Verapamil blocks the afterhyperpolarization but not the spike frequency accommodation of rat CA1 pyramidal cells in vitro

Delayed effects of corticosterone on slow after-hyperpolarization potentials in mouse hippocampal versus prefrontal cortical pyramidal neurons

The rodent stress hormone corticosterone changes neuronal activity in a slow and persistent manner through transcriptional regulation. In the rat dorsal hippocampus, corticosterone enhances the amplitude of calcium-dependent potassium currents that cause a lingering slow after-hyperpolarization (sAHP) at the end of depolarizing events. In this study we compared the putative region-dependency of the delayed effects of corticosterone (approximately 5 hrs after treatment) on sAHP as well as other active and passive properties of layer 2/3 pyramidal neurons from three prefrontal areas, i.e. the lateral orbitofrontal, prelimbic and infralimbic cortex, with the hippocampus of adult mice. In agreement with previous studies, corticosterone increased sAHP amplitude in the dorsal hippocampus with depolarizing steps of increasing amplitude. However, in the lateral orbitofrontal, prelimbic and infralimbic cortices we did not observe any modifications of sAHP amplitude after corticosterone treatment. Properties of single action potentials or % ratio of the last spike interval with respect to the first spike interval, an indicator of accommodation in an action potential train, were not significantly affected by corticosterone in all brain regions examined. Lastly, corticosterone treatment did not induce any lasting changes in passive membrane properties of hippocampal or cortical neurons. Overall, the data indicate that corticosterone slowly and very persistently increases the sAHP amplitude in hippocampal pyramidal neurons, while this is not the case in the cortical regions examined. This implies that changes in excitability across brain regions reached by corticosterone may vary over a prolonged period of time after stress.

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.

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.

The Effect of Calcium Channel Antagonists on Spontaneous and Evoked Epileptiform Activity in the Rat Neocortex In Vitro

Calcium influx through voltage-activated calcium channels may play a crucial role in the propagation and maintenance of seizure activity. We have examined the contribution of various types of calcium currents to epileptogenesis by studying the effects of various calcium channel blockers on epileptiform activity. N-methyl-d-aspartate receptor-mediated epileptiform activity was induced by removal of magnesium ions superfusing the cortex, or by low-frequency stimulation of the underlying white matter. CoCl2, CdCl2 and omega-conotoxin, acting at the N- and L-type calcium channels, significantly reduced epileptiform activity. L-channel antagonists nifedipine and verapamil, and the agonist BAY K 8644, increased spontaneous bursting in cortical wedges, but had no effect upon evoked activity. The T-channel blocker NiCl2 had variable effects on epileptiform activity, whereas phenytoin consistently reduced such activity. These results suggest that calcium influx underlying epileptiform activity in the rat neocortex may occur at least partially via the activation of the N-type calcium channel. However, contributions from other calcium channel types cannot be excluded.

Contribution of L-type calcium channels to epileptiform activity in hippocampal and neocortical slices of guinea-pigs

The aim of the present investigation was to compare the antiepileptic efficacy of the specific L-type calcium channel blocker nifedipine in hippocampal and neocortical slice preparations in the Mg2+-free model of epilepsy. The main findings were as follows. (1) In hippocampal slices, in general, nifedipine (20-80 micromol/l) exerted a suppressive effect both on repetition rate and on area under epileptiform field potentials. This effect was clearly dose dependent. In the majority of cases, this suppression was preceded by an increase, which was transient in nature. Only in the lowest concentration (20 micromol/l) used, in normal K+, instead of a depression, a persistent increase occurred. (2) In neocortical slices, in the majority of experiments, nifedipine (20-80 micromol/l) showed a depressive action only on the area under the epileptiform field potentials. The depressive effect of nifedipine on the area was dose dependent, although to a lesser extent than in the hippocampus. In nearly half of the slices this suppression was preceded by a transient increase. By contrast, the repetition rate of epileptiform field potentials increased transiently in about 20% of the slices followed by a decrease. In the remaining 80% of the slices the repetition rate increased persistently. (3) An elevation of the K+ concentration accentuated the depressive actions of nifedipine only in the hippocampus. In contrast to elevated K+, in both the hippocampus and the neocortex, epileptiform field potentials were not suppressed in all experiments in normal K+. (4) The reversibility of the depressive effects of nifedipine was differential in the two tissue types. In the hippocampus, after suppression of epileptiform field potentials they reappeared in the overwhelming majority of slices. In the neocortex, this was the case in only one experiment. These findings may indicate the existence of L-type calcium channels with a differential functional significance for epileptogenesis and/or the existence of different forms of L-type channels in hippocampal and neocortical tissue. As a whole, the differential effects of L-type calcium channel blockade in the hippocampus and neocortex point to differences in the network properties of the two tissue types.

