Multisite Recordings Research Articles

Neuronal mechanisms of the anoxia-induced network oscillations in the rat hippocampus in vitro.

1. A spindle of fast network oscillations precedes the ischaemia-induced rapid depolarisation in the rat hippocampus in vivo. However, this oscillatory pattern could not be reproduced in slices and the underlying mechanisms remain poorly understood. We have found that anoxia-induced network oscillations (ANOs, 20-40 Hz, lasting for 1-2 min) can be reproduced in the intact hippocampi of postnatal day P7-10 rats in vitro, and we have examined the underlying mechanisms using whole-cell and extracellular field potential recordings in a CA3 pyramidal layer. 2. ANOs were generated at the beginning of the anoxic depolarisation, when pyramidal cells depolarised to subthreshold values. Maximal power of the ANOs was attained when pyramidal cells depolarised to -56 mV; depolarisation above -47 mV resulted in a depolarisation block of pyramidal cells and a waning of ANOs. 3. A multiple unit activity in extracellular field recordings was phase locked to the negative and ascending phases of ANOs. Pyramidal cells recorded in current-clamp mode generated action potentials with an average probability of about 0.05 per cycle. The AMPA receptor-mediated EPSCs and the GABA receptor-mediated IPSCs in CA3 pyramidal cells were also phase locked with ANOs. 4. ANOs were prevented by tetrodotoxin and glutamate receptor antagonists CNQX and APV, and were slowed down by the allosteric GABA(A) receptor modulator diazepam. In the presence of the GABA(A) receptor antagonist bicuculline, ANOs were transformed to epileptiform discharges. 5. In the presence of the A1 adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), the anoxia induced an epileptiform activity and no ANOs were observed. 6. In normoxic conditions, a rise of extracellular potassium to 10 mM induced an epileptiform activity. Increasing extracellular potassium in conjunction with a bath application of the adenosine A1 receptor agonist cyclopentyladenosine induced oscillations similar to ANOs. 7. Multisite recordings along the septo-temporal hippocampal axis revealed that ANOs and anoxic depolarisation originate in the temporal part, and propagate towards the septal pole at a speed of 1.9 mm x min(-1). 8. ANOs were observed starting from P7, i.e. at a developmental stage when the effects of GABA change from depolarisation to hyperpolarisation. 9. These results suggest that the synchronisation of anoxia-induced oscillations relies on synaptic mechanisms; that the inhibition by GABA and adenosine sets the tune for a generation of oscillations and prevents an epileptiform activity; and that a synchronous GABAergic inhibition is instrumental in a phase locking neuronal activity similarly to other types of oscillatory activities in the gamma frequency range.

Techniques for Chronic, Multisite Neuronal Ensemble Recordings in Behaving Animals

Advances in our understanding of neural systems will go hand in hand with improvements in the experimental techniques used to study these systems. This article describes a series of methodological developments aimed at enhancing the power of the methods needed to record simultaneously from populations of neurons over broad regions of the brain in awake, behaving animals. First, our laboratory has made many advances in electrode design, including movable bundle and array electrodes and smaller electrode assemblies. Second, to perform longer and more complex multielectrode implantation surgeries in primates, we have modified our surgical procedures by employing comprehensive physiological monitoring akin to human neuroanesthesia. We have also developed surgical implantation techniques aimed at minimizing brain tissue damage and facilitating penetration of the cortical surface. Third, we have integrated new technologies into our neural ensemble, stimulus and behavioral recording experiments to provide more detailed measurements of experimental variables. Finally, new data analytical techniques are being used in the laboratory to analyze increasingly large quantities of data.

Simultaneous multi-site recordings of neural activity with an inline multi-electrode array and optical measurement in rat hippocampal slices.

