CPP, an NMDA-receptor antagonist, blocks 4-aminopyridine-induced spreading depression episodes but not epileptiform activity in immature rat hippocampal slices

Spreading depression: from serendipity to targeted therapy in migraine prophylaxis

Despite the relatively well-characterized headache mechanisms in migraine, upstream events triggering individual attacks are poorly understood. This lack of mechanistic insight has hampered a rational approach to prophylactic drug discovery. Unlike targeted abortive and analgesic interventions, mainstream migraine prophylaxis has been largely based on serendipitous observations (e.g. propranolol) and presumed class effects (e.g. anticonvulsants). Recent studies suggest that spreading depression is the final common pathophysiological target for several established or investigational migraine prophylactic drugs. Building on these observations, spreading depression can now be explored for its predictive utility as a preclinical drug screening paradigm in migraine prophylaxis.

Methodological approaches to exploring epileptic disorders in the human brain in vitro

Brain surgery, and in particular epilepsy surgery, offers the unique opportunity to study viable human central nervous tissue in vitro. This does not only open a window to address the basic mechanisms underlying human disease, such as epilepsy, but it allows to venture into investigating neurophysiological functions per se. In the present paper, we describe the most commonly used methods in the electrophysiological (and, at least to some extent, also histochemical and molecular) analysis of human tissue in vitro. In addition, we consider the pitfalls and limitations of such studies, in particular regarding the issue of tissue sampling procedures and control experiments.

Background potassium concentrations and epileptiform discharges. II. Involvement of calcium channels

Potassium- and calcium conductances regulate neuronal excitability and epileptiform activity. In this study, the effects of different extracellular potassium concentrations ([K(+)](o)) were investigated on the modulatory effect of the L-type transmembranous calcium currents on epileptiform discharges. The in vitro brain slice technique was used to examine the effects of calcium channel blockers, verapamil and nifedipine, on the repetition rate, amplitude, and duration of epileptiform field potentials (EFP) in the presence of low, physiological, and high background [K(+)](o) in guinea pig hippocampal slices. Epileptiform activity was induced by omission of Mg(2+) from artificial cerebrospinal fluid contained 2, 4, and 8 mM [K(+)](o). Both verapamil and nifedipine suppressed EFP after a transient increase in repetition rate. The extent of EFP frequency rate acceleration significantly increased with reduction of [K(+)](o). The increase in EFP frequency rate induced by application of verapamil and nifedipine was accompanied by a reduction in the EFP amplitude and a reversible increase in the burst discharge duration. The extent of burst discharge prolongation was also significantly higher with decreasing [K(+)](o). Further application of verapamil and nifedipine suppressed the epileptiform burst activity in the presence of different [K(+)](o). The latency of EFP depression was significantly diminished both with increased and decreased background potassium concentrations. The data indicate the importance of the effect of the L-type transmembranous calcium currents on the regulatory effect of background [K(+)](o) on epileptiform burst discharge frequency and duration.

The effects of different 4-aminopyridine and morphine combinations on the intensity of morphine abstinence

The glutamatergic system is deeply involved in the development of opiate dependence and in the manifestation of opiate abstinence syndrome. In this study the effect of the increase in the endogenous glutamate (GLU) release due to 4-aminopyridine (4-AP), a potassium channel blocker, during the development of morphine (M) physical dependence and during the naloxone (NL)-precipitated abstinence syndrome was investigated. For the development of physical dependence M was intraperitoneally (i.p.) injected for 9 days 105 min following i.p. saline administration to a group of rats. In the first 3 days the dose of M was 10 mg x kg(-1). In the second 3 days the initial dose was doubled (20 mg x kg(-1)) and in the last 3 days the dose of M was raised to 40 mg x kg(-1). On day 10, the rats were divided into three groups at random and these three groups were i.p. given saline 105 min before 80 mg x kg(-1)M, 2 mg x kg(-1) 4-AP 105 min after 80 mg x kg(-1) M, and 80 mg x kg(-1) M 105 min before 2 mg x kg(-1) 4-AP, respectively. In a second group of rats, the rats were i.p. given 2 mg x kg(-1) 4-AP 105 min prior to M administration, which was increased every 3 days (10 mg x kg(-1), 20 mg x kg(-1), 40 mg x kg(-1)). On day 10, the rats were divided into two groups whose first injection was saline and 2 mg x kg(-1) 4-AP, respectively. The second injections of both groups after an interval of 105 min following the first one contained 80 mg x kg(-1) M. In contrast, one group of rats received only i.p. saline at every other injection time (the control group). Furthermore, another group of rats was i.p. administered 2 mg x kg(-1) 4-AP once a day, as the first injection. At the second injection time they were i.p. given saline. After a period of 15 min following the last administration on day 10, the rats belonging to all groups were i.p. injected with 2 mg x kg(-1) NL and immediately placed in a metal cage. Body weight loss (g), teeth chattering, rearing, wet-dog shaking, grooming, and jumping were determined or counted for 15 min. Penile erection, defecation, and diarrhoea were separately scored with one point for every individual occurrence, and the total score was named 'total number of others'. The administration of 4-AP before M appeared to intensify the development of dependence, and was most probably due to the Ca2+-induced inactivation of NMDA receptors as a result of excess release of GLU when the 4-AP took effect. The inactivation of NMDA receptors should have acted as a transient blockade of the receptors during the chronic administration period, and as well as after a single administration on day 10 before M injection and before abstinence. The intensification of the abstinence syndrome may be dependent on the excessive GLU released by 4-AP.

