Didier Pinault

Intracellular recordings in thalamic neurones during spontaneous spike and wave discharges in rats with absence epilepsy

1 In vivo extracellular and intracellular recordings were performed from thalamocortical (TC) neurones in a genetic model of absence epilepsy (genetic absence epilepsy rats from Strasbourg) during spontaneous spike and wave discharges (SWDs). 2 Extracellularly recorded single units (n= 14) fired either a single action potential or a high frequency burst of up to three action potentials, concomitantly with the spike component of the spike‐wave complex. 3 Three main events characterized the intracellular activity of twenty‐six out of twenty‐eight TC neurones during SWDs: a small amplitude tonic hyperpolarization that was present throughout the SWD, rhythmic sequences of EPSP/IPSPs occurring concomitantly with the spike‐wave complexes, and a small tonic depolarization at the end of the SWD. The rhythmic IPSPs, but not the tonic hyperpolarization, were mediated by activation of GABAA receptors since they reversed in polarity at ‐68 mV and appeared as depolarizing events when recording with KCl‐filled electrodes. 4 The intracellular activity of the remaining two TC neurones consisted of rhythmic low threshold Ca2+ potentials, with a few EPSP/IPSP sequences present at the start of the SWD. 5 These results obtained in a well‐established genetic model of absence epilepsy do not support the hypothesis that the intracellular activity of TC neurones during SWDs involves rhythmic sequences of GABAB IPSPs and low threshold Ca2+ potentials.

N-Methyl d-Aspartate Receptor Antagonists Ketamine and MK-801 Induce Wake-Related Aberrant γ Oscillations in the Rat Neocortex

BACKGROUND Single subanesthetic doses of ketamine, a non-competitive NMDA receptor (NMDAr) antagonist, induce cognitive impairment, schizophreniform psychosis, hallucinations, and exacerbate schizophrenia symptoms. The neuronal mechanisms underlying transient disruption in NMDAr function are unknown. Disorders of cognition-related coherences of gamma frequency (30-80 Hz) oscillations between cortical areas are a major functional abnormality in schizophrenic patients. Does a single subanesthetic dose of ketamine or MK-801 alter properties of cortical gamma oscillations? METHODS Properties of spontaneously occurring gamma oscillations in the electrocorticogram of the neocortex of freely moving rats (n = 16) were measured before and after subcutaneous administration of a single dose of ketamine (< or = 10 mg/kg), MK-801 (< or = .16 mg/kg), d-amphetamine (< or = 1 mg/kg), apomorphine (< or = 1.6 mg/kg), or vehicle (sodium chloride, .9%). RESULTS The present study gives the first evidence that ketamine and MK-801, both of which induce NMDAr-dependent functional disconnections, dose-dependently increase the power (200%-400%) of wake-related gamma oscillations in the neocortex. Substances that modulate dopaminergic neurotransmission could also increase the gamma power but to a lesser degree. CONCLUSIONS The present findings suggest that abnormal increased synchronization in ongoing gamma oscillations in cortical-related networks might cause dysfunctions of conscious integration, as seen in patients with schizophrenia.

NMDA Receptor Hypofunction Leads to Generalized and Persistent Aberrant γ Oscillations Independent of Hyperlocomotion and the State of Consciousness