Understanding neuronal network function requires multi-site recording. An optical imaging method using voltage-sensitive dyes (VSD) is one such suitable method. It is also important to complement this technique with methods that measure other physiological parameters of neuronal network activity. We aimed to develop a multi-channel electrode method that would easily permit the simultaneous optical imaging of neural activity in vitro. There are several multi-electrode systems; some of these are commercially available, but they are not easy to use and are also expensive. We developed a novel, less expensive, stainless steel inline electrode array that utilizes the standard etching method of circuit board manufacturing. This method, along with widely available computer drawing software, allowed us to produce electrode arrays with uniform quality in electrical characteristics and shapes. The electrode array was designed so that it could easily substitute for single electrode systems without modifying the conventional set-up used for electrophysiological measurements and optical recordings. Simultaneous measurement of evoked neural activities in area CA1 of the rat hippocampal slice with our optical imaging method using the VSD Di-4-ANEPPS is demonstrated in this paper. The electrode array was also used as a stimulating electrode, and the evoked response was measured by the optical recording method. The results demonstrate the advantages of simultaneous recordings obtained by these complementary multi-site recording methods in vitro.

Imaging postsynaptic activities of teleost thalamic neurons at single cell resolution using a voltage-sensitive dye

Optical recording of neuronal activities using voltage-sensitive dyes (VSDs) is a useful method for simultaneous multi-site recording. However, it has been rather difficult to distinguish optical signals from individual, identified cells. We applied the optical recording technique using a high-speed charge coupled device (CCD) imaging system to a teleost thalamic nucleus, corpus glomerulosum (CG) which has a well-defined histological organization and large postsynaptic dendrites. Patch-like dye (di-4-ANEPPS) signals were observed in the dendritic layer of the CG in response to afferent nerve stimulations. These responses were completely blocked by an α-amino-3-hydroxy-5-methyl-4-isoxazole-proprionate (AMPA) receptor antagonist, did not propagate, and the size of the patches were close to that of a single dendritic tip of the ‘large cell’. Thus, we found that these patch-like VSD signals most likely represent postsynaptic potentials at individual dendritic tips of the large cells.

Long-term recordings of networks of immortalized GnRH neurons reveal episodic patterns of electrical activity.

The CNS controls reproduction through pulsatile secretion of gonadotropin-releasing hormone (GnRH). Episodic increases in the firing rate of unidentified hypothalamic neurons have been associated with downstream markers of GnRH secretion. Whether this episodic electrical activity is intrinsic to GnRH neurons, intrinsic to other "pulse generator" neurons that drive GnRH neurons, or a combination of these is unknown. To determine if GnRH neurons display episodic firing patterns in isolation from other cell types, immortalized GnRH neurons (GT1-7 cells) were cultured on multiple microelectrode arrays. Long-term, multi-site recordings of GT1-7 cells revealed repeated episodes of increased firing rate with an interval of 24.8 +/- 1.3 (SE) min that were completely eliminated by tetrodotoxin, a sodium channel blocker. This pattern was comprised of active units that fired independently as well as coincidentally, suggesting the overall pattern of electrical activity in GT1-7 cells emerges as a network property. The A-type potassium-channel antagonist 4-aminopyridine (1 mM) increased both firing rate and GnRH secretion, demonstrating the presence of A-type currents in these cells and supporting the hypothesis that electrical activity is associated with GnRH release. Physiologically relevant episodic firing patterns are thus an intrinsic property of immortalized GnRH neurons and appear to be associated with secretion. The finding that overall activity is derived from the sum of multiple independent active units within a network may have important implications for the genesis of the GnRH secretory pattern that is delivered to the target organ. Specifically, these data suggest not every GnRH neuron participates in each secretory pulse and provide a possible mechanism for the variations in GnRH-pulse amplitude observed in vivo.

Spatiotemporal characterization of rhythmic activity in rat spinal cord slice cultures.

