Ictal Epileptiform Activity in the CA3 Region of Hippocampal Slices Produced by Pilocarpine

Paul A. Rutecki and Yili Yang

Departments of Neurology, Neurosurgery, and Neuroscience, Training Program, University of Wisconsin Medical School Madison, William S. Middleton VA Hospital, Madison, Wisconsin 53705

    ABSTRACT
Abstract
Introduction
Methods
Results
Discussion
References

Rutecki, Paul A. and Yili Yang. Ictal epileptiform activity in the CA3 region of hippocampal slices produced by pilocarpine. J. Neurophysiol. 79: 3019-3029, 1998. Pilocarpine, a muscarinic agonist, produces status epilepticus that is associated with the later development of chronic recurrent seizures. When applied to rat hippocampal slices, pilocarpine (10 µM) produced brief (<200 ms) epileptiform discharges that resembled interictal activity that occurs between seizures, as well as more prolonged synchronous neuronal activation that lasted seconds (3-20 s), and was comparable to ictal or seizures-like discharges. We assessed the factors that favored ictal patterns of activity and determined the biophysical properties of the ictal discharge. The probability of observing ictal discharges was increased when extracellular potassium ([K+]o) was increased from 5 to 7.5 mM. Raising [K+]o to 10 mM resulted in loss of ictal patterns and, in 20 of 34 slices, desynchronization of epileptiform activity. Making the artificial cerebrospinal fluid (ACSF) hyposmotic favored ictal discharges at 5 mM [K+]o, but shifted 7.5 mM [K+]o ACSF patterns to interictal discharges or desynchronized activity. Conversely, increasing osmolality suppressed ictal patterns. The pilocarpine-induced ictal discharges were blocked by atropine (1 µM, n = 5), a muscarinic antagonist, and pirenzepine (1 µM, n = 6), a selective M1 receptor antagonist. Kainate/alpha -amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid receptor blockade stopped all epileptiform activity (n = 8). The N-methyl-D-aspartate antagonist D,L-2-amino-5-phosphonovaleric acid (100 µM, n = 34) did not change the pattern of epileptiform activity but significantly increased the rate of interictal discharges and prolonged the duration of ictal discharges. The ictal discharge was characterized intracellularly by a depolarization that was associated with action potential generation and persisted as a membrane oscillation of 4-10 Hz. The ictal oscillations reversed in polarity at -22.7 ± 2.2 mV (n = 11) with current-clamp recordings and -20.9 ± 3.1 mV (n = 7) with voltage-clamp recordings. The reversal potential of the ictal discharge in the presence of the gamma -aminobutyric acid-A blocker bicuculline (10 µM, n = 6) was -2.2 ± 2.6 mV and was significantly different from that measured without bicuculline. Bicuculline added to 7.5 mM [K+]o and 10 µM pilocarpine did not cause epileptiform activity to change pattern but significantly increased the rate of interictal discharges and prolonged the ictal discharge duration. Both synaptic and nonsynaptic mechanisms are important for the generation of ictal patterns of epileptiform activity. Although the synchronous epileptiform activity produced by pilocarpine required fast glutamate-mediated synaptic transmission, the transition from an interictal to ictal pattern of activity depended on [K+]o and could be influenced by extracellular space.

    INTRODUCTION
Abstract
Introduction
Methods
Results
Discussion
References

When administered systemically to rats, pilocarpine, a muscarinic agonist, produces status epilepticus, a condition of frequent or continuous seizures (Clifford et al. 1987; Turski et al. 1989). Four to 30 days after an episode of pilocarpine-induced status epilepticus, a high proportion of rats will develop recurrent behavioral and electrographic seizures (Cavalheiro et al. 1991). The pilocarpine model of status epilepticus and delayed onset of behavioral seizures mimics the complex partial seizures and associated pathology of hippocampal sclerosis noted in humans with medically intractable complex partial seizures of temporal lobe origin (Cavalheiro et al. 1991; Turski et al. 1989).

Application of pilocarpine to the rat hippocampal-entorhinal slice preparation produces epileptiform discharges that are similar to those associated with interictal spikes and more prolonged episodes of synchronous neuronal activity that resemble seizures or ictal epileptiform discharges (Nagao et al. 1996). The ictal-like discharges appear to arise from the entorhinal cortex, which is more likely to display ictal activity in several different models (Barbarosie and Avoli 1997; Jones and Lambert 1990; Rafiq et al. 1993; Walther et al. 1986).

The cellular physiology of pilocarpine-induced epileptiform activity in the CA3 region has not been characterized nor has the determinants of the transition from an interictal to ictal pattern of epileptiform activity. Using the hippocampal slice preparation without entorhinal cortex, we examined factors that favored ictal patterns of epileptiform activity and investigated the synaptic basis for pilocarpine-induced epileptiform activity in the CA3 region.

