Local pathways of seizure propagation in neocortex

LOCAL PATHWAYS OF SEIZURE PROPAGATION IN NEOCORTEX

Barry W. Connors,* *Department Providence,

of Neuroscience, Rhode

Island;

University

I. II. III. IV. V. VI. VII.

David J. Pinto,* Division

and

of Biology

IDepartment

of Pennsylvania,

and Albert and

Medicine,

of Neurosurgery, Philadelphia,

Introduction Axonal Arbors of the Neocortex Physiology of Local Neocortical Synapses Seizure Initiation and the Minimal Epileptogenic Seizure Propagation: Characteristics Seizure Propagation: Pathways Conclusions References

E. Telfeiant Brown School

University, of Medicine,

Pennsylvania

Volume

I. Introduction

Partial seizures of the neocortex can be initiated by relatively small and localized collections of neurons (Dichter and Ayala, 1987). Anticonvulsant drugs are very successful for treating most cases of partial epilepsy, but they produce unwanted side effects in the large majority of the brain that is not epileptic. For some patients, drugs simply fail to control seizures adequately. In selected cases, surgery can be a dramatically effective and selective therapy for partial seizures (Engel et al., 1997). Unfortunately, the inherent uncertainties of mapping both pathology and function in the cortex often lead to the removal of important, nonepileptic tissue. A better understanding of the cellular basis of epilepsy might lead to more targeted therapeutic strategies. One tactic for preventing the onset of partial seizures is to disconnect, in a literal sense, the offending neurons and so prevent their paroxysmal collusion. This is the rationale behind multiple subpial transection, in which epileptogenic neocortex is scored with parallel cuts, ideally oriented normal to the pia and spaced about 5 mm apart (Morrell, 1997). Although the hope is that such disconnection will actually prevent a localized seizure from starting, it might still serve as a fallback plan by containing the spread of a seizure. Multiple subpial transection is currently being used, on a limited and usually adjunct basis, to treat some INTERNATIONAL NEtiROBIOLOGY,

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Copyright 0 2001 by Academic Prrrr. All I ights of reproduction in any tbrm reserved. 0074.7742101 $35.00

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forms of partial seizure disorders (Sawhney et al., 1995; Rougier et al., 1996; Hufnagel et al., 1997; Smith, 1998). Understanding the optimal requirements for preventing and containing focal seizures by disconnection will require a detailed knowledge of the structure and function of local neocortical circuits. In this review we summarize selected research on the neurons, axons, synaptic connections, and epileptiform activity of the neocortex to illuminate the substrates that mediate seizure propagation.

II. Axonal Arbors of the Neocoriex

One of the most salient facts about the cellular structure of the cortex is that its neurons are extensively interconnected by axons and synapses. These interconnections allow the cortex to perform its normal immediate functions, adapt to short-term contingencies, and form longterm memories. Axonal connections are also essential for abnormal activity, most notably seizures. In fact an old, and still quite reasonable, assumption is that some seizures are actually caused by the presence of too many interconnections between cortical neurons. Selectively pruning these interconnections was the original goal of multiple subpial transection (Morrell, 1997). Here we briefly review the patterns of local interconnections of the neocortex as they relate to the initiation and propagation of seizure discharge (Connors and Amitai, 1995; Connors, 1997). Simple histological stains reveal a six-layered arrangement of the neuronal cell bodies in neocortex. Variations in the thickness and density of the layers and sublayers are the classical means by which one area of neocortex is distinguished from the others. Thus, at first glance the cortex appears to be a stack of broad but thin slabs of packed neurons. A wide range of biochemical and molecular markers can also define the with more specificity than traditional stains. layers, sometimes Determining the patterns of interconnections within the neocortex has always been a more difficult problem. Golgi stains have long been useful for this (Ramon y Cajal, 1909), but they are by nature random and capricious, and tend to stain axons only in tissue from young animals. Far more control and resolution are provided by histochemical methods that label only one or a very small number of axons, combined with optical methods and reconstruction technologies that allow the detailed trajectories of axons to be mapped quantitatively (Heimer and Zaborszky, 1989).