Rat spinal networks generate a spontaneous rhythmic output directed to motoneurons under conditions of increased excitation or of disinhibition. It is not known whether these differently induced rhythms are produced by a common rhythm generator. To investigate the generation and the propagation of rhythmic activity in spinal networks, recordings need to be made from many neurons simultaneously. Therefore extracellular multisite recording was performed in slice cultures of embryonic rat spinal cords grown on multielectrode arrays. In these organotypic cultures most of the spontaneous neural activity was nearly synchronized. Waves of activity spread from a source to most of the network within 35-85 ms and died out after a further 30-400 ms. Such activity waves induced the contraction of cocultured muscle fibres. Several activity waves could be grouped into aperiodic bursts. Disinhibition with bicuculline and strychnine or increased excitability with high K(+) or low Mg(2+) solutions could induce periodic bursting with bursts consisting of one or several activity waves. Whilst the duration and period of activity waves were similar for all protocols, the duration and period of bursts were longer during disinhibition than during increased excitation. The sources of bursting activity were mainly situated ventrally on both sides of the central fissure. The pathways of network recruitment from one source were variable between bursts, but they showed on average no systematic differences between the protocols. These spatiotemporal similarities under conditions of increased excitation and of disinhibition suggest a common spinal network for both types of rhythmic activity.

Biocompatibility of silicon-based arrays of electrodes coupled to organotypic hippocampal brain slice cultures

In this study we examined the passive biocompatibility of a three-dimensional microelectrode array (MEA), designed to be coupled to organotypic brain slice cultures for multisite recording of electrophysiological signals. Hippocampal (and corticostriatal) brain slices from 1-week-old (and newborn) rats were grown for 4–8 weeks on the perforated silicon chips with silicon nitride surfaces and 40 μm sized holes and compared with corresponding tissue slices grown on conventional semiporous membranes. In terms of preservation of the basic cellular and connective organization, as visualized by Nissl staining, Timm sulphide silver-staining, microtubule-associated protein 2 (MAP2) and glial fibrillary acidic protein (GFAP) immunostaining, the slice cultures grown on chips did not differ from conventionally grown slice cultures. Neither were there any signs of astrogliosis or neurodegeneration around the upper recording part of the 47-μm-high platinum-tip electrodes. Slice cultures grown on a separate set of chips with platinum instead of silicon nitride surfaces also displayed normal MAP2 and GFAP immunostaining. The width of the GFAP-rich zone (glia limitans) at the bottom surface of the slice cultures was the same (∼20 μm) in cultures grown on chips with silicon nitride and platinum surfaces and on conventional insert membranes. The slice cultures grown on chips maintained a normal, subfield differentiated susceptibility to the glutamate receptor agonist N-methyl- d-aspartate (NMDA) and the neurotoxin trimethyltin (TMT), as demonstrated by the cellular uptake of propidium iodide (PI), which was used as a reproducible and quantifiable marker for neuronal degeneration. We conclude that organotypic brain slice cultures can grow on silicon-based three-dimensional microelectrode arrays and develop normally with display of normal subfield differentiated susceptibilities to known excito- and neurotoxins. From this it is anticipated that the set-up, designed for recording of electrophysiological parameters, can be used for long-term studies of defined neuronal networks and provide valuable information on both normal, neurotoxicological and neuropathological conditions.

Ordered networks of rat hippocampal neurons attached to silicon oxide surfaces

The control of neuronal cell position and outgrowth is of fundamental interest in the development of applications ranging from cellular biosensors to tissue engineering. We have produced rectangular networks of functional rat hippocampal neurons on silicon oxide surfaces. Attachment and network formation of neurons was guided by a geometrical grid pattern of the adhesion peptide PA22-2 which matches in sequence a part of the A-chain of laminin. PA22-2 was applied by contact printing onto the functionalised silicon oxide surface and was immobilised by hetero-bifunctional cross-linking with sulfo-GMBS. Geometric pattern matching was achieved by microcontact printing using a polydimethylsiloxane (PDMS) stamp. In this way the produced grid pattern ranged from 3 to 20 μm in line width and from 50 to 100 μm in line distances. As shown by atomic force microscopy (AFM), line widths and line distances of the peptide pattern differ less than 0.5 μm from the used PDMS stamp. The height of the layer of immobilised PA22-2 was approximately 3.5 nm implying the layer to be monomolecular. Immobilised PA22-2 was capable of binding anti-PA22-2 antibodies indicating that the function of the peptide was not compromised by immobilisation. Rat hippocampal neurons, cultured at low density in serum-free medium, were applied to the growth matrix of PA22-2-coated substrates and, within 1–3 h of culture, formed a network-like pattern that more or less matched the printed grid. Reliability and reproducibility of neuronal network formation depended on the geometry, line width and node diameter of the grid pattern. The immobilised neurons showed resting membrane potentials comparable with controls and, already after 1 day of culture, were capable of eliciting action potentials. The suitability of the immobilised neurons for the study of man-made neural networks and for multi-site recordings from a functional neuronal network is discussed.

