Potentiation of a slow Ca(2+)-dependent K+ current by intracellular Ca2+ chelators in hippocampal CA1 neurons of rat brain slices

Abstract

1. In hippocampal CA1 neurons of rat brain slices, a Ca(2+)-dependent slow afterhyperpolarization (sAHP) and underlying K+ current (IsAHP) are activated by Ca2+ influx and presumably reflect the time course of the intracellular Ca2+ signal produced by neuronal stimulation. We tested the hypothesis that when exogenous Ca2+ chelators become the predominant mobile Ca2+ buffer in the neuron, they alter the shape of intracellular Ca2+ signals responsible for IsAHP. The nature of this alteration provides insight into the mechanism of IsAHP generation. 2. Derivatives of 1,2-bis-[2-amino phenoxy] ethane N,N,N',N' tetraacetic acid (BAPTA) with different dissociation constants (KDS) for Ca2+ ranging from 0.15 to 7,000 microM were used to test this hypothesis. We also examined the effects of ethylene glycolbis (beta-aminoethyl either)-N,N,N',N'-tetraacetic acid (EGTA), which has a KD similar to that of BAPTA for Ca2+, but which binds and releases Ca2+ 100 times more slowly. When delivered to the cytoplasm by dialysis from a patch pipette, these chelators potentiated, inhibited, or had no effect on IsAHP depending on their concentration, affinity, and binding kinetics. 3. IsAHP decayed exponentially over much of its time course, with a half-decay time of 0.9 +/- 0.1 s (mean +/- SE, n = 22). Immediately after breakthrough into the whole cell configuration, there was an initial period of approximately 5 min during which IsAHP magnitude increased approximately 3.5-fold with no change in time course. Thereafter, the time course and amplitude of IsAHP were stable for > 45 min. 4. Addition of 1 mM of the high-affinity chelators 5,5'-dimethyl BAPTA or BAPTA to the pipette solution first increased the decay time of IsAHP 1.5-fold. However, within 10-15 min after break-through, the current was abolished. Addition of Ca2+ (0.1-1.0 mM) to the patch pipette containing the BAPTA derivatives reduced the ability of a given concentration of high-affinity chelator to inhibit IsAHP and also prolonged the period of IsAHP enhancement. A similar prolongation of the period of enhancement with even less attenuation of IsAHP was apparent with 0.1 mM 5,5'-dimethyl BAPTA and 0.1 mM Ca2+. 5. The intermediate-affinity chelator 4.4'-difluoro BAPTA (1 mM) prolonged the decay phase of the sAHP/IsAHP without attenuating the current. A twofold prolongation of IsAHP also was observed in neurons dialyzed with internal solution containing 3 mM EGTA and 0.3 mM Ca2+. Dialysis with 1 mM of the low-affinity chelators 2-amino-5-fluorophenol-N,N,O-triacetic acid (5-fluoro APTRA) or 5,5'-dinitro BAPTA had no apparent effect on IsAHP. All of the chelators that prolonged the decay phase of IsAHP also induced a rising phase such that a well-defined peak of IsAHP could be discerned at approximately 0.6 s after the end of the stimulus used to evoke the current. 6. Weak stimulation of muscarinic receptors selectively inhibits IsAHP. Thus the uncontaminated time course of IsAHP can be deduced by subtracting currents recorded before and after such muscarinic stimulation. Wit...

Development of Intrinsic Properties and Excitability of Layer 2/3 Pyramidal Neurons During a Critical Period for Sensory Maps in Rat Barrel Cortex

“…Because BAPTA alters Ca 2ϩ chelation and interferes with intrinsic Ca 2ϩ buffers including Ca 2ϩ -activated K ϩ channels (Lancaster and Batchelor 2000), we expected dye perfusion to make immature neurons highly susceptible to switching their spiking behavior. Indeed, moderate concentrations of OGB-1 BAPTA (50-100 M) always induced switching to PS behavior and enhanced the slow AHP (n ϭ 16; data not shown), consistent with previous reports (Borde et al 1995;Jahromi et al 1999;Lorenzon and Foehring 1995;Schwindt et al 1992;Velumian and Carlen 1999;Zhang et al 1995) and with the relationship between spiking behavior and the Ca 2ϩ -dependent slow AHP (Figs. 3 and 5).…”

“…Consistent with previous studies, we found that the amount of SFA of a cell and therefore its spiking pattern could vary during the course of experiments (Borde et al 1995(Borde et al , 2000Lorenzon and Foehring 1995;Zhang et al 1995). Soon after breaking into whole cell configuration (sometimes as early as 2-3 min after break-in), RS cells often displayed increased adaptation, switching eventually (ϳ5-10 min) to PS behavior.…”

“…These cells were healthy by all electrophysiological criteria including stability of resting membrane potential, input impedance, and spike parameters, and all experiments were performed using internal solution based on K ϩ -methylsulfonate, which provides the most stable recording conditions (Velumian et al 1997;Zhang et al 1994). Furthermore, we excluded Ca 2ϩ buffers from the pipette solution, so the change was not due to the effects of BAPTA or other buffers (Lancaster and Batchelor 2000;Lorenzon and Foehring 1995;Schwindt et al 1992;Zhang et al 1995). Finally, stability of spike patterns did not appear to depend on the history of activity: neurons switched spiking behavior even if kept silent during the first few minutes of a recording.…”

Different involvement of type 1, 2, and 3 ryanodine receptors in memory processes

“…However, in central neurons the involvement of RyRs in the reduction of IsAHP appears to be less evident. The CICR mechanism contributes less to the IsAHP (Kumar and Foster 2004;Van de Vrede et al 2007) or does not participate to generation of IsAHP (Zhang et al 1995). The ryanodine retention loss was observed only when the RyR antagonist was administered immediately after the training session.…”

Impact of Cytoplasmic Calcium Buffering on the Spatial and Temporal Characteristics of Intercellular Calcium Signals in Astrocytes

“…For example, the speed with which a chelator binds C a 2ϩ ions (C a 2ϩ association rate) has previously been shown to affect phenomena such as synaptic transmitter release (Adler et al, 1991;Spigelman et al, 1996) and Ca 2ϩ -dependent membrane exitability (Z hang et al, 1995) independently of C a 2ϩ affinity. For example, BAP TA effectively attenuates the release of synaptic transmitter in many preparations in which EGTA (K d , 100 -400 nM) (Harrison and Bers, 1987), a chelator that has a similar C a 2ϩ affinity to BAPTA but that binds C a 2ϩ ions 100 -400 times more slowly (slow K on and K off ), is ineffective (Haraf uji and Ogawa, 1980;Smith et al, 1984;Kao and Tsien, 1988;Adler et al, 1991;Stern, 1992;Winslow et al, 1994;Z hang et al, 1995). To determine whether the effects observed presently on C a 2ϩ waves were affinity-related, rather than being attributable to the C a 2ϩ binding kinetics of the buffers, we tested the effects of EGTA on our preparation.…”