Evidence of increased excitability in GEPR hippocampus preceding development of seizure susceptibility

The genetically epilepsy-prone rat (GEPR) provides a valuable model to study the mechanism of neonatal seizure susceptibility because seizure predisposition in GEPRs is determined by factors present from birth. We have previously shown that reduced afterhyperpolarization (AHP), reduced spike frequency adaptation and increased excitation with repetitive stimulation are present in the adult GEPRs. To investigate whether these abnormalities are present at birth or appear at the time when GEPRs show seizure susceptibility and to elucidate whether these abnormalities were a consequence of seizure experience (the adult rats previously tested were induced to seize in three tests), we studied the membrane and synaptic properties of CA3 hippocampal neurons in preseizing offspring of GEPR-9s (seizure naive GEPRs). Electrophysiological recordings were done in the in vitro brain slice preparation during three different stages of early postnatal development (postnatal day (P) 7-10, P12-15 and P18-28) in GEPRs and compared to age matched control Sprague-Dawley (SD) rats. Reduction in AHP amplitude and duration and reduced inhibitory post synaptic potentials (IPSPs) were observed in the CA3 region in all the three stages tested. Reduction in spike frequency adaptation in 40% of CA3 neurons and reduction in fast AHP occurred in the 3rd and 4th weeks of postnatal development in GEPRs. Therefore, our results suggest that reduced synaptic inhibition and increased membrane excitability in the CA3 circuitry are present from early postnatal development and may represent few of the general cortical features that might eventually contribute to development of enhanced seizure susceptibility in developing GEPRs.

The excitability of CA1 pyramidal cell dendrites is modulated by a local Ca(2+)-dependent K(+)-conductance

Intracellular recordings are made from distal apical dendrites of CA1 pyramidal neurones in the rat hippocampal slice preparation. Injection of a threshold current evoked two predominant firing patterns: fast spiking and compound spiking. Suprathreshold current injection evoked high frequency dendritic spiking followed by a pronounced slow afterhyperpolarization (sAHP(dend)) lasting for several hundred milliseconds, during which spiking was inhibited for a variable period. In fast spiking dendrites, the size of the sAHP(dend) depended on the number and frequency of preceding spikes, whereas, in compound spiking dendrites, it was more closely related to the size and duration of preceding Ca(2+)-spikes. During the peak of the sAHP(dend), the membrane conductance was increased by 56%. The sAHP(dend) was blocked by perfusion with Ca2+ and by intradendritic injection of ethyleneglycol-bis-(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA; 0.01 or 0.2 M), indicating that the activation of the sAHP(dend) depends on a rise in intradendritic Ca2+. The sAHP(dend) was also blocked by low concentrations (0.5-1 microM) of carbachol. The data presented here therefore, provide strong evidence that the sAHP(dend) is due to the activation of a local Ca(2+)-dependent K(+)-conductance. Possible implications of a dendritic Ca(2+)-dependent K(+)-conductance for the integration of synaptic potentials are discussed.

