Spinal and cortical spreading depression enhance spinal cord activity

Spreading Depolarization Induces a Transient Potentiation of Excitatory Synaptic Transmission

Spreading depolarization (SD) is a slowly propagating wave of prolonged activation followed by a period of synaptic suppression. Some prior reports have shown potentiation of synaptic transmission after recovery from synaptic suppression and noted similarities with the phenomenon of long-term potentiation (LTP). Since SD is increasingly recognized as participating in diverse neurological disorders, it is of interest to determine whether SD indeed leads to a generalized and sustained long-term strengthening of synaptic connections. We performed a characterization of SD-induced potentiation, and tested whether distinctive features of SD, including adenosine accumulation and swelling, contribute to reports of SD-induced plasticity. Field excitatory postsynaptic potentials (fEPSPs) were recorded in the hippocampal CA1 subregion of murine brain slices, and SD elicited using focal microinjection of KCl. A single SD was sufficient to induce a consistent potentiation of slope and amplitude of fEPSPs. Both AMPA- and NMDA-receptor mediated components were enhanced. Potentiation peaked ∼20 min after SD recovery and was sustained for ∼30 min. However, fEPSP amplitude and slope decayed over an extended 2-hour recording period and was estimated to reach baseline after ∼3 h. Potentiation was saturated after a single SD and adenosine A1 receptor activation did not mask additional potentiation. Induction of LTP with theta-burst stimulation was not altered by prior induction of SD and molecular mediators known to block LTP induction did not block SD-induced potentiation. Together, these results indicate an intermediate duration potentiation that is distinct from hippocampal LTP and may have implications for circuit function for 1-2 h following SD.

The Critical Role of Spreading Depolarizations in Early Brain Injury: Consensus and Contention

BACKGROUND

When a patient arrives in the emergency department following a stroke, a traumatic brain injury, or sudden cardiac arrest, there is no therapeutic drug available to help protect their jeopardized neurons. One crucial reason is that we have not identified the molecular mechanisms leading to electrical failure, neuronal swelling, and blood vessel constriction in newly injured gray matter. All three result from a process termed spreading depolarization (SD). Because we only partially understand SD, we lack molecular targets and biomarkers to help neurons survive after losing their blood flow and then undergoing recurrent SD.

METHODS

In this review, we introduce SD as a single or recurring event, generated in gray matter following lost blood flow, which compromises the Na⁺/K⁺ pump. Electrical recovery from each SD event requires so much energy that neurons often die over minutes and hours following initial injury, independent of extracellular glutamate.

RESULTS

We discuss how SD has been investigated with various pitfalls in numerous experimental preparations, how overtaxing the Na⁺/K⁺ ATPase elicits SD. Elevated K⁺ or glutamate are unlikely natural activators of SD. We then turn to the properties of SD itself, focusing on its initiation and propagation as well as on computer modeling.

CONCLUSIONS

Finally, we summarize points of consensus and contention among the authors as well as where SD research may be heading. In an accompanying review, we critique the role of the glutamate excitotoxicity theory, how it has shaped SD research, and its questionable importance to the study of early brain injury as compared with SD theory.

GABAergic Mechanisms Can Redress the Tilted Balance between Excitation and Inhibition in Damaged Spinal Networks

Correct operation of neuronal networks depends on the interplay between synaptic excitation and inhibition processes leading to a dynamic state termed balanced network. In the spinal cord, balanced network activity is fundamental for the expression of locomotor patterns necessary for rhythmic activation of limb extensor and flexor muscles. After spinal cord lesion, paralysis ensues often followed by spasticity. These conditions imply that, below the damaged site, the state of balanced networks has been disrupted and that restoration might be attempted by modulating the excitability of sublesional spinal neurons. Because of the widespread expression of inhibitory GABAergic neurons in the spinal cord, their role in the early and late phases of spinal cord injury deserves full attention. Thus, an early surge in extracellular GABA might be involved in the onset of spinal shock while a relative deficit of GABAergic mechanisms may be a contributor to spasticity. We discuss the role of GABA A receptors at synaptic and extrasynaptic level to modulate network excitability and to offer a pharmacological target for symptom control. In particular, it is proposed that activation of GABA A receptors with synthetic GABA agonists may downregulate motoneuron hyperexcitability (due to enhanced persistent ionic currents) and, therefore, diminish spasticity. This approach might constitute a complementary strategy to regulate network excitability after injury so that reconstruction of damaged spinal networks with new materials or cell transplants might proceed more successfully.

