Simultaneous single-cell recording and microdialysis within the same brain site in freely behaving rats: a novel neurobiological method

Letting the little light of mind shine: Advances and future directions in neurochemical detection

Synaptic transmission via neurochemical release is the fundamental process that integrates and relays encoded information in the brain to regulate physiological function, cognition, and emotion. To unravel the biochemical, biophysical, and computational mechanisms of signal processing, one needs to precisely measure the neurochemical release dynamics with molecular and cell-type specificity and high resolution. Here we reviewed the development of analytical, electrochemical, and fluorescence imaging approaches to detect neurotransmitter and neuromodulator release. We discussed the advantages and practicality in implementation of each technology for ease-of-use, flexibility for multimodal studies, and challenges for future optimization. We hope this review will provide a versatile guide for tool engineering and applications for recording neurochemical release.

Collection of biochemical samples with brain-wide electrophysiological recordings from a freely moving rodent

Bridging accumulating insights from microscopic and macroscopic studies in neuroscience research requires monitoring of neuronal population dynamics and quantifying specific molecules or genes from the brain of identical animals. To this end, by minimizing the size and weight of an electrode array, we developed a method that records local field potential signals of multiple brain regions from one side of the hemisphere in a freely moving rodent. At the same time, extracellular cerebrospinal fluid for biochemical assays or a small part of brain tissue samples for gene expression assays are collected from the other side of the hemisphere. This method allows ongoing stable recordings and sample collections for at least two months. The methodological concept is applicable to a wide range of biological reactions at various spatiotemporal scales, allowing us to integrate an idea of physiolomics into existing omics analyses, leading to a new combination of multi-omics approaches.

Construction of an Improved Multi-Tetrode Hyperdrive for Large-Scale Neural Recording in Behaving Rats

Monitoring the activity patterns of a large population of neurons over many days in awake animals is a valuable technique in the field of systems neuroscience. One key component of this technique consists of the precise placement of multiple electrodes into desired brain regions and the maintenance of their stability. Here, we describe a protocol for the construction of a 3D-printable hyperdrive, which includes eighteen independently adjustable tetrodes, and is specifically designed for in vivo extracellular neural recording in freely behaving rats. The tetrodes attached to the microdrives can either be individually advanced into multiple brain regions along the track, or can be used to place an array of electrodes into a smaller area. The multiple tetrodes allow for simultaneous examination of action potentials from dozens of individual neurons, as well as local field potentials from populations of neurons in the brain during active behavior. In addition, the design provides for simpler 3D drafting software that can easily be modified for differing experimental needs.

Photosensitive-polyimide based method for fabricating various neural electrode architectures

An extensive photosensitive-polyimide (PSPI)-based method for designing and fabricating various neural electrode architectures was developed. The method aims to broaden the design flexibility and expand the fabrication capability for neural electrodes to improve the quality of recorded signals and integrate other functions. After characterizing PSPI's properties for micromachining processes, we successfully designed and fabricated various neural electrodes even on a non-flat substrate using only one PSPI as an insulation material and without the time-consuming dry etching processes. The fabricated neural electrodes were an electrocorticogram (ECoG) electrode, a mesh intracortical electrode with a unique lattice-like mesh structure to fixate neural tissue, and a guide cannula electrode with recording microelectrodes placed on the curved surface of a guide cannula as a microdialysis probe. In vivo neural recordings using anesthetized rats demonstrated that these electrodes can be used to record neural activities repeatedly without any breakage and mechanical failures, which potentially promises stable recordings for long periods of time. These successes make us believe that this PSPI-based fabrication is a powerful method, permitting flexible design, and easy optimization of electrode architectures for a variety of electrophysiological experimental research with improved neural recording performance.

An inexpensive drivable cannulated microelectrode array for simultaneous unit recording and drug infusion in the same brain nucleus of behaving rats

Neurons are functionally segregated into discrete populations that perform specific computations. These computations, mediated by neuron-neuron electrochemical signaling, form the neural basis of behavior. Thus fundamental to a brain-based understanding of behavior is the precise determination of the contribution made by specific neurotransmitters to behaviorally relevant neural activity. To facilitate this understanding, we have developed a cannulated microelectrode array for use in behaving rats that enables simultaneous neural ensemble recordings and local infusion of drugs in the same brain nucleus. The system is inexpensive, easy to use, and produces robust and quantitatively reproducible drug effects on recorded neurons.