Modulation by adenine nucleotides of epileptiform activity in the CA3 region of rat hippocampal slices

1. Hippocampal slices (450 microm) generate epileptiform bursts of an interictal nature when perfused with a zero magnesium medium containing 4-aminopyridine (50 microM). The effect of adenine nucleotides on this activity was investigated. 2. ATP and adenosine depressed this epileptiform activity in a concentration-dependent manner, with both purines being equipotent at concentrations above 10 microM. 3. Adenosine deaminase 0.2 u ml(-1), a concentration that annuls the effect of adenosine (50 microM), did not significantly alter the depression of activity caused by ATP (50 microM). 4. 8-Cyclopentyl-1,3-dimethylxanthine (CPT), an A1 receptor antagonist, enhanced the discharge rate significantly and inhibited the depressant effect of both ATP and adenosine such that the net effect of ATP or adenosine plus CPT was excitatory. 5. Several ATP analogues were also tested: alpha, beta-methyleneATP (alpha, beta-meATP), 2-methylthioATP (2-meSATP) and uridine triphosphate (UTP). Only alpha, beta-meATP (10 microM) produced an increase in the frequency of spontaneous activity which suggests a lack of involvement of P2Y or P2U receptors. 6. Suramin and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), P2 receptor antagonists, failed to inhibit the depression produced by ATP (50 microM). The excitatory effect of alpha, beta-meATP (10 microM) was inhibited by suramin (50 microM) and PPADS (5 microM). 7. ATP therefore depresses epileptiform activity in this model in a manner which is not consistent with the activation of known P1 or P2 receptors, suggesting the involvement of a xanthine-sensitive nucleotide receptor. The results are also indicative of an excitatory P2X receptor existing in the hippocampal CA3 region.

Astrocytic neurotransmitter receptors in situ and in vivo

In the brain, astrocytes are associated intimately with neurons and surround synapses. Due to their close proximity to synaptic clefts, astrocytes are in a prime location for receiving synaptic information from released neurotransmitters. Cultured astrocytes express a wide range of neurotransmitter receptors, but do astrocytes in vivo also express neurotransmitter receptors and, if so, are the receptors activated by synaptically released neurotransmitters? In recent years, considerable efforts has gone into addressing these issues. The experimental results of this effort have been compiled and are presented in this review. Although there are many different receptors which have not been identified on astrocytes in situ, it is clear that astrocytes in situ express a number of different receptors. There is evidence of glutamatergic, GABAergic, adrenergic, purinergic, serotonergic, muscarinic, and peptidergic receptors on protoplasmic, fibrous, or specialized (Bergmann glia, pituicytes, Müller glia) astrocytes in situ and in vivo. These receptors are functionally coupled to changes in membrane potential or to intracellular signaling pathways such as activation of phospholipase C or adenylate cyclase. The expression of neurotransmitter receptors by astrocytes in situ exhibits regional and intraregional heterogeneity and changes during development and in response to injury. There is also evidence that receptors on astrocytes in situ can be activated by neurotransmitter(s) released from synaptic terminals. Given the evidence of extra-synaptic signaling and the expression of neurotransmitter receptors by astrocytes in situ, direct communication between neurons and astrocytes via neurotransmitters could be a widespread form of communication in the brain which may affect many different aspects of brain function, such as glutamate uptake and the modulation of extracellular space.

