Processing of information from different sources: spatial synaptic integration in the dendrites of vertebrate CNS neurons

Eye movements of the murine P/Q calcium channel mutant tottering, and the impact of aging

Mice carrying mutations of the gene encoding the ion pore of the P/Q calcium channel (Cacna1a) are an instance in which cerebellar dysfunction may be attributable to altered electrophysiology and thus provide an opportunity to study how neuronal intrinsic properties dictate signal processing in the ocular motor system. P/Q channel mutations can engender multiple effects at the single neuron, circuit, and behavioral levels; correlating physiological and behavioral abnormalities in multiple allelic strains will ultimately facilitate determining which alterations of physiology are responsible for specific behavioral aberrations. We used videooculography to quantify ocular motor behavior in tottering mutants aged 3 mo to 2 yr and compared their performance to data previously obtained in the allelic mutant rocker and C57BL/6 controls. Tottering mutants shared numerous abnormalities with rocker, including upward deviation of the eyes at rest, increased vestibuloocular reflex (VOR) phase lead at low stimulus frequencies, reduced VOR gain at high stimulus frequencies, reduced gain of the horizontal and vertical optokinetic reflex, reduced time constants of the neural integrator, and reduced plasticity of the VOR as assessed in a cross-axis training paradigm. Unlike rocker, young tottering mutants exhibited normal peak velocities of nystagmus fast phases, arguing against a role for neuromuscular transmission defects in the attenuation of compensatory eye movements. Tottering also differed by exhibiting directional asymmetries of the gains of optokinetic reflexes. The data suggest at least four pathophysiological mechanisms (two congenital and two acquired) are required to explain the ocular motor deficits in the two Cacna1a mutant strains.

Reading vascular changes in brain imaging: is dendritic calcium the key?

A key goal in functional neuroimaging is to use signals that are related to local changes in metabolism and blood flow to track the neuronal correlates of mental activity. Recent findings indicate that the dendritic processing of excitatory synaptic inputs correlates more closely than the generation of spikes with brain imaging signals. The correlation is often nonlinear and context-sensitive, and cannot be generalized for every condition or brain region. The vascular signals are mainly produced by increases in intracellular calcium in neurons and possibly astrocytes, which activate important enzymes that produce vasodilators to generate increments in flow and the positive blood oxygen level dependent signal. Our understanding of the cellular mechanisms of functional imaging signals places constraints on the interpretation of the data.

Dendritic sodium spikelets and low-threshold calcium spikes in turtle olfactory bulb granule cells

Active dendritic membrane properties were investigated by whole cell recordings from adult turtle olfactory bulb granule cells. The laminar structure of the olfactory bulb allowed differential polarization of the distal apical dendrites versus the somatic part of the cells by an external electric field. Dendritic depolarization evoked small (approximately 10 mV) all-or-none depolarizing events of approximately 10-ms duration. These spikelets often occurred in bursts at high frequency (< or = 250 Hz); they were present despite the application of synaptic and gap junction antagonists, but were abolished by TTX and intracellularly applied QX314. The spikelets were interpreted as attenuated sodium spikes initiated in different branches of the granule cells dendrites. They occurred spontaneously, but could also be evoked by excitatory postsynaptic potentials (EPSPs) to the distal dendrites. Spikelets initiated by distal excitation could function as prepotentials for full sodium spikes, in part depending on the level of proximal depolarization. Somatic depolarization by the electric field evoked full sodium spikes as well as low-threshold calcium spikes (LTSs). Calcium imaging revealed that the electrophysiologically identified LTS evoked from the soma was associated with calcium transients in the proximal and the distal dendrites. Our data suggest that the LTS in the soma/proximal dendrites plays a major role in boosting excitability, thus contributing to the initiation of sodium spiking in this compartment. The results furthermore suggest that the LTS and the sodium spikes may act independently or cooperatively to regulate dendritic calcium influx.

A cell model study of calcium influx mechanism regulated by calcium-dependent potassium channels in Purkinje cell dendrites

The present study was designed to elucidate the roles of dendritic voltage-gated K+ channels in Ca2+ influx mechanism of a rat Purkinje cell using a computer simulation program. First, we improved the channel descriptions and the maximum conductance in the Purkinje cell model to mimic both the kinetics of ion channels and the Ca2+ spikes, which had failed in previous studies. Our cell model is, therefore, much more authentic than those in previous studies. Second, synaptic inputs that mimic stimulation of parallel fibers and induce sub-threshold excitability were simultaneously applied to the spiny dendrites. As a result, transient Ca2+ responses were observed in the stimulation points and they decreased with the faster decay rate in the cell model including high-threshold Ca2+-dependent K+ channels than in those excluding these channels. Third, when a single synaptic input was applied into a spiny dendrite, Ca2+-dependent K+ channels suppressed Ca2+ increases at stimulation and recording points. Finally, Ca2+-dependent K+ channels were also found to suppress the time to peak Ca2+ values in the recording points. These results suggest that the opening of Ca2+-dependent K+ channels by Ca2+ influx through voltage-gated Ca2+ channels hyperpolarizes the membrane potentials and deactivates these Ca2+ channels in a negative feedback manner, resulting in local, weak Ca2+ responses in spiny dendrites of Purkinje cells.