Background The psychotomimetics ketamine and MK-801, non-competitive NMDA receptor (NMDAr) antagonists, induce cognitive impairment and aggravate schizophrenia symptoms. In conscious rats, they produce an abnormal behavior associated with a peculiar brain state characterized by increased synchronization in ongoing γ (30–80 Hz) oscillations in the frontoparietal (sensorimotor) electrocorticogram (ECoG). This study investigated whether NMDAr antagonists-induced aberrant γ oscillations are correlated with locomotion and dependent on hyperlocomotion-related sensorimotor processing. This also implied to explore the contribution of intracortical and subcortical networks in the generation of these pathophysiological ECoG γ oscillations. Methodology/Principal Findings Quantitative locomotion data collected with a computer-assisted video tracking system in combination with ECoG revealed that ketamine and MK-801 induce highly correlated hyperlocomotion and aberrant γ oscillations. This abnormal γ hyperactivity was recorded over the frontal, parietal and occipital cortices. ECoG conducted under diverse consciousness states (with diverse anesthetics) revealed that NMDAr antagonists dramatically increase the power of basal γ oscillations. Paired ECoG and intracortical local field potential recordings showed that the ECoG mainly reflects γ oscillations recorded in underlying intracortical networks. In addition, multisite recordings revealed that NMDAr antagonists dramatically enhance the amount of ongoing γ oscillations in multiple cortical and subcortical structures, including the prefrontal cortex, accumbens, amygdala, basalis, hippocampus, striatum and thalamus. Conclusions/Significance NMDAr antagonists acutely produces, in the rodent CNS, generalized aberrant γ oscillations, which are not dependent on hyperlocomotion-related brain state or conscious sensorimotor processing. These findings suggest that NMDAr hypofunction-related generalized γ hypersynchronies represent an aberrant diffuse network noise, a potential electrophysiological correlate of a psychotic-like state. Such generalized noise might cause dysfunction of brain operations, including the impairments in cognition and sensorimotor integration seen in schizophrenia.

Cellular interactions in the rat somatosensory thalamocortical system during normal and epileptic 5-9 Hz oscillations.

In Genetic Absence Epilepsy Rats from Strasbourg (GAERS), generalized spike‐and‐wave (SW) discharges (5–9 SW s−1) develop during quiet immobile wakefulness from a natural, medium‐voltage, 5–9 Hz rhythm. This study examines the spatio‐temporal dynamics of cellular interactions in the somatosensory thalamocortical system underlying the generation of normal and epileptic 5–9 Hz oscillations. Paired single‐unit and multi‐unit recordings between the principal elements of this circuit and intracellular recordings of thalamic, relay and reticular, neurones were conducted in neuroleptanalgesied GAERS and control, non‐epileptic, rats. The identity of the recorded neurones was established following juxtacellular or intracellular marking. At least six major findings have emerged from this study. (1) In GAERS, generalized spike‐and‐wave discharges were correlated with synchronous rhythmic firings in related thalamic relay and reticular neurones. (2) Usually, corticothalamic discharges phase‐led related relay and reticular firings. (3) A depolarizing wave emerging from a barrage of EPSPs was the cause of both relay and reticular discharges. (4) In some relay cells, which had a relatively high membrane input resistance, the depolarizing wave had the shape of a ramp, which could trigger a low‐threshold Ca2+ spike. (5) In reticular cells, the EPSP barrage could further trigger voltage‐dependent depolarizations. (6) The epilepsy‐related thalamic, relay and reticular, intracellular activities were similar to the normal‐related thalamic activities. Overall, these findings strongly suggest that, during absence seizures, corticothalamic neurones play a primary role in the synchronized excitation of thalamic relay and reticular neurones. The present study further suggests that absence‐related spike‐and‐wave discharges correspond to hypersynchronous wake‐related physiological oscillations.

Medium-voltage 5–9-Hz oscillations give rise to spike-and-wave discharges in a genetic model of absence epilepsy: in vivo dual extracellular recording of thalamic relay and reticular neurons

