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R Llinás

Publications and source records attributed to R Llinás.

At least 55 records · Page 3Linked to original sources
PubMed1995

The entorhinal cortex entrains fast CA1 hippocampal oscillations in the anaesthetized guinea-pig: role of the monosynaptic component of the perforant path.

Entorhinal inputs reach the hippocampal CA1 field through a trisynaptic circuit involving dentate granule cells and CA3 pyramidal neurons, as well as through a monosynaptic path ending on the distal apical dendrites of CA1 pyramidal cells. The influence of monosynaptic entorhinal inputs onto CA1 operations is poorly understood. In this study, we characterized the involvement of the monosynaptic pathway in the generation of the fast CA1 oscillation bursts (30-60 Hz) that occur in the dorsal hippocampus of anaesthetized guinea-pigs after partial cortex removal. Using multiple-site extracellular and intracellular recording, we found that in this particular preparation, devoid of theta rhythm, fast oscillations are temporally coherent over a large portion of the CA1 region along the hippocampal septotemporal axis. Current source density analysis revealed that fast CA1 oscillations involve two dipoles reflecting synchronous synaptic activities in the stratum lacunosum-moleculare of the hippocampus proper and in the stratum moleculare of the dentate gyrus. These layers constitute the two major termination zones of entorhinal afferents, suggesting that the entorhinal cortex entrains fast CA1 oscillations. This hypothesis was corroborated by the concomitant occurrence of fast oscillation bursts in the entorhinal cortex and CA1 region. Furthermore, fast CA1 oscillations were abolished by lidocaine or tetrodotoxin injections in the entorhinal cortex. Finally, acute interruption of the hippocampal trisynaptic loop did not affect the stratum lacunosum-moleculare dipole recorded extracellularly, but also intracellularly, as high-frequency postsynaptic potentials in CA1 pyramidal cells. These results indicate that the monosynaptic pathway is involved in the genesis of fast CA1 oscillations.

Animals↗
PubMed1995

Serotonin modulation of inferior olivary oscillations and synchronicity: a multiple-electrode study in the rat cerebellum.

Simultaneous recording of complex spikes from multiple Purkinje cells (up to 44) in the rat cerebellum was used to examine the effects of 5-hydroxytryptamine (serotonin, 5-HT) on olivocerebellar function. Microinjection into the inferior olive was found to increase the average firing rate of inferior olivary neurons while slowing their oscillation frequency and increasing the coherence of their oscillations. Indeed, while the normal rostrocaudal band of synchronous activity remained unchanged, the degree of synchrony between Purkinje cell complex spikes within this band was enhanced following the 5-HT injections. Multiple-electrode recordings obtained from crus Ila and vermal lobule Vlb yielded qualitatively similar results; however, the effects on vermal activity were more pronounced. The effects of the 5-HT microinjection decayed with a time course of 75 min. The half-maximum effective concentration of 5-HT was between 10 and 100 microM. Injections of various 5-HT agonists and antagonists demonstrated that a 5-HT type-2A (5-HT2A) receptor is the main mediator for the 5-HT effect, which was very similar to the effect produced by injections of harmaline. However, 5-HT and harmaline appear to have independent mechanisms since the action of harmaline was not blocked by the 5-HT2A antagonist LY53857. A possible role for 5-HT, as a physiological enhancer of the timing of motor function of the olivocerebellar system, is discussed.

Animals↗
PubMed1995

Dynamic organization of motor control within the olivocerebellar system.

What is the role of the cerebellum in motor coordination? Such coordination depends upon the integrity of the inferior olive, a major cerebellar afferent, as its lesion produces ataxic and dysmetric movement abnormalities. Using multiple-microelectrode recordings, we report here that there are domains of Purkinje cell activity that are generated by olivary input during skilled tongue movements in rats. Such activity domains are highly rhythmic and time-locked to movement. Patterns of synchronous olivocerebellar activity are geometrically complex and can change during a sequence of movements. The results support the view that the inferior olive organizes movement in time, by entraining motor-neuronal firing through rhythmic activation of the cerebellum, and in space, by synchronously activating cell ensembles that allow the use of individual muscles. Dynamic repatterning of olivocerebellar synchrony may allow different combinations of muscles to be used for movements intended to have varying spatial structures.

Action Potentials↗
PubMed1995

The concept of calcium concentration microdomains in synaptic transmission.