Neuronal and Glial Membrane Potentials during Sleep and Paroxysmal Oscillations in the Neocortex

This study investigated the fluctuations in the membrane potential of cortical neurons and glial cells during the slow sleep oscillation and spike-wave (SW) seizures. We performed dual neuron-glia intracellular recordings together with multisite field potential recordings from cortical suprasylvian association areas 5 and 7 of cats under ketamine-xylazine anesthesia. Electrical stimuli applied to the cortex elicited responses consisting of a biphasic depolarization in glial cells, which was associated with an EPSP-IPSP sequence in neurons. During the slow (<1 Hz) oscillation, extracellular measurements of the potassium concentration revealed periodic increases with an amplitude of 1-2 mm, similar in shape to glial activities. We suggest that, through their uptake mechanisms, glia cells modulate the neuronal excitability and contribute to the pacing of the slow oscillation. The slow oscillation often evolved into SW paroxysms, mimicking sleep-triggered seizures. This transition was associated with increased coupling between the depolarizing events in neurons and glial cells. During seizures, the glial membrane potential displayed phasic negative events related to the onset of the paroxysmal depolarizing shifts in neurons. These events were not voltage dependent and increased their incidence and amplitude with the development of the seizure. It is suggested that the intraglial transient negativities represent field reflections of synchronized neuronal potentials. We propose that the mechanisms underlying the neuron-glia communication include, besides the traditional neurotransmitter- and ion-mediated pathways, field effects crossing their membranes as a function of the state of the cortical network.

Pulsatile stretch remodels cell-to-cell communication in cultured myocytes.

Mechanical stretch is thought to play an important role in remodeling atrial and ventricular myocardium and may produce substrates that promote arrhythmogenesis. In the present work, neonatal rat ventricular myocytes were cultured for 4 days as confluent monolayers on thin silicone membranes and then subjected to linear pulsatile stretch for up to 6 hours. Action potential upstrokes and propagation velocity (theta) were measured with multisite optical recording of transmembrane voltage of the cells stained with the voltage-sensitive dye RH237. Expression of the gap junction protein connexin43 (Cx43) and the fascia adherens junction protein N-cadherin was measured immunohistochemically in the same preparations. Pulsatile stretch caused dramatic upregulation of intercellular junction proteins after only 1 hour and a further increase after 6 hours (Cx43 signal increased from 0.73 to 1.86 and 2.02% cell area, and N-cadherin signal increased from 1.21 to 2.11 and 2.74% cell area after 1 and 6 hours, respectively). This was paralleled by an increase in theta from 27 to 35 cm/s after 1 hour and 37 cm/s after 6 hours. No significant change in the upstroke velocity of the action potential or cell size was observed. Increased theta and protein expression were not reversible after 24 hours of relaxation. Nonpulsatile (static) stretch produced qualitatively similar but significantly smaller changes than pulsatile stretch. Thus, pulsatile linear stretch in vitro causes marked upregulation of proteins that form electrical and mechanical junctions, as well as a concomitant increase in propagation velocity. These changes may contribute to arrhythmogenesis in myocardium exposed to acute stretch.

Generation and propagation of 4-AP-induced epileptiform activity in neonatal intact limbic structures in vitro.