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Although the three-dimensional pattern of a single neocortical axon most often resembles the branches of an oak tree (hence axonal “arbors”), it is often useful to discuss them as if they looked more like vines that either creep along the ground or climb a flagpole. Axons moving within the plane of the cortical layers (the ground creepers) are referred to as “horizontal” connections, while the climbers provide “vertical” connections. Most real axons have both horizontal and vertical components. In general, there is a strong tendency for the axons in the neocortex to extend along the vertical dimension (Lorente de No, 1949). Many neurons, excitatory and inhibitory, have their dendrites arranged with a dominantly vertical orientation as well. These structural principles are reflected in the functional properties of cortical neurons, which are often more similar among groups of cells arranged vertically than they are among cells scattered across a single horizontal layer (Mountcastle, 1957; Asanuma, 1975; Hubel and Wiesel, 1977). The anatomical and physiological verticality of the neocortex is the basis for the hypothesis that it is composed of closely similar modules-small columns of neurons-that form the irreducible units of neocortical organization (Mountcastle, 1979). Modular organization could provide an efficient and flexible mechanism for adaptation; new cortical areas and functions can presumably evolve by the simple addition of more columnar modules. As the input and output axons of the cortex began to be unraveled (Lorente de No, 1949), it became clear that the basic circuitry between the neocortex and the thalamus, specifically the axon trajectories and synaptic connections between individual neurons, form a strongly vertical array (Gilbert and Wiesel, 1979; Armstrong-James et al., 1992). Thalamocortical axons rise through the deep cortical layers and terminate mainly in layer 4 and the deep aspect of layer 3, where they excite both spiny excitatory neurons (stellate and pyramidal cells) and inhibitory interneurons. Axons of the layer 5 spiny cells course upward to excite pyramidal neurons in the upper layers, which in turn reciprocally excite neurons below in layer 5. Axons of layer 5 cells also form the primary pathway through which the neocortex communicates with the rest of the central nervous system; collaterals of layer 5 cells synapse on cells of layer 6, and these neurons return excitation both to the thalamus and to layer 4. The vertical, columnar organization of neocortex has been a powerful concept, inspiring theories of sensory processing, motor control, and learning, as well as therapies for seizure disorders (Morrell, 1997). But horizontal connections of neocortex are also widespread, complex, essential for normal functions, and readily serve as propagation pathways for seizures (Fig. 1). Most horizontal axons, perhaps 90%, do not extend

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FIG. 1. Summary diagram showing local axonal connections within cortical area 17 of the rat. Axons were traced by making small injections of horseradish peroxidase into progressively deeper cortical layers from A to H. Large dots represent injection sites, and lines represent local axons. Modified with permission, from Burkhalter (1989, Fig. 14).

more than about 1 mm across the cortex (Fisken et al., 1975). Some horizontal axons may travel several millimeters, and the vast majority terminate with excitatory synapses onto both pyramidal cells and inhibitory interneurons. The longest axons are perhaps most important for connecting functionally distinct regions of neocortex: primary sensory areas to various higher-order areas, and back again. An important general principle is that horizontal connections are almost always reciprocal, and the spatial patterns of connections are not homogeneous (Gilbert and Wiesel, 1983; Juliano et al., 1990; Lund et al., 1993). The precise functions of long horizontal axons are not entirely clear. There is mounting evidence that they mediate a variety of plastic changes in the neocortex that are associated with learning, changes of activity in afferent inputs, or lesions of cortical and subcortical structures (Gilbert, 1998; Finnerty et al., 1999). Horizontal axons are also necessary for synchronizing the neural activity of separated regions of cortex, a process that might (Gray, 1999) or might not (Shadlen and Movshon, 1999) have a role in binding together disparate components of information to form more complex, coherent neural representations. On a more local scale, horizontal connections even mediate propagating waves of normal activity through the cortex, which probably serve to integrate information from a wide region of sensory input space (Bringuier et al., 1999).