Dissociation of spikes, synaptic activity, and activity-dependent increments in rat cerebellar blood flow by tonic synaptic inhibition

Functional neuroimaging relies on the robust coupling between neuronal activity, metabolism and cerebral blood flow (CBF) to map the brain, but the physiological basis of the neuroimaging signals is still not well understood. Here we applied a pharmacological approach to separate spiking activity, synaptic activity, and the accompanying changes in CBF in rat cerebellar cortex. We report that tonic synaptic inhibition achieved by topical application of gamma-aminobutyric acid type A (GABAA) (muscimol) or GABAB (baclofen) receptor agonists abolished or reduced spontaneous Purkinje cell spiking activity without affecting basal CBF. The magnitude of CBF responses evoked by climbing fiber stimulation decreased gradually over time after exposure to muscimol, being more pronounced in the superficial than in the deep cortical layers. We provide direct evidence in favor of a laminar-specific regulation of CBF in deep cortical layers, independent of dilatation of surface vessels. With prolonged exposure to muscimol, activity-dependent CBF increments disappeared, despite preserved cerebrovascular reactivity to adenosine and preserved local field potentials (LFP). This dissociation of CBF and LFPs suggests that CBF responses are independent of extracellular synaptic currents that generate LFPs. Our work implies that neuronal and vascular signals evoked by glutamatergic pathways are sensitive to synaptic inhibition, and that local mechanisms independent of transmembrane synaptic currents adjust flow to synaptic activity in distinct cortical layers. Our results provide fundamental insights into the functional regulation of blood flow, showing important interference of GABAA receptors in translating excitatory input into blood flow responses.

Differences in the BOLD fMRI response to direct and indirect cortical stimulation in the rat

Functional MRI (fMRI) exploits a relationship between neuronal activity, metabolism, and cerebral blood flow to functionally map the brain. We have developed a model of direct cortical stimulation in the rat that can be combined with fMRI and used to compare the hemodynamic responses to direct and indirect cortical stimulation. Unilateral electrical stimulation of the rat hindpaw motor cortex, via stereotaxically positioned carbon-fiber electrodes, yielded blood oxygenation level-dependent (BOLD) fMRI signal changes in both the stimulated and homotypic contralateral motor cortices. The maximal signal intensity change in both cortices was similar (stimulated = 3.7 +/- 1.7%; contralateral = 3.2 +/- 1.0%), although the response duration in the directly stimulated cortex was significantly longer (48.1 +/- 5.7 sec vs. 19.0 +/- 5.3 sec). Activation of the contralateral cortex is likely to occur via stimulation of corticocortical pathways, as distinct from direct electrical stimulation, and the response profile is similar to that observed in remote (e.g., forepaw) stimulation fMRI studies. Differences in the neuronal pool activated, or neurovascular mediators released, may account for the more prolonged BOLD response observed in the directly stimulated cortex. This work demonstrates the combination of direct cortical stimulation in the rat with fMRI and thus extends the scope of rodent fMRI into brain regions inaccessible to peripheral stimulation techniques.

Rapid and reversible changes in dendrite morphology and synaptic efficacy following NMDA receptor activation: implication for a cellular defense against excitotoxicity

Postsynaptic neuronal dendrites undergo functional and morphological changes in response to pathologically excessive synaptic activation. Although rapid formation of segmental focal swelling (varicosity) is the most prominent hallmark in such excitotoxic injury, little is known about the pathophysiological function of these structural alterations. We used cultured rat hippocampal slices to evaluate the relationship between the formation of varicosities and subsequent neuronal death. Substantial numbers of segmental dendritic varicosities were observed all over the hippocampus within 5 minutes of exposure to 30 μM NMDA, although neuronal death was detected only in the CA1 region 24 hours after NMDA exposure. Sublethal NMDA concentrations (1-10 μM) induced reversible focal swelling in all hippocampal subregions. NMDA-induced neuronal death was prevented either by NMDA receptor antagonists or by the use of Ca2+-free medium, whereas varicosity formation was virtually independent of Ca2+ influx. Rather, the Ca2+-free conditions per se produced dendritic focal swelling. Also, NMDA-induced varicosity formation was dependent on extracellular Na+ concentration. Thus, we believe that varicosity formation is not causally related to neuronal injury and that the two phenomena are separable and involve distinct mechanisms. Interestingly, dendrite swelling was accompanied by AMPA receptor internalization and a rapid, long-lasting depression in synaptic transmission. Moreover, low Na+ conditions or treatment with ethacrynic acid or proteinase inhibitors, which effectively prevent varicosity formation, aggravated NMDA-induced excitotoxicity, and eliminated the regional specificity of the toxicity. Therefore, the pathological changes in dendrite morphology and function may be associated with an early, self-protective response against excitotoxicity.

