Sources of errors and uncertainties in the assessment of forest soil carbon stocks at different scales-review and recommendations

Spatially explicit knowledge of recent and past soil organic carbon (SOC) stocks in forests will improve our understanding of the effect of human- and non-human-induced changes on forest C fluxes. For SOC accounting, a minimum detectable difference must be defined in order to adequately determine temporal changes and spatial differences in SOC. This requires sufficiently detailed data to predict SOC stocks at appropriate scales within the required accuracy so that only significant changes are accounted for. When designing sampling campaigns, taking into account factors influencing SOC spatial and temporal distribution (such as soil type, topography, climate and vegetation) are needed to optimise sampling depths and numbers of samples, thereby ensuring that samples accurately reflect the distribution of SOC at a site. Furthermore, the appropriate scales related to the research question need to be defined: profile, plot, forests, catchment, national or wider. Scaling up SOC stocks from point sample to landscape unit is challenging, and thus requires reliable baseline data. Knowledge of the associated uncertainties related to SOC measures at each particular scale and how to reduce them is crucial for assessing SOC stocks with the highest possible accuracy at each scale. This review identifies where potential sources of errors and uncertainties related to forest SOC stock estimation occur at five different scales-sample, profile, plot, landscape/regional and European. Recommendations are also provided on how to reduce forest SOC uncertainties and increase efficiency of SOC assessment at each scale.

On parsing the neural code in the prefrontal cortex of primates using principal dynamic modes

Nonlinear modeling of multi-input multi-output (MIMO) neuronal systems using Principal Dynamic Modes (PDMs) provides a novel method for analyzing the functional connectivity between neuronal groups. This paper presents the PDM-based modeling methodology and initial results from actual multi-unit recordings in the prefrontal cortex of non-human primates. We used the PDMs to analyze the dynamic transformations of spike train activity from Layer 2 (input) to Layer 5 (output) of the prefrontal cortex in primates performing a Delayed-Match-to-Sample task. The PDM-based models reduce the complexity of representing large-scale neural MIMO systems that involve large numbers of neurons, and also offer the prospect of improved biological/physiological interpretation of the obtained models. PDM analysis of neuronal connectivity in this system revealed "input-output channels of communication" corresponding to specific bands of neural rhythms that quantify the relative importance of these frequency-specific PDMs across a variety of different tasks. We found that behavioral performance during the Delayed-Match-to-Sample task (correct vs. incorrect outcome) was associated with differential activation of frequency-specific PDMs in the prefrontal cortex.

Design of optimal stimulation patterns for neuronal ensembles based on Volterra-type hierarchical modeling

This paper presents a general methodology for the optimal design of stimulation patterns applied to neuronal ensembles in order to elicit a desired effect. The methodology follows a variant of the hierarchical Volterra modeling approach that utilizes input-output data to construct predictive models that describe the effects of interactions among multiple input events in an ascending order of interaction complexity. The illustrative example presented in this paper concerns the multi-unit activity of CA1 neurons in the hippocampus of a rodent performing a learned delayed-nonmatch-to-sample (DNMS) task. The multi-unit activity of the hippocampal CA1 neurons is recorded via chronically implanted multi-electrode arrays during this task. The obtained model quantifies the likelihood of having correct performance of the specific task for a given multi-unit (spatiotemporal) activity pattern of a CA1 neuronal ensemble during the 'sample presentation' phase of the DNMS task. The model can be used to determine computationally (off-line) the 'optimal' multi-unit stimulation pattern that maximizes the likelihood of inducing the correct performance of the DNMS task. Our working hypothesis is that application of this optimal stimulation pattern will enhance performance of the DNMS task due to enhancement of memory formation and storage during the 'sample presentation' phase of the task.

Modeling neuron-glia interactions: from parametric model to neuromorphic hardware

Recent experimental evidence suggests that glial cells are more than just supporting cells to neurons - they play an active role in signal transmission in the brain. We herein propose to investigate the importance of these mechanisms and model neuron-glia interactions at synapses using three approaches: A parametric model that takes into account the underlying mechanisms of the physiological system, a non-parametric model that extracts its input-output properties, and an ultra-low power, fast processing, neuromorphic hardware model. We use the EONS (Elementary Objects of the Nervous System) platform, a highly elaborate synaptic modeling platform to investigate the influence of astrocytic glutamate transporters on postsynaptic responses in the detailed micro-environment of a tri-partite synapse. The simulation results obtained using EONS are then used to build a non-parametric model that captures the essential features of glutamate dynamics. The structure of the non-parametric model we use is specifically designed for efficient hardware implementation using ultra-low power subthreshold CMOS building blocks. The utilization of the approach described allows us to build large-scale models of neuron/glial interaction and consequently provide useful insights on glial modulation during normal and pathological neural function.