GFAP-positive hippocampal astrocytes in situ respond to glutamatergic neuroligands with increases in [Ca2+]i

It is becoming increasingly clear that astrocytes play very dynamic and interactive roles that are important for the normal functioning of the central nervous system. In culture, astrocytes express many receptors coupled to increases in intracellular calcium ([Ca2+]i). In vivo, it is likely that these receptors are important for the modulation of astrocytic functions such as the uptake of neurotransmitters and ions. Currently, however, very little is known about the expression or stimulation of such astrocytic receptors in vivo. To address this issue, confocal microscopy and calcium-sensitive fluorescent dyes were used to examine the dynamic changes in astrocytic [Ca2+]i within acutely isolated hippocampal slices. Astrocytes were subsequently identified by immunocytochemistry for glial fibrillary acidic protein. In this paper, we present data indicating that hippocampal astrocytes in situ respond to glutamate, kainate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), 1-aminocyclopentane-trans-1,3-dicarboxylic acid (t-ACPD), N-methyl-D-aspartate (NMDA), and depolarization with increases in [Ca2+]i. The increases in [Ca2+]i occurred in both the astrocytic cell bodies and the processes. Temporally the changes in [Ca2+]i were very dynamic, and various patterns ranging from sustained elevations to oscillations of [Ca2+]i were observed. Individual astrocytes responded to neuroligands selective for both ionotropic and metabotropic glutamate receptors with increases in [Ca2+]i. These findings indicate that astrocytes in vivo contain glutamatergic receptors coupled to increases in [Ca2+]i and are able to respond to neuronally released neurotransmitters.

Effects of glutamate, N-methyl-D-aspartate, high potassium, and hypoxia on unit discharges in CA1 area of hippocampal slices of DBA and C57 mice

We studied effects of L-glutamate, N-methyl-D-aspartate (NMDA), high K+, and hypoxia on spontaneous unit discharges in stratum pyramidale of CA1 region of hippocampal slices in DBA and C57 mice aged 3-4 and 5-6 weeks. Application of L-glutamate (0.5-2.0 mM), NMDA (5-20 microM), high K+ (8.5 mM), and a brief period of hypoxia (1 min) to the perfused artificial cerebrospinal fluid (ACSF) all produced different degrees of spontaneous high-frequency discharges from CA1 area of hippocampal slices of both DBA and C57 mice. Two types of responses recorded extracellularly occurred after these manipulations: high-frequency repetitive single spikes and bursts of multiple population spikes. The rate and type of responses from CA1 region of hippocampal slices after these manipulations were different and depended on the strain and age of mice and the nature of manipulations. In general, hippocampal slices from audiogenic seizure-susceptible DBA mice were more sensitive than those from audiogenic seizure-resistant C57 mice, and hippocampal slices from younger animals were more susceptible than those from older ones. Thus, DBA mice aged 3-4 weeks of age were most susceptible and C57 mice aged 5-6 weeks were least susceptible to all these pharmacological, ionic, and hypoxic manipulations. Bursts of multiple population spikes were the most common responses in DBA mice and in younger animals, and repetitive single spikes were the predominant responses in C57 mice and in older animals. In all groups of animals, the average spontaneous discharge rate was highest after L-glutamate perfusion, next highest after NMDA, and lowest after high K+ and hypoxia. The latency of the appearance of spontaneous epileptiform activity from CA1 region of hippocampal slices was long (> 2 min) after NMDA perfusion and short (< 1 min) after L-glutamate, high K+ and hypoxia. The duration of the increased spontaneous discharges was short (-1 min) after L-glutamate perfusion, long (> 3 min) after high K+ and hypoxia, and between short and long after NMDA perfusion. These results suggest that age and strain of animal and nature of stimulus precipitate different patterns of epileptiform activity in CNS.

Are there unifying principles underlying the generation of epileptic afterdischarges in vitro?

To find general principles in the cellular mechanisms of epileptogenesis, one must analyze experimental epilepsy models and determine what exists in common between them. We consider here afterdischarges in hippocampal slices induced using either (1) GABAA blockade (e.g. with bicuculline), (2) a bathing solution lacking Mg2+ ions (low Mg-induced epilepsy), or (3) 4-aminopyridine (4AP). By 'afterdischarge' we mean an event that lasts hundreds of milliseconds or more, involving the synchronous firing of all the neurons in a population, shaped into a long initial burst and a series of one or more secondary bursts, and terminating in a prolonged afterhyperpolarization (AHP). We propose that the following features exists in common between these three experimental epilepsies: (1) recurrent excitatory synaptic connections; (2) sustained dendritic synaptic excitation, mediated by either AMPA or NMDA receptors, or both; (3) an intrinsic cellular response to sustained excitation, consisting of rhythmical dendritic bursts, primarily mediated by Ca spikes. In conclusion, if the picture outlined here proves correct, then the stereotypic appearance of epileptic afterdischarges--consisting of synchronized population bursts in series, whatever the network alteration leading to seizures--does indeed reflect a common set of mechanisms. The mechanisms cannot, apparently, be formulated in simple terms of this receptor or that receptor. Rather, we suggest, the recurrent excitatory synapses are able, under diverse circumstances, collectively to produce sustained dendritic conductances in neuronal populations. Pyramidal neurons, by virtue of their normal intrinsic membrane properties, respond to such sustained conductances with rhythmical bursts. The recurrent synapses, in a dual role, serve to maintain the synchrony of these bursts, and so shape the activity into a synchronized oscillation.