Effects of subdural application of lidocaine in patients with focal epilepsy

Antiepileptic drug (AED) delivery directly into the neocortex has recently been shown to be able to both prevent and terminate focal seizures in rats. The present clinical experiment aimed to test the local effects of lidocaine delivered onto the pia mater adjacent to epileptogenic zones in human patients. Administration of lidocaine resulted in a marked diminishment of spike counts on all patients, with a decremental effect of lidocaine on the faster frequency elements of individual spikes and overall testing epochs. The direct cortical application of lidocaine appears to affect local epileptogenic activity in human patients with intractable focal epilepsy.

Pharmacological Manipulation of Neuronal Ensemble Activity by Reverse Microdialysis in Freely Moving Rats: A Comparative Study of the Effects of Tetrodotoxin, Lidocaine, and Muscimol

To be able to address the question how neurotransmitters or pharmacological agents influence activity of neuronal populations in freely moving animals, the combidrive was developed. The combidrive combines an array of 12 tetrodes to perform ensemble recordings with a moveable and replaceable microdialysis probe to locally administer pharmacological agents. In this study, the effects of cumulative concentrations of tetrodotoxin, lidocaine, and muscimol on neuronal firing activity in the prefrontal cortex were examined and compared. These drugs are widely used in behavioral studies to transiently inactivate brain areas, but little is known about their effects on ensemble activity and the possible differences between them. The results show that the combidrive allows ensemble recordings simultaneously with reverse microdialysis in freely moving rats for periods at least up to 2 wk. All drugs reduced neuronal firing in a concentration dependent manner, but they differed in the extent to which firing activity of the population was decreased and the in speed and extent of recovery. At the highest concentration used, both muscimol and tetrodotoxin (TTX) caused an almost complete reduction of firing activity. Lidocaine showed the fastest recovery, but it resulted in a smaller reduction of firing activity of the population. From these results, it can be concluded that whenever during a behavioral experiment a longer lasting, reversible inactivation is required, muscimol is the drug of choice, because it inactivates neurons to a similar degree as TTX, but it does not, in contrast to TTX, affect fibers of passage. For a short-lasting but partial inactivation, lidocaine would be most suitable.

Elevated lactate suppresses neuronal firing in vivo and inhibits glucose metabolism in hippocampal slice cultures

Glucose is well accepted as the major fuel for neuronal activity, while it remains controversial whether lactate also supports neural activity. In hippocampal slice cultures, synaptic transmission supported by glucose was reversibly suppressed by lactate. To test whether lactate had a similar inhibitory effect in vivo, lactate was perfused into the hippocampi of unanesthetized rats while recording the firing of nearby pyramidal cells. Lactate perfusion suppressed pyramidal cell firing by 87.5+/-8.3% (n=6). Firing suppression was slow in onset and fully reversible and was associated with increased lactate concentration at the site of the recording electrode. In vivo suppression of neural activity by lactate occurred in the presence of glucose; therefore we tested whether suppression of neural firing was due to lactate interference with glucose metabolism. Competition between glucose and lactate was measured in hippocampal slice cultures. Lactate had no effect on glucose uptake. Lactate suppressed glucose oxidation when applied at an elevated, pathological concentration (10 mM), but not at its physiological concentration (1 mM). Pyruvate (10 mM) also inhibited glucose oxidation but was significantly less effective than lactate. The greater suppressive effect of lactate as compared to pyruvate suggests that alteration of the NAD(+)/NADH ratio underlies the suppression of glucose oxidation by lactate. ATP in slice culture was unchanged in glucose (1 mM), but significantly reduced in lactate (1 mM). ATP in slice culture was significantly increased by combination of glucose (1 mM) and lactate (1 mM). These data suggest that alteration of redox ratio underlies the suppression of neural discharge and glucose metabolism by lactate.