We examined the generation, propagation and pharmacology of 4-aminopyridine (4-AP)-induced epileptiform activity (EA) in the intact interconnected limbic structure of the newborn (P0-7) rat in vitro. Whole-cell recordings of CA3 pyramidal cells and multisite field potential recordings in CA3, CA1, dentate gyrus, and lateral and medial entorhinal cortex revealed 4-AP-induced EA as early as P0-1. At this age, EA was initiated in the CA3 region and propagated to CA1, but not to the entorhinal cortex. Starting from P3-4, EA propagated from CA3 to the entorhinal cortex. Along the CA3 septo-temporal axis, EA arose predominantly from the septal pole and spread towards the temporal site. Whereas the onset of 4-AP-induced EA decreased with age from 21.2 +/- 1.6 min at P0-1 to 4.7 +/- 0.63 min at P6-7, the seizure duration increased in the same age groups from 98 +/- 14 s to 269.4 +/- 85.9 s, respectively. The EA was blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) but not by DL-2-amino-5-phosphonovaleric acid (APV), (+)-MK-801 hydrogen maleate (MK-801) or (+/-)-alpha-methyl-4-carboxyphenylglycine (MCPG), suggesting that they were mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate receptor activation. We conclude that: (i) the septal pole of the hippocampal CA3 region plays a central role in the generation of EA in the neonatal limbic system; and (ii) AMPA/kainate receptor-mediated EA can be generated in CA3 already at birth. Therefore, the recurrent collateral synapses and circuits required for the generation of EA are developed earlier than previously suggested on the basis of studies on hippocampal slices.

Functional profile of the giant metacerebral neuron of Helix aspersa: temporal and spatial dynamics of electrical activity in situ.

1. Understanding the biophysical properties of single neurons and how they process information is fundamental to understanding how the brain works. However, action potential initiation and the preceding integration of the synaptic signals in neuronal processes of individual cells are complex and difficult to understand in the absence of detailed, spatially resolved measurements. Multi-site optical recording with voltage-sensitive dyes from individual neurons in situ was used to provide these kinds of measurements. We analysed in detail the pattern of initiation and propagation of spikes evoked synaptically in an identified snail (Helix aspersa) neuron in situ. 2. Two main spike trigger zones were identified. The trigger zones were activated selectively by different sets of synaptic inputs which also produced different spike propagation patterns. 3. Synaptically evoked action potentials did not always invade all parts of the neuron. The conduction of the axonal spike was regularly blocked at particular locations on neuronal processes. 4. The propagating spikes in some axonal branches consistently reversed direction at certain branch points, a phenomenon known as reflection. 5. These experimental results, when linked to a computer model, could allow a new level of analysis of the electrical structure of single neurons.

Integration of low-frequency sleep oscillations in corticothalamic networks

The corticothalamic system acts as a complex network in promoting the various oscillatory patterns (slow oscillation, spindles, delta) that characterize the state of quiet sleep. Local synchronizing mechanisms of any of the above-mentioned oscillations occur at the site of their genesis, thalamic or cortical. These mechanisms are assisted by the wide-range, synchronized occurrence of the cortical slow oscillation, which finally produces the coalesced picture of slow-wave sleep EEG. Multisite, simultaneous intracellular and field potential recordings in cat, as well as EEG recordings in human were performed in order to assess the state of synchrony and the propagation of various sleep rhythms in the corticothalamic network.

Relationship between field potentials and spike activity in rat S1: Multi-site cortical recordings and analysis

We present field potential and multiple single-unit data simultaneously recorded from somatosensory cortex of the anesthetized rat using a multi-site silicon electrode. We find that although single-units fire spontaneously only during troughs of the low-frequency field potential, there is high variability in the timing of their firing. Viewing our results in the context of previous anatomical, physiological, and modeling studies of cerebral cortex, we suggest that local field potentials are better characterized as a recording of local synaptic currents rather than a recording of the superposition of spike activity in populations of neurons. If true, then our data show that similar circuit-level synaptic inputs do not predict the timing of spiking output by the neurons receiving those inputs. These results have important implications for theories of cerebral cortical network function and information coding based on the interpretation of coherence in recorded field potentials.