In humans with absence epilepsy, spike-and-wave discharges develop in the thalamocortical system during quiet immobile wakefulness or drowsiness. The present study examined the initial stage of the spontaneous development of spike-and-wave discharges in Genetic Absence Epilepsy Rats from Strasbourg. Bilateral electrocorticograms were recorded in epileptic and non-epileptic rats under freely moving and undrugged conditions and under neuroleptanalgesia. Short-lasting episodes of medium-voltage 5-9-Hz (mean=6-Hz) oscillations usually emerged spontaneously from a desynchronized electrocorticogram and in bilateral synchrony in both rat strains. These oscillations were distinguishable from sleep spindles regarding their internal frequency, duration, morphology, and moment of occurrence. Spontaneous spike-and-wave discharges developed from such synchronized medium-voltage oscillations, the spike-and-wave complex occurring at the same frequency as the 5-9-Hz wave. Because the thalamus is thought to play a significant role in the generation of spike-and-wave discharges, dual extracellular recording and juxtacellular labelling of relay and reticular neurons were conducted to study the thalamic cellular mechanisms associated with the generation of spike-and-wave discharges. During medium-voltage 5-9-Hz oscillations, discharges of relay and reticular cells had identical patterns in epileptic and non-epileptic rats, consisting of occasional single action potentials and/or bursts (interburst frequency of up to 6-8 Hz) in relay cells, and of rhythmic bursts (up to 12-15 Hz) in reticular neurons, these discharging in the burst mode almost always before relay neurons. The discharge frequency of reticular bursts decelerated to 6 Hz by the beginning of the spike-and-wave discharges. During these, relay and reticular neurons usually fired in synchrony a single action potential or a high-frequency burst of two or three action potentials and a high-frequency burst, respectively, about 12 ms before the spike component of the spike-and-wave complexes. The frequency of these corresponded to the maximal frequency of the thalamocortical burst discharges associated with 5-9-Hz oscillations. The patterns of relay and reticular phasic cellular firings associated with spike-and-wave discharges had temporal characteristics similar to those associated with medium-voltage 5-9-Hz oscillations, suggesting that these normal and epileptic oscillations are underlain by similar thalamic cellular mechanisms. In conclusion, medium-voltage 5-9-Hz oscillations in the thalamocortical loop give rise to spike-and-wave discharges. Such oscillations are not themselves sufficient to initiate spike-and-wave discharges, meaning that genetic factors render thalamocortical networks prone to generate epileptic electrical activity, possibly by decreasing the excitability threshold in reticular cells. While these GABAergic neurons play a key role in the synchronization of glutamatergic relay neurons during seizures, relay cells may participate significantly in the regulation of the recurrence of the spike-and-wave complex. Furthermore, it is very likely that synchronization of relay and reticular cellular discharges during absence seizures is generated in part by corticothalamic inputs.

Single striatofugal axons arborizing in both pallidal segments and in the substantia nigra in primates

The striatofugal fiber system in primates is believed to be composed of separate subsystems terminating in either the external (GPe) or internal (GPi) segment of the globus pallidus, or in the substantia nigra (SN). At variance with this concept is the present demonstration of single biocytin-labeled striatofugal axons that arborize in the three major target structures of the striatum in cynomolgus monkeys. Out of nine single-labeled axons that were analyzed in detail, one terminated exclusively in GPc, another in both GPc and GPi, whereas the rest arborized in GPe, GPi and SN. The axons that branched in the three sites had one preferential recipient structure where they arborized profusely and formed typical woolly fibers. These findings suggest that, in contrast to previous beliefs based on results of retrograde double-labeling studies, most striatofugal axons arborize within more than one striatal target structures in primates.

Dysfunctional Thalamus-Related Networks in Schizophrenia

Thalamus abnormalities are common in neurological and psychiatric illnesses. Therefore, it is essential to understand the properties of the thalamus-related networks. The thalamic reticular nucleus (TRN) is a thin GABAergic layer interface strategically located between the thalamus and the neocortex. It is, at the very beginning of life, an essential neurodevelopmental guide for the accurate build up of reciprocal anatomical glutamatergic connections between the thalamus and neocortex. It is more than the mediator of selective attention. It appears as a combinatorial matrix because it holds and can combine multiple functional modalities. TRN cells work like integrators, thanks to their extraordinary intrinsic electrophysiological properties, under the contextual and leading influence of corticothalamic inputs. The TRN and thalamus principally form 2-neuron open-loop circuits (no reciprocal connection). The major functioning principle of such GABAergic-glutamatergic circuits is lateral inhibition, which is a gold standard device to set up, via differential amplifications, coherent structured thalamocortical activity patterns. Thereby, it selects relevant streams of information and deletes distractors during action, resting states, and information integration, including during consciousness, cognition, emotion, and thought. Disruption of thalamic lateral inhibition may contribute to a lack of coordination in activity between brain regions, as observed in psychiatric disorders like schizophrenia.

Functional stabilization of weakened thalamic pacemaker channel regulation in rat absence epilepsy.