Restorative encoding memory integrative neural device: "REMIND"

Construction and application of a neural prosthesis device that enhances existing and replaces lost memory capacity in humans is the focus of research described here in rodents. A unique approach for the analysis and application of neural population firing has been developed to decipher the pattern in which information is successfully encoded by the hippocampus where mnemonic accuracy is critical. A nonlinear dynamic multi-input multi-output (MIMO) model is utilized to extract memory relevant firing patterns in CA3 and CA1 and to predict online what the consequences of the encoded firing patterns reflect for subsequent information retrieval for successful performance of delayed-nonmatch-to-sample (DNMS) memory task in rodents. The MIMO model has been tested successfully in a number of different contexts, each of which produced improved performance by a) utilizing online predicted codes to regulate task difficulty, b) employing electrical stimulation of CA1 output areas in the same pattern as successful cell firing, c) employing electrical stimulation to recover cell firing compromised by pharmacological agents and d) transferring and improving performance in naïve animals using the same stimulation patterns that are effective in fully trained animals. The results in rodents formed the basis for extension of the MIMO model to nonhuman primates in the same type of memory task that is now being tested in the last step prior to its application in humans.

Dynamic nonlinear modeling of interactions between neuronal ensembles using principal dynamic modes

We present a novel methodology for modeling the interactions between neuronal ensembles that utilizes the concept of Principal Dynamic Modes (PDM) and their associated nonlinear functions (ANF). This new approach seeks to reduce the complexity of the multi-input/multi-output (MIMO) model of the interactions between neuronal ensembles--an issue of critical practical importance in scaling up the MIMO models to incorporate hundreds (or even thousands) of input-output neurons. Global PDMs were extracted from the data using estimated first-order and second-order kernels and singular value decomposition (SVD). These global PDMs represent an efficient "coordinate system" for the representation of the MIMO model. The ANFs of the PDMs are estimated from the histograms of the combinations of PDM output values that lead to output spikes. For initial testing and validation of this approach, we applied it to a set of data collected at the pre-frontal cortex of a non-human primate during a behavioral task (Delayed Match-to-Sample). Recorded spike trains from Layer-2 neurons were viewed as the "inputs" and from Layer-5 neurons as the outputs. Model prediction performance was evaluated by means of computed Receiver Operating Characteristic (ROC) curves. The results indicate that this methodology may greatly reduce the complexity of the MIMO model without significant degradation of performance.

Modeling of nonlinear nonstationary dynamic systems with a novel class of artificial neural networks

This paper introduces a novel neural-network architecture that can be used to model time-varying Volterra systems from input-output data. The Volterra systems constitute a very broad class of stable nonlinear dynamic systems that can be extended to cover nonstationary (time-varying) cases. This novel architecture is composed of parallel subnets of three-layer perceptrons with polynomial activation functions, with the output of each subnet modulated by an appropriate time function that gives the summative output its time-varying characteristics. The paper shows the equivalence between this network architecture and the class of time-varying Volterra systems, and demonstrates the range of applicability of this approach with computer-simulated examples and real data. Although certain types of nonstationarities may not be amenable to this approach, it is hoped that this methodology will provide the practical tools for modeling some broad classes of nonlinear, nonstationary systems from input-output data, thus advancing the state of the art in a problem area that is widely viewed as a daunting challenge.

Improving bioassay sensitivity for neurotoxins detection using volterra based third order nonlinear analysis

Based on a novel analytical method for analyzing short-term plasticity (STP) of the CA1 hippocampal region in vitro, a screening tool for the detection and classification of unknown chemical compounds affecting the nervous system was recently introduced [1], [2]. The recorded signal consisted of evoked population spike in response to Poisson distributed random train impulse stimuli. The developed analytical approach used the first order Volterra kernel and the Laguerre coefficients of the second order Volterra model as classification features [3]. The biosensor showed encouraging results, and was able to classify out of sample compounds correctly [2]. We have taken an exploratory step to investigate the advantage of introducing a third order model [4]. DAP5, an NMDA channel blocker, did not show major changes in the second order kernel and in its corresponding Laguerre coefficients. Data were reanalyzed using a third order model. DAP5 showed discernable changes in the third order kernel as well as in the some of the corresponding Laguerre coefficients. Hence, the third order Volterra based model has the potential to improve the sensitivity and the discriminatory power of the proposed bioassay.