The potential of substance P to initiate and perpetuate cortical spreading depression (CSD) in rat in vivo

The tachykinin substance P (SP) increases neuronal excitability, participates in homeostatic control, but induces brain oedema after stroke or trauma. We asked whether SP is able to induce cortical spreading depression (CSD) which often aggravates stroke-induced pathology. In anesthetized rats we applied SP (10⁻⁵, 10⁻⁶, 10⁻⁷, or 10⁻⁸ mol/L) to a restricted cortical area and recorded CSDs there and in remote non-treated areas using microelectrodes. SP was either applied in artificial cerebrospinal fluid (ACSF), or in aqua to perform a preconditioning. Plasma extravasation in cortical grey matter was assessed with Evans Blue. Only SP dissolved in aqua induced self-regenerating CSDs. SP dissolved in ACSF did not ignite CSDs even when excitability was increased by acetate-preconditioning. Aqua alone elicited as few CSDs as the lowest concentration of SP. Local pretreatment with 250 nmol/L of a neurokinin 1 receptor antagonist prevented the SP-induced plasma extravasation, the initiation of CSDs by 10⁻⁵ mol/L SP diluted in aqua, and the initiation of CSDs by aqua alone, but did not suppress KCl-induced CSD. Thus neurokinin 1 receptor antagonists may be used to explore the involvement of SP in CSDs in clinical studies.

The facilitating effect of unfavorable lactation on the potentiation of electrocorticogram after spreading depression in awake and anesthetized adult rats

OBJECTIVES

Cortical spreading depression (CSD) is a brain excitability-related phenomenon that can be affected by unfavorable conditions of lactation and by anesthetic agents. We have previously demonstrated that after CSD the electrocorticogram (ECoG) amplitude increases significantly (ECoG potentiation). Here, we investigated this potentiation in awake and anesthetized adult rats that were previously suckled among different lactation conditions.

METHODS

Newborn rats were suckled in litters with 6 pups or 12 pups (L₆ or L₁₂ condition, respectively). At adulthood, we evaluated the ECoG potentiation after CSD at two cortical points (one point near, and another remote to the CSD-eliciting site). The amplitude of the ECoG waves was averaged with the support of an algorithm implemented in Matlab™ software. In both L₆ and L₁₂ condition, awake animals were compared with anesthetized groups that received either tribromoethanol- or urethane + chloralose-anesthesia.

RESULTS

L₁₂ rats presented significantly lower body- and brain weights than L₆ rats (P < 0.01), indicating a nutritional deficiency. The anesthetized L₆ groups presented with ECoG potentiation (P < 0.05) only in the near cortical recording point (28.0% and 32.6% increase for the tribromoethanol and urethane + chloralose groups, respectively), whereas the L₁₂ groups displayed ECoG potentiation in both near (67.0% and 55.0%) and remote points (37.0% and 20.0%), in comparison with the baseline values (before CSD).

DISCUSSION

The results suggest a facilitating effect of unfavorable lactation on the potentiation of ECoG after spreading depression in anesthetized adult rats. The potential implications for the human neurological health remain to be investigated.

Effects of garlic extract on spreading depression: In vitro and in vivo investigations

OBJECTIVES

The potential use of garlic for prevention and treatment of different types of headaches has been suggested by several medieval literatures. Cortical spreading depression (CSD), a propagating wave of neuroglial depolarization, was established as a target for anti-migraine drugs. This study was designed to investigate the effect of garlic extract on CSD in adult rats.

METHODS

CSD was induced by KCl microinjection in the somatosensory cortex. The effects of five different concentrations of garlic oil (1-500 μl/l) were tested on different characteristic features of CSD in necocortical slices. In in vivo experiments, the effects of garlic oil on electrophysiological and morphological changes induced by CSD were investigated.

RESULTS

Garlic oil in a dose-dependent manner decreased the amplitude of CSD but not its duration and velocity in neocortical brain slices. Garlic oil at concentration of 500 μl/l reversibly reduced the amplitude of the field excitatory post-synaptic potentials and inhibited induction of long-term potentiation in the third layer of neocortical slices. In in vivo studies, systemic application of garlic oil (1 ml/l) for three consecutive days reduced the amplitude and repetition rate of CSD. Garlic oil also prevented of CSD-induced reactive astrocytosis in the neocortex.