    METHODS
Abstract
Introduction
Methods
Results
Discussion
References

Male Sprague-Dawley rats (150-400 g) were killed after ether anesthesia. The brain was removed quickly and transferred to iced artificial cerebrospinal fluid (ACSF). The hippocampus was isolated, and transverse 500-µm-thick slices were prepared using a McIllwain tissue chopper. The slices then were transferred to an interface chamber and superfused with ACSF bubbled with 95% O2 and 5% CO2 maintained at 32-34°C. The ACSF composition was (in mM) 124 NaCl, 2.5 KCl, 1.25 NaH2PO4, 2 CaCl2, 2 MgSO4, 26 NaHCO3, and 10 glucose. Slices were incubated for >1 h before epileptiform activity was produced by bath application of 10 µM pilocarpine. In most cases, when pilocarpine was used to produce epileptiform activity, the extracellular [K+]o was raised to >= 5 mM.

Extracellular recordings were made from the cell body layer of the CA3c and b regions of the hippocampus using 2-10 MOmega glass microelectrodes filled with 2 M NaCl. Intracellular recordings were made with microelectrodes of 20-60 MOmega resistance and filled with either 2 M potassium acetate (KAc) or 2 M cesium acetate (CsAc) and 50-100 mM QX-314, a lidocaine derivative. The latter mixture was used to depolarize the neurons by blocking potassium conductances and also retarded the generation of sodium spikes that interfered with voltage clamping and measurements of synaptic potential amplitudes in current clamp. Intracellular recordings wereobtained using an Axoclamp 2A amplifier in both a bridge and switch-clamp mode. When using a switching mode, the electrode was monitored with a separate oscilloscope so that sampling rate and capacitance compensation could be adjusted appropriately. The sampling rate used was between 3 and 7 kHz. The extracellular recording and intracellular current and voltage output was digitized using a Digidata 1200 interface (AXON Instruments).

In the hippocampal slice preparation, the application of most convulsants produces epileptiform discharges that resemble the paroxysmal depolarizing shift that occurs in vivo during electroencephalographic interictal epileptiform spikes (Matsumoto and Ajmone Marsan 1964a; Schwartzkroin and Prince 1978). We defined spontaneously occurring extracellular discharges as interictal if they were relatively brief (<200 ms) field discharges that occurred spontaneously at rates of <2 Hz and usually <1 Hz. Ictal discharges were defined as lasting >3 s and consisted of a rhythmic discharge of >2 Hz and usually between 4 and 10 Hz (Fig. 1). When rare interictal discharges and background unit extracellular fields were noted, the slice was defined as showing desynchronized activity. Measurements of interictal frequency were made either from digitized data or chart recordings. Ictal discharges were characterized by the time between the end of one discharge and the onset of the next as well as the duration of the ictal activity (Fig. 2B). These measurements were made from chart recordings or digitized data. The reversal potential and associated conductance for the ictal discharge was measured by evaluating the first two or three depolarizations of the discharge. The voltage-voltage or current-voltage data then were fitted to a straight line by linear regression to determine the reversal potential and conductance for voltage-clamp data.


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FIG. 1. A: pilocarpine-induced epileptiform activity in 7.5 mM [K+]o artificial cerebrospinal fluid (ACSF) resembled interictal and ictal discharges. Pairs of intracellular and simultaneous extracelluar recordings showing patterns of epileptiform activity at different time scales. At the resting membrane potential of -54 mV, there were spontaneously occurring action potentials and interictal discharges followed by an afterhyperpolarization that occurred with an extracellular discharge. An interictal discharge is shown (bottom left). A more prolonged intra- and extracellular discharge occurred near the end of the trace, and the discharge resembled an ictal pattern of epileptiform activity. Right: ictal discharge at a faster time scale. Intracellular recording demonstrated that a prolonged depolarization with action potential generation was associated with the extracellular discharge that was accompanied by a DC negative shift. Traces on the right show that after the initial component of the discharge, there was an intracellular membrane oscillation that occurred between 4 and 6 Hz. Action potentials were truncated by the digitization rate of 1 kHz. Intracellular recordings were made with an electrode filled with 2 M potassium acetate. B: [K+]o alters the pattern of pilocarpine-induced epileptiform activity in the CA3 region. Extracellular recordings from the same slice made during bath application of 5, 7.5, and 10 mM [K+]o ACSF containing 10 µM pilocarpine. In 5 mM [K+]o, the pattern of activity is interictal, characterized by recurrent 70- to 95-ms duration synchronous activity occurring at 0.28 Hz. At 7.5 mM [K+]o, the epileptiform activity takes on a pattern that is ictal with recurrent 8-10 s long synchronous activity that oscillated at 4-11 Hz, riding on a slow DC negative shift. In this slice, when the [K+]o was increased to 10 mM, the extracellular activity desynchronized and the interictal activity became sporadic and of lower amplitude.


