Excitation Of Local Neurons Research Articles

N°183 – Local neuronal excitation and global inhibition during epileptic fast ripples in humans

N°183 – Local neuronal excitation and global inhibition during epileptic fast ripples in humans

Local neuronal excitation and global inhibition during epileptic fast ripples in humans.

Understanding the neuronal basis of epileptic activity is a major challenge in neurology. Cellular integration into larger scale networks is all the more challenging. In the local field potential, interictal epileptic discharges can be associated with fast ripples (200-600 Hz), which are a promising marker of the epileptogenic zone. Yet, how neuronal populations in the epileptogenic zone and in healthy tissue are affected by fast ripples remain unclear. Here, we used a novel 'hybrid' macro-micro depth electrode in nine drug-resistant epileptic patients, combining classic depth recording of local field potentials (macro-contacts) and two or three tetrodes (four micro-wires bundled together) enabling up to 15 neurons in local circuits to be simultaneously recorded. We characterized neuronal responses (190 single units) with the timing of fast ripples (2233 fast ripples) on the same hybrid and other electrodes that target other brain regions. Micro-wire recordings reveal signals that are not visible on macro-contacts. While fast ripples detected on the closest macro-contact to the tetrodes were always associated with fast ripples on the tetrodes, 82% of fast ripples detected on tetrodes were associated with detectable fast ripples on the nearest macro-contact. Moreover, neuronal recordings were taken in and outside the epileptogenic zone of implanted epileptic subjects and they revealed an interlay of excitation and inhibition across anatomical scales. While fast ripples were associated with increased neuronal activity in very local circuits only, they were followed by inhibition in large-scale networks (beyond the epileptogenic zone, even in healthy cortex). Neuronal responses to fast ripples were homogeneous in local networks but differed across brain areas. Similarly, post-fast ripple inhibition varied across recording locations and subjects and was shorter than typical inter-fast ripple intervals, suggesting that this inhibition is a fundamental refractory process for the networks. These findings demonstrate that fast ripples engage local and global networks, including healthy tissue, and point to network features that pave the way for new diagnostic and therapeutic strategies. They also reveal how even localized pathological brain dynamics can affect a broad range of cognitive functions.

Neuropeptide Y in the rostral ventromedial medulla reverses inflammatory and nerve injury hyperalgesia in rats via non-selective excitation of local neurons

Chronic pain reflects not only sensitization of the ascending nociceptive pathways, but also changes in descending modulation. The rostral ventromedial medulla (RVM) is a key structure in a well-studied descending pathway, and contains two classes of modulatory neurons, the ON-cells and the OFF-cells. Disinhibition of OFF-cells depresses nociception; increased ON-cell activity facilitates nociception. Multiple lines of evidence show that sensitization of ON-cells contributes to chronic pain, and reversing or blocking this sensitization is of interest as a treatment of persistent pain. Neuropeptide Y (NPY) acting via the Y1 receptor has been shown to attenuate hypersensitivity in nerve-injured animals without affecting normal nociception when microinjected into the RVM, but the neural basis for this effect was unknown. We hypothesized that behavioral anti-hyperalgesia was due to selective inhibition of ON-cells by NPY at the Y1 receptor. To explore the possibility of Y1 selectivity on ON-cells, we stained for the NPY-Y1 receptor in the RVM, and found it broadly expressed on both serotonergic and non-serotonergic neurons. In subsequent behavioral experiments, NPY microinjected into the RVM in lightly anesthetized animals reversed signs of mechanical hyperalgesia following either nerve injury or chronic hindpaw inflammation. Unexpectedly, rather than decreasing ON-cell activity, NPY increased spontaneous activity of both ON- and OFF-cells without altering noxious-evoked changes in firing. Based on these results, we conclude that the anti-hyperalgesic effects of NPY in the RVM are not explained by selective inhibition of ON-cells, but rather by increased spontaneous activity of OFF-cells. Although ON-cells undoubtedly facilitate nociception and contribute to hypersensitivity, the present results highlight the importance of parallel OFF-cell-mediated descending inhibition in limiting the expression of chronic pain.

Critical Thresholds for Cerebrovascular Reactivity: Fact or Fiction?

