Effects of calcium channel agonist and antagonists on calcium-dependent events in CA1 hippocampal neurons

The Molecular Basis for the Calcium-Dependent Slow Afterhyperpolarization in CA1 Hippocampal Pyramidal Neurons

Neuronal signal transmission depends on the frequency, pattern, and timing of spike output, each of which are shaped by spike afterhyperpolarizations (AHPs). There are classically three post-spike AHPs of increasing duration categorized as fast, medium and slow AHPs that hyperpolarize a cell over a range of 10 ms to 30 s. Intensive early work on CA1 hippocampal pyramidal cells revealed that all three AHPs incorporate activation of calcium-gated potassium channels. The ionic basis for a fAHP was rapidly attributed to the actions of big conductance (BK) and the mAHP to small conductance (SK) or Kv7 potassium channels. In stark contrast, the ionic basis for a prominent slow AHP of up to 30 s duration remained an enigma for over 30 years. Recent advances in pharmacological, molecular, and imaging tools have uncovered the expression of a calcium-gated intermediate conductance potassium channel (IK, KCa3.1) in central neurons that proves to contribute to the slow AHP in CA1 hippocampal pyramidal cells. Together the data show that the sAHP arises in part from a core tripartite complex between Cav1.3 (L-type) calcium channels, ryanodine receptors, and IK channels at endoplasmic reticulum-plasma membrane junctions. Work on the sAHP in CA1 pyramidal neurons has again quickened pace, with identified contributions by both IK channels and the Na-K pump providing answers to several mysteries in the pharmacological properties of the sAHP.

Calcium-induced calcium release and type 3 ryanodine receptors modulate the slow afterhyperpolarising current, sIAHP, and its potentiation in hippocampal pyramidal neurons

The slow afterhyperpolarising current, sI_AHP, is a Ca²⁺-dependent current that plays an important role in the late phase of spike frequency adaptation. sI_AHP is activated by voltage-gated Ca²⁺ channels, while the contribution of calcium from ryanodine-sensitive intracellular stores, released by calcium-induced calcium release (CICR), is controversial in hippocampal pyramidal neurons. Three types of ryanodine receptors (RyR1-3) are expressed in the hippocampus, with RyR3 showing a predominant expression in CA1 neurons. We investigated the specific role of CICR, and particularly of its RyR3-mediated component, in the regulation of the sI_AHP amplitude and time course, and the activity-dependent potentiation of the sI_AHP in rat and mouse CA1 pyramidal neurons. Here we report that enhancement of CICR by caffeine led to an increase in sI_AHP amplitude, while inhibition of CICR by ryanodine caused a small, but significant reduction of sI_AHP. Inhibition of ryanodine-sensitive Ca²⁺ stores by ryanodine or depletion by the SERCA pump inhibitor cyclopiazonic acid caused a substantial attenuation in the sI_AHP activity-dependent potentiation in both rat and mouse CA1 pyramidal neurons. Neurons from mice lacking RyR3 receptors exhibited a sI_AHP with features undistinguishable from wild-type neurons, which was similarly reduced by ryanodine. However, the lack of RyR3 receptors led to a faster and reduced activity-dependent potentiation of sI_AHP. We conclude that ryanodine receptor-mediated CICR contributes both to the amplitude of the sI_AHP at steady state and its activity-dependent potentiation in rat and mouse hippocampal pyramidal neurons. In particular, we show that RyR3 receptors play an essential and specific role in shaping the activity-dependent potentiation of the sI_AHP. The modulation of activity-dependent potentiation of sI_AHP by RyR3-mediated CICR contributes to plasticity of intrinsic neuronal excitability and is likely to play a critical role in higher cognitive functions, such as learning and memory.