SEIZURE

PROPAGATION

III. Physiology

of local

PATHWAYS

Neocortical

531

Synapses

Knowing the locations of synapses and the patterns of their axonal connections tells only part of the story about a neural network. It is also essential to know how those synapses behave. All chemical synapses use a highly conserved collection of molecular machinery (Bennett and Scheller, 1994), yet their functional properties can vary tremendously (Thomson and Deuchars, 1997; Connors et al., 2000). In neocortex, a varied collection of synapses release a variety of neurotransmitter types, display distinctive forms of short-term dynamics, are modulated by both intrinsic and extrinsic forces, and exhibit long-term changes due to ongoing activity. The short-term dynamics displayed by excitatory synapses in neocortex are diverse. When activated at moderate frequencies, the synapses from thalamic axons onto layer 4 neurons initially tend to be strong, but quickly depress to much smaller amplitudes (Gil et al., 1997). Quite the opposite tendency is displayed by the excitatory synapses that interconnect the spiny stellate cells of the cortex with the low-threshold spiking (LTS) type of inhibitory interneuron (Reyes et al., 1998; Gibson et al., 1999). In this case, when the synapses are activated at low frequency they are so unreliable and weak that they might often be mistaken for no connection at all. When the presynaptic spiny cell fires at frequencies of 20 to 40 Hz, however, its synapses onto LTS cells facilitate and, within 5 to 10 pulses, the strength of the synapses can increase manyfold. Dynamics also vary widely for the inhibitory synapses of the cortex. Some of the same interneurons whose excitatory input synapses show facilitation also show the strongest facilitation of their output (inhibitory) synapses (Beierlein and Connors, 1999; Gupta et al., 2000). The dynamics of cortical synapses can be modified in several ways. A variety of neurotransmitter receptors, mostly of the G-protein-coupled variety, reside on presynaptic terminals and regulate transmitter release. Acting on these receptors, a transmitter may regulate its own release rate, as y-aminobutyric acid (GABA) does when its levels rise high enough to activate presynaptic GABA-B autoreceptors (Deisz and Prince, 1989). Release may also be regulated by sources extrinsic to cortex. For example acetylcholine, most of which comes from the basal forebrain, may act on muscarinic cholinergic receptors that reside on both glutamatergic and GABAergic synaptic terminals intrinsic to neocortical layer 4 (Gil et al., 1997; Beierlein and Connors, 1999). Additionally, the complement of presynaptic receptors, and thus the potential for synapse regulation, varies from pathway to pathway. GABA-B receptors, for example, do not

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regulate the release of glutamate from thalamocortical synapses, although they do suppress its release from intracortical synapses (Gil et al., 1997). Finally, the functions of neocortical synapses may change as a result of experience and activity. Numerous studies suggest that synapses can strengthen or weaken following chronic changes in sensory experience or learning (Kirkwood et al., 1996; Rioult-Pedotti et al., 1998). This may be the result of alterations in the pattern, frequency, timing, or synchrony of synaptic activity. The short-term dynamics of neocortical synapses are also subject to regulation by sensory activity (Finnerty et al., 1999). The role of synapse dynamics in seizure generation has rarely been considered, but it is likely to play a role in their time course and propagation patterns, and might provide a new site for seizure control.

IV. Seizure

Initiation

and

the Minimal

Epileptogenic

Volume

Clinical observations long ago led to the conclusion that seizures sometimes arise from localized, apparently small regions of cerebral cortex (Taylor, 1931). Just how small can the epileptogenic volume of tissue be? The answer is of general theoretical interest, and can tell us something about the patterns of connections in cortical networks (Traub and Miles, 1991). It may also be of therapeutic relevance, and was an important consideration of Morrell in formulating the idea of multiple subpial transection (Morrell et al., 1989). Seizures, by their nature, involve interacting populations of neurons. If the substrate of this interaction, axons and their synaptic terminals, could be cut, then seizures might be curtailed. The key questions are: Which are the most critical connections? What is the minimal amount of cutting necessary to disrupt seizures? How important are those connections for normal cortical functions? Previous studies of experimental seizure foci suggested to Morrell that the “minimal epileptic volume” was rather large-a vertical column of tissue no less than about 5 mm on a side. His rationale for multiple subpial transection was that separating horizontal axons at intervals of about 5 mm would not simply stop the propagation of a seizure, but would prevent its initiation by “compromising the capacity of cortex to develop the synchrony that is necessary for engendering epileptiform events to begin with” (Morrell, 1997). A number of studies suggest that the minimum amount of cortex necessary to generate seizure-like activity is quite small, certainly much smaller than a 5 mm” column. By applying convulsant drugs such as