Spatially resolved non-invasive chemical stimulation for modulation of signalling in reconstructed neuronal networks

Functional coupling of reconstructed neuronal networks with microelectronic circuits has potential for the development of bioelectronic devices, pharmacological assays and medical engineering. Modulation of the signal processing properties of on-chip reconstructed neuronal networks is an important aspect in such applications. It may be achieved by controlling the biochemical environment, preferably with cellular resolution. In this work, we attempt to design cell-cell and cell-medium interactions in confined geometries with the aim to manipulate non-invasively the activity pattern of an individual neuron in neuronal networks for long-term modulation. Therefore, we have developed a biohybrid system in which neuronal networks are reconstructed on microstructured silicon chips and interfaced to a microfluidic system. A high degree of geometrical control over the network architecture and alignment of the network with the substrate features has been achieved by means of aligned microcontact printing. Localized non-invasive on-chip chemical stimulation of micropatterned rat cortical neurons within a network has been demonstrated with an excitatory neurotransmitter glutamate. Our system will be useful for the investigation of the influence of localized chemical gradients on network formation and long-term modulation.

Patterning chemical stimulation of reconstructed neuronal networks

A spatially resolved delivery of substances integrated with cell culture substrates shows promise for application in pharmacological assays, bioanalytical studies on cell signaling pathways and cell-based biosensors, where control over the extracellular biochemical environment with a cellular resolution is desirable. In this work, we studied a biohybrid system where rat embryonic cortical neuronal networks are reconstructed on microstructured silicon chips and interfaced to microfluidics. The design of cell-cell and cell-medium interactions in confined geometries is presented. We developed an aligned microcontact printing technique (AmicroCP) for poly(lysine)-extracellular matrix proteins on microstructured chips, which allows a high degree of geometrical control over the network architecture and alignment of the neuronal network with the microfluidic features of a substrate. Spatially resolved on-chip delivery of compounds with a cellular resolution is demonstrated by chemical stimulation of patterned rat cortical neurons within a network with a number of solutions of excitatory neurotransmitter glutamate delivered via microfluidics. The combination of the system described with a patch-clamp technique allowed both modulation of the biochemical environment on a cellular level and the monitoring of electrophysiological properties in the reconstructed rat embryonic cortical networks changed by this microenvironment.

The effects on place cells of local scopolamine dialysis are mimicked by a mixture of two specific muscarinic antagonists

Using a dialysis probe near CA1 hippocampal recording electrodes, we infused nonspecific (scopolamine) and specific (methoctramine, pirenzepine) antagonists of muscarinic cholinergic transmission to determine their effects on the positional firing properties of place cells. Both low (0.5 mM) and high (2.0 or 3.0 mM) scopolamine significantly decreased in-field firing rate, increased the ratio of out-of-field to in-field rate, and reduced the smoothness of rate maps, while tending to increase out-of-field rate. Thus, local nonspecific muscarinic blockade mimicked the effects seen with intracerebroventricular application, suggesting that blockade of receptors local to the recorded cells plays an essential role. Unexpectedly, dialysis of scopolamine reduced locomotor activity, again duplicating the effects of intracerebroventricular administration. Most effects of methoctramine (1.0 mM), which blocks presynaptic m2 and m4 receptors, were initially strong but then diminished over hours. Methoctramine produced a significant increase only in out/in ratio and out-of-field rate, whereas it tended to increase in-field rate and monotonically decrease smoothness. Pirenzepine (3.0 mM), which blocks postsynaptic m1 receptors, produced a significant increase only in out/in ratio, whereas it tended to increase out-of-field rate and decrease in-field rate; all these effects were monotonic with respect to time. A mixture of methoctramine plus pirenzepine recapitulated the place-cell effects of scopolamine, although neither the mixture nor its separate components affected behavior. We conclude that the effects of scopolamine on place cells likely result from a combination of blockade of postsynaptic m1 receptors, leading to reduced excitability, with blockade of presynaptic m2 and m4 receptors, leading to increased out-of-field firing.

