Muscarinic depression of synaptic transmission in the epileptogenic GABA withdrawal syndrome focus.

The GABA withdrawal syndrome (GWS) is a model of local status epilepticus consecutive to the interruption of a prolonged GABA infusion into the rat somatomotor cortex. Bursting patterns in slices from GWS rats include intrinsic bursts of action potentials (APs) induced by intracellular depolarizing current injection and/or paroxysmal depolarization shifts (PDSs) induced by white matter stimulation. Possible changes in the effects of cholinergic drugs after in vivo induction of GWS were investigated on bursting cells (n = 30) intracellularly recorded in neocortical slices. In GWS slices, acetylcholine (Ach, 200-1000 microM) or carbachol (Cch, 50 microM) applications increased the number of bursts induced by depolarizing current injection while synaptically induced PDSs were significantly diminished (by 50-60%) or even blocked independently of the cholinergic-induced depolarization. The intrinsic burst facilitation and PDS depression provoked by Ach or Cch were mimicked by methyl-acetylcholine (mAch, 100-400 microM, n = 11), were reversed by atropine application (1-50 microM, n = 3), and were not mimicked by nicotine (50-100 microM, n = 4), indicating the involvement of muscarinic receptors. In contrast, in nonbursting cells from the same epileptic area (n = 42) or from equivalent area in control rats (n = 24), a nonsignificant muscarinic depression of EPSPs was induced by Cch and Ach. The mAch depression of excitatory postsynaptic potential (EPSPs) was significantly lower than that seen for PDSs in GWS rats. None of the cholinergic agonists caused bursting appearance in these cells. Therefore the present study demonstrates a unique implication of muscarinic receptors in exerting opposite effects on intrinsic membrane properties and on synaptic transmission in epileptiform GWS. Muscarinic receptor mechanisms may therefore have a protective role against the development and spread of epileptiform activity from the otherwise-activated epileptic focus.

Origin and propagation of spontaneous electrographic sharp waves in the in vitro turtle brain: a model of neuronal synchronization.

OBJECTIVES: Neuronal synchronization is a basic feature in the generation of epileptiform discharges. Spontaneous large sharp waves (LSWs) can be recorded in the turtle brain in vitro, indicating the synchronous activation of large neuronal populations. The aim of this study was to analyze the spatial and temporal distribution of LSWs within the brain; the participation of glutamate in LSWs generation was also investigated. METHODS: Extracellular field potentials were recorded in vivo (n = 4) and in vitro (n = 36). LSWs were recorded from cerebral cortex, optic tectum, and thalamus. RESULTS: LSWs were recorded from cerebral cortex, optic tectum and thalamus. No LSWs were observed in cerebellum and brain stem. In some experiments, LSWs could be recorded only from medial cortex. Latency studies demonstrated that, within each hemisphere, medial cortex led the generation of LSWs; in addition, isolated medial cortex could sustain LSWs. Intracortical laminar field potentials in medial cortex indicated that LSWs generate mainly in the molecular layer, probably at pyramidal cell dendrites. Pharmacological experiments demonstrated that NMDA and non-NMDA glutamate receptors are involved in LWSs generation. CONCLUSIONS: These results suggest that turtle medial cortex is the pacemaker area for LSWs generation and it can be a useful model to study cellular and circuital mechanisms of neuronal synchronization.

The cerebral hemisphere of the turtle in vitro. An experimental model with spontaneous interictal-like spikes for the study of epilepsy.

Slice in vitro preparations have been useful to study the cellular basis of some epilepsy related phenomena. However, the cellular mechanisms that generate ictal activity remain poorly understood. Therefore, an experimental in vitro model capable of generating seizure-like activity might contribute to the study of the cellular basis of seizures. The outstanding resistance to hypoxia of turtles enabled us to develop an in vitro preparation that keeps all the cortical neural circuitry intact. A whole cerebral hemisphere of the turtle Chrysemys d'orbigny was isolated (n = 45) and simultaneous electrographic and intracellular recordings were performed in the medial cortex. The electrographic activity was composed by a non-rhythmic, low-voltage (10-20 microV) activity interrupted by spontaneous large (50-700 microV) sharp waves (LSWs). The cellular counterpart of the LSWs was often a burst of action potentials that resembled the paroxysmal depolarisation shift (PDS). Bicuculline (20-40 microM, n = 20) increased the interictal-like activity and in some preparations (3 out of 20) provoked seizure-like events. Complex bursting activity and a slow afterhyperpolarisation were cellular events observed during seizures. We propose that this model might be a valuable tool for the study the cellular mechanisms involved in the transition from the interictal to the ictal activities.