Activity-dependent plasticity in the adult auditory brainstem

Over the past few years we have studied the plasticity of the adult auditory brainstem in the rat following unilateral changes to the pattern of sensory activation, either by intracochlear electrical stimulation or by deafening. We discovered that modifications to afferent activity induced changes in the molecular composition and cellular morphology throughout the auditory brainstem, including its major centers: the cochlear nucleus complex, the superior olivary complex, and the inferior colliculus. The time window studied ranged from 2 h to over 1 year following induction of changes to afferent activity. The molecular markers employed include the NMDA receptor subunit type 1, the cAMP response element binding protein (CREB), the immediate early gene products c-Fos, c-Jun and Egr-1, the growth and plasticity-associated protein GAP-43 and its mRNA, the calcium binding protein calbindin, the cell adhesion molecule integrin-alpha(1), the microtubule-associated protein MAP-1b, and the neurofilament light chain (NF-L). As a consequence of the specific electrical stimulation of the auditory afferents or the loss of hearing, a cascade of events is triggered that apparently modifies the integrative action and computational abilities of the central auditory system. An attempt is made to relate the diverse phenomena observed to a common molecular signaling network that is suspected to bridge sensory experience to changes in the structure and function of the brain. Eventually, a thorough understanding of these events will be essential for the specific diagnosis of patients, optimal timing for implantation, and suitable parameters for running of a cochlear implant or an auditory brainstem implant in humans. In this report an overview of the results obtained in the past years in our lab is presented, flanked by an introduction into the history of plasticity research and a model proposed for intracellular signal cascades related to activity-dependent plasticity.

Input-output functions of mammalian motoneurons

Our intent in this review was to consider the relationship between the biophysical properties of motoneurons and the mechanisms by which they transduce the synaptic inputs they receive into changes in their firing rates. Our emphasis has been on experimental results obtained over the past twenty years, which have shown that motoneurons are just as complex and interesting as other central neurons. This work has shown that motoneurons are endowed with a rich complement of active dendritic conductances, and flexible control of both somatic and dendritic channels by endogenous neuromodulators. Although this new information requires some revision of the simple view of motoneuron input-output properties that was prevalent in the early 1980's (see sections 2.3 and 2.10), the basic aspects of synaptic transduction by motoneurons can still be captured by a relatively simple input-output model (see section 2.3, equations 1-3). It remains valid to describe motoneuron recruitment as a product of the total synaptic current delivered to the soma, the effective input resistance of the motoneuron and the somatic voltage threshold for spike initiation (equations 1 and 2). However, because of the presence of active channels activated in the subthreshold range, both the delivery of synaptic current and the effective input resistance depend upon membrane potential. In addition, activation of metabotropic receptors by achetylcholine, glutamate, noradrenaline, serotonin, substance P and thyrotropin releasing factor (TRH) can alter the properties of various voltage- and calcium-sensitive channels and thereby affect synaptic current delivery and input resistance. Once motoneurons are activated, their steady-state rate of repetitive discharge is linearly related to the amount of injected or synaptic current reaching the soma (equation 3). However, the slope of this relation, the minimum discharge rate and the threshold current for repetitive discharge are all subject to neuromodulatory control. There are still a number of unresolved issues concerning the control of motoneuron discharge by synaptic inputs. Under dynamic conditions, when synaptic input is rapidly changing, time- and activity-dependent changes in the state of ionic channels will alter both synaptic current delivery to the spike-generating conductances and the relation between synaptic current and discharge rate. There is at present no general quantitative expression for motoneuron input-output properties under dynamic conditions. Even under steady-state conditions, the biophysical mechanisms underlying the transfer of synaptic current from the dendrites to the soma are not well understood, due to the paucity of direct recordings from motoneuron dendrites. It seems likely that resolving these important issues will keep motoneuron afficiandoes well occupied during the next twenty years.

An approach to the complexity of the brain

The establishment of ordered neuronal connections is supposed to take place under the control of specific cell adhesion molecules (CAM) which guide neuroblasts and axons to their appropriate destination. The extreme complexity of the nervous system does not provide a favorable medium for the development of deterministic connections. Simon's [112] theorems offer a mean to approach the high level of complexity of the nervous system. The basic tenet is that complex systems are hierarchically organized and decomposable. Such systems can arise by selective trial and error mechanisms. Subsystems in complex systems only interact in an aggregate manner, and no significant information is lost if the detail of aggregate interactions is ignored. A number of nervous activities, which qualify for these requirements, are shown. The following sources of selection are considered: internal and external feedbacks, previous experience, plasticity in simple structures, and the characteristic geometry of dendrites. The role played by CAMs and other membrane-associated molecules is discussed in the sense that they are either inductor molecules that turn on different homeobox genes, or downstream products of genes, or both. These molecules control cellular and tissular differentiation in the developing brain creating sources of selection required for the trial and error process in the organization of the nervous tissue.