Flexible polyimide probes with microelectrodes and embedded microfluidic channels for simultaneous drug delivery and multi-channel monitoring of bioelectric activity

The study of intracellular communication requires devices that can not only monitor the bioelectric activity, but also control and observe the biochemical environment at the cellular level. This paper reports on the development and characterisation of implantable polyimide microprobes that allow simultaneous, selective chemical delivery/probing and multi-channel recording/stimulation of bioelectric activity. The key component of the system is a flexible polyimide substrate with embedded microchannels that is batch-fabricated combining polyimide micromachining and a lamination technique. The devices provide platinum microelectrodes on both sides of the polyimide substrate with an active surface between 20 microm x 20 microm and 50 microm x 50 microm. The embedded microchannels permit highly localised drug delivery or probing at the tip of the device via channel outlets adjacent to the microelectrodes. The microelectrodes were characterised by electrical impedance spectroscopy and the microchannels were studied in microflow experiments. Two different fluid delivery schemes were explored in two different designs. The first device type consists of a simple combination of microchannels and microelectrodes on one substrate. Liquids are ejected at the tip of the device by pressure injection techniques. The second device was inspired by the so-called U-tube concept allowing for highly localised delivery of controlled amounts of liquids in the picoliters range. Thus, the influence of chemical compounds on the electrical activity of cells can be studied with high temporal and spatial resolution. The flexible, implantable devices can be used for studying the chemical and electrical information exchange and communication of cells in in vivo and in vitro experiments.

The use of a remote-controlled minivalve, carried by freely moving animals on their head, to achieve instant pharmacological effects in intracerebral drug-perfusion studies

Intracerebral drug-perfusion studies in animals can be very efficiently performed with the 'reverse-dialysis' procedure. In this procedure, drugs are delivered into the brain via an intracerebrally implanted microdialysis probe. Traditionally, in reverse-dialysis studies the flow of control and drug solutions in the microdialysis site is alternated by large and heavy valves placed far from the experimental animal. In this arrangement, the drugs travel from the fluid-alternating device for a long (20--60 min) period before reaching the brain. This can obscure the onset of drug action, makes it difficult to deliver drugs into the extracellular space during short-lasting behavioral episodes, and considerably limits the number of drug solutions that can be perfused within an experimental session. This report describes the use of a miniature (15 mm long and 8 mm diameter), lightweight (1.4 g) minivalve (patent pending) for combined neuronal recording--intracerebral microdialysis studies in freely moving rats. The device is activated remotely and carried by the animals on their head. This allows the experimenter to alternate the control and drug solutions in the intracerebral recording/dialysis site rapidly and to detect the drug-induced neuronal firing pattern changes instantly, without interfering with the animal's behavior. It is demonstrated that with this novel device the onset of drug actions on hippocampal neurons can be clearly defined and that these actions occur within 2 min after minivalve activation. Furthermore, it is demonstrated that the minivalve allows one to test a large number of drug solutions, successively, within the same experimental session. The described protocol offers a high-throughput method for testing the neuron-specific pharmacological effects of intracerebrally perfused drugs during various behaviors.

Evidence for the ability of hippocampal neurons to develop acute tolerance to ethanol in behaving rats

BACKGROUND

The cellular mechanisms underlying acute tolerance to alcohol are unclear. This study aimed to determine whether hippocampal neurons have the ability to develop acute tolerance to alcohol in behaving rats.

METHODS

Intrahippocampal microdialysis was performed in freely behaving rats, and the firing of single neurons in the dialysis area was recorded. The control microdialysis fluid, artificial cerebrospinal fluid (ACSF), was replaced with 1 M ethanol in ACSF for a 30 min period. One hour later, the ethanol perfusion was repeated. To test the functional integrity of the microdialysis probe in situ, each microdialysis session was completed with recording the effect of a 10-20 min perfusion of 500 microM N-methyl-D-aspartate (NMDA). The extracellular concentration profile of ethanol during intrahippocampal microdialysis with 1 M ethanol was estimated in a separate study in anesthetized rats. The ethanol content was measured in tissue slices surrounding the probe with gas chromatography (GC), and the generated data were analyzed with a mathematical model for microdialysis to estimate the concentration of ethanol at the recording site.

RESULTS

The predominant effect of the first intrahippocampal microdialysis with ethanol was a decrease in firing rate in both pyramidal cells and interneurons. In contrast, such firing rate decrease did not develop during the second ethanol perfusion. Subsequent NMDA perfusion still induced robust changes in the electrical activity of the neurons. The estimated extracellular ethanol concentration at the recording site was 45-70 mM.

CONCLUSION

This study revealed that hippocampal neurons have the ability to develop acute tolerance to a single exposure of clinically relevant concentrations of ethanol in behaving rats, without influences from the rest of the body.