Ever since the initial measurements of presynaptic calcium currents it has been evident that calcium triggers transmitter release quite rapidly. Several models indicate, as did the initial voltage clamp measurements, that the calcium concentration triggering such release could be very high at the entry site and that this concentration should be very short lasting. In order to determine this time course, calcium entry was studied at the squid giant synapse by imaging light emission from n-aequorin-J, intracellularly injected into the presynaptic terminal. The imaging utilized a video system capable of acquiring 4000 frames per sec. The results indicate that the calcium entry, triggered by action potentials, reaches a peak within 200 musec and has an overall duration of close to 800 musec, closely matching the duration of the presynaptic calcium current determined by voltage clamp results under similar conditions.

Animals↗
PubMed1995

Conscious and pre-conscious processes as seen from the standpoint of sleep-waking cycle neurophysiology.

The literature on state-dependent fluctuations in thalamocortical activities indicates that in electrophysiological terms, waking and paradoxical sleep are fundamentally identical states, with the provision that the handling of sensory information is altered in REM sleep. The central paradox of REM sleep, namely the apparent lack of cognitive responsiveness to sensory stimulation in spite of increased thalamocortical responsiveness to sensory stimuli, will lead us to hypothesize that the processing of sensory inputs in REM sleep is similar to that underlying preconscious processing of sensory inputs in the waking state. This will lead to a general discussion of the role of fast (approximately equal to 40 Hz) thalamocortical oscillations and temporal binding in sensory processing and conscious experience.

Arousal↗
PubMed1995

Time resolved calcium microdomains and synaptic transmission.

The time course for the calcium entry that triggers release was studied at the squid giant synapse by imaging light emission from n-aequorin-J intracellularly injected into the presynaptic terminal. The imaging utilized a video system capable of acquiring 4000 frames per second. The results indicate that the calcium entry triggered by action potentials reaches a peak within 200 microseconds and has an overall duration of close to 800 microseconds, closely matching the duration of the presynaptic calcium current determined by voltage clamp results under similar conditions.

Aequorin↗
PubMed1995

Pharmacological characterization of the voltage-dependent Ca2+ channels present in synaptosomes from rat and chicken central nervous system.

The voltage-dependent calcium channels present in mammalian and chicken brain synaptosomes were characterized pharmacologically using specific blockers of L-type channels (1,4-dihydropyridines), N-type channels (omega-conotoxin GVIA), and P-type channels [funnel web toxin (FTX) and omega-agatoxin IVA]. K(+)-induced Ca2+ uptake by chicken synaptosomes was blocked by omega-conotoxin GVIA (IC50 = 250 nM). This toxin at 5 microM did not block Ca2+ entry into rat frontal cortex synaptosomes. FTX and omega-agatoxin IVA blocked Ca2+ uptake by rat synaptosomes (IC50 = 0.17 microliter/ml and 40 nM, respectively). Likewise, in chicken synaptosomes, FTX and omega-agatoxin IVA affected Ca2+ uptake, FTX (3 microliters/ml) exerted a maximal inhibition of 40% with an IC50 similar to the one obtained in rat preparations, whereas with omega-agatoxin IVA saturation was not reached even at 5 microM. In chicken preparations, the combined effect of saturating concentrations of FTX (1 microliter/ml) and different concentrations of omega-conotoxin GVIA showed no additive effects. However, the effect of saturating concentrations of FTX and omega-conotoxin GVIA was never greater than the one observed with omega-conotoxin GVIA. We also found that 60% of the Ca2+ uptake by rat and chicken synaptosomes was inhibited by omega-conotoxin MVIID (1 microM), a toxin that has a high index of discrimination against N-type channels. Conversely, nitrendipine (10 microM) had no significant effect on Ca2+ uptake in either the rat or the chicken. In conclusion, Ca2+ uptake by rat synaptosomes is potently inhibited by different P-type Ca2+ channel blockers, thus indicating that P-type channels are predominant in this preparation.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1995

Introduction of magnetoencephalography to stereotactic techniques.

Magnetoencephalography (MEG), a noninvasive functional brain mapping technique, was used for preoperative localization of the sensorimotor cortex in patients harboring lesions involving these eloquent regions. Prior to surgery, MEG source locations were transferred onto high-resolution MRI pictures which were then used for preoperative evaluation, risk analysis, and planning. We have developed a process to transform the MEG-derived sensorimotor localization coordinates into the COMPASS stereotactic coordinate system. Thus the MEG-derived functional information is incorporated into the stereotactic database, enabling the simultaneous visualization of functional and anatomical data. This information can be used for the selection of cases and in planning safe approaches for computer-assisted volumetric resections. The integration of MEG and stereotactic neurosurgery also allows a more precise comparison between MEG and intraoperative direct electrocorticographic mapping (ECoG). Seven patients were studied with good correlation between MEG and intraoperative mapping. In 4, the correlation was only based on gross visual comparison between intraoperative identification of the gyrus pattern and MEG photographs. The availability of the MEG coordinates in the stereotactic system, however, allows a more precise correlation between MEG and ECoG. In all 3 patients studied in this manner, the MEG coordinates (pinpointed to a precise cortical representation of a few millimeters) overlapped with ECoG results. In summary, we compared functional MEG data to intraoperative ECoG and conclude that the introduction of MEG into stereotactic neurosurgery can provide precise functional and anatomic information for image-guided surgical planning and resection.