Biochemical networks in nervous systems: expanding neuronal information capacity beyond voltage signals

In addition to synaptically mediated signals that are based on changes in membrane potential, neurons also generate and receive many types of signals that involve biochemical pathways, some of which are independent of voltage. Although networks of biochemical pathways have often been thought of as being only neuromodulatory, recent computational and experimental studies have highlighted how these pathways can also integrate and transfer information themselves. Interactions between biochemical pathways involving positive and negative feedback loops allow biochemical signals to exhibit emergent properties, most notably bistability and oscillations. New and evolving techniques, including real-time imaging of second messengers, hold the promise of illuminating information processing that cannot be detected using microelectrodes, and revealing how 'biochemical integration' might contribute to the computational abilities of the nervous system.

Time-sharing contributions of A- and D-type K+ channels to the integration of high-frequency sequential excitatory post synaptic potentials at a model dendrite in rats

A- and D-type K+ channels (KA and KD channels) have unique physiological properties that play important roles in the integration of excitatory post synaptic potentials (EPSPs) in neuronal dendrites. These functions were analyzed using a computer program, NEURON, to simulate high-frequency sequential synaptic inputs, that can induce long-term potentiation (LTP). We paid close attention to the stability of the reduction of sequential EPSPs. When either KA or KD channels were included in models, the EPSP reduction ratios were less stable than containing both KA and KD channels. When both KA and KD channels were present in the model, the variance of EPSP reduction ratios was significantly smaller in comparison with that in the presence of either KA or KD channels alone. We thus concluded that the co-existence of KA and KD channels is necessary to produce stable EPSPs during the high-frequency synaptic stimulation necessary for induction of LTP.

The wired network as a learning paradigm for normal and abnormal brain neuronal communication

The brain is a highly sophisticated assembly of neuronal networks for interaction with the internal and external environment. Fundamentally, the neuronal communication process is analogous structurally and functionally to the electrical (wire-mediated) network. In particular, both have coupled information-processing and conduction properties. We suggest that the electrical system can be used as a learning paradigm in brain research and clinical practice. Our model shows how the study of wire-mediated networks may be of benefit in tracing overt psychiatric manifestations to intrinsic biological faults in brain circuitry.

Spike-dependent calcium influx in dendrites of the cricket giant interneuron

Identified wind-sensitive giant interneurons in the cricket's cercal sensory system integrate cercal afferent signals and release an avoidance behavior. A calcium-imaging technique was applied to the giant interneurons to examine the presence of the voltage-dependent Ca(2+) channels (VDCCs) in their dendrites. We found that presynaptic stimuli to the cercal sensory nerve cords elevated the cytosolic Ca(2+) concentration ([Ca(2+)](i)) in the dendrites of the giant interneurons. The dendritic Ca(2+) rise coincided with the spike burst of the giant interneurons, and the rate of Ca(2+) rise depended on the frequency of the action potentials. These results suggest that the action potentials directly caused [Ca(2+)](i) increase. Observation of the [Ca(2+)](i) elevation induced by depolarizing current injection demonstrates the presence of the VDCCs in the dendrites. Although hyperpolarizing current injection into the giant interneuron suppressed action potential generation, EPSPs could induce no [Ca(2+)](i) increase. This result means that ligand-gated channels do not contribute to the synaptically stimulated Ca(2+) elevation. On the other hand, antidromically stimulated spikes also increased [Ca(2+)](i) in all cellular regions including the dendrites. And bath application of a mixture of Ni(2+), Co(2+), and Cd(2+) or tetrodotoxin inhibited the [Ca(2+)](i) elevation induced by the antidromic stimulation. From these findings, we suppose that the axonal spikes antidromically propagate and induce the Ca(2+) influx via VDCCs in the dendrites. The spike-dependent Ca(2+) elevation may regulate the sensory signals processing via second-messenger cascades in the giant interneurons.

Roles of ion channels in EPSP integration at neuronal dendrites

Many different kinds of voltage-gated ion channels (Na+ channels, K+ channels, Ca2+ channels) exist at neuronal dendrites. Integration of dendritic electric signals (excitatory postsynaptic potentials (EPSPs), inhibitory postsynaptic potentials (IPSPs) and action potentials) and/or non-electric signals (Ca2+ and second messengers) occurs in restricted dendritic compartments consisting of spines and adjacent fine dendrites. Voltage-gated ion channels at neuronal dendrites play crucial roles in the integration of dendritic signals. Dendritic signals, in turn, play important roles in the modulation of local dendritic physiological functions (e.g. input-specific synaptic plasticity, long-term potentiation (LTP) and long term depression (LTD)). A combined experimental and theoretical approach is a good way to clarify the biophysical behaviors of dendritic ion channels. Analyses of dendritic ion channels can open the door to a new wave of discoveries about EPSP integration at neuronal dendrites.