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FIG. 2. Effects of muscarinic and DL-2-amino-5-phosphonovaleric acid (APV) antagonists on ictal activity produced by pilocarpine and 7.5 mM [K+]o ACSF. A: in this slice, the ictal discharges lasted 15 s and occurred about every 10 s. Twenty minutes after bath application of 1 µM pirenzepine, a M1 muscarinic antagonist, ictal discharge duration decreased and the interval between discharges increased. By 40 min after pirenzepine exposure, the pattern became interictal. B: N-methyl-D-aspartate (NMDA) antagonist APV did not alter the ictal pattern of epileptiform activity. Top: control recording; Insets: interictal and ictal activity at different time scales. Bottom: activity from the same slice after bath application of 100 µM D,L-APV. Ictal duration was measured as the time from the onset of >2 Hz activity until it stopped. Interictal interval was measured as the time from the last discharge of the ictal episode to the onset of the next discharge. In this slice, the ictal duration in control was 8.4 s and the interictal interval was 56 s. In the presence of APV, the ictal discharge became slightly longer (10 s) and the interictal interval was 50 s. These findings were similar to mean changes found in 12 slices that displayed ictal activity before and after APV (see text).

Bicuculline methiodide (BMI, 10 µM) or DL-2-amino-5-phosphonovaleric acid (APV, 100 µM) was added to the ACSF to evaluate the effects of gamma -aminobutyric acid-A (GABAA) or N-methyl-D-aspartate (NMDA) receptor activation on the pattern of epileptiform activity. The ACSF osmolality was altered by either adding 20 mM mannitol to increase osmolality or, to decrease osmolality, the concentrations of sodium chloride, sodium bicarbonate, and glucose were reduced by 20%. The pattern of the epileptiform activity, the frequency of interictal discharges, and the duration and time between ictal discharges measured were assessed before and after changes in osmolality.

Data were compared with Student's t-test when normally distributed and with a Mann-Whitney rank sum test when variance was not equal. For comparisons of discharge pattern classification that occurred with manipulations of the bathing ACSF, chi 2 testing was used. Significance was defined as P < 0.05.

    RESULTS
Abstract
Introduction
Methods
Results
Discussion
References

Pilocarpine-induced epileptiform activity and [K+]o

Pilocarpine (10 µM) added to the bathing saline produced two patterns of epileptiform activity. The first pattern was similar to that produced by a variety of convulsants and consisted of a rhythmic synchronous discharge occurring at <1 Hz that could be recorded extracellularly. The intracellular correlate of the epileptiform discharge was a depolarizing potential of <200 ms duration that resembled interictal activity observed in vivo (Matsumoto and Ajmone Marsan 1964a). On occasion, a longer period of neuronal synchronization (>3 s) with an associated extracellular field discharge occurred and was characterized by a rhythmic field oscillation of >2 Hz and resembled ictal discharges recorded in vivo during a seizures (Matsumoto and Ajmone Marsan 1964b) (Fig. 1A).

Interictal discharges could occur between ictal discharges (Fig. 1A), or the ictal discharge could occur without any preceding interictal activity (Fig. 2A). Both discharge types were initiated in the CA3 region with propagation to the CA1 region of the hippocampus. Only a volume conducted potential was recorded in the dentate granule cell layer.

The interictal and ictal discharges were more likely to occur when [K+]o was elevated. At a [K+]o of 2.5 mM, interictal discharges were observed in <20% of slices and ictal discharges were not noted. In eight preparations in which all the slices from one hippocampus were recorded in 5 mM [K+]o ACSF (between 10 and 13 slices per hippocampus, total slices = 93), 30.4 ± 6.1% (mean ± SE) of the slices per preparation demonstrated desynchronized activity, 58.2 ± 8.1% demonstrated an interictal pattern, and 11.4 ± 3.3% displayed an ictal pattern. In 7.5 mM [K+]o ACSF, 16.8 ± 5.6% of slices were desynchronized, 41.2 ± 13.8% of slices had interictal burst activity, and 42.0 ± 8.3% displayed ictal activity (n = 5 preparations, 59 slices). The percentage of slices demonstrating ictal activity in 7.5 mM [K+]o ACSF was significantly higher than in 5 mM [K+]o ACSF (P = 0.0062, Mann Whitney rank sum test).

In slices bathed in 7.5 mM [K+]o that displayed epileptiform activity, an increase in [K+]o to 10 mM caused 20 of 34 slices to become desynchronized without a clear interictal or ictal discharge recorded extracellularly. Seven slices that displayed interictal discharges continued to have this pattern with an increase of [K+]o to 10 mM, and seven slices that had an ictal pattern in 7.5 mM [K+]o reverted to an interictal pattern. The difference in proportion of slices displaying epileptiform patterns was significantly different by chi 2 test (P < 0.0001) and showed that 10 mM [K+]o ACSF was less likely to promote ictal discharges.