DISCUSSION

Garlic oil suppresses CSD, likely via inhibition of synaptic plasticity, and prevents its harmful effects on astrocyte. Further studies are required to identify the exact active ingredient(s) of garlic oil that inhibit CSD and may have the potential to use in treatment of CSD-related disorders.

Anesthetic agents modulate ECoG potentiation after spreading depression, and insulin-induced hypoglycemia does not modify this effect

Cortical spreading depression (CSD) is characterized by reversible reduction of spontaneous and evoked electrical activity of the cerebral cortex. Experimental evidence suggests that CSD may modulate neural excitability and synaptic activity, with possible implications for long-term potentiation. Systemic factors like anesthetics and insulin-induced hypoglycemia can influence CSD propagation. In this study, we examined whether the post-CSD ECoG potentiation can be modulated by anesthetics and insulin-induced hypoglycemia. We found that awake adult rats displayed increased ECoG potentiation after CSD, as compared with rats under urethane+chloralose anesthesia or tribromoethanol anesthesia. In anesthetized rats, insulin-induced hypoglycemia did not modulate ECoG potentiation. Comparison of two cortical recording regions in awake rats revealed a similarly significant (p<0.05) potentiation effect in both regions, whereas in the anesthetized groups the potentiation was significant only in the recording region nearer to the stimulating point. Our data suggest that urethane+chloralose and tribromoethanol anesthesia modulate the post-CSD potentiation of spontaneous electrical activity in the adult rat cortex, and insulin-induced hypoglycemia does not modify this effect. Data may help to gain a better understanding of excitability-dependent mechanisms underlying CSD-related neurological diseases.

Malnutrition and REM-sleep Deprivation Modulate in Rats the Impairment of Spreading Depression by a Single Sub-convulsing Dose of Pilocarpine

This study aimed to investigate the effect of a single injection of pilocarpine upon the phenomenon of cortical spreading depression (SD), in adult rats submitted to early malnutrition and/or to REM-sleep deprivation for 72h prior to the SD-recordings. The SD was recorded continuously for 3-4h in 13 well-nourished (W) and 15 early-malnourished (M) adult rats. One to two hours after the beginning of the recording session, a sub-convulsing intraperitoneal (i.p.) injection of pilocarpine (190mg/kg) was applied and its effects on SD were studied during the rest of the recording session. Pilocarpine reduced markedly the ECoG amplitudes in all animals and decreased the SD velocity of propagation in the M-, but not in the W-rats, as compared with the pre-drug values for the same animals. In additional 9W- and 10 M-animals, REM-sleep deprivation was induced during the 72 h preceding the SD-recording session. This condition enhanced the pilocarpine effects on SD in the W-, but not in the M-rats, as compared to the respective non-deprived (ND) groups. The results indicate an important acute cholinergic influence on SD, acting by means of pilocarpine-activated muscarinic receptors. This effect seems to be differentially modulated by sleep deprivation and malnutrition.

Sequential changes in neuronal activity in single neocortical neurons after spreading depression

BACKGROUND

Cortical spreading depression (CSD) has an important role in migraine with aura. Prolonged neuronal depression is followed by a late excitatory synaptic plasticity after CSD.

METHOD

Intra- and extracellular recordings were performed to investigate the effect of CSD on intracellular properties of mouse neocortical tissues in the late excitatory period.

RESULTS

During CSD, changes in the membrane potentials usually began with a relatively short hyperpolarization followed by an abrupt depolarization. These changes occurred roughly at the same time point after CSD as the beginning of the negative extracellular deflection. Forty-five minutes after CSD, neurons showed significantly smaller amplitude of afterhyperpolarization and a reduced input resistance. Depolarization and hyperpolarization of the cells by constant intracellular current injections in this period significantly changed the frequency of the action potentials.

CONCLUSION

These data indicate higher excitability of the neocortical neurons after CSD, which can be assumed to contribute to hyperexcitability of neocortical tissues in patients suffering from migraine.

Effect of cortical spreading depression on basal and evoked traffic in the trigeminovascular sensory system

AIM

To use an animal model to test whether migraine pain arises peripherally or centrally.

