slices. In addition to confirming the insensitivity to 100 nM apamin and 1 mM TEA, we have shown that the sI(AHP) is also insensitive to 100 nM charybdotoxin but is inhibited by 100 microM D-tubocurarine. Concentrations of nifedipine (10 microM) and nimodipine (3 microM) that maximally inhibit L-type calcium channels reduced the sI(AHP) by 30 and 50%, respectively. However, higher concentrations of nimodipine (10 microM) abolished the sI(AHP), which can be partially explained by an effect on action potentials. Both nifedipine and nimodipine at maximal concentrations were found to reduce the HVA calcium current in freshly dissociated neurons to the same extent. The N-type calcium channel inhibitor, omega-conotoxin GVIA (100 nM), irreversibly inhibited the sI(AHP) by 37%. Together, omega-conotoxin (100 nM) and nifedipine (10 microM) inhibited the sI(AHP) by 70%. 10 microM ryanodine also reduced the sI(AHP) by 30%, suggesting a role for calcium-induced calcium release. It is concluded that activation of the sI(AHP) in cultured hippocampal pyramidal cells is mediated by a rise in intracellular calcium involving multiple pathways and not just entry via L-type calcium channels.

Inhibition of voltage-gated calcium channels by fluoxetine in rat hippocampal pyramidal cells

Fluoxetine, an antidepressant which is used world-wide, is a prominent member of the class of selective serotonin re-uptake inhibitors. Recently, inhibition of voltage-gated Na(+) and K(+) channels by fluoxetine has also been reported. We examined the effect of fluoxetine on voltage-gated calcium channels using the patch-clamp technique in the whole-cell configuration. In hippocampal pyramidal cells, fluoxetine inhibited the low-voltage-activated (T-type) calcium current with an IC(50) of 6.8 microM. Fluoxetine decreased the high-voltage-activated (HVA) calcium current with an IC(50) between 1 and 2 microM. Nifedipine and omega-conotoxin GVIA inhibited the HVA current by 24% and 43%, respectively. Fluoxetine (3 microM), applied in addition to nifedipine or omega-conotoxin, further reduced the current. When fluoxetine (3 microM) was applied first neither nifedipine nor omega-conotoxin attenuated the remaining component of the HVA current. This observation indicates that fluoxetine inhibits both L- and N-type currents. In addition, fluoxetine inhibited the HVA calcium current in carotid body type I chemoreceptor cells and pyramidal neurons prepared from prefrontal cortex. In hippocampal pyramidal cells high K(+)-induced seizure-like activity was inhibited by 1 microM fluoxetine; the mean burst duration was shortened by an average of 44%. These results provide evidence for inhibition of T-, N- and L-type voltage-gated calcium channels by fluoxetine at therapeutically relevant concentrations.

Cell-attached measurements of the firing threshold of rat hippocampal neurones

1. The cell-attached configuration of the patch-clamp technique was used to assess resting membrane potential and firing threshold of CA1 pyramidal cells and interneurones of rat hippocampal slices. 2. Resting potential was inferred from the reversal potential of voltage-gated K+ currents with symmetrical intracellular and pipette K+ concentrations. Its mean value was -74 +/- 9 mV for silent interneurones (mean +/- s.d.; n = 17) and -84 +/- 7 mV for silent pyramidal cells (n = 8). Spontaneous action currents occurred in thirteen out of thirty-two interneurones and two out of ten pyramidal cells. In active cells, membrane potential values fluctuated by up to 20 mV, due in part to the large hyperpolarizations that followed an action current. 3. Membrane potential values determined from K+ current reversal were 13 +/- 6 mV more hyperpolarized than those measured in whole-cell recordings from the same neurones (n = 8), probably due to a Donnan equilibrium potential between pipette and cytoplasm. 4. Firing threshold of silent cells was determined by elevating external K+ until action currents were generated, while membrane potential was monitored from the cell-attached K+ current reversal. Spike threshold was attained at -49 +/- 8 mV for interneurones (n = 17) and at -60 +/- 8 mV for pyramidal cells (n = 8). Increasing external Ca2+ from 2 to 4 mM shifted the neuronal voltage threshold by +5 mV, without affecting resting potential. 5. For comparison with these values, we examined how the rate of membrane polarization influenced firing threshold in whole-cell records. Ramp current injections, of duration 15-1500 ms, revealed that current threshold followed a classical strength-duration relationship. In contrast voltage threshold, determined from current injection or by elevating extracellular K+, varied little with the rate of membrane polarization. 6. The state of activation and inactivation of Na+ and K+ currents might contribute to the stability of the voltage threshold. Cell-attached records showed that 79 +/- 10 % of Na+ channels and 64 +/- 10 % of K+ channels were available for activation at resting potential in silent cells (n = 8). As cells were depolarized to threshold, Na+ current availability was reduced to 23 +/- 10 %, and K+ current availability to 31 +/- 12 %. 7. The speed of transition into the inactivated states also appears to contribute to the invariance of threshold for all but the fastest depolarizations. At potentials close to threshold, the rate of inactivation of Na+ and K+ followed a double exponential time course, such that Na+ currents were 62 % inactivated and K+ currents were 63 % inactivated within 15 ms.