Responses to repetitive afferent activity of rat solitary complex neurons isolated in brainstem slices.

The response of postsynaptic solitary complex neurons to repetitive stimulation (20-50 Hz) of the tractus solitarius were investigated by intracellular recordings in a brainstem slice preparation. Short duration stimuli (0.5 s) elicited increases in synaptic activity and short-term potentiation of synaptic potentials, both of which lasted approximately 1 min, plus a 10 s repolarization suppressed in the presence of glutamate ionotropic receptors antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) and 2-D-amino-7-phosphonoheptanoic acid (AP7, 50 microM). Longer (5 s) stimuli elicited 2-10 min depolarizations accompanied by membrane resistance increases and unaffected by glutamate ionotropic receptors antagonists. Our study reveals several mechanisms by which rhythmic visceral afferents may exert a tonic control of postsynaptic solitary complex neurons.

Inhibitory effects of excitatory amino acids on pyramidal cells of the in vitro turtle medial cortex.

The electroencephalogram of the in vitro brain of the turtle Chrysemys d' orbigny shows spontaneous random large sharp waves (LSWs) which may be compared to interictal spikes. In order to evaluate the role of excitatory amino acids (EAAs)--in particular through the N-methyl-D-aspartate (NMDA) receptor--in the generation of LSWs, the bath application of NMDA and its antagonists 3-((+/-)-2-carboxypiperazin-4y)-propyl-1-phosphonic acid (CPP) and DL-2-amino-5-phosphonovaleric acid (APV), was performed in the whole open hemisphere (WOH) in vitro. Field recordings in WOH showed that both CPP and APV unexpectedly increased LSW amplitude. Consistently, NMDA in the bath suppressed the LSWs. Iontophoretically applied glutamate, kainate and NMDA produced a hyperpolarization of intracellularly recorded medial cortex pyramidal cells both in WOH and in slices. The EAA-induced hyperpolarization was tetrodotoxin (TTX) and bicuculline sensitive and reversed close to -70 mV. It would therefore seem to be due to the activation of gamma-aminobutyric acid (GABA) interneurons. The NMDA could also produce an excitation of pyramidal cells--always following a previous inhibitory phase. In some cases rhythmic bursting discharges or plateau potentials were observed. These NMDA effects were mainly elicited by a direct effect on pyramidal cells. A long-lasting hyperpolarizing response following the NMDA excitatory phase was also observed. This long-lasting response was an intrinsic property of pyramidal cells since it was TTX resistant. This study demonstrates that GABAergic interneurons from the turtle medial cortex can be activated by EAAs, a mechanism that can account for the effects of NMDA antagonists on LSWs.

Electroencephalogram in vitro and cortical transmembrane potentials in the turtle Chrysemys d'orbigny.

An experimental stable model of an in vitro turtle brain (Chrysemys d'orbigny) was developed in order to compare electrographic activity (EEG) with transmembrane potentials. Two preparations were used: a whole intact hemisphere and a whole open hemisphere. The latter permitted easier impalement of cortical neurons through the ependymal surface. The EEG characteristics were similar to those described in turtles in vivo. The EEG was nonrhythmic (rhythmicity coefficient less than 0.40). The power spectrum presented a high energy band between 1 and 3 Hz, decreasing progressively towards the higher frequencies. Total power of the EEG was one order of magnitude greater than the system noise. Random large amplitude sharp waves (22-300 microV, 500-1,900 ms) were recorded spontaneously. Hypoxia produced an increase in frequency and amplitude of the large sharp waves, without modification of either EEG background activity or membrane potentials. Physostigmine provoked the disappearance of the large sharp waves, an effect reversed by atropine. The addition of TTX to the medium provoked the abolition of the EEG, although spikes and plateaus determined by Ca2+ conductances persisted. The power spectra band of maximum relative potency was 0.8-2.5 Hz for both EEG and slow membrane potentials.

Limbic epilepsy induced in the rat by dendrotoxin, a polypeptide isolated from the green mamba (Dendroaspis angusticeps) venom.