METHODS

We monitored the spontaneous and evoked activity of second-order trigeminovascular neurons in rats to test whether traffic increased following a potential migraine trigger (cortical spreading depression, CSD) and by what mechanism any such change was mediated.

RESULTS

Neurons (n = 33) responded to stimulation of the dura mater and facial skin with A-δ latencies. They were spontaneously active with a discharge rate of 6.1 ± 6.4 discharges s(-1). Injection of 10 µg lignocaine into the trigeminal ganglion produced a fully reversible reduction of the spontaneous discharge rate of neurons. Neuronal discharge rate returned to normal by 90 min. Lignocaine reduced the evoked responses of neurons to dural stimulation to 37% and to facial skin stimulation to 53% of control. Induction of CSD by cortical injection of KCl increased the spontaneous discharge rate of neurons from 2.9 to 16.3 discharges s(-1) at 20 min post CSD. Injection of 10 µg lignocaine into the trigeminal ganglion at this time failed to arrest or reverse this increase. Injection of lignocaine prior to the initiation of CSD failed to prevent the subsequent development of CSD-induced increases in discharge rates.

CONCLUSIONS

These results suggest that there is a continuous baseline traffic in primary trigeminovascular fibres and that CSD does not act to increase this traffic by a peripheral action alone - rather, it must produce some of its effect by a mechanism intrinsic to the central nervous system. Thus the pain of migraine may not always be the result of peripheral sensory stimulation, but may also arise by a central mechanism.

Potentiation of spontaneous and evoked cortical electrical activity after spreading depression: in vivo analysis in well-nourished and malnourished rats

Cortical spreading depression (CSD) is influenced by brain excitability and is related to neurological diseases, such as epilepsy. In vitro evidence indicates that neuronal electrical activity is potentiated after CSD. Malnutrition can cause electrophysiological changes in the brain, both in animals and in humans. Here, we investigated in vivo whether CSD potentiates the amplitude of electrocorticogram (ECoG) and of transcallosal evoked responses in adult well-nourished (W), early-malnourished (M), and food-restricted rats. ECoG amplitudes were compared before and after CSD, at two parietal regions (designated the anterior and posterior regions). In the anterior region, post-CSD amplitudes of the ECoG waves were 13-23% higher (P < 0.05) than the pre-CSD values in all groups. In the posterior region, amplitudes increased 22% in the M group only (P < 0.05). In a fourth CSD-free group, ECoG amplitude did not change during the four recording hours. Transcallosal electrically evoked cortical responses also increased 21.5 ± 9.6% and 41.8 ± 28.5%, after CSD, in the W and M conditions, respectively, as compared to pre-CSD values. The data support the hypothesis of an in vivo CSD potentiation on cortical excitability as recorded by spontaneous and evoked electrical activity and modulation by nutritional status.

Evaluation of cutaneous allodynia following induction of cortical spreading depression in freely moving rats

BACKGROUND

Cortical spreading depression (CSD) is a wave of depolarization followed by depression of bioelectrical activity that slowly propagates through the cortex. CSD is believed to be the underlying mechanism of aura in migraine; however, whether CSD can elicit pain associated with migraine headache is unclear.

METHODS

Awake, freely moving rats were monitored for both CSD events and behavioral responses resulting from dural-cortical pinprick and/or KCl injection to the occipital cortex.

RESULTS

We observed tactile allodynia of the face and hindpaws, as well as enhanced Fos expression within the trigeminal nucleus caudalis (TNC) following CSD induced by KCl injection into the cortex, but not by pinprick. Application of KCl onto the dura elicited cutaneous allodynia and increased Fos staining in the TNC but did not elicit CSD events.

CONCLUSIONS

These data suggest that sustained activation of trigeminal afferents that may be required to establish cutaneous allodynia may not occur following CSD events in normal animals.

Animal models of headache: from bedside to bench and back to bedside

In recent years bench-based studies have greatly enhanced our understanding of headache pathophysiology, while facilitating the development of new headache medicines. At present, established animal models of headache utilize activation of pain-producing cranial structures, which for a complex syndrome, such as migraine, leaves many dimensions of the syndrome unstudied. The focus on modeling the central nociceptive mechanisms and the complexity of sensory phenomena that accompany migraine may offer new approaches for the development of new therapeutics. Given the complexity of the primary headaches, multiple approaches and techniques need to be employed. As an example, recently a model for trigeminal autonomic cephalalgias has been tested successfully, while by contrast, a satisfactory model of tension-type headache has been elusive. Moreover, although useful in many regards, migraine models are yet to provide a more complete picture of the disorder.