Single-cell recording from the brain of freely moving monkeys

Single-cell recording from the brain of non-human primates has traditionally been performed in monkeys seated in a primate chair. However, this arrangement makes long-term recordings difficult, causes stress that may confound the data, and prevents the manifestation of natural behaviors. Extending our previous neurophysiological studies in non-human primates (Ludvig et al. Brain Res. Protocols 2000;5:75-85), we have developed a method for recording the electrical activity of single hippocampal neurons in freely moving squirrel monkeys (Saimiri sciureus). The recording sessions lasted for up to 6 h, during which the monkeys moved freely around on the walls and the floor of a large test chamber and collected food pellets. Stable action potential waveforms were readily kept throughout the sessions. The following factors proved to be critical in this study: (a) selecting squirrel monkeys for the experiments, (b) using a driveable bundle of microwires for the recordings, (c) using a special recording cable, (d) implanting the microwires into the brain without causing neurological deficits, and (e) running the recording sessions in a special test chamber. The described method allows long-term extracellular recordings from the brain of non-human primates, without the stress of chairing, during a wide range of natural behaviors. Using this model, new insights can be obtained into the unique firing repertoire of the neurons of the primate brain.

Time course and effective spread of lidocaine and tetrodotoxin delivered via microdialysis: an electrophysiological study in cerebral cortex

Microdialysis is a useful tool for administering drugs into localized regions of brain tissue, but the diffusion of drugs from the probe has not been systematically examined. Lidocaine (10%) and tetrodotoxin (TTX, 10 microM), drugs typically used in neural inactivation studies, were infused through a microdialysis probe into raccoon somatosensory cortex while evoked responses were recorded at four electrodes equally spaced 0.5--2.0 mm from the probe. The decreases in evoked response amplitude as a function of time and distance from the probe were used as functional measures to describe the time course and spread of the drugs. TTX inactivated distant sites more quickly and to a greater extent than lidocaine. Responses recovered within approximately 40 min after termination of lidocaine, but did not recover for at least 2 h after TTX. Based on these measurements, we estimated that, at the concentrations used, lidocaine has a maximal spread of 2.1 mm, while TTX could spread as far as 4.8 mm from the microdialysis probe. However, in terms of significant inactivation of neuronal activity, lidocaine and TTX have an effective spread of 1 and 2 mm, respectively.

Analysis of amino acids and catecholamines, 5-hydroxytryptamine and their metabolites in brain areas in the rat using in vivo microdialysis

In vivo microdialysis, using dialysis probes inserted into discrete brain areas and subsequent analysis of neurotransmitters and related substances in the dialysates (usually with HPLC), has yielded a great deal of important information about the actions of psychotropic drugs and endogenous neurotransmitter systems and about the functional interactions between various brain areas. This paper reviews the principles involved in in vivo microdialysis, its advantages and disadvantages, and recent innovations in methodology and applications. The first section includes brief discussions of principles and applications of dialysis, use of anesthetized versus conscious freely moving animals, and methods used to determine the neural origin of neurotransmitters in the dialysate. The subsequent sections provide detailed descriptions, based largely on our own studies in rats, of stereotaxic surgery, in vivo microdialysis, and dialysate analysis, with an emphasis on amino acids and biogenic amines and their metabolites. A discussion of methodological problems which may be encountered in the analysis of amino acids and biogenic amines is also included.

Computational modeling of entorhinal cortex

Computational modeling provides a means for linking the physiological and anatomical characteristics of entorhinal cortex at a cellular level to the functional role of this region in behavior. We have developed detailed simulations of entorhinal cortical neurons and networks, with an emphasis on the role of acetylcholine in entorhinal cortical function. Computational modeling suggests that when acetylcholine levels are high, this sets appropriate dynamics for the storage of stimuli during performance of delayed matching tasks. In particular, acetylcholine activates a calcium-sensitive nonspecific cation current which provides an intrinsic cellular mechanism which could maintain neuronal activity across a delay period. Simulations demonstrate how this phenomena could underlie entorhinal cortex delay activity as described in previous unit recordings. Acetylcholine also induces theta rhythm oscillations which may be appropriate for timing of afferent input to be encoded in hippocampus and for extraction of individual stored sequences from multiple stored sequences. Lower levels of acetylcholine may allow sharp wave dynamics which can reactivate associations encoded in hippocampus and drive the formation of additional traces in hippocampus and entorhinal cortex during consolidation.

