Jasper's Basic Mechanisms of the Epilepsies Jasper's Basic Mechanisms of the Epilepsies Perturbations of Dendritic Excitability in Epilepsy

Cha-Min Tang1 Scott M. Thompson2 1 Departments of Neurology and Physiology, University of Maryland School of Medicine; Baltimore VA, Medical Center 2 Department of Physiology, University of Maryland School of Medicine

Abstract The dendritic arbor is the site of complex interactions between synaptic excitation and intrinsic excitability. It has become apparent recently, that the dendritic arbor cannot be viewed as a passive antenna that simply receives and relays synaptic input to the cell body. Instead, dendrites express an abundance of voltage-gated channels that are capable of initiating regenerative spikes and actively regulate the local dendritic resting membrane potential. Active properties can be expressed as back- propagating action potentials along the main apical trunk and as localized spikes confined to individual terminal dendritic segments. The notion of the dendritic arbor as a highly active structure has profound implications for the generation of epilepsy. This chapter will focus on recent data on perturbations to dendritic intrinsic excitability associated with epileptic conditions. An attempt will be made to understand how hyperexcitability may be the result of maladaptive homeostatic mechanism. Topics to be addressed relevant to the functional reorganization of dendrites include activity-dependent down regulation of IA (Kv4.2), epilepsy induced down regulation of Ih (HCN1 and HCN2), and deafferentation induced down regulation of SK-type potassium channels. Other topics to be addressed include the concept of electrical compartmentalization within the dendritic arbor and the recruitment of NMDA receptors as part of intrinsic excitability. A clearer understanding of dendritic mechanisms of neuronal hyperexcitability may offer novel insights for therapeutic interventions.

The dendrite is where the thousands of excitatory and inhibitory synaptic inputs are received by the . But rather than just being a simple antenna, the dendrite is also where these inputs actively interact with intrinsic conductances. The interactions are complex and still incompletely understood. The synaptic inputs are distributed in time and space, and each of the many intrinsic dendritic conductances are also distributed in their own unique spatial pattern. The interactions lead to signal transformations whose significance may be best appreciated in terms of elementary steps in signal processing and computation. Under pathological conditions, changes to these interactions may result in aberrant excitability and contribute to neurological disease.

Rather than compiling a list of dendritic conductances and their linkages with epilepsy, which is done in other chapters of this book, the focus of this chapter is to integrate these results with an emphasis on how perturbations of the elementary steps in dendritic integration affect the way process their inputs and promote aberrant neuronal excitability.

A brief history of the active dendrite Our view of the role of dendrites in epilepsy has evolved with increased understanding of the roles dendrites serve in normal physiology. For most of the last century, the dendritic arbor was viewed as a structure that gathered and faithfully funneled inputs to the soma and axon hillock, where non-linear processing - in the form of action potential initiation - was thought to reside. This view of the dendrite as a passive antenna would not suggest that the dendrite Page 2

plays a pivotal role in neuronal hyperexcitability. The possibility that apical dendrites of pyramidal neurons are active was first demonstrated by Spencer and Kandel when they described events they called “fast prepotentials” (1). Similar “dendritic spikes” were reported

Jasper's Basic Mechanisms of the Epilepsies Jasper's Basic Mechanisms of the Epilepsies from neocortical neurons by Purpura (2). The concept of the active dendrite was further advanced with demonstration of calcium electrogenesis in dendrites by Llinas, Schwartzkroin, Wong, Prince, and Sakmann (3–6). Magee and Johnston further extended the concept of the active dendrite by demonstrating high levels of expression of sodium channels throughout the apical trunk in hippocampal pyramidal neurons (7). Simultaneously, Stuart and Sakmann and others showed that action potential can propagate bidirectionally in the dendritic arbor (8). This bidirectional signaling provided the means for coincidence detection, which was postulated to serve an important role in triggering acute and long term changes in excitability such as spike timing dependent plasticity (STDP) (9). Several others in Johnston’s and Sakmann’s groups also investigated the properties of calcium electrogenesis at the apical tuft region of layer 5 pyramidal neurons, showing that these calcium channels help amplify synaptic inputs from the more distal branches (10–12) and that back propagating action potentials (bAPs) can trigger calcium spikes localized to the apical tuft (13). Next, Schiller and colleagues demonstrated the ability of NMDA receptors to generate local spikes on tertiary basal dendrites of cortical pyramidal neurons (14). The capacity of NMDA receptors for regenerative depolarization under appropriate conditions provided an additional mechanism of excitability that is unique to dendrites and not observed in other excitable membranes such as axons and muscle. Thus, over a span of 20 to 30 years, the view of the dendrite as a passive antenna evolved to that of a highly excitable structure that rivaled the soma. This change in perspective also led to a greater interest and understanding of the role of dendritic excitability in epilepsy.