A group of 19 slices were exposed to ascending concentrations of [K+]o (5, 7.5, and 10 mM in ACSF containing pilocarpine), and the pattern of the epileptiform activity was characterized as being interictal or ictal. In some slices, a pattern of desynchronized unit or small field discharges was noted and classified as a desynchronized pattern of activity. The ictal pattern of epileptiform activity was most common in 7.5 mM [K+]o. At 5 mM [K+]o, the predominant pattern was interictal, and at 10 mM [K+]o, over half the slices displayed a desynchronized pattern (Table 1, Fig. 1B).

 
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TABLE 1. Epileptiform activity pattern produced by pilocarpine at different [K+]o

Muscarinic and glutamatergic pharmacology of ictal discharges

Atropine (1 µM, n = 5) and the more selective M1 antagonist pirenzepine (1 µM, n = 6) stopped the occurrence of ictal discharges produced by pilocarpine in 7.5 mM [K+]o ACSF (Fig. 2A). Interictal discharges continued following the muscarinic antagonists as occurs when high [K+]o ACSF is applied in the absence of a convulsant (Rutecki et al. 1985).

Blocking AMPA/kainate receptors with 6-cyano-7-nitroquinoxaline-2,3-dione (20 µM, n = 8) resulted in a gradual loss of all spontaneously occurring epileptiform activity in either 5 or 7.5 mM [K+]o saline with pilocarpine. Antagonism of the NMDA class of glutamate receptor by bath application of 100 µM D,L-APV did not block the occurrence of ictal discharges (Fig. 2B). Although three slices changed from an ictal to an interictal pattern in the presence of APV, four slices changed from an interictal to an ictal pattern with APV application, and there was no significant change in the proportion of slices displaying ictal patterns after APV (n = 34 slices, ictal pattern in 16 slices before and in 17 slices after APV application). The duration of the ictal discharges became significantly longer after APV application (7.17 ± 0.87 vs. 8.79 ± 0.94 s, P = 0.0074, Student's paired t-test). The interval between ictal discharges was not altered by APV (128 ± 27 vs. 105 ± 42 s). In slices that only displayed interictal discharges, APV increased the rate of discharges from 0.35 ± 0.03 to 0.46 ± 0.05 (P = 0.008).

Intracellular recordings of ictal discharge and synaptic properties

The intracellular correlate of the ictal discharge consisted of a sustained depolarization with repetitive depolarizations that corresponded to the extracellular field rhythmic 4-10 Hz oscillation (Figs. 1 and 3). When recorded with potassium acetate-filled electrodes, the initial depolarization was associated with fast sodium spikes that evolved to a pattern of oscillations with associated small spikes (Fig. 1). To investigate the synaptic potentials that contribute to the ictal discharge, we recorded from CA3 pyramidal neurons using electrodes filled with CsAcetate (2 M) and QX-314 (50-100 mM). The electrode solution allowed for the membrane to be depolarized by blocking potassium channels and inhibited sodium spike and persistent sodium currents from contributing to the depolarization.


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FIG. 3. A: current-clamp recording of a CA3 neuron from a slice displaying ictal discharges produced by 10 µM pilocarpine in 7.5 mM [K+]o ACSF. Membrane oscillations that occurred during the ictal discharge were depolarizing at -85 mV and hyperpolarizing at -2 mV. Bottom: oscillation recorded intracellulary is shown at a faster sweep speed. Electrode was filled with 2 M CsAcetate and 100 mM QX-314. B: relationship between the amplitude of the 1st 2-3 membrane oscillations (VIDS) was plotted as a function of membrane potential yielding a reversal potential of -30.6 mV. , fitted to the data by linear regression.

Current-clamp recordings demonstrated that the ictal discharge oscillation could be reversed by membrane depolarization (Fig. 3). The oscillations reversed at the same potential and were superimposed on a slower potential shift that had a reversal near that of the oscillations. The mean reversal for the phasic oscillating potentials associated with the ictal discharges was -22.7 ± 2.2 mV (n = 11) suggesting a contribution of an inhibitory component to the synaptic potential. The amplitude of the individual oscillations decreased the longer the discharge lasted and increased near the end of the discharge (Fig. 3). Using single-electrode voltage-clamp techniques, the mean reversal potential of the current that drove the oscillations was -20.9 ± 3.1 mV (n = 7 neurons, Fig. 4). The associated conductance was 79.5 ± 24.8 nS. The synaptic current that sustained the oscillation was greatest at the beginning and end of the ictal discharge (Fig. 4).