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penicillin directly to the cortical surface while carefully measuring neural activity, early experiments in the visual cortex of whole animals implied that epileptiform activity could emanate from a column extending vertically through all cortical layers, but perhaps no more than about 125 km in diameter (Gabor et al., 1979; Reichenthal and Hocherman, 1979). By using focal microinjections of convulsant drugs into the cortical depth, Ebersole and Chatt (1986) were able to show that even this was an overestimate; drug treatment of not much more than a column diameter’s worth of one neuronal layer, in this case layer 4, was enough to trigger epileptogenesis. A diminutive size for the minimal epileptogenic volume is also implied by studies of slices of neocortex maintained in vitro (Connors, 1984). Epileptiform activity can be induced in rodent neocortex by treating it with low doses of GABA-A receptor antagonists (to reduce synaptic inhibition), or by bathing it in solutions containing low [Mg”] (to boost Nmethyl-n-aspartate (NMDA) receptor-dependent excitatory activity). Under these conditions, narrow slices consisting of layer 5 alone, with horizontal dimensions of only a few hundred micrometers, are able to support both the initiation and propagation of epileptiform activity (Silva et al., 1991; Telfeian and Connors, 1998). Consistent with this, when the inhibitory neurotransmitter GABA was focally applied to cortex in minute amounts, it most easily prevented or slowed epileptiform activity when applied to layer 5 (Telfeian and Connors, 1998). The reasons for the peculiar epileptogenicity of layer 5 have been extensively reviewed (Connors and Amitai, 1993; Amitai and Connors, 1995; Connors, 1997), and probably involve the presence of a particularly excitable and interconnected set of pyramidal cells. Optical recordings using voltage-sensitive dyes have recently been done in low-[Mg’+]-treated neocortical slices (Tsau et nl., 1999). The images implied that epileptiform initiation sites could arise in any cortical layer from 1 to 6; however, because the method is relatively insensitive, and presumably requires a large number of active neurons to generate a detectable signal, it might easily have missed the relatively small number of cells that triggered the initial event. There are other means by which neocortex can be made acutely epileptogenic, but they do not necessarily require any more neurons to do so. For example, application of drugs that activate kainic acid receptors induce phasic, highly synchronized epileptiform events that absolutely require the neurons of layers 213, but do not require neurons in deeper layers (Flint and Connors, 1996). Both supragranular neuronrequiring activity (i.e., kainate) and infragranular neuron-requiring activity (i.e., low-[Mg’+] solutions or high doses of GABA-A antangonists) occur in roughly similar-sized fragments of cortex. These studies, and

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others in the hippocampus (Miles and Wong, 1983), establish that a remarkably small number of excitatory neurons, perhaps no more than 1000, are capable of initiating seizure-like activity under extreme conditions. In fact, studies in slices have not yet established the minimum number of neurons necessary to form an epileptogenic aggregate, and it could be far fewer than 1000. The ultimate small neuronal network was studied by Segal (1991). He grew hippocampal pyramidal neurons in culture and showed that even a single, strongly self-innervating excitatory cell could generate long-lasting “seizure”-like activity when triggered by a brief stimulus. The studies in cortical slices suggest that therapeutic strategies relying on disconnection, i.e., cutting some axons to decouple groups of neurons, could be impractical because of a requirement for a spacing between cuts that simply cannot be achieved. But slice studies do not establish how cortical networks might behave in vivo, when they are still embedded in a much larger circuit and a different environment. They also do not address the more important clinical issue: does disconnection of cortical regions on a relatively large scale (i.e., every 5 mm or so) significantly reduce the incidence of clinical seizure initiation, or the severity of a focal seizure once it starts. 2 Few studies have addressed this directly. A recent study of kainic acid seizures in rabbits found that multiple subpial transection did not prevent epileptic activity in the seizure focus, although it did suppress seizure propagation ipsilaterally once it started (Hashizume and Tanaka, 1998). Clearly more basic research is necessary to understand how the chronic seizure initiation process in cortex in vivo is influenced by subpial transections.