A modeling paradigm incorporating parametric and non-parametric methods

A novel parametric/non-parametric modeling paradigm was defined and used in characterization of synaptic transmission. In this paradigm, parametric and nonparametric techniques were incorporated in a complementary manner. Non-parametric method was used to generalize experimental data and extract system input/output properties. It provided a quantitative and intuitive way to validate a parametric model with respect to general, complete input patterns. Biological processes or mechanisms missed by the conventional parametric modeling approach were revealed and subsequently included into the modified parametric model.

17beta-Estradiol potentiates field excitatory postsynaptic potentials within each subfield of the hippocampus with greatest potentiation of the associational/commissural afferents of CA3

We sought to determine the impact of 17beta-estradiol throughout the hippocampal trisynaptic pathway and to investigate the afferent fiber systems within CA1 and CA3 in detail. To achieve this objective, we utilized multielectrode arrays to simultaneously record the field excitatory postsynaptic potentials from the CA1, dentate gyrus, and CA3 of rat hippocampal slices in the presence or absence of 100 pM 17beta-estradiol. We confirmed our earlier findings in CA1, where 17beta-estradiol significantly increased field excitatory postsynaptic potentials amplitude (20%+/-3%) and slope (22%+/-7%). 17beta-Estradiol significantly potentiated the field excitatory postsynaptic potentials in dentate gyrus, amplitude (15%+/-4%) and slope (17%+/-5), and in CA3, amplitude (15%+/-4%) and slope (19%+/-5%). Using a high-density multielectrode array, we sought to determine the source of potentiation in CA1 and CA3 by determining the impact of 17beta-estradiol on the apical afferents and the basal afferents within CA1 and on the mossy fibers and the associational/commissural fibers within CA3. In CA1, 17beta-estradiol induced a modest increase in the amplitude (7%+/-2%) and slope (9%+/-3%) following apical stimulation with similar magnitude of increase following basal stimulation amplitude (10%+/-2%) and slope (12%+/-3%). In CA3, 17beta-estradiol augmented the mossy fiber amplitude (15%+/-3%) and slope (18%+/-6%) and the associational/commissural fiber amplitude (31%+/-13%) and slope (40%+/-15%). These results indicate that 17beta-estradiol potentiated synaptic transmission in each subfield of the hippocampal slice, with the greatest magnitude of potentiation at the associational/commissural fibers in CA3. 17beta-Estradiol regulation of CA3 responses provides a novel site of 17beta-estradiol action that corresponds to the density of estrogen receptors within the hippocampus. The implications of 17beta-estradiol potentiation of the field potential in each of the hippocampal subfields and in particular CA3 associational/commissural fibers for memory function and clinical assessment are discussed.

General methodology for nonlinear modeling of neural systems with Poisson point-process inputs

This paper presents a general methodological framework for the practical modeling of neural systems with point-process inputs (sequences of action potentials or, more broadly, identical events) based on the Volterra and Wiener theories of functional expansions and system identification. The paper clarifies the distinctions between Volterra and Wiener kernels obtained from Poisson point-process inputs. It shows that only the Wiener kernels can be estimated via cross-correlation, but must be defined as zero along the diagonals. The Volterra kernels can be estimated far more accurately (and from shorter data-records) by use of the Laguerre expansion technique adapted to point-process inputs, and they are independent of the mean rate of stimulation (unlike their P-W counterparts that depend on it). The Volterra kernels can also be estimated for broadband point-process inputs that are not Poisson. Useful applications of this modeling approach include cases where we seek to determine (model) the transfer characteristics between one neuronal axon (a point-process 'input') and another axon (a point-process 'output') or some other measure of neuronal activity (a continuous 'output', such as population activity) with which a causal link exists.