To the Editor, We read with interest the recent publication by Sorrentino et al. [1] entitled: ‘‘Critical Thresholds for Cerebrovascular Reactivity after Traumatic Brain Injury’’. The study is based on the hypothesis that bedside real-time calculation of the pressure-reactivity index (PRx) allows a continuous estimation of cerebral pressure autoregulation [1–3]. From measurements of hydrostatic pressure alone the authors define threshold levels for survival as well as favorable outcome in patients with severe traumatic brain lesions. Pressure autoregulation is present in many tissues but it is most pronounced in the brain and the kidney. The main ‘‘physiological purpose’’ of pressure autoregulation is probably to keep intracapillary hydrostatic pressure relatively constant [4]. Changes in precapillary resistance (R) underlying cerebral pressure autoregulation are calculated from cerebral perfusion pressure (CPP) divided by cerebral blood flow (F): R = (Pa ICP)/F [5]. Accordingly, to describe cerebral vasoreactivity it is necessary to assess not only arterial blood pressure (Pa) and intracranial pressure (ICP) but also F. The authors behind the PRx method claim that cerebrovascular pressure reactivity may be estimated by observing the response of ICP to changes in Pa. PRx is in this study determined by calculating the correlation coefficient between 30 consecutive data points resulting from the time averaging of ICP and Pa signals with a width of the moving-average window of 8 s [1]. However, in our opinion it is obvious from the equation above that cerebrovascular resistance cannot be calculated unless cerebral blood flow is also assessed. The method of PRx is based on the assumption that the observed change in Pa is the primary and independent cause of the variations in the other three monitored variables. In the validity study of the PRx method Pa was increased by i.v. vasopressor infusion and F was measured utilizing PET-technique [3]. In this defined, experimental situation it is possible that the increase in Pa is the primary and independent cause of the variations in the other variables. Under clinical conditions this is, however, often not the case. During neurocritical care fluctuations in pain, sedation, stress level, local neuronal excitation, etc., will cause changes in cerebral energy metabolism resulting in variations in precapillary vasoconstriction and cerebral blood flow that are not a function of simultaneously occurring changes in Pa. All these sources of error are neglected when PRx is interpreted as a measure of cerebral pressure autoregulation. As the Cambridge group has also suggested that PRx is correlated to cerebral energy metabolism [6] it seems unlikely that this index might be used for continuous estimation pressure autoregulation as well. In their analyses of prognostic threshold levels the authors use mean values for the entire monitoring period for each parameter: ICP, CPP, and PRx. In the study, the monitoring periods vary between 6 h and 20 days. The authors comment on this fact in the section of limitations. However, in our opinion the consequences are more serious than mentioned by the authors and may invalidate the threshold levels arrived at. In patients with brain trauma mortality is usually caused by an increase in ICP and patients who die within the first few days of admission usually have a high ICP. However, patients who initially may have equally high ICP (in this C.-H. Nordstrom (&) T. H. Nielsen Department of Neurosurgery, Odense University Hospital, Odense, Denmark e-mail: carl-henrik.nordstrom@med.lu.se

Bold claims for optogenetics

In a recent Letter to Nature, Lee and colleagues combined optogenetic stimulation with functional magnetic resonance imaging (ofMRI) to examine the relationship between pyramidal-cell spiking and the blood oxygenation level dependent (BOLD) signal. To do so, they injected an adeno-associated viral vector into the primary motor cortex (M1) of adult rats to drive the expression of channelrhodopsin (ChR2) in cortical projection neurons, thus making them sensitive to light. The authors then used combined light stimulation and functional magnetic resonance imaging (fMRI) to examine the effects of selective activation of the light-sensitive pyramidal cells on the BOLD signal, as well as to probe the value of this methodology for mapping brain connectivity. They found that excitation of these neurons induced positive BOLD signals both in the injected M1 region and in remote target thalamic nuclei receiving direct projections from that region, and concluded that ofMRI reliably links positive BOLD signals with increased local neuronal excitation. However, their analysis neglects the almost immediate activation of other circuits that could lead to the generation of BOLD signals through local perisynaptic rather than spiking activity. Their experiments therefore do not pin down the identity of the specific neuronal signals that give rise to the BOLD signal.