The slow afterhyperpolarization: a target of β1-adrenergic signaling in hippocampus-dependent memory retrieval

In rodents, adrenergic signaling by norepinephrine (NE) in the hippocampus is required for the retrieval of intermediate-term memory. NE promotes retrieval via the stimulation of β1-adrenergic receptors, the production of cAMP, and the activation of both protein kinase A (PKA) and the exchange protein activated by cAMP. However, a final effector for this signaling pathway has not been identified. Among the many targets of adrenergic signaling in the hippocampus, the slow afterhyperpolarization (sAHP) is an appealing candidate because its reduction by β1 signaling enhances excitatory neurotransmission. Here we report that reducing the sAHP is critical for the facilitation of retrieval by NE. Direct blockers of the sAHP, as well as blockers of the L-type voltage-dependent calcium influx that activates the sAHP, rescue retrieval in mutant mice lacking either NE or the β1 receptor. Complementary to this, a facilitator of L-type calcium influx impairs retrieval in wild-type mice. In addition, we examined the role of NE in the learning-related reduction of the sAHP observed ex vivo in hippocampal slices. We find that this reduction in the sAHP depends on the induction of persistent PKA activity specifically in conditioned slices. Interestingly, this persistent PKA activity is induced by NE/β1 signaling during slice preparation rather than during learning. These observations suggest that the reduction in the sAHP may not be present autonomously in vivo, but is likely induced by neuromodulatory input, which is consistent with the idea that NE is required in vivo for reduction of the sAHP during memory retrieval.

Bidirectional pattern-specific plasticity of the slow afterhyperpolarization in rats: role for high-voltage activated Ca2+ channels and I h

A burst of action potentials in hippocampal neurons is followed by a slow afterhyperpolarization (sAHP) that serves to limit subsequent firing. A reduction in the sAHP accompanies acquisition of several types of learning, whereas increases in the sAHP are correlated with cognitive impairment. The present study demonstrates in vitro that activity-dependent bidirectional plasticity of the sAHP does not require synaptic activation, and depends on the pattern of action potential firing. Whole-cell current-clamp recordings from CA1 pyramidal neurons in hippocampal slices from young rats (postnatal days 14-24) were performed in blockers of synaptic transmission. The sAHP was evoked by action potential firing at gamma-related (50 Hz, gamma-AHP) or theta frequencies (5 Hz, theta-AHP), two firing frequencies implicated in attention and memory. Interestingly, when the gamma-AHP and theta-AHP were evoked in the same cell, a gradual potentiation of the gamma-AHP (186 ± 31%) was observed that was blocked using Ca(2+) channel blockers nimodipine (10 μm) or ω-conotoxin MVIIC (1 μm). In experiments that exclusively evoked the sAHP with 50 Hz firing, the gamma-AHP was similarly potentiated (198 ± 44%). However, theta-burst firing pattern alone resulted in a decrease (65 ± 19%) of the sAHP. In these experiments, application of the h-channel blocker ZD7288 (25 μm) selectively prevented enhancement of the gamma-AHP. These data demonstrate that induction requirements for bidirectional AHP plasticity depend on the pattern of action potential firing, and result from distinct mechanisms. The identification of novel mechanisms underlying AHP plasticity in vitro provides additional insight into the dynamic processes that may regulate neuronal excitability during learning in vivo.

Deletion of the L-type calcium channel Ca(V) 1.3 but not Ca(V) 1.2 results in a diminished sAHP in mouse CA1 pyramidal neurons

Trains of action potentials in CA1 pyramidal neurons are followed by a prolonged calcium-dependent postburst afterhyperpolarization (AHP) that serves to limit further firing to a sustained depolarizing input. A reduction in the AHP accompanies acquisition of several types of learning and increases in the AHP are correlated with age-related cognitive impairment. The AHP develops primarily as the result of activation of outward calcium-activated potassium currents; however, the precise source of calcium for activation of the AHP remains unclear. There is substantial experimental evidence suggesting that calcium influx via voltage-gated L-type calcium channels (L-VGCCs) contributes to the generation of the AHP. Two L-VGCC subtypes are predominately expressed in the hippocampus, Ca(V) 1.2 and Ca(V) 1.3; however, it is not known which L-VGCC subtype is involved in generation of the AHP. This ambiguity is due in large part to the fact that at present there are no subunit-specific agonists or antagonists. Therefore, using mice in which the gene encoding Ca(V) 1.2 or Ca(V) 1.3 was deleted, we sought to determine the impact of alterations in levels of these two L-VCGG subtypes on neuronal excitability. No differences in any AHP measure were seen between neurons from Ca(V) 1.2 knockout mice and controls. However, the total area of the AHP was significantly smaller in neurons from Ca(V) 1.3 knockout mice as compared with neurons from wild-type controls. A significant reduction in the amplitude of the AHP was also seen at the 1 s time point in neurons from Ca(V) 1.3 knockout mice as compared with those from controls. Reductions in both the area and 1 s amplitude suggest the involvement of calcium influx via Ca(V) 1.3 in the slow AHP (sAHP). Thus, the results of our study demonstrate that deletion of Ca(V) 1.3, but not Ca(V) 1.2, significantly impacts the generation of the sAHP.