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FIG. 4. A: extra- and intracellular recordings from a CA3 pyramidal neuron displaying ictal activity. Intracellular traces represent current recordings made with a single-electrode voltage clamp. Bottom traces are the oscillation depicted at a faster time base. This is the same neuron as depicted in Fig. 3. B: I-V relationship of the 1st 2 inward currents measured at the onset of the 8 Hz activity revealed a synaptic conductance of 98 nS with a reversal potential of -35 mV. , linear regression fit to the data. The negative reversal potential points to a mixed inhibitory-excitatory synaptic conductance that underlies the 8 Hz activity during the ictal discharge.

Effect of GABAA blockade on the ictal discharge

GABA-mediated synaptic potentials appear to be important for generation of theta and gamma oscillations (Traub et al. 1996b; Ylinen et al. 1995); however, GABAA blockade with bicuculline methiodide (BMI, 10 µM) did not promote the transition from an interictal to an ictal pattern or cause ictal patterns to become interictal in character (Fig. 5A). The rate of interictal discharges was increased by BMI from 0.66 ± 0.13 to 1.00 ± 0.15 Hz (n = 11, P < 0.02). BMI did not change the period between ictal discharges (45.7 ± 13.1 vs. 44.6 ± 13.1 s, n = 13) but did significantly increase the duration of the ictal discharge from 7.5 ± 0.4 to 9.5 ± 0.9 s (P < 0.005).


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FIG. 5. Blockade of gamma -aminobutyric acid-A (GABAA) inhibition by bicuculline methiodide (BMI) did not change the pattern of epileptiform activity but changed the reversal potential of the oscillations associated with the ictal discharge. A: extracellular recordings before and after bath application of 10 µM BMI showed that the ictal pattern persisted during BMI although the duration of the ictal discharge lengthened from 6.75 to 8.24 s. Ictal durations were measured from the start of >= 2 Hz activity to the end of the field activity. B: current-clamp recordings from a CA3 neuron during ictal discharges in 10 µM BMI and pilocarpine in 7.5 mM [K+]o ACSF. In these conditions, the reversal potential of the membrane oscillations associated with the ictal discharge was near 0 mV (-2 mV in this neuron), consistent with a significant contribution of GABAA-mediated inhibition to the reversal potential of the oscillation measured in the absence of BMI.

Because the reversal potential of the ictal discharge was negative to the expected reversal potential of excitatory postsynaptic potentials (EPSPs, 0 mV), the effects of GABAA blockade with BMI on the ictal discharge were investigated. The reversal potential of the depolarizing oscillations of the ictal discharge measured in current clamp was determined to assess the contribution of GABAA synaptic activation to the oscillation. In the presence of BMI, the mean reversal potential was -2.2 ± 2.6 mV (n = 6), a value significantly different than the -22.7 mV obtained for neurons in the absence of BMI (P < 0.0001, Student's independent t-test, Fig. 5B).

Alteration of osmolality and ictal patterns

The osmolality of the bathing ACSF was lowered by diluting the ACSF by 20% using a solution that maintained [K+]o, [Ca2+]o, [Mg2+]o, and [pilocarpine] but lacked other components of the control ACSF. Decreasing osmolality was expected to cause cellular swelling and reduce the extracellular volume (Chebabo et al. 1995), and in conditions where synaptic transmission is blocked, to enhance prolonged synchronous activation of dentate granule neurons (Dudek et al. 1990; Roper et al. 1992). We hypothesized that a decrease in extracellular space could enhance nonsynaptic interactions among neurons and promote ictal patterns in the CA3 region. In ACSF containing 5 mM [K+]o and 10 µM pilocarpine, 12.5% of slices demonstrated an ictal pattern of epileptiform discharges and 87.5% demonstrated an interictal pattern (n = 48, Fig. 6A). Lowering osmolality of the ACSF resulted in a significant increase in proportion of slices displaying ictal activity (47.9%, P = 0.025 by chi 2, Figs. 6A and 7A). The discharge rate in the slices that continued to demonstrate interictal discharges in hyposmotic ACSF was not different from the control 5 mM [K+]o and pilocarpine ACSF (Fig. 6C). In the six slices that demonstrated ictal discharges before changing to dilute ACSF, the interval between discharges was decreased from 212.3 ± 44.0 to 26.6 ± 2.9 s (P = 0.006, Fig. 6B), although there was no significant change in the ictal discharge duration (5.2 ± 0.3 to 3.7 ± 0.5 s, Fig. 6B).