A Micro-Electrode Array device coupled to a laser-based system for the local stimulation of neurons by optical release of glutamate

Optical stimulation is a promising approach to investigate the local dynamic responses of cultured neurons. In particular, flash photolysis of caged compounds offers the advantage of allowing the rapid change of concentration of either extracellular or intracellular molecules, such as neurotransmitters or second messengers, for the stimulation or modulation of neuronal activity. We describe here the use of an ultra-violet (UV) laser diode coupled to an optical fibre for the local activation of caged compounds combined with a Micro-Electrode Array (MEA) device. Local uncaging was achieved by UV irradiation through the optical fibre previously positioned by using a red laser diode. The size of the stimulation was determined using caged fluorescein, whereas its efficacy was tested by studying the effect of uncaging the neurotransmitter glutamate. Uncaged glutamate evoked neuronal responses that were recorded using either fluorescence measurements or electrophysiological recordings with MEAs, thus showing the ability of our system to induce local neuronal excitation. This method allows overcoming the limitations of the MEA system related to unfocused electrical stimulation and induction of electrical artefacts. In addition, the coupling of a UV laser diode to an optical fibre allows a precise local stimulation and a quick change of the stimulation point.

Self-organization of globally continuous and locally distributed information representation

A number of findings suggest that the preferences of neighboring neurons in the inferior temporal (IT) cortex of macaque monkeys tend to be similar. However, a recent study reports convincingly that the preferences of neighboring neurons actually differ. These findings seem contradictory. To explain this conflict, we propose a new view of information representation in the IT cortex. This view takes into account sparse and local neuronal excitation. Since the excitation is sparse, information regarding visual objects seems to be encoded in a distributed manner. The local excitation of neurons coincides with the classical notion of a column structure. Our model consists of input layer and output layer. The main difference from conventional models is that the output layer has local and random intra-layer connections. In this paper, we adopt two rings embedded in three-dimensional space as an input signal space, and examine how resultant information representation depends on the distance between two rings that is denoted as D. We show that there exists critical value for the distance D c. When D> D c, the output layer becomes able to form the column structure, this model can obtain the distributed representation within the column. While the output layer acquires the conventional information representation observed in the V1 cortex when D< D c. Moreover, we consider the origin of the difference between information representation of the V1 cortex and that of the IT cortex. Our finding suggests that the difference in the information representations between the V1 and the IT cortices could be caused by difference between the input space structures.

Neurons of a Limited Subthalamic Area Mediate Elevations in Cortical Cerebral Blood Flow Evoked by Hypoxia and Excitation of Neurons of the Rostral Ventrolateral Medulla

Sympathoexcitatory reticulospinal neurons of the rostral ventrolateral medulla (RVLM) are oxygen detectors excited by hypoxia to globally elevate regional cerebral blood flow (rCBF). The projection, which accounts for >50% of hypoxic cerebral vasodilation, relays through the medullary vasodilator area (MCVA). However, there are no direct cortical projections from the RVLM/MCVA, suggesting a relay that diffusely innervates cortex and possibly originates in thalamic nuclei. Systematic mapping by electrical microstimulation of the thalamus and subthalamus revealed that elevations in rCBF were elicited only from a limited area, which encompassed medial pole of zona incerta, Forel's field, and prerubral zone. Stimulation (10 sec train) at an active site increased rCBF by 25 +/- 6%. Excitation of local neurons with kainic acid mimicked effects of electrical stimulation by increasing rCBF. Stimulation of the subthalamic cerebrovasodilator area (SVA) with single pulses (0.5 msec; 80 microA) triggered cortical EEG burst-CBF wave complexes with latency 24 +/- 5 msec, which were similar in shape to complexes evoked from the MCVA. Selective bilateral lesioning of the SVA neurons (ibotenic acid, 2 microg, 200 nl) blocked the vasodilation elicited from the MCVA and attenuated hypoxic cerebrovasodilation by 52 +/- 12% (p < 0.05), whereas hypercarbic vasodilation remained preserved. Lesioning of the vasodilator site in the basal forebrain failed to modify SVA-evoked rCBF increase. We conclude that (1) excitation of intrinsic neurons of functionally restricted region of subthalamus elevates rCBF, (2) these neurons relay signals from the MCVA, which elevate rCBF in response to hypoxia, and (3) the SVA is a functionally important site conveying vasodilator signal from the medulla to the telencephalon.

GABAA and excitatory amino acid receptors in dorsomedial hypothalamus and heart rate in rats.