Control of IsAHP in mouse hippocampus CA1 pyramidal neurons by RyR3-mediated calcium-induced calcium release

In several neuronal preparations, the ryanodine-sensitive calcium store was reported to participate in the generation of slow afterhyperpolarization currents (IsAHP) involved in spike frequency adaptation. We show that calcium release from the ryanodine-sensitive calcium store is a major determinant of the triggering of IsAHP in mouse CA1 pyramidal neurons. Whole-cell patch clamp recordings in hippocampus slices show that the intracellular calcium stores depletion using an inhibitor of the endoplasmic reticulum Ca2+-ATPase (5 microM cyclopiazonic acid), as well as the specific blockade of ryanodine receptors (100 microM ryanodine) both reduced the IsAHP by about 70%. Immunohistology, using an anti-RyR3 specific antibody, indicates that RyR3 expression is particularly enriched in the CA1 apical dendrites (considered as the most important site for sAHP generation). We show that our anti-RyR3 antibody acts as a functional RyR3 antagonist and induced a reduction in IsAHP by about 70%. The additional ryanodine application (100 micro M) did not further affect IsAHP, thus excluding RyR2 in IsAHP activation. Our results argue in favor of a specialized function of RyR3 in CA1 pyramidal cells in triggering IsAHP due to their localization in the apical dendrite.

Mechanisms underlying activation of the slow AHP in rat hippocampal neurons

The firing of a train of action potentials in hippocampal pyramidal neurons is terminated by an afterhyperpolarization (AHP) that displays two main components; the medium AHP (I(mAHP)), lasting a few hundred milliseconds and the slow AHP (I(sAHP)), that has a duration of several seconds. It is unclear how much of I(mAHP) is dependent on the entry of calcium ions (Ca(2+)), whereas it is accepted that I(sAHP) is caused by activation of Ca(2+)-activated potassium channels. There has been controversy regarding the subcellular localization and mechanism of activation of these channels. Whole-cell recordings from CA1 neurons in the hippocampal slice preparation showed that inhibition of L-type, but not N-, P/Q-, T- and R-type Ca(2+) channels, reduced both I(mAHP) and I(sAHP). Inhibition of both AHP components by L-type Ca(2+) channel antagonists was not complete, with I(sAHP) being significantly more sensitive than I(mAHP). Somatic extracellular ionophoresis of BAPTA during I(sAHP) caused a transient inhibition, but had no effect on I(mAHP). Cell-attached patch recordings from the soma of CA1 neurons within a slice displayed channels that produced an ensemble waveform reminiscent of I(sAHP) when the patch was subjected to a train of action potential waveforms. The channels were Ca(2+)-activated, exhibited a limiting slope conductance of 19 pS and were not observed in dendritic membrane patches. These data demonstrate that the I(sAHP) is somatic in origin and arises from continued Ca(2+) entry through functionally co-localized L-type channels.