V. Seizure

Propagation:

Characteristics

The spatiotemporal patterns of propagating seizures have been charted on a gross scale (centimeters) by using electroencephalographic procedures (Lemieux et al., 1984) and on a somewhat finer scale (millimeters) by using arrays of cortical surface recordings (Petsche et al., 1974; Goldensohn and Salazar, 1986). These methods, however, are too crude to see the influence of the most striking substructures in the neocortex, its so-called columns, which have dimensions of about 100 to 1000 km. Various optical recording methods have been developed for visualizing cortical activity in vivo, but the usual techniques combine either fine spatial detail and poor temporal resolution (intrinsic optical signals) or good temporal resolution and low spatial resolution, and often

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tissue toxicity (voltage-sensitive dyes) (Lieke et al., 1989). New dyes and imaging technology are improving the situation (Shoham et al., 1999), but optical imaging has only rarely been used to study the intact human cortex and its seizures (Haglund, 1997). The most detailed views of seizure propagation, on the scale of columns and laminae, come from experiments using microelectrode arrays or imaging of voltage-sensitive dyes in cortical slices in vitro. Slices can be cut in a variety of planes and configurations, but the conventional approach is to slice normal to the cortical surface, usually in the coronal plane. All slices are constrained to a maximal thickness of about 0.5 mm, but they can be arbitrarily wide. When coronal slices from rodents are treated with high doses of GABA-A receptor antagonists or with low-[Mg’+] medium, synchronous waves triggered from either end of the slice propagate reliably across the entire extent of the cortex. Near the epileptiform initiation site, activity propagates along the vertical dimension much faster than it does horizontally (Sutor et al., 1994). Estimates of their average horizontal propagation velocity, measured with electrode arrays or optical methods, are quite similar, ranging from about 0.03 to 0.1 m/s (Chervin et al., 1988; Wong and Prince, 1990; Wadman and Gutnick, 1993; Tanifuji et al., 1994; Albowitz and Kuhnt, 1995; Golomb and Amitai, 1997; Wu et al., 1999; Laaris et al., 1999). If GABA-A receptors are only partially blocked, propagation velocity may be a bit slower (Chagnac-Amitai and Connors, 1989a), although titrating the concentration of GABA-A antagonist in single slices suggests that velocity is relatively independent of the degree of disinhibition (Pinto and Connors, 1999). When human neocortex is surgically resected and further sliced, then treated with GABA-A receptor blockers in vitro, its epileptiform activity is very similar to that of rodents, including an average propagation velocity of about 0.04 m/s (Albowitz et al., 1998). Horizontal propagation across the neocortex is rarely uniform. Careful measurements in slices of rat somatosensory and cat visual cortices showed periodicities with a spatial frequency of about 1 mm-’ (Fig. 2) (Chervin et al., 1988). In some cases an epileptiform wave apparently jumped across a small region of tissue. Propagation patterns are also directional, and in many slices the local velocity patterns for the two directions were even antiphasic, i.e., those places where the waves had the highest velocity from later-to-medial tended to be where the medial-to-laterally directed waves moved most slowly. Spatial inhomogeneities of propagation have been seen in various other types of neocortex, using several methods of measurement (Wadman and Gutnick, 1993; Albowitz and Kuhnt, 1995; Wu et al., 1999). Propagation patterns may be specific for the architectonic area across which they spread.

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Fro. 2. Periodicity and directionality in the propagation of epileptiform discharges across cat area 17. Slices of cortex were maintained in vitro, and made epileptogenic by bath application of high doses of a GABA-A receptor antagonist. (A) Graph of the “latency difference” (essentially the reciprocal of local velocity) against horizontal distance across the cortical slice. Solid and dashed lines depict the spatial profile of l/velocity in the two directions (arrows). (B) Power spectrum of propagation data shows that the peak spatial frequency peaks at about 1 mm-‘. Reproduced, with permission, from Chervin et nl. (1988).

When waves traveled across the rat visual areas their speed was relatively uniform and nondirectional as they crossed area 17, but they became sharply irregular and direction dependent as they moved across the adjacent area 18 (Chervin et al., 1988). In some studies the sites at which waves shifted velocity most sharply were very stable from trial to trial over long periods (Chervin et al., 1988; Wadman and Gutnick, 1993), but there are reports of such sites varying over time (Wu et al., 1999). Propagation patterns are strongly influenced by the state of synaptic circuitry in the cortex. GABAergic inhibition is perhaps the most powerful modifier. When GABA-A receptors are only partially blocked with an antagonist, leaving most inhibition intact, epileptiform waves are

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much more variable than when inhibition is completely eliminated (Chagnac-Amitai and Connors, 1989a). In the presence of inhibition, propagation can be a fragile affair, and waves will sweep easily across the cortex on one trial only to propagate partially and fail at a specific site on the next trial. Often a wave reaches a site at which its propagation slows dramatically, then speeds up again after it pushes past that site. These sites of hesitation also tend to be places where waves can, after a variable delay, be reflected back to propagate across the cortex they have just traversed. Clearly synaptic inhibition has an important role in sculpting the spatiotemporal patterns of epileptiform propagation. The presence of distinct types of inhibitory networks in the neocortex, each with its own membrane and synapse dynamics (Gibson et ul., 1999; Gupta et al., 2000), suggests that different patterns of seizure activity may preferentially engage different types of interneurons.