Propagation of spontaneous synchronized activity in cortical slice cultures recorded by planar electrode arrays

The spatial propagation of synchronized activity in cortical slice cultures was characterized by multi-site extracellular recording. Spontaneous activity was studied in normal culture medium, and in bicuculline- or kainic acid-containing media. A common feature in all these conditions was that activity was generated first in superficial layers (i.e., layer I/II) before spreading over the whole area of the slice. In culture medium or bicuculline-containing medium, the initiation site of the activity was not constant and showed a large variety of patterns of horizontal propagation. Kainic acid induced epileptiform activity, consisting of intense initial bursts followed by repetitive after-discharges. Though the patterns of spatial propagation of the bursts were variable as in the other conditions, the after-discharges followed a constant path. Cross-correlation analysis indicated that the network moved in a graded fashion to a steady state during the sequence of after-discharges.

Olfactory inputs activate the medial entorhinal cortex via the hippocampus.

The lateral and medial regions of the entorhinal cortex differ substantially in terms of connectivity and pattern of activation. With regard to olfactory input, a detailed and extensive physiological map of the olfactory projection to the entorhinal cortex is missing, even if anatomic studies suggest that the olfactory afferents are confined to the lateral and rostral entorhinal region. We studied the contribution of the medial and lateral entorhinal areas to olfactory processing by analyzing the responses induced by lateral olfactory tract stimulation in different entorhinal subfields of the in vitro isolated guinea pig brain. The pattern of synaptic activation of the medial and lateral entorhinal regions was reconstructed either by performing simultaneous multisite recordings or by applying current source density analysis on field potential laminar profiles obtained with 16-channel silicon probes. Current source density analysis demonstrated the existence of a direct monosynaptic olfactory input into the superficial 300 microm of the most rostral part of the lateral entorhinal cortex exclusively, whereas disynaptic sinks mediated by associative fibers arising from the piriform cortex were observed at 100-350 microm depth in the entire lateral aspect of the cortex. No local field responses were recorded in the medial entorhinal region unless a large population spike was generated in the hippocampus (dentate gyrus and CA1 region) by a stimulus 3-5x the intensity necessary to obtain a maximal monosynaptic response in the piriform cortex. In these conditions, a late sink was recorded at a depth of 600-1000 microm in the medial entorhinal area (layers III-V) 10.6 +/- 0.9 (SD) msec after a population spike was simultaneously recorded in CA1. Diffuse activation of the medial entorhinal region was also obtained by repetitive low-intensity stimulation of the lateral olfactory tract at 2-8 Hz. Higher or lower stimulation frequencies did not induce hippocampal-medial entorhinal cortex activation. These results suggest that the medial and the lateral entorhinal regions have substantially different roles in processing olfactory sensory inputs.

Body surface mapping of counterclockwise and clockwise typical atrial flutter: a comparative analysis with endocardial activation sequence mapping

OBJECTIVESThis study was directed at developing spatial 62-lead electrocardiogram (ECG) criteria for classification of counterclockwise (CCW) and clockwise (CW) typical atrial flutter (Fl) in patients with and without structural heart disease.BACKGROUNDElectrocardiographic classification of CCW and CW typical atrial Fl is frequently hampered by inaccurate and inconclusive scalar waveform analysis of the 12-lead ECG.METHODSElectrocardiogram signals from 62 torso sites and multisite endocardial recordings were obtained during CCW typical atrial Fl (12 patients), CW typical Fl (3 patients), both forms of typical Fl (4 patients) and CCW typical and atypical atrial Fl (1 patient). All the Fl wave episodes were divided into two or three successive time periods showing stable potential distributions from which integral maps were computed.RESULTSThe initial, intermediate and terminal CCW Fl wave map patterns coincided with: 1) caudocranial activation of the right atrial septum and proximal-to-distal coronary sinus activation, 2) craniocaudal activation of the right atrial free wall, and 3) activation of the lateral part of the subeustachian isthmus, respectively. The initial, intermediate and terminal CW Fl wave map patterns corresponded with : 1) craniocaudal right atrial septal activation, 2) activation of the subeustachian isthmus and proximal-to-distal coronary sinus activation, and 3) caudocranial right atrial free wall activation, respectively. A reference set of typical CCW and CW mean integral maps of the three successive Fl wave periods was computed after establishing a high degree of quantitative interpatient integral map pattern correspondence irrespective of the presence or absence of organic heart disease.CONCLUSIONSThe 62-lead ECG of CCW and CW typical atrial Fl in man is characterized by a stereotypical spatial voltage distribution that can be directly related to the underlying activation sequence and is highly specific to the direction of Fl wave rotation. The mean CCW and CW Fl wave integral maps present a unique reference set for improved clinical detection and classification of typical atrial Fl.