Electrical compartmentalization of dendrites: the intersection of form and function The organization of this chapter is based on the premise that dendrites serve signal processing and computational needs that are fundamental to the function of nervous system. Because non- trivial computation generally requires some form of non-linear operation, it is informative to examine the source of non-linearity in dendritic integration. One obvious source of non- linearity is active conductances. A less obvious but important determinant of nonlinearity is electrical compartmentalization.

Electrical compartmentalization is like the parenthesis in mathematics. It separates those variables that are included in an operation from those that are excluded. The ability to segregate variables and “bind” them for non-linear operations is as fundamental to orderly signal processing as it is in mathematics. At the level of the dendrite, binding of synaptic inputs can be implemented by electrical compartmentalization within different regions of the dendritic arbor and individual dendritic segments (16–18). Electrical compartmentalization can be functionally defined as a condition in which local interactions within a confined space occur quasi-independently from the influence of the rest of the cell. At rest, all dendritic regions are close to isopotential. During activity, a region of the dendritic arbor (i.e., a “compartment”) may be at a membrane potential that is significantly different from the potential of other nearby dendrites and the cell body. For example, nearly synchronous activity in a set of synaptic inputs that arrive on a single dendritic segment can trigger a variety of active electrical responses, such as plateau potentials (16). These responses functionally bind together the original inputs and are amplified and transmitted to the soma reliably. In contrast, inputs arriving on different dendritic branches, or arrive with insufficient temporal synchrony to allow such binding, will fail to trigger active dendritic responses; consequently, they will not be amplified and will exert lesser influence on the output of the neuron.

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Electrical compartmentalization is determined by three factors: the passive electronic properties of the dendritic arbor; the expression of active conductances on these structures; and the temporal-spatial pattern of excitation. Factors favoring compartmentalization within the

Jasper's Basic Mechanisms of the Epilepsies Jasper's Basic Mechanisms of the Epilepsies dendritic tree are high output impedance of the dendritic segment and high expression of regenerative voltage-dependent conductances. Electrical compartmentalization can be dynamic, and changes in the extent of ongoing synaptic activity(both excitatory and inhibitory) can change the impedance of the dendrite. Because the voltage-dependent blockade of NMDA receptors by Mg2+ can effectively add a non-linear excitability to the dendritic, the tempo- spatial pattern of synaptic excitation within a dendritic domain is a strong determinant of the extent of the compartment.

Two functional dendritic compartments will be discussed in detail in the context of epileptogenesis. One is the compartment comprising of the individual terminal dendritic branches, which receive >80% of synaptic inputs to pyramidal neurons (19). The other is the region on the distal apical trunk, near the base of the apical dendritic tuft, which has a high propensity for generating calcium spikes (6,20).

Experimental observation has shown that distal and proximal dendritic compartments express distinct intrinsic conductances. In fact, dendritic information processing can be modeled as a two layer system of distal and apical domains, each with distinct functions that are connected to the soma (15).

The terminal dendrite as an electrical compartment The terminal dendrite of cortical and hippocampal pyramidal neurons has attributes that favor electrical compartmentalization. Contrary to frequently assumed belief, the dendritic arbor of pyramidal neurons does not mimic the arbor of a typical tree. Whereas the diameter of branches of a tree gradually taper with each division and become progressively shorter, the diameter of the terminal dendritic branch of pyramidal neurons is markedly thinner than its parent branch and the terminal branch is typically the longest segment of the dendritic arbor (20). Interestingly, the diameter of the terminal dendrite is not tapered, as distinct from the apical trunk or the branches of tree; it is <1um in diameter both where it joins the main apical trunk and at its distal tip 100 um away. This geometry is well suited for electrical compartmentalization, because synaptic currents are gathered from a large surface area within a high resistance anatomically defined region. Thus, even a relatively modest level of synaptic current flowing into the long terminal dendritic process produces a significant depolarization over a considerable portion of its length. This depolarization can then secondarily trigger the activation of voltage-dependent sodium and calcium conductances, and the unblocking of glutamate bound - but Mg2+-blocked - NMDA receptors (14,16). The result is a regenerative spike in the thin terminal dendrite that appears as a slow plateau potential at the soma (Figure 1A). Calcium imaging suggests that the spike originates from and is largely confined to the terminal dendritic compartment (Figure 1B).