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FIG. 6. Hyposmotic ACSF alters pilocarpine-induced epileptiform activity. A: at a [K+]o of 5 mM, making the ACSF hyposmotic significantly increased the proportion of slices demonstrating ictal patterns of epilpetiform activity from 12.5 to 47.9% (n = 48 slices, P = 0.025 by chi 2 *). Decreasing the osmolality of the 7.5 mM [K+]o ACSF caused a shift of slices that displayed synchronous patterns of epileptiform activity to desynchronized patterns. In 7.5 mM [K+]o and pilocarpine ACSF, 58% of slices displayed interictal patterns and 42% ictal patterns. In hyposmotic ACSF, the proportion significantly shifted so that 36.2% demonstrated interictal activity, 30.5% ictal activity, and 33.3% became desynchronized (Desyn, n = 69 slices, P < 0.0001 by chi 2 *). B: hyposmotic ACSF did not change the duration of ictal discharges that were present in control and hyposmotic ACSF. Interval between ictal discharges did become significantly shorter in hyposmotic ACSF(*P = 0.006 for 5 mM and P = 0.005 for 7.5 mM [K+]o ACSF). C: rate of interictal discharges in slices that displayed this pattern before and after hyposmotic ACSF did not significantly change.


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FIG. 7. Effects of osmolality of the ACSF on epileptiform pattern. A: in control 5 mM [K+]o ACSF with pilocarpine, the epileptiform activity produced in this slice was an interictal pattern. When the ACSF was made hyposmotic, the pattern became ictal. B: ictal pattern of epileptiform activity produced by 7.5 [K+]o and pilocarpine was converted to an interictal pattern by the addition of 20 mM mannitol to the ACSF.

Of 69 slices that displayed epileptiform activity in 7.5 mM [K+]o and pilocarpine, 58% had an interictal pattern of activity and 42% an ictal pattern (Fig. 6A). When the ACSF was made hyposmotic, a significant shift in epileptiform activity pattern occurred. In the hyposmotic ACSF, 36.2% of slices had an interictal pattern, 30.5% had and ictal pattern, and in 33.3% of slices epileptiform activity stopped (P < 0.0001 by chi 2). The 23 slices that stopped displaying epileptiform activity consisted of 11 that had an ictal pattern and 12 with an initial interictal pattern before solution change. In the 18 slices that continued to display a interictal pattern, there was not a significant change in rate (0.39 ± 0.2 vs. 0.47 ± 0.07 Hz after, Fig. 6C). In the 11 slices that continued to demonstrate an ictal pattern, no change in the duration of the discharge was observed (12.0 ± 3.5 vs. 10.6 ± 4.9 s); however, the interval between ictal discharges shortened significantly from 93 ± 15 s to 36 ± 5 s (Fig. 6B).

The effect of increased osmolality on epileptiform pattern was investigated by adding 20 mM mannitol, a membrane impermeable sugar, to the bathing ACSF. Before adding 20 mM mannitol, 69% of 55 slices displayed an ictal pattern of activity in 7.5 mM [K+]o and pilocarpine and 31% an interictal pattern. After mannitol, one slice that had displayed interictal activity stopped having spontaneously occurring discharges and six slices with ictal patterns changed to an interictal pattern (Fig. 7B, not significant by chi 2). Mannitol significantly slowed the discharge rate in slices that displayed an interictal pattern (0.47 ± 0.03 to 0.31 ± 0.06 Hz, P = 0.001). Mannitol also changed the characteristics of the ictal pattern. The duration of the ictal discharge significantly shortened from 8.8 ± 0.4 to 7.7 ± 0.4 s (P = 0.01, n = 32). The interval between ictal discharges significantly lengthened from 36.9 ± 4.1 to 85.4 ± 19.3 s (P = 0.005).

    DISCUSSION
Abstract
Introduction
Methods
Results
Discussion
References

Patterns of epileptiform activity in the slice

In general, the CA3 region of the hippocampus is the most susceptible to interictal discharge generation (Schwartzkroin and Prince 1978); however, ictal discharges have been noted under certain conditions. More prolonged discharges (>3 s) have been noted in the CA3 region of slices from younger animals when the bathing solution contains penicillin (Smith et al. 1995; Swann et al. 1993), 4-aminopyridine (Avoli et al. 1996; Chestnut and Swann 1988), or low [Mg2+]o (Anderson et al. 1986). In the CA1 region, increases in [K+]o produces ictal discharges that appear to occur in this area because of the tight packing of neurons and a relatively small extracellular space (Jensen and Yarri 1988; McBain et al. 1990; Traynelis and Dingledine 1988, 1989). In slices from adult animals, the CA3 region is relatively resistant to more prolonged discharges. When the entorhinal cortex is included in the slice and its projections are intact, the dentate granule neurons and the CA3 region demonstrate prolonged discharges that resemble ictal activity and are driven by ictal activity generated in entorhinal cortex (Barbarosie and Avoli 1997; Jones and Lambert 1990; Nagao et al. 1996; Walther et al. 1986).