We have previously shown that microinjection of drugs that interfere with the function of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) into the hypothalamus produces cardiorespiratory and behavioral changes resembling those seen in emotional stress. The purpose of this study was to determine whether excitatory amino acids (EAAs) can produce a cardiovascular response similar to that caused by the GABAA receptor antagonist bicuculline methiodide (BMI) when microinjected at the same hypothalamic site in urethan-anesthetized rats and to clarify the precise locus of action of these agents. N-methyl-D-aspartic acid (NMDA, 0.68-6.8 pmol/50 nl) and kainic acid (KA, 0.47-4.7 pmol/50 nl) produced dose-related increases in heart rate and blood pressure when injected at sites in the dorsomedial hypothalamus reactive to BMI (20 pmol/50 nl). Higher doses of NMDA (68 pmol), however, failed to elicit consistent increases in heart rate and blood pressure when injected at these same sites. The effects of NMDA were selectively blocked by the NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid, whereas the effects of KA were selectively blocked by the non-NMDA EAA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione. These results demonstrate that 1) blockade of inhibitory amino acid receptors or stimulation of EAA receptors in the dorsomedial nucleus of the hypothalamus produces tachycardic and pressor responses in urethan-anesthetized rats and 2) use of high doses of EAAs may be an unreliable method of evoking local neuronal excitation in certain regions of the central nervous system.

Limitations of the technique of pressure microinjection of excitatory amino acids for evoking responses from localized regions of the CNS

The aim of this study, performed on anaesthetized cats and rabbits, was to test the assumption that pressure microinjections of excitatory amino acids cause long-lasting excitation of neurones located close to the injection site. Unitary action potentials on antidromic field potentials were recorded from respiratory or ‘reticular’ neurones in the medulla oblongata and from phrenic motoneurones at different distances from the injection site. Injection of 10–150 nl (5–150 nmol) of l-glutamate or dl-homocysteic acid into these areas resulted in complex and widespread neuronal events. Generally, more distant neurones (500–1300 μm) were excited for variable periods of time (3–15 min), while neurones in the vicinity of the injection site (0–500 μm) showed, after a brief period of excitation time, a long-lasting (up to 30 min) decrease in excitability or silencing of discharge, probably due to a depolarizing block and disturbances in the ionic composition of the extracellular space. These findings show that interpretation of physiological responses following such injections should not be based on an assumption of local neuronal excitation. Some recommendations regarding the use of this technique are made.

Cardiovascular responses elicited by stimulation of neurons in the central amygdaloid nucleus in awake but not anesthetized rats resemble conditioned emotional responses

Cardiovascular responses elicited by electrical stimulation of the central amygdaloid nucleus were examined in awake and anesthetized rats. Stimulation through chronically implanted electrodes evoked increases in arterial pressure and heart rate in awake, freely behaving rats. The responses, which were dependent upon the frequency and the intensity of the stimulus, were not consistently related to the presence of evoked amygdaloid afterdischarges or to evoked behavioral reactions. Following induction of anesthesia, stimuli delivered to the same rats through the same fixed electrodes produced decreases in blood pressure and heart rate. Microinjection of l-glutamate into the amygdala of freely behaving rats also elicited increases in arterial pressure and heart rate, indicating that the cardiovascular changes evoked by electrical stimuli are due to excitation of local neurons rather than fibers of passage. The timing and pattern of the response elicited by electrical stimulation of the amygdala in the awake but not anesthetized rat closely corresponds with that evoked by an acoustic conditioned emotional stimulus.

A novel vasodepressor response elicited from the rat cerebellar fastigial nucleus: the fastigial depressor response

We sought to determine by using kainic acid (KA), an agent stimulating neuronal cell bodies but not fibers, whether the elevations in arterial pressure (AP) and heart rate (HR) elicited by electrical stimulation of the cerebellar fastigial nucleus (FN) are due to excitation of local neurons in FN or fibers passing through the area. Rats were anesthetized (chloralose 40 mg/kg), paralyzed (tubocurarine) and artificially ventilated. Electrical stimulation of FN (50 Hz, 10–100 μA) increased AP and HR. In contrast restricted microinjections of KA (0.05–5 nmol/100 nl) in FN elicited a sustained, dose-dependent decrease in AP and HR. The response is not due to diffusion of KA to adjacent sites. Excitation by KA of neurons intrinsic to FN elicits a heretofore unrecognized vasodepressor response, the fastigial depressor response.