Aberrant function of pacemaker currents (Ih), carried by hyperpolarization‐activated cation non‐selective (HCN) channels, affects neuronal excitability and accompanies epilepsy, but its distinct roles in epileptogenesis and chronic epilepsy are unclear. We probed Ih function and subunit composition during both pre‐ and chronically epileptic stages in thalamocortical (TC) neurones of the Genetic Absence Epilepsy Rat from Strasbourg (GAERS). Voltage gating of Ih was unaltered in mature somatosensory TC cells, both in vivo and in vitro. However, the enhancement of Ih by phasic, near‐physiological, cAMP pulses was diminished by ∼40% and the half‐maximal cAMP concentration increased by ∼5‐fold. This decreased responsiveness of Ih to its major cellular modulator preceded epilepsy onset in GAERS, persisted throughout the chronic state, and was accompanied by an enhanced expression of the cAMP‐insensitive HCN1 channel mRNA (> 50%), without changes in the mRNA levels of HCN2 and HCN4. To assess for alterations in TC cell excitability, we monitored the slow up‐regulation of Ih that is induced by Ca2+‐triggered cAMP synthesis and important for terminating in vitro synchronized oscillations. Remarkably, repetitive rebound Ca2+ spikes evoked normal slow Ih up‐regulation in mature GAERS neurones; that sufficed to attenuate spontaneous rhythmic burst discharges. These adaptive mechanisms occurred upstream of cAMP turnover and involved enhanced intracellular Ca2+ accumulation upon repetitive low‐threshold Ca2+ discharges. Therefore, HCN channels appear to play a dual role in epilepsy. Weakened cAMP binding to HCN channels precedes, and likely promotes, epileptogenesis in GAERS, whereas compensatory mechanisms stabilizing Ih function contribute to the termination of spike‐and‐wave discharges in chronic epilepsy.

Acute administration of typical and atypical antipsychotics reduces EEG gamma power, but only the preclinical compound LY379268 reduces the ketamine-induced rise in gamma power

A single non-anaesthetic dose of ketamine, a non-competitive NMDA receptor (NMDAR) antagonist with hallucinogenic properties, induces cognitive impairment and psychosis, and aggravates schizophrenia symptoms in patients. In conscious rats an equivalent dose of ketamine induces key features of animal models of acute psychosis, including hyperlocomotor activity, deficits in prepulse inhibition and gating of auditory evoked potentials, and concomitantly increases the power of ongoing spontaneously occurring gamma (30-80 Hz) oscillations in the neocortex. This study investigated whether NMDAR antagonist-induced aberrant gamma oscillations could be modulated by acute treatment with typical and atypical antipsychotic drugs. Extradural electrodes were surgically implanted into the skull of adult male Wistar rats. After recovery, rats were subcutaneously administered either clozapine (1-5 mg/kg, n=7), haloperidol (0.05-0.25 mg/kg; n=8), LY379268 (a preclinical agonist at mGluR2/3 receptors: 0.3-3 mg/kg; n=5) or the appropriate vehicles, and 30 min later received ketamine (5 mg/kg s.c.). Quantitative measures of EEG gamma power and locomotor activity were assessed throughout the experiment. All three drugs significantly reduced the power of baseline EEG gamma oscillations by 30-50%, an effect most prominent after LY379268, and all inhibited ketamine-induced hyperlocomotor activity. However, only pretreatment with LY379268 attenuated trough-to-peak ketamine-induced gamma hyperactivity. These results demonstrate that typical and atypical antipsychotic drugs acutely reduce cortical gamma oscillations, an effect that may be related to their clinical efficacy.

Thalamic reticular input to the rat visual thalamus: a single fiber study using biocytin as an anterograde tracer.

This study describes the axonal projections of single thalamic reticular (TR) neurons within the visual thalamus in rats. Experiments were performed under urethane anesthesia and reticular cells were labeled by extracellular or juxtacellular microiontophoretic applications of biocytin. The axonal arborizations of 19 TR cells projecting to the dorsal lateral geniculate nucleus (DLG) or to the lateral dorsal/lateral posterior complex (LD/LP) were reconstructed from serial horizontal sections. It was found that single TR cells projected within the limits of a single thalamic nucleus, either the DLG or the LD/LP complex, where their terminal fields formed rostrocaudally oriented rods (length: approximately 800 microns; diameter: approximately 100 microns) densely packed with grape-like boutons and varicosities. In addition, none of the labeled TR cells possessed recurrent axonal collaterals that ramified within the reticular complex itself. The functional implications of these morphological data for the synchronization of thalamic oscillations are discussed.