We found that in the presence of pilocarpine, increasing [K+]o could produce ictal discharges in the CA3 region. Similar patterns have been described when pilocarpine is bath applied to a entorhinal-hippocampal slice; however, when the entorhinal projection to the hippocampus is cut, the CA3 region reverts to an interictal pattern (Nagao et al. 1996). Prolonged synchronous activity has been noted with carbachol, a nonhydrolizable cholinergic agonist that activates both nicotinic and muscarinic receptors. This pattern was felt to represent a rhythmic slow activity analogous to the theta rhythm observed in vivo and, in some cases, in the slice (Bland et al. 1988; MacVicar and Tse 1989). This rhythmic slow activity occurs primarily in slices from younger animals (MacVicar and Tse 1989). Carbachol also produces synchronized rhythmic bursts in CA3 neurons of guinea pig hippocampal slices when ionotropic glutamate and GABA receptors are blocked (Bianchi and Wong 1994). Because pilocarpine is a potent convulsant, we believe that the activity produced by pilocarpine is more akin to epileptiform activity rather than to theta activity.

Synaptic characteristics of ictal activity

As seen with other cholinergic-induced synchronized activity (Bianchi and Wong 1994; MacVicar and Tse 1989; Nagao et al. 1996), atropine and pirenzepine, muscarinic antagonists, blocked the ictal pattern of discharges. Furthermore, all epileptiform activity was blocked by kainate/AMPA antagonism, supporting the hypothesis that local fast excitatory synaptic interactions among CA3 neurons drive the sustained oscillation that occurs during the ictal discharge. In some models of more prolonged epileptiform discharges, NMDA-mediated synaptic transmission appears to drive so-called secondary discharges (Traub et al. 1993, 1996a). In the current model and the rhythmic slow activity produced by carbachol, NMDA blockade did not impair generation of prolonged synchronous discharges (MacVicar and Tse 1989). We found that NMDA blockade increased the rate of interictal discharges as described by others (Nagao et al. 1996), and the duration of ictal discharges was prolonged when NMDA receptors were blocked. These results may be explained by a decrease in the amount of calcium influx during epileptiform discharges and a resultant decrease in calcium activated potassium repolarizing currents (Zorumski et al. 1989).

The intracellular phasic component of the ictal discharge could be reversed in polarity supporting the hypothesis that synaptic transmission provides the drive for the oscillation. The reversal potential was around -20 mV, suggesting that the large synaptic potential was composed of both inhibitory and excitatory synaptic components. Furthermore, GABAA receptor blockade changed the reversal potential to near 0 mV, the reversal potential of fast glutamatergic synapses, consistent with a GABAA contribution to the ictal discharge. The pattern of activity was not qualitatively changed by bicuculline, but GABAA receptor activation did contribute to the timing of interictal discharges and duration of ictal discharges. When bicuculline was present, the interictal discharge rate increased significantly and the ictal discharges became significantly longer. Enhancement of GABAA synaptic transmission appears to depress pilocarpine ictal discharges. Propofol, an anesthetic that prolongs GABAA-mediated currents (Orser et al. 1995), blocks the occurrence of pilocarpine-induced ictal activity (Rasmussen et al. 1996).

Pilocarpine decreases CA3 paired-pulse inhibition activated by stratum radiatum stimulation (Sayin and Rutecki 1997). Carbachol decreases evoked monosynaptic inhibitory transmission mediated by GABAA receptors, although spontaneously occurring inhibitory postsynaptic potentials are increased by muscarinic activation (Behrends and ten Bruggencate 1993; Pitler and Alger 1992). GABAB receptor blockade did not change the rhythmic slow activity produced by carbachol (MacVicar and Tse 1989), and we did not observe a significant effect of GABAB block with 5-hydroxy-saclofen (100-500 µM) on pilocarpine-induced ictal discharges (unpublished observations).

Muscarine also causes presynaptic inhibition of excitatory transmission in the hippocampus (Williams and Johnston 1990). Modeling studies demonstrate that if excitatory conductance is increased beyond a certain level, the network will desynchronize (Traub et al. 1992). The presynaptic inhibitory properties of muscarinic receptors may help prevent excessive excitation or inhibition from desynchronizing the CA3 synaptic network. Postsynaptic excitatory actions of muscarine that decrease potassium currents and favor activation of a calcium-dependent cationic current (see further) are expected to enhance spontaneous synaptic potentials and action potential generation.