VI.

Seizure

Propagation:

Pathways

What accounts for the irregular spatial patterns of seizure propagation? The hypothesis we currently favor is that they reflect spatial inhomogeneities of the excitatory connections along the horizontal dimension. There are certainly other possibilities. If the cortex had regional variations in the intrinsic excitability of its neurons (e.g., if there were local clusters of intrinsically bursting pyramidal neurons) (ChagnacAmitai et al., 1989b, 1990), then propagation might proceed in fits and starts, but this alone could not account for direction-dependent patterns. Subcortical influences on propagation patterns have obviously been removed in most slice experiments, although they might be an important consideration in viva. Inhibitory circuits are unlikely to be important for most of the studies cited above, for the simple reason that GABA-A receptors were strongly blocked in most of them. Still, in vivo and during slice experiments that leave most inhibition intact (Chagnac-Amitai and Connors, 1989b), regional variations in inhibitory circuitry could yield direction-dependent irregularities in propagation only if inhibitory axons had asymmetric connections along the horizontal dimension. Such general asymmetries of inhibitory networks have been difficult to detect, although axonal arbors of some individual inhibitory neurons are distinctly asymmetric (Kisvarday et al., 1994). There is also some evidence that the horizontal excitatory axons that engage inhibitory neurons can be very asymmetric (Weliky et al., 1995).

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The most direct evidence that excitatory, glutamatergic synapses are involved in epileptiform propagation comes from experiments in which the critical transmitter receptors, those of the a-amino-5-hydroxy-3methyl-4-isoxazolepropionic acid (AMPA) subtype, are blocked with a specific antagonist. When the concentration of antagonist is slowly increased, propagation velocity progressively decreases and its spatial patterns become more complex (Fig. 3). At some critically slow speed that is about 30 to 40% of the control value, propagation fails entirely (Golomb and Amitai, 1997). Blockade of the NMDA subtype of glutamate receptor shortens the duration of epileptiform discharges, but has no effect on their propagation velocity when AMPA receptors are functional. If, however, AMPA receptors are blocked then NMDA receptors are capable of sustaining propagation that proceeds at about 20% of the control velocity (Telfeian and Connors, 1999). What are the properties of the excitatory axons that mediate horizontal propagation? Several studies have addressed this issue using physiological methods. Careful measurements of propagation have shown that, for a particular sample of cortex, the spatially irregular patterns are very stable over time and from trial to trial, particularly when GABA-A receptors are strongly blocked (Chervin et al., 1988; Wadman and Gutnick, 1993; Pinto and Connors, 1999). This suggests that the substrate of propagation has a “hard-wired” quality to it. We propose that the periodicity and directionality of propagation reflect spatial variations in the neural hard wires-the horizontal excitatory axons. There is ample anatomical evidence for dramatic local variations in the length, density, symmetry, and continuity of axonal connections and their synaptic terminations in neocortex (Rockland and Lund, 1983; Isseroff et nl., 1984). As suggested by the AMPA receptor blockade experiments, as well as computational models (Chervin et al., 1988; Golomb and Amitai, 1997), velocity depends strongly on the strength of the hori-

FIG. 3. Propagation of epileptiform waves recorded by a linear 16-electrode array oriented horizontally across a neocortical slice. These graphs plot horizontal distance against the time following a trigger stimulus applied to a slice treated with a GABA-A receptor antagonist. The voltage at each local electrode is represented by shading, linearly interpolated across the array, with white lines representing a relatively large voltage level. The top plot shows the nonuniformity of propagation, with regions of faster and slower velocity, and some spatial skipping over the last half of the measured region. In the bottom plot the slice has been treated with a very low dose of AMPA receptor antagonist (0.5 p,M DNQX) to suppress synaptic excitation. Propagation velocity is slightly slowed, and its pattern becomes more complex and splintered. Reproduced, with permission, from Pinto and Connors (unpublished observations).