Regulation of action potential size and excitability in substantia nigra compacta neurons: sensitivity to 4-aminopyridine

Slow, pacemaker-like firing is due to intrinsic membrane properties in substantia nigra compacta (SNc) neurons in vitro. How these properties interact with afferent synaptic inputs is not fully understood. In this study, intracellular recordings from SNc neurons in brain slices showed that spontaneous action potentials (APs) were attenuated when generated from lower than normal threshold. Such APs were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and could be related to non-N-methyl-D-aspartate (NMDA) receptor-mediated spontaneous excitatory postsynaptic potentials (EPSPs). The AP attenuation was reproduced by stimulus-evoked EPSPs and by current injections to the soma. APs evoked from holding potentials between -40 and -60 mV were reduced in width by Cd(2+) (0. 2 mM). Tetraethylammonium chloride (TEA, 10 mM) or 4-aminopyridine (4-AP, 5 mM) increased the AP width. However, at more negative holding potentials, Cd(2+) and TEA were inefficacious, whereas 4-AP enlarged the AP, partly via induction of a Cd(2+)-sensitive component. A monophasic afterhyperpolarization (AHP), following attenuated APs, was little affected by either Cd(2+) or TEA, but inhibited by 4-AP, which induced an additional, slow component, sensitive to Cd(2+) or apamin (100 nM). The AP delay showed a discontinuous relation to the amplitude or slope of the injected current (delay shift), which was sensitive to low doses of 4-AP (0. 05 mM). The initial time window before the delay shift was longer than the rise time of EPSPs. It is suggested that a 4-AP-sensitive current prevents or postpones discharge during slow depolarization's, but allows direct excitation by fast EPSPs. Fast excitation leads to AP attenuation, primarily due to strong activation of 4-AP-sensitive current. This seems to cause inhibition of the Ca(2+) current during the AP and reduction of Ca(2+)-dependent K(+) currents. Together, these properties are likely to influence the excitability and the local, somatodendritic effects of the AP, in a manner that discriminates between firing induced by the intrinsic pacemaker mechanism and fast synaptic potentials.

Supralinear summation of synaptic inputs by an invertebrate neuron: dendritic gain is mediated by an "inward rectifier" K(+) current

Dendritic processing of glutamatergic synaptic inputs was investigated in the anterior pagoda cell of leech. We observed that below spike threshold, the amplitude of individual EPSPs decreased with hyperpolarization and that simultaneous stimulation of pairs of synaptic inputs leads to the supralinear summation of EPSPs. Voltage-clamp measurements revealed a hyperpolarization-activated, Ba(2+)-sensitive, fast, noninactivating K(+) conductance that depends on the external [K(+)]. These features are those of an "inward rectifier," Kir. Microsurgery experiments, in combination with electrophysiological measurements, revealed an inhomogeneous spatial distribution of the Kir conductance. Furthermore, on surgical removal of the neurites that contain the Kir conductance, the amplitude of EPSPs from the remaining synaptic inputs increased with hyperpolarization. A model cell, with the Kir conductance as the sole voltage-dependent conductance, reproduced qualitatively the observed voltage dependence of individual EPSPs as well as the supralinear summation of EPSP pairs.

Modification of activity-dependent increases of cerebral blood flow by excitatory synaptic activity and spikes in rat cerebellar cortex

1. Mechanisms of activity-dependent increases in cerebral blood flow (CBF) were examined in rat cerebellar cortex using the laser Doppler flow technique and extracellular recordings of single unit activity and field potentials. 2. Stimulation of the monosynaptic climbing fibre system evoked long-lasting complex spikes in Purkinje cells, and extracellular field potentials with a characteristic profile that indicated contributions from both passive and active membrane mechanisms. The concomitant CBF increases were reproducible at fairly short intervals, and suggest that both synaptic activity and spikes may contribute to increased CBF. 3. Stimulation of the disynaptic parallel fibre system inhibited the spiking activity in Purkinje cells, while the postsynaptic activity increased as indicated by the simultaneously recorded field potential. Nevertheless, CBF always increased. The inhibition of spike firing activity was partly dependent on GABAergic transmission, but may also relate to the intrinsic membrane properties of Purkinje cells. 4. The CBF increases evoked by parallel or climbing fibre stimulation were highly correlated to the sum of neural activities, i.e. the negativity of field potentials multiplied by the stimulus frequency. This suggests a robust link between extracellular current flow and activity-dependent increases in CBF. 5. AMPA receptor blockade attenuated CBF increases and field potential amplitudes, while NMDA receptor antagonism did not. This is consistent with the idea that the CBF responses are of neuronal origin. 6. This study has shown that activity-dependent CBF increases evoked by stimulation of cerebellar parallel fibres are dependent on synaptic excitation, including excitation of inhibitory interneurones, whereas the net activity of Purkinje cells, the principal neurones of the cerebellar cortex, is unimportant for the vascular response. For the climbing fibre system, not only synaptic activity but also the generation of complex spikes from Purkinje cells contribute to the increases in CBF. The strong correlation between CBF and field potential amplitudes suggests that extracellular ion fluxes contribute to the coupling of brain activity to blood flow.