Factors that favor transformation of epileptiform patterns

The main factor that favored the transition to an ictal pattern of epileptiform activity was the level of [K+]o. An increase in extracellular [K+]o much above 5 mM will produce spontaneously occurring interictal epileptiform activity in the CA3 region, which activates similar activity in the CA1 region (Rutecki et al. 1985). Ictal discharges in CA3 region of the immature hippocampus produced by 4-aminopyridine are associated with a rise in [K+]o that appears to help maintain prolonged synchronization (Avoli et al. 1996). At a [K+]o of 8.5 mM and a large periodic synaptic input, the CA1 region will independently change its epileptiform pattern to resemble an ictal pattern (Traynelis and Dingledine 1988). The ictal pattern that is generated in CA1 is associated with a precipitous rise of [K+]o above 8.5 mM and a decrease in extracellular space (Traynelis and Dingledine 1989). The extracellular volume is smallest in the CA1 region, ~12%, compared with the CA3 region where it's ~18% (McBain et al. 1990). Increasing [K+]o to 8.5 mM further decreases extracellular space in both the CA3 and CA1 region (McBain et al. 1990). Increasing extracellular space by increasing extracellular osmolality retards ictal discharges in CA1 in the high [K+]o model (Traynelis and Dingledine 1989) and decreases prolonged synchronization in dentate and CA1 in the absence of synaptic transmission (Dudek et al. 1990; Roper et al. 1992).

Our model of ictal activity in the CA3 subfield produced by pilocarpine and elevated [K+]o behaved in a similar fashion to the ictal discharges that occur in the CA1 region in the absence of pilocarpine. Increasing extracellular space by using mannitol to increase osmolality decreased the occurrence and duration of ictal like discharges and slowed the rate of interictal discharges produced in pilocarpine and 7.5 mM [K+]o. Conversely, lowering the osmolality and decreasing extracellular space converted interictal patterns to ictal patterns of activity. Ictal discharges occurred became more frequently after a change to a hyposmotic ACSF.

Lowering osmolality and decreasing extracellular space has a number of effects that include limited spatial buffering of ions and neurotransmitters, enhanced ephaptic interactions among neurons, and increased number of spontaneously occurring EPSPs and burst firing in CA3 pyramidal neurons (Andrew et al. 1989; Jensen et al. 1994; Jensen and Yaari 1997; McBain et al. 1990; Saly and Andrew 1993). Our findings that at 10 mM [K+]o, pilocarpine was less likely to produce ictal patterns of activity than at 7.5 mM [K+]o and that a similar desynchronization of ictal patterns of activity was produced by hyposmotic 7.5 mM [K+]o ACSF argue that extracellular [K+]o is the main factor in favoring the ictal occurrence in the CA3 region when pilocarpine is present. Without pilocarpine, interictal discharges produced by high [K+]o gradually slow and desynchronize at concentrations >10 mM (Rutecki et al. 1985). Excessive depolarization of CA3 pyramidal neurons may cause inactivation of currents required for appropriate patterns of action potential generation that help synchronize network activity.

A number of factors may explain the action of pilocarpine to promote ictal activity in the CA3 region. One may be that pilocarpine, like increasing [K+]o or decreasing osmolality, enhances spontaneously occurring EPSPs. In a model of high [K+]o interictal activity in the CA3 region, spontaneously occurring EPSPs appear to be the main factor that drives synchronization (Chamberlin et al. 1990; Traub and Dingledine 1990). Similar conclusions where made using a model of rhythmic slow activity produced by carbachol (Traub et al. 1992). Another effect of pilocarpine is to promote the occurrence of a plateau potential in CA1 neurons that appears to be produced by a calcium-activated nonselective cation conductance interacting with high-threshold calcium channels (Fraser and MacVicar 1996). Such an intrinsic excitability property produced by pilocarpine in CA3 pyramidal neurons could explain how a prolonged synchronous activity driven by synaptic interactions occurs in the CA3 region of the hippocampus. Depolarization and elevated [K+]o would enhance the occurrence of such a plateau potential.

The ictal epileptiform patterns produced by pilocarpine appear to result from multiple factors that depend on synaptic and nonsynaptic mechanisms. Fast glutamatergic ionotropic receptors drive the prolonged ictal oscillation. GABAA receptor activation contributes to, but is not required for, ictal patterns of epileptiform activity. The prolonged synchronized synaptic drive responsible for the ictal discharge is also dependent on nonsynaptic mechanisms that include [K+]o and a decrease in extracellular space.

    ACKNOWLEDGEMENTS

  We thank Drs. Lew Haberly Peter Lipton, Bill Lytton, and Tom Sutula for comments on drafts of this manuscript.

  This study was supported by Veteran Affairs research and the National Institute of Neurological Disorders and Stroke (NS-28580).

    FOOTNOTES

   Present address of Y. Yang: Dept. of Neurology, Children's Hospital, Harvard Medical School, 300 Longwood Ave., Boston, MA 02115.

  Address for reprint requests: P. A. Rutecki, Dept. of Neurology, Neurosurgery, and Neuroscience Training Program, University ot Wisconsin Medical School, William S. Middleton VA Hospital, 2500 Overlook Tr., Madison, WI 53705.

  Received 7 October 1997; accepted in final form 18 February 1998.

    REFERENCES
Abstract
Introduction
Methods
Results
Discussion
References

0022-3077/98 $5.00 Copyright ©1998 The American Physiological Society