Brain Mapping↗
PubMed1994

The inositol high-polyphosphate series blocks synaptic transmission by preventing vesicular fusion: a squid giant synapse study.

Presynaptic injection of inositol 1,3,4,5-tetraphosphate, inositol 1,3,4,5,6-pentakisphosphate, or inositol 1,2,3,4,5,6-hexakisphosphate--which we denote here the inositol high-polyphosphate series (IHPS)--is shown to block synaptic transmission when injected into the preterminal of the squid giant synapse. This effect is not produced by injection of inositol 1,4,5-trisphosphate. The synaptic block is characterized by a time course in the order of 15-45 min, depending on the injection site in the preterminal fiber; the fastest block occurs when the injection is made at the terminal release site. Presynaptic voltage clamp during transmitter release demonstrates that IHPS block did not modify the presynaptic inward, calcium current. Analysis of synaptic noise at the postsynaptic axon shows that both the evoked and spontaneous transmitter release are blocked by the IHPS. Tetanic stimulation of the presynaptic fiber at frequencies of 100 Hz indicates that block is accompanied by gradual reduction of the postsynaptic response, demonstrating that the block interferes with vesicular fusion rather than with vesicular docking. These results, in combination with the recently demonstrated observation that the IHPS bind the C2B domain in synaptotagmin [Fukada, M., Aruga, J., Niinobe, M., Aimoto, S. & Mikoshiba, K. (1994) J. Biol. Chem. 269, 29206-29211], suggest that IHPS elements are involved in vesicle fusion and exocytosis. In addition, a scheme is proposed in which synaptotagmin triggers transmitter release directly by promoting the fusion of synaptic vesicles with the presynaptic plasmalemma, in agreement with the very rapid nature of transmitter release in chemical synapses.

Animals↗
PubMed1994

Human oscillatory brain activity near 40 Hz coexists with cognitive temporal binding.

Spontaneous oscillatory electrical activity at a frequency near 40 Hz in the human brain and its reset by sensory stimulation have been proposed to be related to cognitive processing and to the temporal binding of sensory stimuli. These experiments were designed to test this hypothesis and to determine specifically whether the minimal interval required to identify separate auditory stimuli correlates with the reset of the 40-Hz magnetic signal. Auditory clicks were presented at varying times, while magnetic activity was recorded from awake human subjects. Experimental and modeling results indicate a stimulus-interval-dependent response with a critical interval of 12-15 ms. At shorter intervals only one 40-Hz response, to the first stimulus, was observed. With longer intervals, a second 40-Hz wave abruptly appeared, which coincided with the subject's perception of a second distinct auditory stimulus. These results indicate that oscillatory activity near 40 Hz represents a neurophysiological correlate to the temporal processing of auditory stimuli. It also supports the view that 40-Hz activity not only relates to primary sensory processing, but also could reflect the temporal binding underlying cognition.

Adult↗
PubMed1994

Transmission at the squid giant synapse was blocked by tetanus toxin by affecting synaptobrevin, a vesicle-bound protein.

1. The effect of whole tetanus toxin (TeTX) and of its light chain (TeTX L-chain) on transmitter release was determined by presynaptic pressure-injection in the squid giant synapse. 2. The results indicate that whole TeTX does not modify transmission while the L-chain blocks transmission within 20-30 min. This block does not involve changes in the sodium or potassium conductances responsible for spike generation or the voltage-dependent presynaptic calcium current responsible for transmitter release. 3. Western blotting of protein fractions from the squid optic lobe demonstrated the presence of a protein which reacted with specific antibodies against mammalian synaptobrevin, a vesicular protein. In addition, this protein was enzymatically cleaved by the L-chain component of the toxin in a similar fashion to its mammalian counterpart. 4. These results demonstrate that TeTX L-chain toxin acts directly on a squid synaptobrevin and prevents synaptic release probably by interfering with the docking-fusion synaptic vesicles at the active zone.