Cellular electrophysiological changes in the hippocampus of freely behaving rats during local microdialysis with epileptogenic concentration of N-methyl-D-aspartate

N-methyl-D-aspartate (NMDA) receptor dysfunctions are thought to be involved in the pathophysiology of seizures of hippocampal origin. While the cellular effects of excessive NMDA receptor stimulation have been widely studied in vitro, no data are available on the sequence of cellular electrophysiological events that follow the overstimulation of hippocampal NMDA receptors in awake, behaving subjects. Therefore, the present study addressed this problem. Intrahippocampal microdialysis with 500 microM NMDA was performed in freely behaving rats, and the electrical activity of single neurons in the dialysis area were monitored. In all recorded neurons (n = 9), regardless of their type, NMDA induced a long-lasting electrical silence preceded in most cells by a brief but robust firing rate increase. During these firing rate increases, place cells lost the spatial selectivity of their discharges, and a gradual reduction in the amplitude of the action potentials was also observed. Remarkably, electroencephalographic (EEG) seizures developed exclusively after the appearance of cellular electrical silence in the recording/dialysis site. The NMDA-induced electrophysiological changes were reversible. This study demonstrates that the combined single-cell recording-intracerebral microdialysis technique can be readily used for inducing focal epileptiform events in the hippocampus and monitoring the induced cellular electrophysiological events in behaving animals.

Delivering drugs, via microdialysis, into the environment of extracellularly recorded hippocampal neurons in behaving primates

Hippocampal neurons in primates have been extensively studied with electrophysiological and neuroanatomical methods. Much less effort has been devoted to examining these cells with contemporary pharmacological techniques. Therefore, we modified a recently developed integrative technique (N. Ludvig, P.E. Potter, S.E. Fox, Simultaneous single-cell recording and microdialysis within the same brain site in freely behaving rats: a novel neurobiological method, J. Neurosci. Methods 55 (1994) 31-40 [9] ) for cellular neuropharmacological studies in behaving monkeys. A driveable microelectrode-microdialysis probe guide assembly was implanted stereotaxically into the left hippocampus of squirrel monkeys (Saimiri sciureus) under isoflurane anesthesia. The assembly was covered with a protective cap. After 3 weeks of postsurgical recovery and behavioral training, the experimental subject was seated in a primate chair. For 4-5 h, single-cell recording and microdialysis were simultaneously performed in the hippocampal implantation site. The technique allowed the recording of both complex-spike cells and fast-firing neurons without the use of head restraint. The control microdialysis solution, artificial cerebrospinal fluid (ACSF), was replaced with either 1 M ethanol or 500 microM N-methyl-D-aspartate (NMDA) for 10-30 min intervals. The ethanol perfusions principally suppressed the firing of the neurons in the dialysis area. The NMDA perfusions initially increased the firing of local neurons, then caused electrical silence. These drug delivery/cell recording sessions were performed with 1-4 day intersession intervals over a 1-month period. The described method provides a tool to elaborate the pharmacology of primate hippocampal neurons during behavior and without the confounding effects of systemic drug administrations.

Adenosinergic modulation of rat basal forebrain neurons during sleep and waking: neuronal recording with microdialysis

1. The cholinergic system of the basal forebrain (BF) is hypothesized to play an important role in behavioural and electrocortical arousal. Adenosine has been proposed as a sleep-promoting substance that induces sleep by inhibiting cholinergic neurons of the BF and brainstem. However, adenosinergic influences on the activity of BF neurons in naturally awake and sleeping animals have not been demonstrated. 2. We recorded the sleep-wake discharge profile of BF neurons and simultaneously assessed adenosinergic influences on wake- and sleep-related activity of these neurons by delivering adenosinergic agents adjacent to the recorded neurons with a microdialysis probe. Discharge rates of BF neurons were recorded through two to three sleep-wake episodes during baseline (artificial cerebrospinal fluid perfusion), and after delivering an adenosine transport inhibitor (s-(p-nitrobenzyl)-6-thioinosine; NBTI), or exogenous adenosine, or a selective adenosine A1 receptor antagonist (8-cyclopentyl-1, 3-dimethylxanthine; CPDX). 3. NBTI and adenosine decreased the discharge rate of BF neurons during both waking and non-rapid eye movement (NREM) sleep. In contrast, CPDX increased the discharge rate of BF neurons during both waking and NREM sleep. These results suggest that in naturally awake and sleeping animals, adenosine exerts tonic inhibitory influences on BF neurons, supporting the hypothesized role of adenosine in sleep regulation. 4. However, in the presence of exogenous adenosine, NBTI or CPDX, BF neurons retained their wake- and sleep-related discharge patterns, i.e. still exhibited changes in discharge rate during transitions between waking and NREM sleep. This suggests that other neurotransmitters/neuromodulators also contribute to the sleep-wake discharge modulation of BF neurons.