Dendrotoxin was isolated from green mamba (Dendroaspis angusticeps) venom and its effects on motor behavior and cortical and subcortical bioelectrical activity were studied in the rat. In chronic experiments, free moving rats injected i.p. with dendrotoxin, presented motor behavior similar to that described in rat amygdaloid epilepsy and bioelectrical signs of epilepsy beginning at the amygdala were observed. In acute experiments, rats anaesthetized with urethane were intracerebrally or intracerebroventricularly injected with dendrotoxin, which produced bioelectrical signs of epilepsy. Following intracerebroventricular injection, signs of epileptic discharge were first observed at the dorsal hippocampus. When dendrotoxin was microinjected in the amygdala or the hippocampus, the seizures appeared at the injection sites with a shorter latency and the bioelectrical epileptic signs lasted longer than when injections were given in non-limbic structures, such as the globus pallidus or the mesencephalic reticular formation. Dendrotoxin is a very powerful toxin that acts effectively at the level of the limbic system.

[Modification of frontal and occipital cortical excitability provoked by trains of flashes in Papio papio (author's transl)]. / Modifications de l'excitabilite corticale (frontale et occipitale) par des trains d'eclairs chez le singe Papio papio.

The experimental conditions necessary for obtaining an evoked paroxysmal response from the frontal cortex were studied in the baboon Papio papio. The trigger stimulus was comprised of an isolated flash preceded by a train of intermittent light stimulation (SLI). Two conditions were necessary for the appearance of paroxysmal responses: a subconvulsant dose of DL-allylglycine had to be injected at least 3 h previous to recording, and a sufficient number of SLI trains had to be presented to the animal. The paroxysmal responses disappeared as soon as SLI trains were stopped. At the same time, modifications in the evoked occipital potential continue, although these do not become paroxysmal. These modifications appear either simultaneously with or previous to the paroxysmal frontal response.

Excitability of bulbar respiratory neurones: a study using microiontophoretic applications of depolarizing agents.

The discharge of cat's bulbar respiratory neurones (RN) was shown to be modulated by periodic depressions which are characterized by their ability to reduce the effectiveness of microiontophoretically applied depolarizing agents: L-glutamate, acetylcholine and potassium. From the observation of cycle triggered time histograms (CTH), it appeared that these depressions have a determined and invariable phase relationship within the respiratory cycle. They were demonstrated in RN histologically located between and including the nucleus of the tractus solitarius and the nucleus ambiguus. Reproducibility and dose/response relationship of L-glutamate-induced depolarizations enabled an estimation of the functional effectiveness of these periodic depressions. In spontaneously phasic or 'silent' RN, depressions were demonstrated in the majority of cases (71%). Strongest depressions prevented spontaneous and L-glutamate-induced firing. Slighter depressions did not completely abolish L-glutamate effectiveness but reduced it by 20-90%. Conversely, in the majority of spontaneously tonic units (68%) depressions were not identified since the L-glutamate effect remained unchanged throughout the respiratory cycle. Four types of these respiration-related depressions were differentiated on the basis of their length, their phase relation to the respiratory cycle and their potentiation in barbiturate-anaesthetized preparations. A first type suppressed L-glutamate-evoked firing throughout inspiration; it was found in late-expiratory neurones. Two other types of depressions had a more restricted duration in the cycle: one was restricted to a portion of inspiration and was found in early-expiratory neurones; the other restricted to the beginning of expiration, was found in a special group of inspiratory neurones. A fourth type of inhibition was weaker and actively prolonged throughout expiration: it was found in another group of inspiratory neurones including the respiratory neurones located at the level of the nucleus of the tractus solitarius. These periodic depressions are interpreted in terms of synaptic inhibition; it is proposed that they play a major role in the functional organization of the respiratory centers at the bulbar level.

Relationships of hippocampal theta cycles with bar pressing during self-stimulation.

Previous reports have demonstrated a relationship between hippocampal rhythmical slow activity (theta) and movement, and have suggested that rhythmical movements tend to occur during certain phase of theta. Therefore, the relationships between theta cycles and voluntary motor activity were investigated. Bar pressing for electrical brain stimulation provided the reference to relate hippocampal activity. Ongoing theta increased in amplitude and frequency before and after pressing. Periodic waves preceding and following pressing. These waves only appear if the presses occur at particular phases of the theta cycles. They gradually disappeared during sessions lasting 4 to 8 hr. Light weights ( less than 90 g) added to the lever did not alter theta, but heavier weights (greater than 90 g) produced averages without periodic waves and with movement-related potentials. Introduction of a delay between pressing and electrical stimulation delayed evoked potentials, while averaging indicated that periodical waves persisted before and after bar pressing. Total lesions of the septum or superior fornix abolished theta and increased the frequency of self stimulation. In animals with partial lesions, theta reappeared during pressing. The above results indicating that the rats tended to lever press during particular phases of theta, suggest that phase-locked theta may be a corollary of motor mechanisms, and perhaps of the timing of motor responses.