Roles of A- and D-type K channels in EPSP integration at a model dendrite

We examined the roles of A- and D-type K channels in the integration of EPSPs, particularly EPSP reduction, in a model dendrite using a computer simulation program (NEURON), instead of conventional electrophysiological methods which present technical difficulties. We obtained two important results: (1) KD channels play a crucial role in every synaptic input event, and (2) KA channels affect reduction of the second EPSP only when the second synaptic input is applied with a short interval (<100 ms). These results suggest that KD channels, which have not yet been analyzed experimentally, play a major role in synaptic integration in dendrites by altering their cable properties.

Integration of excitatory postsynaptic potentials in dendrites of motoneurons of rat spinal cord slice cultures

We examined the attenuation and integration of spontaneous excitatory postsynaptic potentials (sEPSPs) in the dendrites of presumed motoneurons (MNs) of organotypic rat spinal cord cultures. Simultaneous whole cell recordings in current-clamp mode were made from either the soma and a dendrite or from two dendrites. Direct comparison of the two voltage recordings revealed that the membrane potentials at the two recording sites followed each other very closely except for the fast-rising phases of the EPSPs. The dendritic recording represented a low-pass filtered version of the somatic recording and vice versa. A computer-assisted method was developed to fit the sEPSPs with a generalized alpha-function for measuring their amplitudes and rise times (10-90%). The mean EPSP peak attenuation between the two recording electrodes was determined by a maximum likelihood analysis that extracted populations of similar amplitude ratios from the fitted events at each electrode. For each pair of recordings, the amplitude attenuation ratio for EPSP traveling from dendrite to soma was larger than that traveling from soma to dendrite. The linear relation between mean ln attenuation and distance between recording electrodes was used to map 1/e attenuations into units of distance (micron). For EPSPs with typical time course traveling from the somatic to the dendritic recording electrode, the mean 1/e attenuation corresponded to 714 micron for EPSPs traveling in the opposite direction, the mean 1/e attenuation corresponded to 263 micron. As predicted from cable analysis, fast EPSPs attenuated more in both the somatofugal and somatopetal direction than did slow EPSPs. For EPSPs with rise times shorter than approximately 2.0 ms, the attenuation factor increased steeply. Compartmental computer modeling of the experiments with biocytin-filled and reconstructed MNs that used passive membrane properties revealed amplitude attenuation ratios of the EPSP traveling in both the somatofugal and somatopetal direction that were comparable to those observed in real experiments. The modeling of a barrage of sEPSPs further confirmed that the somato-dendritic compartments of a MN are virtually isopotential except for the fast-rising phase of EPSPs. Large, transient differences in membrane potential are locally confined to the site of EPSP generation. Comparing the modeling results with the experiments suggests that the observed attenuation ratios are adequately explained by passive membrane properties alone.

Intracellular correlates of acquisition and long-term memory of classical conditioning in Purkinje cell dendrites in slices of rabbit cerebellar lobule HVI

Intradendritic recordings in Purkinje cells from a defined area in parasaggital slices of cerebellar lobule HVI, obtained after rabbits were given either paired (classical conditioning) or explicitly unpaired (control) presentations of tone and periorbital electrical stimulation, were used to assess the nature and duration of conditioning-specific changes in Purkinje cell dendritic membrane excitability. We found a strong relationship between the level of conditioning and Purkinje cell dendritic membrane excitability after initial acquisition of the conditioned response. Moreover, conditioning-specific increases in Purkinje cell excitability were still present 1 month after classical conditioning. Although dendritically recorded membrane potential, input resistance, and amplitude of somatic and dendritic spikes were not different in cells from paired or control animals, the size of a potassium channel-mediated transient hyperpolarization was significantly smaller in cells from animals that received classical conditioning. In slices of lobule HVI obtained from naive rabbits, the conditioning-related increases in membrane excitability could be mimicked by application of potassium channel antagonist tetraethylammonium chloride, iberiotoxin, or 4-aminopyridine. However, only 4-aminopyridine was able to reduce the transient hyperpolarization. The pharmacological data suggest a role for potassium channels and, possibly, channels mediating an IA-like current, in learning-specific changes in membrane excitability. The conditioning-specific increase in Purkinje cell dendritic excitability produces an afterhyperpolarization, which is hypothesized to release the cerebellar deep nuclei from inhibition, allowing conditioned responses to be elicited via the red nucleus and accessory abducens motorneurons.