Genetic and hormonal factors modulate spreading depression and transient hemiparesis in mouse models of familial hemiplegic migraine type 1

Familial hemiplegic migraine type 1 (FHM1) is an autosomal dominant subtype of migraine with aura that is associated with hemiparesis. As with other types of migraine, it affects women more frequently than men. FHM1 is caused by mutations in the CACNA1A gene, which encodes the alpha1A subunit of Cav2.1 channels; the R192Q mutation in CACNA1A causes a mild form of FHM1, whereas the S218L mutation causes a severe, often lethal phenotype. Spreading depression (SD), a slowly propagating neuronal and glial cell depolarization that leads to depression of neuronal activity, is the most likely cause of migraine aura. Here, we have shown that transgenic mice expressing R192Q or S218L FHM1 mutations have increased SD frequency and propagation speed; enhanced corticostriatal propagation; and, similar to the human FHM1 phenotype, more severe and prolonged post-SD neurological deficits. The susceptibility to SD and neurological deficits is affected by allele dosage and is higher in S218L than R192Q mutants. Further, female S218L and R192Q mutant mice were more susceptible to SD and neurological deficits than males. This sex difference was abrogated by ovariectomy and senescence and was partially restored by estrogen replacement, implicating ovarian hormones in the observed sex differences in humans with FHM1. These findings demonstrate that genetic and hormonal factors modulate susceptibility to SD and neurological deficits in FHM1 mutant mice, providing a potential mechanism for the phenotypic diversity of human migraine and aura.

Cortical spreading depression modulates synaptic transmission of the rat lateral amygdala

Clinical and pathophysiological evidence connects migraine and the amygdala. Cortical spreading depression (CSD) plays a causative role in the generation of aura symptoms. However, the role of CSD in the pathophysiology of other symptoms of migraine needs to be investigated. An in vitro brain slice technique was used to investigate CSD effects on tetanus-induced long-term potentiation (LTP) in the lateral amygdala (LA) of the combined rat amygdala-hippocampus-cortex slices. More than 75% of CSD induced in temporal cortex propagated to LA. Induction of CSD in combined amygdala-hippocampus-cortex slices in which CSD propagated from neocortex to LA significantly augmented LTP in LA. LTP was inhibited when CSD travelled only in the neocortical tissues. Separation of the amygdala from the remaining neocortical part of the slice, in which CSD propagation was limited to the neocortex, increased LTP close to the control levels. Pharmacological manipulations of the slices, in which CSD reached LA, revealed the involvement of NMDA and AMPA glutamate subreceptors as well as dopamine D2 receptors in the enhancement of LTP in LA. However, neither blocking of GABA receptors nor activation of dopamine D1 receptors affected LTP in these slices. The results indicate the disturbances of LA synaptic transmission triggered by propagation of CSD. This perturbation of LA synaptic transmission induced by CSD may relate to some symptoms occurring during migraine attacks.

Spreading Depression Enhances Human Neocortical Excitability in vitro

Cortical spreading depression (CSD) plays a role in migraine with aura. However, studies of the neuronal effects of CSD in human cortex are scarce. Therefore, in the present study, the effects of CSD on the field excitatory postsynaptic potentials (fEPSP) and the induction of long-term potentiation (LTP) were investigated in human neocortical slices obtained during epilepsy surgery. CSD significantly enhanced the amplitude of fEPSP following a transient suppressive period and increased the induction of LTP in the third layer of neocortical tissues. These results indicate that CSD facilitates synaptic excitability and efficacy in human neocortical tissues, which can be assumed to contribute to hyperexcitability of neocortical tissues in patients suffering from migraine.