Animals↗
PubMed1994

Non-lamellar propagation of entorhinal influences in the hippocampal formation: multiple electrode recordings in the isolated guinea pig brain in vitro.

Experiments were carried out to study the spatiotemporal organization of medial entorhinal inputs to the hippocampal system. They were performed in the isolated guinea pig brain in vitro preparation as it provides easy access to the medial entorhinal cortex (mEC) which is difficult to reach in vivo. Multiple simultaneous field potential recordings along the septotemporal extent of the dentate granular layer revealed that the mEC projection to the dentate gyrus (DG) is organized topographically. Thus, stimulation of the caudal regions of the mEC elicited population spikes (PSs) in the septal pole of the DG while successively more rostral stimulation sites activated progressively more temporal sectors of the DG. However, threshold mEC stimuli never elicited PSs over more than one-third of the DG. In the CA1 pyramidal layer, only trisynaptic PSs were evoked by the mEC stimulation (latency > 20 ms at 30 degrees C). However, PSs were widely distributed in the transverse and longitudinal axes of the hippocampus and, irrespective of the mEC stimulation site, the latency of CA1 PSs gradually increased from the CA3/CA1 border toward the subiculum. By contrast, in the longitudinal axis, each segment of the CA1 region responded at a shorter latency to stimulation of a given rostrocaudal level of the mEC. Septal CA1 levels responded at shorter latencies to caudal mEC stimulation sites while more temporal CA1 levels responded at shorter latencies to rostral mEC stimulation sites. When stimulated at threshold stimulation intensity, the initial CA1 response propagated to the rest of the CA1 field with a conduction velocity of 0.5-0.9 m/s.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1994

Neuromagnetic studies of the lip area of primary somatosensory cortex in humans: evidence for an oscillotopic organization.

Magnetic trigeminal somatosensory responses from human subjects were recorded using a 14-channel magnetoencephalographic system. Sensory stimuli comprising a 15-ms vibration at frequencies of 50 Hz, 150 Hz and 250 Hz were given at randomized interstimulus intervals. Using a single dipole model, the neuronal sources of the evoked responses were determined, and mapped onto magnetic resonance images of each subject. Source localization analysis was based on the main peak of the averaged signal (M55). All of the sources were located deep in the anterior bank of the postcentral gyrus, corresponding to area 3b of somatosensory cortex SI. In all cases, the source for the upper lip was significantly higher in the vertical axis (0.6-1.1 cm) than for the lower lip, while the lower lip stimulation produced a larger response than the upper lip. Furthermore, statistically significant differences were found between the locations of the dipoles evoked by different frequency stimulation. The location of the response shifted with change in stimulation frequency, showing a trend among all subjects with medial shift between 150 and 250 Hz for both upper and lower lip. The accuracy of source localization calculated from magnetic fields ranged between +/- 0.9 and +/- 3.0 mm (SEM). These results demonstrate (1) that a large area of the somatosensory cortex is utilized for lip representation and (2) that the spatial displacement of the trigeminal somatosensory response may be related to the discrimination of frequency.

Adult↗
PubMed1994

Serotonergic and cholinergic inhibition of mesopontine cholinergic neurons controlling REM sleep: an in vitro electrophysiological study.