Long-term neural recording characteristics of wire microelectrode arrays implanted in cerebral cortex

This paper describes a detailed protocol for obtaining chronic, multi-site unit recordings in cerebral cortex of awake animals for periods of three months or more. The protocol includes details for making relatively simple and inexpensive implantable multichannel electrodes that consist of arrays of separate microwires. The results reported in this paper suggest that a viable implant will have discriminable unit activity on about 80% of the electrodes, resulting in, on average, the simultaneous unit recording of upwards of 60 units during a daily recording session. The active electrodes during one recording session tend to remain active in subsequent recording sessions for several weeks. Using the methods described here, implants have been constructed which incorporate several different electrode materials, coatings, sizes, and electrode separation within a single array. These microwire electrode arrays provide the basic technology for obtaining unit recordings for several months. This provides a model system for studying biocompatibility of neural implants, which is a critical component for the development of neural implants that have an indefinite working span. Themes: Sensory Systems Topics: Auditory systems: central physiology

Simultaneous multi-site recordings and iontophoretic drug and dye applications along the trigeminal system of anesthetized rats

A multi-electrode system that permits simultaneous recordings from multiple neurons and iontophoretic applications at two or three different brain sites during acute experiments is described. This system consists of two or three microdrive terminals, each of which includes four electrodes that can be moved independently and used for both extracellular recordings and microiontophoretic drug administration. Drug applications were performed during standard extracellular recordings of multiple single-units via specialized combined electrodes (CEs), which enable ejection of neuroactive substances and recording of neuronal activity from the same electrode. With this system, we were able to successfully record simultaneously from different levels (brainstem, thalamus, and cortex) of the vibrissal ascending pathway of the anesthetized rat. Herein, examples of simultaneous recordings from the brainstem and thalamus and from the thalamus and cortex are presented. An effect of iontophoretic applications of agonists and antagonists of metabotropic glutamate receptors (mGluRs) in the thalamus is demonstrated, and the extent of drug diffusion in the barrel cortex is demonstrated with biocytin. This new multi-electrode system will facilitate the study of transformations of sensory information acquired by the whiskers into cortical representations.

Multisite optical recording of excitability in the enteric nervous system.

A multisite optical recording technique consisting of an array of 464 photodiodes was used to measure dynamic changes in transmembrane potentials (Vm) of guinea-pig and mouse enteric neurones stained with the voltage-sensitive dye Di-8-ANEPPS. Optical recordings of Vm changes in enteric neurones which were evoked by depolarizing current pulses or synaptic activation mirrored the Vm changes measured intracellularly in the same neurone. Action potentials had fractional change in fluorescence of -0.09 +/- 0.06% and their peak to peak noise level was 20 +/- 14% of the action potential amplitude. Optical recordings after electrical stimulation of interganglionic nerve strands revealed slow EPSPs, nicotinergic supra- and subthreshold fast EPSPs as well as propagation of action potentials along interganglionic strands. Local application of acetylcholine onto a single ganglion induced reproducibly and dose dependently action potential discharge demonstrating the feasibility of neuropharmacological studies. The optical mapping made it possible to record action potentials simultaneously in a large number of neurones with high spatiotemporal resolution that is unattainable by conventional techniques. This technique presents a powerful tool to study excitability spread within enteric circuits and to assess differential activation of enteric populations in response to a number of stimuli which modulate neuronal activity directly or through synaptic mechanisms.