Cortical catecholamine changes and seizures induced by 4-aminopyridine in awake rats, studied with a dual microdialysis-electrical recording technique

We describe a rotatory electrical device that permits the simultaneous microdialysis and electroencephalographic (EEG) recording, by means of bipolar electrodes attached to the microdialysis probe, in two brain regions of awake rats. Using this device, we have found that the microdialysis infusion of 4-aminopyridine (4-AP) in the motor cerebral cortex produces intense behavioral convulsions and EEG seizures in both the infused and the contralateral cortex. This convulsant action is accompanied by a remarkable increase of extracellular dopamine (about 15-fold), norepinephrine (2.4-fold) and vanillylmandelic acid (1.8-fold) concentration in the infused cortex. Delayed increases of these amines were observed also in the contralateral cortex. The results suggest that 4-AP induces the release of catecholamines either through a direct effect on nerve endings or as a consequence of seizures.

In vivo neurochemical correlates of cognitive processes: methodological and conceptual challenges

The advent of the use of in vivo microdialysis and voltammetry techniques in behaving animals has ushered in a great deal of research on the neurochemistry of cognition. While studies exploring the relationship between neurotransmitter release and cognitive processing are quite feasible, a number of methodological and conceptual issues pose challenges to the interpretation of experimental results. These challenges include: 1) a demonstration that the behavioral task highlights the particular cognitive construct under study; 2) a determination of the role of non-cognitive variables (i.e. transfer effects, sensory stimulation, motivational variables, and motor activity) in affecting transmitter release, and 3) a recognition of the value of a distributed systems approach to studying the neurochemistry of cognition. This review summarizes the data on the validity of microdialysis and voltammetry as correlates of neurotransmitter release and then illustrates the impact that the above challenges can have on the conclusions drawn from various studies.

Release of vasopressin within the brain contributes to neuroendocrine and behavioral regulation

In addition to its peripheral secretion from the neurohypophysis, the neuropeptide vasopressin (VP) is released within the mammalian brain from probably all parts of the neuronal membrane. In particular the development of brain microdialysis in vivo together with blood microdialysis or blood sampling provides the advantage of being able to reliably compare the dynamic release patterns into different compartments of the organism. The central VP release within hypothalamic (e.g., supraoptic, paraventricular and suprachiasmatic nuclei) and limbic (e.g., septum, amygdala) rat brain areas is stimulated by a variety of substances and stressors, including interleukin-1 beta, social defeat and forced swimming. Furthermore, it is characterized by positive and negative feedback mechanisms and the capacity of the VP system for co-ordinated or independent release, the latter being observed, for example, during social defeat. This emotional stressor, in contrast to exposure to a novel cage, increased VP release within the supraoptic nucleus, but not into plasma. This failure to release VP peripherally could be observed also during forced swimming, despite a dramatic rise in plasma osmolality and a markedly stimulated central release. In another series of experiments we studied the effects of centrally-released VP on cognitive and emotional aspects of behavior using reverse microdialysis for antagonist administration during the behavioral tests and antisense targeting to downregulate either VP or its local V1 receptor subtype. In this way, centrally (in particular septally) released VP could be shown to be causally involved in short-term memory and anxiety-related behavior. Furthermore, VP release within the hypothalamic paraventricular nucleus is likely to provide a negative tonus on the activity of the hypothalamic-pituitary-adrenocortical axis. This neuroendocrine effect together with cognitive, emotional and immunological effects of centrally released VP is thought to be essential to ensure adequate behavior of the animal during challenging situations and to contribute to the development of efficient coping strategies.