Hierarchical model of the population dynamics of hippocampal dentate granule cells

A hierarchical modeling approach is used as the basis for a mathematical representation of the population activity of hippocampal dentate granule cells. Using neural field equations, the variation in time and space of dentate granule cell activity is derived from the summed synaptic potential and summed action potential responses of a population of granule cells evoked by monosynaptic excitatory input from entorhinal cortical afferents. In this formulation of the problem, we have considered a two-level hierarchy: the synapses of entorhinal cortical axons define the first level of organization, and dentate granule cells, which include these synapses, define the second, higher level of organization. The model is specified by two state field variables, for membrane potential and for synaptic efficacy, respectively, with both evolving according to different time scales. The two state field variables introduce new parameters, physiological and anatomical, which characterize the dentate from the point of view of neuronal and synaptic populations: (1) a set of geometrical constraints corresponding to the morphological properties of granule cells and anatomical characteristics of entorhinal-dentate connections; and (2) a set of neuronal parameters corresponding to physiological mechanisms. Assuming no interaction between granule cells, i.e., neither ephaptic nor synaptic coupling, the model is shown to be mathematically tractable and allows solution of the field equations leading to the determination of activity. This treatment leads to the definition of two state variables, volume of stimulated synapses and firing time, which describe observed activity. Numerical simulations are used to investigate the populational characterization of the dentate by individual parameters: (1) the relationship between the conditions of stimulation of active perforant path fibers, e.g., stimulating intensity, and activity in the granule cell layer; and (2) the influence of geometry on the generation of activity, i.e., the influence of neuron density and synaptic density-connectivity. As an example application of the model, the granule cell population spike is reconstructed and compared with experimental data.

Projection of the magnocellular red nucleus to the region of the accessory abducens nucleus in the rabbit

The projection of the magnocellular red nucleus (RNm) to the region of the accessory abducens nucleus (AABD) was traced in rabbit using the bidirectional tracer wheat germ agglutinin-horseradish peroxidase (WGA-HRP). In one set of animals, recordings of antidromic responses from RNm neurons elicited by electrical stimulation of the rubrospinal tract were used to localize injections of WGA-HRP for orthograde labeling of RNm terminals. In a different set of animals, horseradish peroxidase was injected into the retractor bulbi muscle to retrogradely label motoneurons of the AABD. The positions of RNm fibers and terminals were examined and compared to the locations and distribution of AABD cell bodies and labeled dendrites. Analyses revealed that along the entire rostrocaudal extent of the AABD, RNm efferents terminate primarily lateral to, or in the lateral aspects of, labeled motoneurons. For the rostral AABD, RNm efferents terminate only lateral to the nucleus. Although the terminals are not positioned to contact cell bodies of the AABD, they could overlap with dendrites that extend in the lateral direction. RNm efferents terminate more extensively within the posterior AABD, overlapping within both dendritic and cell body regions of the nucleus. Even in this posterior region, however, RNm efferents were distributed primarily over the lateral half of the nucleus. These data show that RNm can monosynaptically influence the AABD, through primarily its lateral and posterior aspects. Our findings also show that a major target of RNm efferents is the reticular cell population located lateral to the AABD, suggesting that the RNm also may affect AABD motoneuronal output indirectly through its projection to reticular cells.

Differential effect of TEA on long-term synaptic modification in hippocampal CA1 and dentate gyrus in vitro

The effectiveness of tetraethylammonium (TEA) and high-frequency stimulation (HFS) in inducing long-term synaptic modification is compared in CA1 and dentate gyrus (DG) in vitro. High-frequency stimulation induces long-term potentiation (LTP) at synapses of both perforant path-DG granule cell and Schaffer collateral-CA1 pyramidal cell pathways. By contrast, TEA (25 mM) induces long-term depression in DG while inducing LTP in CA1. The mechanisms underlying the differential effect of TEA in CA1 and DG were investigated. It was observed that T-type voltage-dependent calcium channel (VDCC) blocker, Ni2+ (50 microM), partially blocked TEA-induced LTP in CA1. A complete blockade of the TEA-induced LTP occurred when Ni2+ was applied together with the NMDA receptor antagonist, D-APV. The L-type VDCC blocker, nifidipine (20 microM), had no effect on CA1 TEA-induced LTP. In DG of the same slice, TEA actually induced long-term depression (LTD) instead of LTP, an effect that was blocked by D-APV. Neither T-type nor L-type VDCC blockade could prevent this LTD. When the calcium concentration in the perfusion medium was increased, TEA induced a weak LTP in DG that was blocked by Ni2+. During exposure to TEA, the magnitude of field EPSPs was increased in both CA1 and DG, but the increase was substantially greater in CA1. Tetraethylammonium application also was associated with a large, late EPSP component in CA1 that persisted even after severing the connections between CA3 and CA1. All of the TEA effects in CA1, however, were dramatically reduced by Ni2+. The results of this study indicate that TEA indirectly acts via both T-type VDCCs and NMDA receptors in CA1 and, as a consequence, induces LTP. By contrast, TEA indirectly acts via only NMDA receptors in DG and results in LTD. The results raise the possibility of a major synaptic difference in the density and/or distribution of T-type VDCCs and NMDA receptors in CA1 and DG of the rat hippocampus.