Intracellular recordings were obtained from neurons of the laterodorsal tegmental and pedunculopontine tegmental nuclei in a brain-slice preparation. The action of exogenously applied 5-hydroxytryptamine and acetylcholine was studied on NADPH-diaphorase-labeled cells which contain nitric oxide synthase and are presumed to be cholinergic. Our results indicated that these cells were hyperpolarized by both 5-hydroxytryptamine and acetylcholine; the ionic mechanism of this inhibition was investigated using current and voltage clamp methods. Cells voltage-clamped at resting membrane potential exhibited a net outward current and an increased membrane conductance during 5-hydroxytryptamine and acetylcholine mediated inhibition. The membrane hyperpolarization and outward current generated by this paradigm reversed near the expected K equilibrium potential and was blocked by low concentrations of extracellular Ba. The 5-hydroxytryptamine- and acetylcholine-dependent currents showed inward rectification and the reversal potential shifted in the depolarizing direction by about 15 mV for a doubling of extracellular K, indicating that both 5-hydroxytryptamine and acetylcholine activate inwardly rectifying, potassium-selective conductances. The 5-hydroxytryptamine-evoked hyperpolarization was antagonized by spiperone and mimicked by (+)8-hydroxy-2-(Di-N-propylamino)-tetralin suggesting the presence of a 5-hydroxytryptamine1A receptor while the acetylcholine-evoked hyperpolarization was blocked by atropine and only high concentrations of pirenzepine, suggesting a muscarinic M2 receptor. The outward currents evoked by 5-hydroxytryptamine and acetylcholine were not additive, suggesting that both receptors are coupled to an overlapping pool of K channels as has been observed in several systems in which receptors are coupled to effectors by G-proteins. These results indicate that the dominant actions of 5-hydroxytryptamine and acetylcholine relate to the inhibition of mesopontine cholinergic neurons via activation of an overlapping pool of inwardly rectifying K channels. Cholinergic neurons of these nuclei are thought to play an instrumental role in the induction and maintenance of rapid eye movement sleep. It has been previously hypothesized that acetylcholine would be excitatory and that 5-hydroxytryptamine would be inhibitory to these cells in the context of rapid eye movement sleep. [McCarley R. and Massaquoi S. (1986) Am. J. Physiol. 251, R1011-R1029; McCarley R. W. et al. (1975) Science 189, 58-60]. Our results are consistent with the proposed inhibitory action of 5-hydroxytryptamine but indicate recurrent input to cholinergic neurons would be inhibitory. Accordingly, models of the neural substrate underlying rapid eye movement sleep production need to be changed to reflect this inhibitory action of acetylcholine on cholinergic neurons.

Acetylcholine↗
PubMed1994

Time-resolved imaging of Ca(2+)-dependent aequorin luminescence of microdomains and QEDs in synaptic preterminals.

Localized elevation of intracellular free calcium [Ca2+]i concentration serves as the trigger for a wide variety of physiological processes, e.g., neurotransmitter release at most chemical synapses (1-3). The details of the mechanisms that regulate these processes are still unresolved (3-6), but they must involve precise temporal sequences of molecular events initiated by a transient localized elevation of Ca2+ concentration (i.e., a Ca2+ microdomain [3,7-15]). A microdomain is defined as an autonomous compartment of minimal spatio-temporal volume within which a signaled process can occur (8, 10, 12). A quantum emission domain (QED) is a quantal signal element (3, 16, 17). The concept of a QED was first applied to Ca2+ signaling at the synaptic preterminal (3, 4) and for large-diameter mitotic cells (16, 17). The concept of Ca2+ microdomains was tested by labeling preterminals of squid giant synapses with low-sensitivity aequorin (a photoprotein that emits a photon upon binding Ca2+ [18, 19]). That work confirmed earlier modeling efforts (10, 16) and showed that, upon depolarization, the [Ca2+]i profile reaches 200-300 microM within the microdomains, and that these [Ca2+]i profiles are composed of groups of short-lived 0.5 microns diameter QEDs. In those records, obtained with 2:1 interlacing devices operating at the RS-170 standard, QEDs appeared as striped dots or chevrons rather than as solid dots, indicating that a QED lasted less than 16.6 ms (one video field), and thus establishing the need for higher sampling rates to better characterize the QED.(ABSTRACT TRUNCATED AT 250 WORDS)

Aequorin↗
PubMed1994

High-resolution measurement of the time course of calcium-concentration microdomains at squid presynaptic terminals.

Transmitter release is considered to be a secretory event triggered by localized calcium influx which, by binding to a low-affinity Ca2+ site at the presynaptic active zone, initiates vesicular exocytosis (1-7). In previous experiments with aequorin-loaded presynaptic terminals we visualized, upon tetanic presynaptic stimulation, small points of light produced by calcium concentration microdomains of about 300 microM (5). These microdomains had a diameter of about 0.5 microns (5) and covered 5-10% of the total presynaptic membrane with an average density of 8.4 microns2 per 100 microns2, corresponding closely to the size and distribution of the active zones in that junction (6, 7). To understand in more detail the nature of these concentration microdomains, we obtained rapid video images (400/s) after injecting the photoprotein n-aequorin-J into the presynaptic terminals of squid giant synapses. Using that experimental approach, we determined that microdomains evoked by presynaptic spike activation had a duration of about 800 microseconds. Spontaneous quantum emission domains (QEDs) observed at about the same locations as the microdomains were smaller in amplitude, shorter in duration, and less frequent. These results illustrate the time course of the calcium concentration profiles responsible for transmitter release. Their extremely short duration compares closely with that of calcium current flow during a presynaptic action potential and indicates that, as theorized in the past (6-8), intracellular calcium concentration at the active zone remains high only for the duration of transmembrane calcium flow.

Action Potentials↗