Figure 1. Compartmentalized and spiking property of terminal dendrites

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(A) Progressive increase in the strength of glutamate photolysis stimulus directed at a thin terminal dendrite leads to a non-linear response. (B) Calcium imaging with Fluo-3 during such a plateau potential reveals that the electrogenesis is largely confined to the terminal dendritic compartment.

Jasper's Basic Mechanisms of the Epilepsies Jasper's Basic Mechanisms of the Epilepsies The regenerative depolarization of these thin distal processes is mediated by a combination of NMDA and voltage-gated channels. The relative contribution of NMDA and voltage-gated calcium conductances is hard to determine precisely. We will therefore use the term plateau potential to refer these regenerative all-or-none depolarizations. The impedance mismatch of the terminal dendrite where it joins the main apical trunk results in a marked attenuation of the distal depolarization - which allows the terminal dendrite to function in a quasi-independent manner from the apical trunk.

Perturbations affecting the thin distal electrical compartment What factors limit the generation of dendritic plateau potentials? The answer includes both extrinsic and intrinsic conductances. Modeling of the excitability of the thin terminal dendrites shows that the level of GABAergic inhibition is a very potent modulator of plateau potential generation (21). A local inhibitory current 20-fold lower in amplitude than that of the current underlying the plateau potential is sufficient to prevent its initiation. Further, the impact of a conductance on excitability of the dendrite will depend, in part, on the level of its expression relative to the input impedance/leak conductance of the electrical compartment. The steepness of the negative slope of the current-voltage activation relationship of the NMDA and voltage- dependent Ca2+ channels (VDCC) that underlie plateau potential generation are also critical. One possible explanation for the extraordinary sensitivity of the plateau potential to inhibition is that the negative slope of the NMDA and VDCC current-voltage activation relationship is not steep compared to sodium channels at the axon initial segment. Intrinsic inhibitory voltage- dependent conductances may similarly exert powerful control over the excitability of the terminal dendrites. These conductances include IA and Ih, which are activated at or near rest, and SK-type calcium-activated K+ channels, which are activated subsequent to depolarization. All three channel types are found in abundance in the thin distal dendrites. This distribution of channels creates a system which is normally well balanced, but is vulnerable to over-excitation when inhibition is reduced. Thus, factors that attenuate IA, Ih, and ISK will lead to aberrant excitability of distal dendrites which subsequently feed into the burst generating apical trunk compartment of the dendritic arbor.

Attenuation of IA lowers the threshold for regenerative spiking + IA is a rapidly inactivating, voltage-gated K conductance that is mediated by Kv4.2 potassium channels in dendrites. Its expression on apical trunk dendrites has been directly measured by patch clamp methods and is found to increase with distance from the soma, up to 22 at least 350 um ( ). On the thin terminal dendrite, however, the expression of IA must be inferred by indirect means because it is not possible to record from these thin structures directly. Fluorescent monitoring of intracellular calcium levels in response to somatically evoked bAPs 23 has provided a convenient indirect measure for the presence of IA ( ). Under control conditions, the bAP-evoked calcium signal in thin oblique dendrites was similar to that observed in the apical trunk. After application of the IA blocker, 4-aminopyridine (4AP) the calcium signal was markedly increased in the oblique dendrites to a much greater extent than 23 in the trunk dendrite ( ). This observation confirms that IA is expressed on the oblique dendrites. But it may lead to an over estimation of the density of IA in the oblique dendrite, because IA can exert highly non-linear control over the generation of NMDA and calcium channel dependent spikes (Figure 2). Because the threshold for NMDA spike generation is significantly lower in the oblique dendrites compared to the main apical trunk due to cable properties, increased sensitivity to IA as indirectly monitored by calcium influx does not necessarily mean there is increased expression of IA in the oblique dendrites. Regardless of the absolute density of IA on the thin terminal dendrites, IA exerts powerful control over