A novel network for nonlinear modeling of neural systems with arbitrary point-process inputs

This paper address the issue of nonlinear model estimation for neural systems with arbitrary point-process inputs using a novel network that is composed of a pre-processing stage of a Laguerre filter bank followed by a single hidden layer with polynomial activation functions. The nonlinear modeling problem for neural systems has been attempted thus far only with Poisson point-process inputs and using cross-correlation methods to estimate low-order nonlinearities. The specific contribution of this paper is the use of the described novel network to achieve practical estimation of the requisite nonlinear model in the case of arbitrary (i.e. non-Poisson) point-process inputs and high-order nonlinearities. The success of this approach has critical implications for the study of neuronal ensembles, for which nonlinear modeling has been hindered by the requirement of Poisson process inputs and by the presence of high-order nonlinearities. The proposed methodology yields accurate models even for short input-output data records and in the presence of considerable noise. The efficacy of this approach is demonstrated with computer-simulated examples having continuous output and point-process output, and with real data from the dentate gyrus of the hippocampus.

Intraocular pharmacokinetics and safety of a humanized monoclonal antibody in rabbits after intravitreal administration of a solution or a PLGA microsphere formulation

Poly(lactic-co-glycolic) acid (PLGA) bioresorbable microspheres are used for controlled-release drug delivery and are particularly promising for ocular indications. The objective of the current study was to evaluate the pharmacokinetics and safety of a recombinant human monoclonal antibody (rhuMAb HER2) in rabbits after bolus intravitreal administration of a solution or a PLGA-microsphere formulation. On Day 0, forty-eight male New Zealand white rabbits (2.3-2.6 kg) were immobilized with intramuscular ketamine/xylazine, and the test materials were injected directly into the vitreous compartment. Group 1 animals received rhuMAb HER2 in 50:50 lactide: glycolide PLGA microspheres; Group 2 animals received rhuMAb HER2 in solution (n = 24/group). The dose for each eye was 25 microg (50 microl). After dosing, animals were sacrificed at 2 min, and on 1, 2, 4, 7, 14, 23, 29, 37, 44, 50, and 56 days (n = 2/timepoint/group). Safety assessment included direct ophthalmoscopy, clinical observations, body weight, and hematology and clinical chemistry panels. At necropsy, vitreous and plasma were collected for pharmacokinetics and analysis for antibodies to rhuMAb HER2, and the vitreal pellet (Group 1) was prepared for histologic evaluation. All animals completed the study per protocol-both treatments were well tolerated, and no suppurative or mixed inflammatory cell reaction was observed in the vitreal samples (Group 1) at any of the time points examined. Antibodies to rhuMAb HER2 were detected in plasma samples by Day 7 in both treatment groups, but infrequently in vitreous samples. There were no safety implications associated with this immune response. The in vitro characterization of the PLGA microspheres provided reasonable projections of the in vivo rhuMAb HER2 release kinetics (Group 1). The total amount of antibody that was released was similar in vitro (25.9%) and in vivo (32.4%). RhuMAb HER2 (Group 2) was cleared slowly from the vitreous compartment, with initial and terminal half-lives of 0.9 and 5.6 days, respectively. The volume of distribution approximated the vitreous volume in a rabbit eye.