Differential roles of two types of voltage-gated Ca2+ channels in the dendrites of rat cerebellar Purkinje neurons

The distribution and function of voltage-gated Ca2+ channels in Purkinje neurons in rat cerebellar slices were studied using simultaneous Ca2+ imaging and whole-cell patch clamp recording techniques. Voltage-gated Ca2+ channels were activated by applying depolarizing voltage steps through the pipette attached at the soma in a voltage-clamp mode in the presence of tetrodotoxin. Poor space clamp due to extensive arborization of the dendrites allowed the dendrites to fire Ca2+ spikes. Ca2+ imaging with Fura-2 injected through the pipette, showed a steady [Ca2+]i increase at the soma and transient, spike-linked [Ca2+]i jumps in the dendrites. omega-Agatoxin-IVA (200 nM) abolished the depolarization-induced Ca2+ spikes, the spike-linked [Ca2+]i increase in the dendrites, and the steady [Ca2+]i increase at the soma. omega-Conotoxin-GVIA (5 microM) and nifedipine (3 microM) had no significant effect on the depolarization-induced responses. In the presence of 4-aminopyridine(2 mM) and omega-Agatoxin-IVA, transient [Ca2+]i increases remained in the dendrites. Low concentrations of Ni2+(100 microM) reversibly suppressed this [Ca2+]i increase. The voltage for half-maximal activation and inactivation of this component were lower than -50 mV and -31 mV, respectively. In normal conditions, low concentration of Ni2+ slowed the onset of the Ca2+ spike without changing the time course of the spikes or the amplitude of the accompanying [Ca2+]i increase. These results show that omega-Agatoxin-IVA-sensitive Ca2+ channels are distributed both in the soma and the dendrites, and are responsible for dendritic Ca2+ spikes, whereas low-voltage activated, Ni2+-sensitive Ca2+ channels are distributed in the whole dendrites including both thick and fine branches, and provide boosting current for spike generation.

5-Hydroxytryptamine2A serotonin receptors in the primate cerebral cortex: possible site of action of hallucinogenic and antipsychotic drugs in pyramidal cell apical dendrites

To identify the cortical sites where 5-hydroxytryptamine2A (5-HT2A) serotonin receptors respond to the action of hallucinogens and atypical antipsychotic drugs, we have examined the cellular and subcellular distribution of these receptors in the cerebral cortex of macaque monkeys (with a focus on prefrontal areas) by using light and electron microscopic immunocytochemical techniques. 5-HT2A receptor immunoreactivity was detected in all cortical layers, among which layers II and III and layers V and VI were intensely stained, and layer IV was weakly labeled. The majority of the receptor-labeled cells were pyramidal neurons and the most intense immunolabeling was consistently confined to their parallelly aligned proximal apical dendrites that formed two intensely stained bands above and below layer IV. In double-label experiments, 5-HT2A label was found in calbindin D28k-positive, nonphosphorylated-neurofilament-positive, and immuno-negative pyramidal cells, suggesting that probably all pyramidal cells express 5-HT2A receptors. 5-HT2A label was also found in large- and medium-size interneurons, some of which were immuno-positive for calbindin. 5-HT2A receptor label was also associated with axon terminals. These findings reconcile the data on the receptor's cortical physiology and localization by (i) establishing that 5-HT2A receptors are located postsynaptically and presynaptically, (ii) demonstrating that pyramidal neurons constitute the major 5-HT2A-receptor-expressing cells in the cortex, and (iii) supporting the view that the apical dendritic field proximal to the pyramidal cell soma is the "hot spot" for 5-HT2A-receptor-mediated physiological actions relevant to normal and "psychotic" functional states of the cerebral cortex.

Visualization of calcium influx through channels that shape the burst and tonic firing modes of thalamic relay cells

Thalamic neurons have two firing modes: "tonic" and "burst." During burst mode, both low-threshold (LT) and high-threshold (HT) calcium channels are activated, while in tonic mode, only the HT-type of calcium channel is activated. The calcium signals associated with each firing mode were investigated in rat thalamic slices using whole cell patch clamping and confocal calcium imaging. Action potentials were induced by direct current injection into thalamic relay cells loaded with a fluorescent calcium indicator. In both tonic and burst firing modes, large calcium signals were recorded throughout the soma and proximal dendrites. To map the distribution of the channels mediating these calcium fluxes, LT and HT currents were independently activated using specific voltage-clamp protocols. We focused on the proximal region of the cell (up to 50 microm from the soma) because it appeared to be well clamped. For a voltage pulse of a given size, the largest calcium signals were observed in the proximal dendrites with smaller signals occurring in the soma and nucleus. This was true for both LT and HT signals. Rapid imaging, using one-dimensional linescans, was used to more precisely localize the calcium influx. For both LT and HT channels, calcium influx occurred simultaneously throughout all imaged regions including the soma and proximal dendrites. The presence of sizable calcium signals in the dendrites, soma, and nucleus during both firing modes, and the presence of LT calcium channels in the proximal dendrite where sensory afferents synapse, have implications for both the electrical functioning of relay cells and the transmission of sensory information to cortex.