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regenerative excitation. Indeed, when NMDA receptor-mediated glutamate responses in thin oblique dendrites are examined in the presence of an AMPA receptor antagonist (Figure 2B), the effect of removing the inhibitory influence of IA (by applying 4AP) has a much more

Jasper's Basic Mechanisms of the Epilepsies Jasper's Basic Mechanisms of the Epilepsies dramatic effect on regenerative spike initiation than on AMPA mediated depolarization (Figure 2A).

Figure 2. Disparate effect of IA on regenerative and non-regenerative dendritic excitation A. A hippocampal CA1 oblique dendrite in an acute hippocampal slice is stimulated by photolysis of caged glutamate at two intensities in the presence of a NMDA receptor antagonist (AP5) (black). The stimulation is then repeated in the presence of 4AP (red). B. An oblique dendrite is similarly stimulated at two intensities in the presence of an AMPA receptor antagonist (NBQX) (black) and then repeated in the presence of 4AP (red).

IA is a highly regulated conductance in dendrites. Its activity is altered by the recent history of activation, by the baseline membrane potential, and by the activity of a number of protein kinases ( 24–26). IA inactivates rapidly at voltages close to the resting membrane potential. High frequency inputs will therefore inactivate IA and release the regenerative capacity of the terminal dendrites from its inhibitory influence. High frequency inputs are thus far more 27 effective at triggering plateau potentials ( ). Tight regulation of IA provides hints to its central role in controlling excitability within the dendritic arbor and the development of short- and 28 long-term plasticity ( ). Most recent attention has been focused on the role played by IA in the apical trunk in regulating the extent to which somatic action potentials can back propagate into the distal dendritic arbor. In addition to changes in bAP propagation, however, it is important to remember that changes in the properties of IA can also promote epilepsy by altering the compartmentalization and regenerative activity of distal dendrites.

Factors that attenuate or downregulate the activity of IA are well recognized as proconvulsants. 29 Blockade of IA with 4AP is a well established in vitro model of epilepsy ( ). Local application of 4AP to the distal of CA1 pyramidal neurons cause the afterhyperpolarization to change to a afterdepolarization and the normal single evoked spike to change to bursts of 30 spikes ( ). In the pilocarpine model of TLE, attenuation of IA and increased excitability of pyramidal cell dendrites is believed to be secondary to an ERK-dependent phosphorylation of the Kv4.2 channels (31). In another study of the chronic phase of the Li-pilocarpine animal model, there is a down regulation of Kv4.2 channel expression (32). Surgical tissue obtained from patients with hippocampal sclerosis showed decreased immunoreactivity to Kv4.2 in the dendritic region of the CA1 and CA3 (33), but increased immunoreactivity in the soma of the surviving neurons. Unfortunately, the limited resolution of the light microscopic methods does not allow a determination of whether the decrease was localized to the apical trunk or the thin tertiary branches, or whether it was due to an overall loss of dendrites.

While these studies show solid correlation between attenuated dendritic IA and epilepsy, one is still faced with the classic dilemma of not knowing whether the change in IA is the cause or

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the result of epilepsy. It remains important to determine whether the perturbation is localized to the apical trunk, to the terminal dendrite, or throughout the dendritic arbor. The action of IA on the back propagation of action potentials and the interaction between the soma and apical Jasper's Basic Mechanisms of the Epilepsies Jasper's Basic Mechanisms of the Epilepsies tuft is well documented and will be discussed later. Whether perturbation of IA on the terminal dendrites contributes significantly to the epileptic conditions described above is not know. The observations shown in Figure 2 provide one plausible proconvulsant mechanism. IA can regulate the threshold for regenerative NMDA and calcium spikes. This issue requires further investigation, for if hyperexcitability were to originate from the terminal dendrite, it would involve mechanisms that are distinct from action potentials propagating backwards from the apical trunk and soma.