Application of a novel modeling method to the nonstationary properties of potentiation in the rabbit hippocampus

This paper presents the first application of a novel methodology for nonstationary nonlinear modeling to neurobiological data consisting of extracellular population field potentials recorded from the dendritic layer of the dentate gyrus of the rabbit hippocampus under conditions of stimulus-induced potentiation. The experimental stimulus was a Poisson random sequence with a mean rate of 5 impulses/s applied to the perforant path, which was sufficient to induce a progressive potentiation of perforant path-evoked granule cell response. The modeling method utilizes a novel artificial neural network architecture, which is based on the general time-varying Volterra model. The artificial neural network is composed of parallel subnets of three-layer perceptrons with polynomial activation functions, with the output of each subnet modulated by an appropriate time function that models the system nonstationarities and gives the summative output its time-varying characteristics. For the specific application presented herein these time functions are sigmoidal functions with trainable slopes and inflection points. A possible mapping between the nonstationary components of the model and the mechanisms underlying potentiation changes in the hippocampus is discussed.

17beta-estradiol enhances NMDA receptor-mediated EPSPs and long-term potentiation

Gonadal steroid hormones influence CNS functioning through a variety of different mechanisms. To test the hypothesis that estrogen modulates synaptic plasticity in the hippocampus, in vitro hippocampal slices from 2-mo-old Sprague-Dawley male rats were used to determine the effect of 17beta-estradiol on both N-methyl-D-aspartate (NMDA) receptor-mediated excitatory postsynaptic potentials (EPSPs) through intracellular recordings and long-term potentiation (LTP) through extracellular recordings. Intracellular EPSPs and extracellular field EPSPs (fEPSPs) were recorded from CA1 pyramidal cells by stimulating Schaffer collateral fibers. In intracellular experiments, slices were perfused with medium containing bicuculline (5 microM) and low Mg2+ (0.1 mM) to enhance the NMDA receptor-mediated currents and 6, 7-dinitroquinoxaline-2,3-dione (DNQX) (10 microM) to block the alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprianate (AMPA) receptor-mediated component. The effects of 17beta-estradiol on NMDA receptor-mediated activity were excitatory; concentrations >10 nM induced seizure activity, and lower concentrations (1 nM) markedly increased the amplitude of NMDA-mediated EPSPs (both the first and second responses increased during paired pulse stimulation by 180 and 197%, respectively). In extracellular experiments, slices perfused with 17beta-estradiol (100 pM) exhibited a pronounced, persisting, and significant enhancement of LTP of both the fEPSP slope (192%) and fEPSP amplitude (177%) compared with control slices (fEPSP slope = 155%; fEPSP amplitude = 156%) 30 min after high-frequency stimulation. These data demonstrate that estrogen enhances NMDA receptor-mediated currents and promotes an enhancement of LTP magnitude.

Spatial distribution of potentiated synapses in hippocampus: dependence on cellular mechanisms and network properties

Long-term potentiation (LTP) of synaptic transmission, studied intensively in reduced brain preparations such as hippocampal brain slices, is the leading candidate for the cellular/molecular basis of learning and memory. Serious consideration of LTP as underlying information storage in the intact brain, however, requires understanding how LTP can be induced selectively at specific synaptic sites in a neural system when the mechanisms underlying LTP are regulated by other structural and functional properties of the same neural system. In the studies reported here, we tested the hypothesis that different patterns of activity within the same population of entorhinal cortical afferents could lead to a selective potentiation of spatially distinct populations of synapses across different regions of the hippocampus, including those activated multisynaptically. We focused specifically on potentiation of direct, monosynaptic entorhinal input to dentate granule cells, which expresses an NMDA receptor-dependent LTP, and on potentiation of indirect, disynaptic entorhinal input to CA3 pyramidal cells, which is transmitted by the mossy fiber projection of dentate granule cells and expresses an NMDA receptor-independent LTP. The principal findings of these experiments show that lower stimulation frequencies (10-20 Hz) of entorhinal cortical axons selectively induce LTP of mossy fiber input to CA3 transsynaptically via excitation of dentate granule cells, and that patterns of stimulation of that mimic neuronal firing in the entorhinal cortex during endogenous theta rhythm (five-impulse bursts at 200 Hz, interburst intervals of 200 msec) induce LTP both monosynaptically for input to dentate granule cells and transsynaptically for mossy fiber input to CA3.