Evidence for decreased calcium dependent potassium conductance in hippocampal CA3 neurons of genetically epilepsy-prone rats

The genetically epilepsy-prone rat (GEPR) has become an important model to study genetic predisposition to epilepsy involving not only the brainstem but also forebrain structures. Previous work in CA1 hippocampal cells showed a reduction in spike frequency adaptation and only subtle changes in slow afterhyperpolarization (AHP). As important differences exist in calcium dependent potentials in the CA1 and CA3 hippocampal cells, we compared the membrane properties of hippocampal CA3 cells in GEPRs and Sprague-Dawley (SD) rats. There was no significant difference in the resting membrane potential, input resistance, charging time constant or rheobase between GEPRs and SD rat neurons. The action potential amplitude and the width at half maximal amplitude did not differ. A marked reduction in spike frequency adaptation accompanied by a very significant reduction in AHP was seen in the GEPR rats. Since calcium dependent potassium conductance produces both spike frequency adaptation and AHP, our results suggest that this conductance is reduced in the GEPR CA3 neurons.

Role of NMDA receptors and voltage-activated calcium channels in an in vitro model of cerebral ischemia

In an in vitro model of cerebral ischemia we investigated the functional consequences of repeated hypoxias and the potential protective effect of the N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphonovaleric acid (D-APV) and the calcium channel blocker verapamil in preventing the expression of pathophysiological activity. Rat neocortical slices were exposed to nitrogen for 2-13 min and the hypoxia-induced functional modifications were monitored in layer II/III by recording the extracellular DC potential, the extracellular calcium concentration ([Ca2+]o) and the stimulus-evoked synaptic responses. Hypoxia caused a reversible 2.4-24.6 mV negative shift in the extracellular DC potential associated with a [Ca2+]o decrease from 1.2 to 0.2 mM and a complete loss of synaptic responsiveness. Repeating hypoxias induced an increase in the amplitude of this anoxic depolarization (AD) and a significant decrease in the AD onset latency. Synaptic responses partially recovered at 20 and 60 min intervals between subsequent hypoxic periods, indicating that the initial AD did not induce any short-term irreparable functional deficits. Verapamil (50 microM) caused an increase in the AD onset latency. However, in comparison to untreated controls, verapamil induced a reduction of excitatory and inhibitory responses during hypoxia probably by blocking voltage-activated calcium conductances. In addition, verapamil did not have any significant effect on the hypoxia-induced reduction of [Ca2+]o. Bath application of D-APV (30 microM) prevented the significant reduction in the AD onset latency to the second hypoxia, but had no significant effect on the AD amplitude and duration. The hypoxia-induced decrease in [Ca2+]o was not altered after addition of D-APV to the bathing medium. These data indicate that the influx of calcium through voltage-activated calcium channels and the NMDA receptor-gated ionophore does not significantly contribute to the massive depolarization observed under hypoxic conditions.

Electrophysiology and burst-firing of rat subicular pyramidal neurons in vitro: a comparison with area CA1

Intracellular recordings were made from subicular and CA1 neurons in slices of the ventral hippocampal and parahippocampal region of the rat. All of the subicular cells that were stained by intracellular injection of biocytin were pyramidal in form. Although most electrophysiological properties were similar in the two areas, in response to depolarising current injection, the majority of subicular cells displayed a distinctive pattern of burst-firing which was rarely seen in CA1 cells. Burst-firing was voltage sensitive but was not abolished by blocking excitatory synaptic transmission, suggesting that it is an intrinsic membrane property of subicular pyramidal cells.

Trans-ACPD-induced phosphoinositide hydrolysis and modulation of hippocampal pyramidal cell excitability do not undergo parallel developmental regulation

The selective metabotropic glutamate receptor agonist, trans-1-aminocyclopentane-1,3-dicarboxylic acid (trans-ACPD), stimulates phosphoinositide hydrolysis and elicits a number of electrophysiological responses in the hippocampus. If these effects are mediated by the same receptor subtype, they should undergo parallel developmental regulation. Therefore, we compared the phosphoinositide hydrolysis response and the electrophysiological responses to trans-ACPD at two different developmental stages. Trans-ACPD-stimulated phosphoinositide hydrolysis was significantly greater in hippocampal slices from immature (6-11-day-old) rats than from adults. In contrast, trans-ACPD elicited decreases in spike frequency adaptation and in the amplitude of the slow afterhyperpolarization in roughly equal percentages of immature and adult CA1 pyramidal cells. Similar results were obtained using the putative endogenous agonist, glutamate. These data support the hypothesis that certain electrophysiological effects of trans-ACPD are mediated by a metabotropic glutamate receptor that is distinct from the phosphoinositide hydrolysis-linked glutamate receptor.