Downregulation of Ih increases input impedance

Ih is a mixed sodium and potassium cation conductance activated by membrane hyperpolarization and gated by intracellular cyclic-nucleotides. Ih is mediated by the HCN family of ion channels, which are highly expressed on distal dendrites of CA1 and cortical 34 35 pyramidal neurons ( , ). In conjunction with potassium channels, Ih controls the resting membrane potential especially for dendrites of pyramidal neurons further away from the soma (36). Activation of this conductance leads to membrane depolarization with a reversal potential near -30mV. Because its activation would be expected to move the membrane potential towards -30 mV, it may be considered excitatory. But Ih also significantly reduces the input impedance of the distal dendrites, thereby decreasing the efficacy of excitatory synaptic inputs. In this respect, Ih may be considered a stabilizing or inhibitory conductance. These opposing actions on membrane excitability have led to conflicting opinions as to the role of Ih in epilepsy. Whether the net effect of Ih is an increased or decreased excitability depends on a combination of factors, including the preexisting input impedance, resting membrane potential, membrane time constant, timing of synaptic inputs, neuronal subtype, and the age of the animal (37; see review chapter by Poolos in this edition). Like IA, Ih may serve different roles in different locations of the neurons. Because of the high baseline input impedance of the distal dendritic compartment, the stabilizing action of increased shunting by Ih is likely to be more important than its depolarizing action. Ih -mediated decreases in input impedance attenuates the depolarization of glutamatergic inputs and speeds the decay of excitatory synaptic events. The net result is reduced summation of EPSPs (34,38).

A large body of evidence supports the idea that Ih is predominately a stabilizing conductance with respect to epilepsy. In the kainate model of temporal lobe epilepsy, decreased Ih conductance is linked to the latent period of epileptogenesis (39). In a hippocampal deafferentation model of epilepsy created by lesioning the entorhinal cortex, there is a decrease in expression of HCN1 channels (40). Also consistent with this notion is the observation that 41 42 two anticonvulsants, lamotrogine and acetazolamide, both enhance Ih ( , ), although questions remain about whether it is their action on Ih that provides them with their anti- 43 convulsant actions ( ). The more general question of whether promoting Ih is pro- or anti- convulsant does not have a simple answer. Different seizure types respond differently to Ih modulation. In the rat febrile seizure model, Ih is enhanced and its blockade reduces 43 hyperexcitability ( ). Interestingly, Ih has also been implicated in thalamically-generated absence epilepsy. In a genetic model of absence epilepsy in rats, a rapid decline in the 44 expression of HCN1 channels precedes the onset of seizures ( ). Similarly, enhancing Ih prevents experimentally induced thalamic hyperexcitability (45).

Another possibility is that Ih serves as a shunt to maintain the membrane potential of distal dendrite at a steady value and decreases its time constant. This action may be beneficial by acting as a bias current to regulate the voltage-dependent block of NMDA receptors by Mg2+. Because of the high affinity of NMDA receptors for glutamate, subthreshold synaptic

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excitation of terminal dendrites results in an appreciable fraction of NMDA receptors on that branch that are in the bound-but-blocked state. In this state, the NMDA receptor is not conducting current; however, it reverts to a conducting state if the membrane becomes

Jasper's Basic Mechanisms of the Epilepsies Jasper's Basic Mechanisms of the Epilepsies depolarized sufficiently. Ih is ideally suited for such a depolarizing purpose. Because the bound- but-blocked state of the NMDA receptor contains information about the recent activity at that receptor (dating back hundreds of milliseconds), it may provide an energy-efficient and high capacity mechanism for short term memory (46).

Down regulation of SK channels leads to prolonged spikes in terminal dendrites

IA and Ih play a role in limiting dendritic depolarization and the initiation of regenerative dendritic events such as plateau potentials. SK type Ca2+ activated K+ channels, in contrast, are critical for termination of plateau potentials. Although single Ca2+ activated K+ channels have not been observed in recordings from the main apical trunk, there is evidence that Ca2+ activated K+ channels are expressed on oblique and the thin terminal dendrites. SK-type channels have also been detected in dendritic spines, where they are activated by NMDA receptor-mediated Ca2+ influx (47). Activation of voltage-dependent calcium channels, and relief of NMDA channel block by Mg2+ during the depolarization of the plateau potential, lead to large and relatively long-lasting elevations of the intracellular concentration of Ca2+ at the site of active initiation (16). This elevated Ca2+ activates SK-type channels that are responsible for termination of the plateau potential (27). Addition of either the toxin, apamin, a selective SK channel blocker, or chelation of intracellular calcium, markedly increases the duration of the plateau potential, without affecting its amplitude.