Relationships between intracellular calcium and afterhyperpolarizations in neocortical pyramidal neurons

We examined the effects of recent discharge activity on [Ca2+]i in neocortical pyramidal cells. Our data confirm and extend the observation that there is a linear relationship between plateau [Ca2+]i and firing frequency in soma and proximal apical dendrites. The rise in [Ca2+] activates K+ channels underlying the afterhyperpolarization (AHP), which consists of 2 Ca(2+)-dependent components: the medium AHP (mAHP) and the slow AHP (sAHP). The mAHP is blocked by apamin, indicating involvement of SK-type Ca(2+)-dependent K+ channels. The identity of the apamin-insensitive sAHP channel is unknown. We compared the sAHP and the mAHP with regard to: 1) number and frequency of spikes versus AHP amplitude; 2) number and frequency of spikes versus [Ca2+]i; 3) IAHP versus [Ca2+]i. Our data suggest that sAHP channels require an elevation of [Ca2+]i in the cytoplasm, rather than at the membrane, consistent with a role for a cytoplasmic intermediate between Ca2+ and the K+ channels. The mAHP channels appear to respond to a restricted Ca2+ domain.

Dependence on morphine leads to a prominent sharing among the different mechanisms of long-term potentiation in the CA1 region of rat hippocampus

Here, we examined chronic exposure to morphine to determine if this treatment shifted LTP mechanism in the CA1 field in vitro. Long-term potentiation (LTP) of population spikes induced by a 200 Hz theta pattern primed bursts (PBs) stimulation. Verapamil was used to isolate NMDA-dependent LTP. In control slices, a 200 Hz tetanus induced a compound potentiation, consisted of two pharmacologically separable components: nmdaLTP and vdccLTP. LTP in slices taken from morphine dependent rats was completely abolished by either APV or verapamil. These data suggest that morphine dependence in rats does not interfere with the induction and maintenance of hippocampal CA1 LTP. While in control rats both NMDA and voltage-dependent Ca(2+) channel (VDCC) antagonists must have been used concurrently to prevent the induction of LTP, in morphine-dependent rats, each of the antagonist could prevent the LTP induction suggesting a tighter coupling between these two calcium influx regulating processes.

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.

Gating properties of single SK channels in hippocampal CA1 pyramidal neurons

The activation of small-conductance calcium-activated potassium channels (SK) has a profound effect on membrane excitability. In hippocampal pyramidal neurons, SK channel activation by Ca2+ entry from a preceding burst of action potentials generates the slow afterhyperpolarization (AHP). Stimulation of a number of receptor types suppresses the slow AHP, inhibiting spike frequency adaptation and causing these neurons to fire tonically. Little is known of the gating properties of native SK channels in CNS neurons. By using excised inside-out patches, a small-amplitude channel has been resolved that was half-activated by approximately 0.6 microM Ca2+ in a voltage-independent manner. The channel possessed a slope conductance of 10 pS and exhibited nonstationary gating. These properties are in accord with those of cloned SK channels. The measured Ca2+ sensitivity of hippocampal SK channels suggests that the slow AHP is generated by activation of SK channels from a local rise of intracellular Ca2+.

Selective activation of Ca2+-activated K+ channels by co-localized Ca2+ channels in hippocampal neurons

Calcium entry through voltage-gated calcium channels can activate either large- (BK) or small- (SK) conductance calcium-activated potassium channels. In hippocampal neurons, activation of BK channels underlies the falling phase of an action potential and generation of the fast afterhyperpolarization (AHP). In contrast, SK channel activation underlies generation of the slow AHP after a burst of action potentials. The source of calcium for BK channel activation is unknown, but the slow AHP is blocked by dihydropyridine antagonists, indicating that L-type calcium channels provide the calcium for activation of SK channels. It is not understood how this specialized coupling between calcium and potassium channels is achieved. Here we study channel activity in cell-attached patches from hippocampal neurons and report a unique specificity of coupling. L-type channels activate SK channels only, without activating BK channels present in the same patch. The delay between the opening of L-type channels and SK channels indicates that these channels are 50-150 nm apart. In contrast, N-type calcium channels activate BK channels only, with opening of the two channel types being nearly coincident. This temporal association indicates that N and BK channels are very close. Finally, P/Q-type calcium channels do not couple to either SK or BK channels. These data indicate an absolute segregation of coupling between channels, and illustrate the functional importance of submembrane calcium microdomains.