The effect of estrogen and progesterone on spreading depression in rat neocortical tissues

Although gender differences in the incidence of migraine with aura appear to be related to high circulating levels of ovarian hormones, the underlying mechanisms are not yet fully understood. Several studies have suggested a major role for spreading depression (SD) in the pathogenesis and symptomatology of migraine with aura. To investigate a possible role of SD in the association of high female hormones and attacks of migraine with aura, the effects of beta-estradiol and progesterone on SD were studied in rat neocortical tissues. Application of both hormones enhanced the repetition rate as well as the amplitude of SD in neocortical slices treated with hypotonic artificial cerebrospinal fluid. beta-Estradiol and progesterone also dose dependently increased the amplitude of SD induced by KCl microinjection. Both hormones exhibited a pronounced, persisting, and significant enhancement of long-term potentiation of the field excitatory postsynaptic potential in the neocortical tissues. The changes in SD characteristics in the presence of estrogen and progesterone may responsible for increased migraine with aura attacks associated by high female hormones. These hormones may exert their effects on SD via facilitation of synaptic transmission.

Dynamic neuronal responses in cortical and thalamic areas during different phases of formalin test in rats

Although formalin-induced activity in primary afferent fibers and spinal dorsal horn is well described, the forebrain neural basis underlying each phase of behavior in formalin test has not yet been clarified. The present study was designed to investigate the cortical and thalamic neuronal responses and interactions among forebrain areas during different phases after subcutaneous injection of formalin. Formalin-induced neuronal activities were simultaneously recorded from primary somatosensory cortex (SI), anterior cingulate cortex (ACC) and medial dorsal (MD) and ventral posterior (VP) thalamus during different phases (i.e., first phase, interphase, second phase and third recovery phase starting from 70 min after injection) of formalin test, using a multi-channel, single-unit recording technique. Our results showed that, (i) unlike the responses in primary afferent fibers and spinal dorsal horn, many forebrain neurons displayed monophasic excitatory responses in the first hour after formalin injection, except a small portion of neurons which exhibited biphasic responses; (ii) the response patterns of many cortical and thalamic neurons changed from excitatory to inhibitory at the end of the second phase; (iii) the direction of information flow also changed dramatically, i.e., from cortex to thalamus and from the medial to the lateral pathway in the first hour, but reversed in phase 3. These results indicate that the changes of activity pattern in forebrain networks may underlie the emerging and subsiding of central sensitization-induced pain behavior in the second phase of formalin test.

Effect of cortical spreading depression on synaptic transmission of rat hippocampal tissues

Cortical spreading depression (CSD) is believed to be a putative neuronal mechanism underlying migraine aura and subsequent pain. In vitro and ex vivo/in vitro brain slice techniques were used to investigate CSD effects on the field excitatory postsynaptic potentials (fEPSP) and tetanus-induced long-term potentiation (LTP) in combined rat hippocampus-cortex slices. Induction of CSD in combined hippocampus-cortex slices in which DC negative deflections did not propagate from neocortex to hippocampus significantly augmented fEPSP amplitude and LTP in the hippocampus. Propagation of CSD to the hippocampus resulted in a transient suppression followed by reinstatement of fEPSP with amplitude of pre-CSD levels. LTP was inhibited when DC potential shifts were recorded in the hippocampus. Furthermore, CSD was induced in anaesthetized rats and, thereafter, hippocampal tissues were examined in vitro. LTP was significantly enhanced in hippocampal slices obtained from ipsilateral site to the hemisphere in which CSD was evoked. The results indicate the disturbances of hippocampal synaptic transmission triggered by propagation of CSD. This perturbation of hippocampal synaptic transmission induced by CSD may relate to some symptoms occurring during migraine attacks, such as amnesia and hyperactivity.

Epilepsy surgery: perioperative investigations of intractable epilepsy

Recent advances in our understanding of the basic mechanisms of epilepsy have derived, to a large extent, from increasing ability to carry out detailed studies on patients surgically treated for intractable epilepsy. Clinical and experimental perioperative studies divide into three different phases: before the surgical intervention (preoperative studies), on the intervention itself (intraoperative studies), and on the period when the part of the brain that has to be removed is available for further investigations (postoperative studies). Before surgery, both structural and functional neuroimaging techniques, in addition to their diagnostic roles, could be used to investigate the pathophysiological mechanisms of seizure attacks in epileptic patients. During epilepsy surgery, it is possible to insert microdialysis catheters and electroencephalogram electrodes into the brain tissues in order to measure constituents of extracellular fluid and record the bioelectrical activity. Subsequent surgical resection provides tissue that can be used for electrophysiological, morphological, and molecular biological investigations. To take full advantage of these opportunities, carefully designed experimental protocols are necessary to compare the data from different phases and characterize abnormalities in the human epileptic brain.