We have obtained evidence that alterations in SK channel function may contribute to epileptogenesis after traumatic brain injury. In many neurological conditions associated with abnormal excitability, such as posttraumatic epilepsy, neural damage leads to cellular degeneration and loss of nerve tracts. In addition, rapid acceleration/deceleration of the brain leads to axonal injury due to damaging shear forces. One consequence of these injuries is to produce chronic partial deafferentation of large populations of neurons. In a study of the effects of chronic deafferentation resulting from Schaffer collateral transections in hippocampal slice cultures, we observed increased excitability in area CA1 beginning with a delay of several days after transection. Comparable hyperexcitability is observed in the neocortex after deafferentation and axonal injury produced by cortical undercuts (48). Hyperexcitability in denervated CA1 cells is accompanied by a marked prolongation of plateau potentials due to a functional down-regulation of repolarizing SK channels (49). The molecular mechanisms remain unclear, but decreases in mRNA levels or protein expression could not be detected, suggesting a posttranslational regulation of channel conductance, trafficking, or Ca2+ sensitivity. Interestingly, SK channel enhancers such as EBIO have recently been shown to be effective against an in vitro model of epilepsy (50)

Why does prolongation of plateau potentials lead to hyperexcitability? EPSPs occurring in distal thin and oblique dendrites have a relatively low probability of triggering action potentials, compared to main apical dendrites (50). Plateau potentials in terminal apical dendrites or oblique dendrites elicit action potentials in <10% of trials in normally innervated CA1 cells. Similarly, strong activation of temporoammonic inputs to distal apical dendrites in str. lacunosum/molecular is relatively ineffective in triggering somatic action potentials (51). Both pharmacological prolongation of plateau potentials with SK channel blockers and deafferentation-induced prolongation of plateau potentials facilitate action potential initiation markedly (27). One week after deafferentation, >80% of terminal apical and oblique dendrites display action potentials as the result of plateau potential initiation. Although action potential initiation can occur in apical trunk dendrites, the threshold for initiation is higher than at the soma and axon hillock (52). Facilitation of action potential initiation by prolonged plateau

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potentials in chronically deafferented dendrites results from a lowering of the effective threshold for dendritic initiation of action potentials. In a population of neurons coupled by recurrent excitatory synapses, such as the hippocampus, such maladaptive plasticity will be

Jasper's Basic Mechanisms of the Epilepsies Jasper's Basic Mechanisms of the Epilepsies amplified in a feed forward, synergistic manner and promote hyperexcitability and epileptiform discharge.

The apical trunk as an independent electrical compartment From a morphologic standpoint, the apical trunk, with its direct high conductance connection to the soma, is an unlikely structure to behave as an independent electrical compartment. But dual electrode recordings have clearly shown that the distal apical trunk can generate electrical behavior that is independent of the soma (53). In contrast to the thin terminal dendritic compartment, which is created in a large part by its passive cable properties, electrical compartmentalization of the apical tuft at the distal apical trunk is largely created by the distribution of active intrinsic conductances. The high expression of VDCCs near the apical tuft allows regenerative calcium current produced there to overwhelm and escape the electrotonic control of the soma. The expression of VDCCs is lower in the region between the tuft and the soma in layer 5 pyramidal neurons (6). This spatially restricted expression, combined with the tapering geometry of the apical trunk, normally limits the ability of calcium spikes generated at the apical tuft to propagate regeneratively to the soma. The distinct electrical excitability of the thin terminal dendrites and the apical tuft in response to focal photolysis is illustrated in Figure 2 (16). These dendritic calcium spikes, even when confined to the apical tuft, are still able to drive action potential firing at the soma.

Figure 3. Distal and proximal dendritic compartment Photolysis of caged glutamate directed at the terminal dendrite evokes a spatially restricted plateau potential. Photolysis directed at the main apical trunk near the base of the tuft evokes higher amplitude depolarizations that are dominated by calcium spikes. Wong and Stewart showed that depolarizing current injections into the apical trunk of guinea- pig CA1 pyramidal neurons initiated burst firing, whereas current injection at the soma elicited single action potentials (20). These observations show the electrical independence of the apical trunk and its special role in driving burst firing that are particularly powerful in recruiting downstream neurons within a neural network.