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zontal connections that mediate propagation. Thus, at a particular point along the epileptiform propagation path, a high density of horizontal axons projecting from medial to lateral would favor high velocity in that direction; a wave propagating in the opposite direction that encountered a much lower density of axons projecting from lateral to medial across the same narrow cortical column would have a relatively much lower velocity. A dramatic feature of neocortical anatomy is that many areas exhibit highly discontinuous, “patchy,” horizontal connections (Gilbert and Weisel, 1983). Patchy connections could also contribute to irregularities of propagation by allowing neurons at the wavefront of the epileptiform discharge to project their synchronous activity forward, beyond the immediately adjacent tissue. Indeed, propagation patterns are sometimes discontinuous, as the discharge momentarily jumps past a small region of cortex before propagating backward into it (Chervin et al., 1988). Are there particular axons that mediate epileptiform propagation? Anatomy shows us that horizontally directed axons spread through every cortical layer, from layer 1 downward into the white matter (Fig. 1). It is possible that any substantial subset of these axons, regardless of laminar location, might sustain propagation. If, however, there were a favored pathway, its specific disconnection could provide a more refined, less traumatic, and more effective alternative to traditional multiple subpial transection. Studies in vitro have provided interesting results. When GABA-A-mediated inhibition is eliminated, epileptiform propagation is remarkably robust and often proceeds successfully past vertical cuts that have severed all but thin bridges of cortex no thicker than a single layer (Albowitz and Kuhnt, 1995; Telfeian and Connors, 1998). It seems that, indeed, axons at any cortical depth can mediate propagation as long as there is some minimal number of them. But as we pointed out above, eliminating inhibition entirely may not be a fair model of clinical seizures. When inhibition in slices was reduced only enough to induce epileptiform activity, and cuts were made vertically or horizontally to transect selective pathways, it became clear that there was a preferred pathway of propagation-through layer 5 (Fig. 4) (Telfeian and Connors, 1998). Recall that this is also the layer that is most strongly implicated in the epileptiform initiation process (Connors and Amitai, 1995). When injured, pyramidal cells of layer 5 can even react by sprouting new axonal connections that might facilitate seizure initiation and spread (Prince, 1999). In summary, a variety of evidence implies that the same set of pyramidal neurons that initiates epileptiform activity can, via its horizontally directed axons, also efficiently mediate seizure propagation.

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FIG. 4. Dependence of propagation on circuitry in layer 5. Neocortical slices were treated with a low dose of CABA-A receptor antagonist. The diagram (top) shows schematically how slices were successively transected horizontally either from the top down or the bottom up. After each cut, propagation distance was measured in the remaining fragment of slice. The graph (bottom) plots data compiled from 10 slices. It shows that propagation proceeded across the entire width of the slices, until the cuts removed part or all of layer 5. Reproduced, with permission, from Telfeian and Connors (1998).

VII.

Conclusions

The experiments reviewed here suggest that a small subset of pyramidal neurons may be responsible for the initiation and propagation of experimental epileptiform activity in neocortex. If we assume that similar types of neurons reside in some types of human epileptic cortex, we can imagine potentially new therapeutic strategies that might be more selective or effective than some currently available drugs and surgery. One possibility is a much more precise and targeted form of subpial transection. If pathways in layer 5 are indeed the preferred route for seizures, then transecting just the deep layers might preserve cortical

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functions that depend on supragranular pathways. Such precision is beyond current surgical techniques, but could become feasible as new, perhaps optical, alternatives to the surgical knife become available. It could be particularly powerful when paired with very fine-scale, noninvasive functional mapping of the brain (Kim et al., 2000). More elegant methods might make use of the fact that subtypes of cortical neurons have distinctive patterns of gene expression, and that their axonal growth can be manipulated locally by molecular strategies (Ghosh, 1999). If cortical wiring is the substrate for seizure generation, then selective rewiring of the cortex might eventually be the most effective anticonvulsant.

Acknowledgments

and

These studies were supported in part the Burroughs-Wellcome Fund.

by the

National

Institutes

of Health

(NS25983)

References

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