Effect of eugenol on spreading depression and epileptiform discharges in rat neocortical and hippocampal tissues

Eugenol, an aromatic molecule derived from several plants, has been receiving examination for clinical relevance in epilepsy and headache. To investigate the neurophysiologic properties of the action of eugenol, its effects on epileptiform field potentials elicited by omission of extracellular Mg2+, spreading depression induced by KCl microinjection, electrically evoked field potentials, and long-term potentiation were tested in rat neocortical and hippocampal tissues. Eugenol (10-100 micromol/l) dose-dependently and reversibly suppressed both epileptiform field potentials and spreading depression Eugenol also reversibly decreased the amplitude of the field postsynaptic potentials evoked in CA1 area of hippocampus and the third layer of neocortex. Eugenol significantly reduced the long-term potentiation by approximately 30% compared with controls. Thus, eugenol can suppress epileptiform field potentials and spreading depression, likely via inhibition of synaptic plasticity. The results indicate the potential for eugenol to use in the treatment of epilepsy and cephalic pain.

Long-range oscillatory Ca2+waves in rat spinal dorsal horn

Synchronous activity of large populations of neurons shapes neuronal networks during development. However, re-emergence of such activity at later stages of development could severely disrupt the orderly processing of sensory information, e.g. in the spinal dorsal horn. We used Ca2+ imaging in spinal cord slices of neonatal and young rats to assess under which conditions synchronous activity occurs in dorsal horn. No spontaneous synchronous Ca2+ transients were detected. However, increasing neuronal excitability by application of 4-aminopyridine after pretreatment of the slice with blockers of (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate, gamma-aminobutyric acid (GABA)(A) and glycine receptors evoked repetitive Ca2+ waves in dorsal horn. These waves spread mediolaterally with a speed of 1.0 +/- 0.1 mm/s and affected virtually every dorsal horn neuron. The Ca2+ waves were associated with large depolarizing shifts of the membrane potential of participating neurons and were most likely synaptically mediated because they were abolished by blockade of action potentials or N-methyl-D-aspartate (NMDA) receptors. They were most pronounced in the superficial dorsal horn and absent from the ventral horn. A significant proportion of the Ca2+ waves spread to the contralateral dorsal horn. This seemed to be enabled by disinhibition as primary afferent-induced dorsal horn excitation crossed the midline only when GABA(A) and glycine receptors were blocked. Interestingly, the Ca2+ waves occurred under conditions where AMPA/kainate receptors were blocked. Thus, superficial dorsal horn NMDA receptors are able to sustain synchronous neuronal excitation in the absence of functional AMPA/kainate receptors.

Spreading depression in vivo potentiates electrically-driven responses in frog optic tectum

This is the first description of an in vivo potentiation phenomenon associated to spreading depression (SD) in the frog optic tectum. Field potential responses electrically-elicited from the optic tract and recorded in the optic tectum disappeared during KCl-elicited SD and recovered 10-20 min thereafter. Post-SD responses reached amplitudes 10-30% higher than their pre-SD values (P<0.05), indicating a potentiation effect. Current source density analysis of the tectal depth profiles of field-potential responses, as well as the calculation of the post-SD intratectal conductance changes, also supported the potentiation phenomenon. This in vivo potentiation lasted for 40-90 min, suggesting a post-SD enhancement of synaptic transmission, which may be important in understanding mechanisms of brain disfunctions like epilepsy.

Spreading depression enhances the spontaneous epileptiform activity in human neocortical tissues

Spreading depression (SD) is a well-known phenomenon in animal models of experimental epilepsy. However, the interaction of SD with epileptiform activity in human neuronal tissues is not clear. The aim of the present study was to investigate the effect of SD on synchronous rhythmic sharp field potentials in human neocortical slices. Spreading depression was elicited in human neocortical slices that exhibited sharp potentials. Extracellular field potentials were recorded from the third and fifth layers. SD significantly enhanced the repetition rate and amplitude of spontaneous rhythmic potentials in all tested slices. The results indicate that SD may facilitate the synchronization of different foci of rhythmic sharp field potentials and increase the excitability in human brain tissue.