Perturbations of the apical trunk compartment With the demonstration of dendritic calcium spikes, considerable attention was given to the linkage between these calcium spikes and burst firing patterns that are closely associated with epileptiform discharges. In particular, Wong and Prince (54) noticed the parallels between the

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dendritic calcium spikes and the intrinsic bust discharges displayed by CA3 pyramidal cells in acutely prepared hippocampal brain slices, in which a burst of 2–10 high frequency action potentials rides on a slow depolarizing envelope of ca. 20 mV in amplitude and about 100 ms

Jasper's Basic Mechanisms of the Epilepsies Jasper's Basic Mechanisms of the Epilepsies in duration. This discharge bore considerable resemblance to the so-called paroxysmal depolarization shift, the discharge displayed by pyramidal cells during interictal EEG activity in various models of epilepsy (55). Performing the first direct recordings from dendrites, Wong and Prince (56) showed that burst discharges can be elicited from CA1 cell dendrites in response to direct depolarization, much like those described previously in the large dendrites of alligator Purkinje cells by Llinas and colleagues (3). Nevertheless, these dendritic bursts are not normally elicited by orthodromic synaptic stimuli. Wong and Prince showed that the short latency hyperpolarization produced by the feed-forward IPSP prevents synaptically-evoked bursting under normal conditions. Convulsants, such as penicillin, bicuculline, or picrotoxin, diminish this IPSP (because they are GABAA receptor antagonists) and thereby disinhibit this endogenous burst capacity of the apical dendrites.

Computer modeling studies suggested that intrinsically bursting neurons are pivotal in the generation of epileptiform activity (57,58). In vitro studies of convulsant-induced discharge in the neocortex also showed that cells with intrinsic burst firing were critical for the initiation of epileptiform events (59–61). In the pilocarpine model of temporal lobe epilepsy, 54% of CA1 pyramidal cells, which normally fire in a regular mode, are persistently converted to a bursting mode after an episode of SE induced by the convulsant (62). Evidence suggests that T-type calcium channels located in the apical dendrites is a driver of de novo burst firing in the pilocarpine model of hippocampal epilepsy. Burst firing in this model could be suppressed by focally applying the putative T-type calcium channel blocker, Ni2+, to the apical dendrites, but not to the soma. Severing the distal apical dendrites ~150 μM from the pyramidal layer also abolished this activity. (63)

Coupling between different dendritic compartments Coupling between the soma, apical tuft, and terminal dendritic compartments is mediated by both passive and active mechanisms. Due to the asymmetric geometry of the dendritic arbor, passive cable theory predicts that the reliability of regenerative spikes propagation will be different depending on the direction of propagation (64). The smaller diameter of higher order dendritic branches creates a significant impedance mismatch at the branch point. Signals leaving thin branches would experience a significant drop in impedance as they enter the larger branch. This mismatch would result in significant voltage attenuation, whereas little attenuation would be expected for signals traveling from the larger branch towards the thinner branch. Thus, under normal conditions, short duration plateau potentials in the terminal dendrites are ineffective in generating somatic action potentials (16), and sodium and calcium spikes generated at the apical tuft may not always propagate to the soma. In contrast to poor orthodromic propagation, cable theory predicts that back propagation of action potentials from the soma towards the dendrites should be more reliable. Contrary to this prediction, however, direct patch-clamp recordings from the distal apical tufts of pyramidal neurons revealed that bAPs only partially propagate into the dendritic arbor. The bulk of evidence suggests that two dendritic conductances, IA and Ih, actively regulate the extent of bAP invasion of distal dendrites, thereby explaining the discrepancy between the predication of cable theory and actual experimental observation. The fast kinetics of IA and the Kv4.2 potassium channel is 22 well suited to attenuate the fast kinetics of the bAP ( ). In contrast, the slower kinetics of Ih and the HCN family of channels has been shown to be important in the regulation of bursts of bAPs in layer 5 pyramidal neurons (65). It is important to note, however, that this understanding has been challenged by a recent report in which voltage-sensitive dyes were used to monitor membrane depolarizations. With this technique, robust invasion of bAPs into the very tips of thin terminal dendrites was observed (66).