4-Aminopyridine-induced spreading depression episodes in immature hippocampus: developmental and pharmacological characteristics

Spontaneous spreading depression episodes were studied in CA1 and CA3 areas of immature hippocampal slices (two to 30 days postnatally) during 4-aminopyridine (50 microM) perfusion. Spreading depression occurred in the CA3 area of 34% of all slices tested (two to 30 days postnatally). The duration and frequency of the spreading depression field potentials changed with development. In the CA3 area, their duration decreased from 169 +/- 22 s (n = 17, postnatal days to to 10) to 55 +/- 7 s (n = 10, postnatal days 21-30), their rate of occurrence increased from four episodes per hour (0.0011 +/- 0.0001 Hz, n = 11, postnatal days two to 10) to 6.5 episodes per hour (0.0018 +/- 0.0003 Hz, n = 8, postnatal days 21-30), while their amplitude remained stable (10-30 mV). Spreading depression d.c. potential shift originated closer to CA1 than CA3. Furthermore, spreading depression field potentials had greater magnitude (amplitude and duration) in CA1. Spreading depressions were reversibly blocked by the N-methyl-D-aspartate receptor antagonist 3,3-(2-carboxy-piperazine-4-yl)-propyl-1-phosphonate (CPP, 1-5 microM, n = 15), but were not affected by 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX, 2-5 microns, n = 11), which is a non-N-methyl-D-aspartate receptor antagonist. The GABAA receptor antagonist bicuculline methiodide (3-10 microM) initially favored and then blocked spreading depression in 79% of the slices tested (n = 16). In addition, bicuculline impaired spreading depression propagation from CA1 to CA3. 4-Aminopyridine also induced the appearance of other types of spontaneous activity, such as ictal and interictal-like epileptiform discharges. The effects of 3,3-(2-carboxy-piperazine-4-yl)-propyl-1-phosphonate, 6-cyano-7-nitro-quinoxaline-2,3-dione and bicuculline on epileptiform activity were opposite to those on spreading depression. Our findings demonstrate that spreading depression can occur as early as two days postnatally and that the characteristics of this phenomenon change with maturation. These results also indicate that 4-aminopyridine-induced spreading depression episodes and epileptiform activity are mediated by the activation of different types of excitatory amino acid receptors. Finally, spreading depression is influenced by blockade of the GABAA receptor.

Tetraethylammonium-induced epileptiform activity in young and adult rat hippocampus

Extracellular field potential recordings were used to study the epileptiform activity evoked by tetraethylammonium (TEA) in the CA3 subfield of hippocampal slices obtained from young (12-18 day-old) and adult (> 60-day-old) rats. During TEA application (5-10 mM), young slices generated both ictal-like (duration: up to 28 s, rate of occurrence 1-3 x 10(-2) s-1) and interictal-like (duration: 1.5-2 s; rate of occurrence: 1-3 x 10(-1) s-1) activity. In adult slices only interictal-like activity was induced by TEA (3-10 mM). Depending on the concentrations of TEA, these events lasted 80-600 ms and occurred at 5-60 x 10(-2) s-1. Both the N-methyl-D-aspartate (NMDA) receptor antagonist 3-3(2-carboxypiperazine-4-yl)propyl-1-phosphonate (5-10 microM; CPP) and the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (5-10 microM; CNQX) were necessary to suppress ictal-likeand interictal-like discharges in young slices. By contrast, interictal-like activity in adult slices was reduced and eventually blocked by CNQX (0.5-3 microM) alone. Furthermore the pattern of epileptiform discharges seen in young slices was modified by CPP (i.e. decrease in the rate of occurrence of ictal events and reduction in the duration of interictal discharges), while the activity recorded in adult slices was resistant to this NMDA antagonist. Bicuculline methiodide (5 microM; BMI) enhanced the duration of epileptiform activities in both young and adult slices. Our data demonstrate that the epileptiform discharges induced by TEA in the CA3 subfield of the rat hippocampus display age-dependent patterns of activity.(ABSTRACT TRUNCATED AT 250 WORDS)