Neural activity in SII modifies sensory evoked potentials in SI in awake rats

The function of the projection from the secondary somatosensory cortex (SII) to the primary somatosensory cortex (SI) in rats was investigated by recording sensory evoked potentials (SEP) in SI during glutamate activation and lidocaine blockade of SII. In anesthetized animals, glutamate stimulation of SII decreased SEP latency and increased SEP amplitude, whereas no changes were evident during lidocaine blockade of SII. In awake animals, a second, later component of the SEP appeared. This second component was almost completely eliminated during lidocaine blockade of SII. We conclude that the projection from SII to SI in rats slightly facilitates the SEP response in anesthetized animals and is responsible for a major portion of the late component of the SEP in awake animals.

Microdialysis-coupled place cell detection in the hippocampus: a new strategy for the search for cognition enhancer drugs

1. The MPCD method in freely moving rats is a new neuroscience technique. It is able to detect the location-specific firing of hippocampal place cells, and to deliver, via microdialysis, various drug solutions into the extracellular environment of the detected neurons. Place cells are critical elements of the neural system in brain which governs cognitive processes. It is emphasized in this article that effective cognition enhancer drugs must selectively and significantly affect the firing of these cells. 2. By using MPCD, it is possible to recognize drug combinations which can increase the location-specific firing of place cells to an optimal level. This paper proposes that such pharmacological action facilitates engram-creation in extrahippocampal cortical areas, improving cognitive functions. Thus, an MPCD-based research strategy may lead to the rational development of a new generation of cognition enhancer drugs for the treatment of learning and memory disorders, including Alzheimer's disease (AD).

Manipulation of pyramidal cell firing in the hippocampus of freely behaving rats by local application of K+ via microdialysis

In this study, microdialysis was performed in the hippocampus of freely behaving rats, and the firing of pyramidal cells, including place cells, was recorded at the site of the microdialysis probe. For 10-min periods, the artificial cerebrospinal fluid (ACSF) in the microdialysis system was replaced with ACSF containing 50 mM K+ (high K+ solution). Complementary in vitro tests determined that microdialysis with such high K+ solution produced an outflow of 5% of the perfused K+ from the microdialysis probe. Application of K+ with this method into the CA1 region significantly increased the firing of the local pyramidal cells, including place cells, during both movement and sleep. On average, K+ exposures increased the firing rate of the neurons to 306% and 448% of the control firing rate during movement and sleep, respectively. After the termination of the K+ outflow, the cells continued to discharge for 5-30 min with a significantly higher frequency than before the K+ challenge. This phenomenon also occurred in both behavioral states. During the period of enhanced firing, the out-of-field firing rate of the recorded place cells was dramatically increased. It was also found that during the K+ applications, otherwise silent pyramidal cells often became electrically active. The K(+)-induced firing modifications were usually not accompanied by behavioral or EEG changes. The data raise the possibility that transient elevations in the extracellular K+ concentration contribute to the ionic/molecular processes which are responsible for plastic firing pattern modifications in hippocampus. Pharmacological manipulation of place cells with the described method offers a new strategy to understand the molecular bases of spatial memory.

The suppressant effect of ethanol, delivered via intrahippocampal microdialysis, on the firing of local pyramidal cells in freely behaving rats

Intrahippocampal microdialysis was performed on 14 freely behaving rats, and the firing of pyramidal cells within the dialysis area was recorded. In one group of rats, the microdialysis was conducted only with artificial cerebrospinal fluid (ACSF) for 2-4 h. In this control group, the recorded neurons displayed normal firing patterns. In another group, ACSF was replaced for 30-60 min with various concentrations of ethanol to deliver this drug via the microdialysis probe into the cell recording area. Ethanol at the concentration of 5% (w/v) significantly and reversibly suppressed the firing of the recorded neurons. The marked firing rate alterations were not accompanied with apparent changes in the hippocampal EEG activity or the behavior of the rats, indicating localized drug actions. These data demonstrate for the first time that in the physiologically functioning brain, ethanol exerts principally a suppressant effect on the electrical activity of hippocampal pyramidal cells.