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What functional advantages might the tight regulation of AP back propagation in the dendritic arbor serve? Two ideas have been proposed: control of synaptic plasticity and control of dendritic excitability. Precise coincident activation of a presynaptic glutamatergic synaptic

Jasper's Basic Mechanisms of the Epilepsies Jasper's Basic Mechanisms of the Epilepsies input and the firing of an action potential of the post-synaptic neuron underlie a form of synaptic plasticity called spike timing-dependent plasticity (STDP) (9,67). A robust AP back propagation enables the somatic depolarization of the post-synaptic cell to reach the synaptic site. It has been suggested that regulation of the coupling between the soma and the apical tuft via the 24–26 bAP is one means for regulating synaptic plasticity ( ). As discussed above, IA limits synaptic plasticity by regulating AP backpropagation. Conversely, synaptic plasticity is accompanied by regulation of IA. The Kv 4.2 channels underlying IA are phosphorylated by a number of activity-dependent kinases, which shift their activation to more depolarized potentials and effectively decreases their activation (24).

Robust AP back propagation into the distal dendrite may also trigger calcium spikes in the apical tuft and plateau potentials in the thin terminal dendrites (12,65). Bursts of 4–5 bAP over a narrow frequency range (10–20 Hz) are particularly effective in eliciting large calcium spikes in the apical tufts of layer 5 pyramidal neurons (12). These calcium spikes at the apical tuft can, in turn, lead to burst firing at the axon hillock.

Perturbations of coupling between the soma and the apical tuft

As discussed earlier in this chapter, conditions associated with attenuated IA or Ih are strongly linked to epilepsy. These studies postulated that the key mechanism responsible for the epilepsy was an increase in somato-dendritic coupling. In support of this hypothesis, dual recording from the soma and the apical tuft were used to measure the increased somato-dendritic coupling and the lowering of the frequency threshold for generating dendritic calcium spikes in the rat absence epilepsy model due to decreased HCN1 channels (44). Similarly, in the pilocarpine model of temporal lobe epilepsy, with its decreased expression of Kv4.2 and increased phosphorylation of the Kv4.2 channels, evidence was obtained indicating that there is increased penetration of bAPs into the dendritic arbor (31). Despite these findings, it is still not possible to definitively conclude that altered somato-dendritic coupling is the sole consequence of altered IA and Ih. Loss of IA and Ih will also increase coupling between the terminal dendrite and apical tuft compartments, directly increase the intrinsic excitability of all dendritic compartments, and increase coupling from the apical tuft to the soma. To address these issues, it will be interesting to focally apply blockers of these two conductances to different regions of the dendritic arbor and/or examine dendritic excitability in response to orthodromic, physiological synaptic stimuli.

The epileptic neuron vs the epileptic network While it is generally accepted that epilepsy exists in many forms and has many different pathophysiological mechanisms, opinions over the years have shifted between two extreme emphases: the epileptic neuron and the epileptic network. One would think that epilepsy associated with dendritic hyperexcitability would clearly place this mechanism in the epileptic neuron category. But this categorization can still be problematic because the changes in the expression of dendritic conductances could be a primary event – but also could be secondary to changes in dendritic input. In certain examples, such as in the rat absence epilepsy model due to decreased HCN1 expression, it is possible to determine which event comes first (44). But in other cases, it is not clear whether the changes in IA expression associated with epilepsy is the result of repeated seizures. In yet other cases, such as the downregulation of SK channels following deafferentation, the change in dendritic excitability is secondary to injury, but the dendrite is still the primary source of aberrant hyperexcitability, so aberrant excitability in this model would fall best in the epileptic neuron category. This issue is reminiscent of the controversy as to the origin of the paroxysmal depolarization shift. One school of thought

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postulated that the paroxysmal depolarization shift is an abnormal intrinsic dendritic event (56), whereas others postulated that it is secondary to a giant synaptic potential (68). After 30 years, this issue, and many others relating to the role of dendrites in epilepsy, remain

Jasper's Basic Mechanisms of the Epilepsies Jasper's Basic Mechanisms of the Epilepsies incompletely understood.

Controversies in epilepsy were an important early driving force in the study of dendrites. We now hope that recent rapid advances in techniques to study dendritic excitability leads to a better understanding of many epilepsies.

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