Suppression of 4-aminopyridine-induced paroxysmal depolarizing shift in rat amygdaloid neurons by diltiazem

The effects of organic Ca2+ channel blocker, diltiazem, on the epileptiform activity induced by 4-aminopyridine (4-AP) were studied in rat amygdaloid slices using intracellular recording techniques. Application of 4-AP (0.5 mM) resulted in spontaneous and evoked epileptiform activity which consisted of an initial burst followed by a number of afterdischarges. The initial burst began with rapidly rising action potentials superimposed on a large depolarizing wave termed paroxysmal depolarizing shift (PDS). Diltiazem reversibly suppressed the amplitude and duration of PDS in a concentration-dependent manner. The IC50, estimated from the graph of the concentration-response relationship, was approximately 60 microM. These results demonstrate that a calcium current sensitive to diltiazem is involved in the generation of PDS and suggest that PDS is based on giant synaptic conductance as well as endogenous calcium currents.

Potassium currents in hippocampal pyramidal cells

The hippocampal pyramidal cells provide an example of how multiple potassium (K) currents co-exist and function in central mammalian neurones. The data come from CA1 and CA3 neurones in hippocampal slices, cell cultures and acutely dissociated cells from rats and guinea-pigs. Six voltage- or calcium(Ca)-dependent K currents have so far been described in CA1 pyramidal cells in slices. Four of them (IA, ID, IK, IM) are activated by depolarization alone; the two others (IC, IAHP) are activated by voltage-dependent influx of Ca ions (IC may be both Ca- and voltage-gated). In addition, a transient Ca-dependent K current (ICT) has been described in certain preparations, but it is not yet clear whether it is distinct from IC and IA. (1) IA activates fast (within 10 ms) and inactivates rapidly (time constant typically 15-50 ms) at potentials positive to -60 mV; it probably contributes to early spike-repolarization, it can delay the first spike for about 0.1 s, and may regulate repetitive firing. (2) ID activates within about 20 ms but inactivates slowly (seconds) below the spike threshold (-90 to -60 mV), causing a long delay (0.5-5 s) in the onset of firing. Due to its slow recovery from inactivation (seconds), separate depolarizing inputs can be "integrated". ID probably also participates in spike repolarization. (3) IK activates slowly (time constant, tau, 20-60 ms) in response to depolarizations positive to -40 mV and inactivates (tau about 5s) at -80 to -40 mV; it probably participates in spike repolarization. (4) IM activates slowly (tau about 50 ms) positive to -60 mV and does not inactivate; it tends to attenuate excitatory inputs, it reduces the firing rate during maintained depolarization (adaptation) and contributes to the medium after-hyperpolarization (mAHP); IM is suppressed by acetylcholine (via muscarinic receptors), but may be enhanced by somatostatin. (5) IC is activated by influx of Ca ions during the action potential and is thought to cause the final spike repolarization and the fast AHP (although ICT may be involved). Like IM, it also contributes to the medium AHP and early adaptation. It differs from IAHP by being sensitive to tetraethylammonium (TEA, 1 mM), but insensitive to noradrenaline and muscarine. Large-conductance (BK; about 200 pS) Ca-activated K channels, which may mediate IC, have been recorded. (6) IAHP is slowly activated by Ca-influx during action potentials, causing spike-frequency adaptation and the slow AHP. Thus, IAHP exerts a strong negative feedback control of discharge activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Membrane currents in hippocampal neurons

This chapter reviews properties and functions of endogenous ionic currents in hippocampal neurones. Currents considered are: Na currents INa(fast) and INa(slow); Ca currents; K currents--delayed rectifier IK(DR), transient IK(A), 'delay' current IK(D) and M current IK(M); inward rectifiers IQ, IK(IR) and ICl(V); Ca-activated currents IK(Ca) (IC and IAHP), ICl(Ca) and Ication(Ca); Na-activated currents; and anoxia-induced currents.