Interactions between associative synaptic potentials in the piriform cortex of the in vitro isolated guinea pig brain

The interaction between synaptic potentials generated by the activation of separate sets of associative fibres was investigated in the piriform cortex of an in vitro isolated guinea pig brain preparation. Restricted regions of the piriform cortex served by separate contingents of afferent fibres of the lateral olfactory tract were isolated surgically. The activity generated by these patches of cortex in response to afferent stimulation propagates to remote cortical regions along cortico-cortical associative fibres. Current source density (CSD) analysis of field potential laminar profiles evoked by lateral olfactory tract stimulation confirmed that the synaptic sinks induced by distinct associative fibre contingents converge on the apical dendrites of piriform cortex neurons in the superficial lb layer. Pairing between potentials evoked by activation of two separate sets of associative fibres resulted in an almost linear summation when the two responses coincided. For interstimulus intervals of <100 ms, heterosynaptic pairing of independent associative inputs induced a facilitation of the conditioned associative potential, which correlated with an increase in the associative sink located in layer lb, as demonstrated by CSD analysis. The evaluation of the pairing intervals suggests that the heterosynaptic facilitation of the conditioned associative potentials may be due to the summation of local and remote associative synaptic events. It is concluded that separate associative inputs converge on the apical dendrites of piriform cortex pyramidal neurons to generate synaptic potentials through the activation of spatially close but independent synapses. The role of associative synaptic integration in the functional organization of the olfactory cortex is discussed.

The excitability of CA1 pyramidal cell dendrites is modulated by a local Ca(2+)-dependent K(+)-conductance

Intracellular recordings are made from distal apical dendrites of CA1 pyramidal neurones in the rat hippocampal slice preparation. Injection of a threshold current evoked two predominant firing patterns: fast spiking and compound spiking. Suprathreshold current injection evoked high frequency dendritic spiking followed by a pronounced slow afterhyperpolarization (sAHP(dend)) lasting for several hundred milliseconds, during which spiking was inhibited for a variable period. In fast spiking dendrites, the size of the sAHP(dend) depended on the number and frequency of preceding spikes, whereas, in compound spiking dendrites, it was more closely related to the size and duration of preceding Ca(2+)-spikes. During the peak of the sAHP(dend), the membrane conductance was increased by 56%. The sAHP(dend) was blocked by perfusion with Ca2+ and by intradendritic injection of ethyleneglycol-bis-(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA; 0.01 or 0.2 M), indicating that the activation of the sAHP(dend) depends on a rise in intradendritic Ca2+. The sAHP(dend) was also blocked by low concentrations (0.5-1 microM) of carbachol. The data presented here therefore, provide strong evidence that the sAHP(dend) is due to the activation of a local Ca(2+)-dependent K(+)-conductance. Possible implications of a dendritic Ca(2+)-dependent K(+)-conductance for the integration of synaptic potentials are discussed.

Spatial synaptic integration in Purkinje cell dendrites

Synaptic integration occurs within a framework of synaptic connections, and cell type-specific, intrinsic and transmitter-gated ion channels. These components are differentially distributed over the somato-dendritic membrane. Recent results from Purkinje cells and pyramidal cells exemplify some of these mechanisms of spatial synaptic integration. This paper focusses on the cerebellar Purkinje cell. In these neurons, the amplitude and distribution of single climbing fibre and parallel fibre EPSP-evoked Ca2+ influx were regulated by the transient outward, IA-like current in the distal (spiny) dendrites. The synaptically evoked Ca2+ influx was graded from a local response involving only a few terminal spiny dendrites to a propagated Ca2+ spike. The climbing fibre-evoked Ca2+ influx in the spiny dendrites was finely graded by parallel fibre-induced depolarization. Climbing fibre and parallel fibre-evoked Ca2+ influx elicited a short lasting afterhyperpolarization that affected subsequent dendritic Ca2+ influx. In addition, inhibitory synaptic input controlled dendritic Ca2+ influx. Interaction between information from different sources along the dendrites is thus controlled by intrinsic potassium conductances and IPSPs. Different electrophysiological properties are found in the cerebellar neurons. Thus, Golgi cells, stellate cells and granule cells seem to integrate on a shorter intrinsic timescale than do Purkinje cells, the output neuron of the cerebellar cortex. The specific mechanisms by which different types of presynaptic neurons specifically innervate a given dendritic compartment remain to be elucidated, but recent results provide some experimental evidence of a differential distribution of cell adhesion molecules between the axonal and the somato-dendritic membrane, suggesting one mechanism contributing to the ordered distribution of synapses during synaptogenesis.