Beta-adrenergic stimulation selectively inhibits long-lasting L-type calcium channel facilitation in hippocampal pyramidal neurons

L-type calcium channels are abundant in hippocampal pyramidal neurons and are highly clustered at the base of the major dendrites. However, little is known of their function in these neurons. Single-channel recording using a low concentration of permeant ion reveals a long-lasting facilitation of L-type channel activity that is induced by a depolarizing prepulse or a train of action potential waveforms. This facilitation exhibits a slow rise, peaking 0.5-1 sec after the train and decaying over several seconds. We have termed this behavior "delayed facilitation," because of the slow onset. Delayed facilitation results from an increase in opening frequency and the recruitment of longer duration openings. This behavior is observed at all membrane potentials between -20 and -60 mV, with the induction and magnitude of facilitation being insensitive to voltage. beta-Adrenergic receptor activation blocks induction of delayed facilitation but does not significantly affect normal L-type channel activity. Delayed facilitation of L-type calcium channels provides a prolonged source of calcium entry at negative membrane potentials. This behavior may underlie calcium-dependent events that are inhibited by beta-adrenergic receptor activation, such as the slow afterhyperpolarization in hippocampal neurons.

Age-dependent changes in voltage-gated calcium channels and ATP-dependent potassium channels in Fischer 344 rats

1. Radioligand binding and 45Ca2+ uptake measurements quantitated ion channel numbers and properties in brain membranes from Fischer 344 rats at 6, 12, 18 and 30 months of age. 2. Decreases in 1,4-dihydropyridine density occurred in striatum, hippocampus and cortex with a decreased affinity. 3. Decreases in w-conotoxin binding occurred in hippocampus and striatum with an increase in affinity. 4. K+ depolarization-mediated 45Ca2+ uptake decreased only in striatum at 18 months. 5. Decreases in glibenclamide binding occurred in cortex and cerebellum at 12-30 months. 6. No changes in 1,4-dihydropyridine binding occurred with age in heart, but glibenclamide binding density was significantly decreased at 30 months.

Different effects of omega-conotoxin GVIA at excitatory and inhibitory synapses in rat CA1 hippocampal neurons

The nature of the coupling mechanism of presynaptic calcium channels involved in the release of neurotransmitters in the mammalian central nervous system is unknown. Using intracellular recordings from CA1 neurons in the rat hippocampal slice preparation, we show that the N-type calcium channels antagonist omega-conotoxin GVIA (omega-CgTx) blocks partially the excitatory (EPSP) and totally the inhibitory (IPSP) synaptic transmission in CA1 hippocampal pyramidal neurons. In addition, the inhibitory effect of omega-CgTx on IPSPs is strongly depressed by intrahippocampal injection of PTX, while the effect on EPSP is not. The results suggest that the nature or the regulation of calcium channels might be different, depending on the location of these channels on excitatory or inhibitory terminals.

Presynaptic inhibitory effect of baclofen on hippocampal inhibitory synaptic transmission involves a pertussis toxin-sensitive G-protein

The involvement of a pertussis toxin (PTX)-sensitive G-protein in the activation of presynaptic GABAB receptor is controversial. In the present study, we reinvestigated the problem using intracellular recordings from CA1 neurons in rat hippocampus slices. We showed that the presynaptic inhibitory effect of baclofen is mediated differently at excitatory and inhibitory synapses. Excitatory (e.p.s.p.) and inhibitory (i.p.s.p.) postsynaptic potentials were strongly antagonized by baclofen in control rats. Three days after administration of PTX into the stratum radiatum of the hippocampus, the inhibitory effect of baclofen on i.p.s.p. was antagonized. In contrast, the inhibitory effect on e.p.s.p. was partly maintained. These results suggest that different sub-types of GABAB receptors exist on nerve terminals with different transduction mechanisms. GABAB receptors located on GABAergic inhibitory terminals are linked to a PTX-sensitive G-protein, whereas those located on excitatory terminals could consist of a PTX-sensitive type and a PTX-insensitive type. In addition, we showed that part of the inhibitory effect of baclofen at excitatory synapses is independent of omega-conotoxin (omega-CgTx)-sensitive N-type Ca2+ channels.