Novel expression mechanism for synaptic potentiation: alignment of presynaptic release site and postsynaptic receptor

A combination of experimental and modeling approaches was used to study cellular-molecular mechanisms underlying the expression of short-term potentiation (STP) and long-term potentiation (LTP) of glutamatergic synaptic transmission in the hippocampal slice. Electrophysiological recordings from dentate granule cells revealed that high-frequency stimulation of perforant path afferents induced a robust STP and LTP of both (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and N-methyl-D-aspartic acid (NMDA) receptor-mediated synaptic responses. However, the decay time constant for STP of the AMPA receptor-mediated excitatory postsynaptic potential was approximately 6 min, whereas the decay time constant for STP of the NMDA receptor-mediated excitatory postsynaptic potential was only 1 min. In addition, focal application of agonists during the expression of STP revealed that the magnitude of conductance change elicited by NMDA application was significantly enhanced, whereas the magnitude of conductance change elicited by application of AMPA remained constant. These findings are most consistent with a postsynaptic mechanism of STP and LTP. Different putative mechanisms were evaluated formally using a computational model that included diffusion of glutamate within the synaptic cleft, different kinetic properties of AMPA and NMDA receptor/channels, and geometric relations between presynaptic release sites and postsynaptic receptor/channels. Simulation results revealed that the only hypothesis consistent with experimental data is that STP and LTP reflect a relocation of AMPA receptor/channels in the postsynaptic membrane such that they become more closely "aligned" with presynaptic release sites. The same mechanism cannot account for STP or LTP of NMDA receptor-mediated responses; instead, potentiation of the NMDA receptor subtype is most consistent with an increase in receptor sensitivity or number.

Transport and fate of trifluoroacetate in upland forest and wetland ecosystems

Although trifluoroacetate (TFA), a breakdown product of chlorofluorocarbon replacements, is being dispersed widely within the biosphere, its ecological fate is largely unknown. TFA was added experimentally to an upland, northern hardwood forest and to a small forest wetland ecosystem within the Hubbard Brook Experimental Forest in New Hampshire. Inputs of TFA were not transported conservatively through these ecosystems; instead, significant amounts of TFA were retained within the vegetation and soil compartments. More TFA was retained by the wetland ecosystem than by the upland forest ecosystem. Using simulation modeling, TFA concentrations were predicted for soil and drainage water until the year 2040.

Differential expression of short-term potentiation by AMPA and NMDA receptors in dentate gyrus

Both alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and N-methyl-D-aspartate (NMDA) glutamatergic receptor subtypes in hippocampus have been shown to express long-term potentiation (LTP), a form of synaptic modification believed to be involved in memory formation. Because of their postsynaptic localization, any differential expression of LTP by the two receptor subtypes would strongly support the existence of a postsynaptic mechanism of LTP expression. In this study, electrophysiological recordings from dentate granule cells were used to compare the potentiation of AMPA and NMDA receptor-mediated responses occurring during the initial phase of LTP, typically identified as STP. Results revealed that high-frequency stimulation (HFS) of perforant path afferents induces a robust STP of both AMPA and NMDA receptor-mediated components of granule cell EPSPs (referred to as AMPA STP and NMDA STP, respectively). Although STP for both receptor subtypes decayed to an asymptotic, steady-state level of LTP and could be induced repetitively, there were substantial differences in several aspects of AMPA and NMDA STP dynamics. STP of the AMPA receptor reached its peak magnitude approximately 30 sec after HFS and decayed with a time constant of approximately 6 min. In contrast, peak magnitude of NMDA STP always appeared immediately after HFS and decayed with a time constant of only 1 min. Single-pulse stimulation of perforant path afferents paired with postsynaptic depolarization also induced LTP of both AMPA and NMDA components. When this induction paradigm was used, however, only the AMPA component showed significant STP. Our results demonstrate that AMPA and NMDA receptors exhibit markedly different degrees of activity-dependent, short-term modifiability, with the possibility that STP of the NMDA receptor reflects primarily post-tetanic potentiation (PTP). In addition, our results strongly suggest that the mechanisms underlying STP of the AMPA receptor are postsynaptic in origin.

Dynamic synapse: a new concept of neural representation and computation

Presynaptic mechanisms influencing the probability of neurotransmitter release from an axon terminal, such as facilitation, augmentation, and presynaptic feedback inhibition, are fundamental features of biological neurons and are cardinal physiological properties of synaptic connections in the hippocampus. The consequence of these presynaptic mechanisms is that the probability of release becomes a function of the temporal pattern of action potential occurrence, and hence, the strength of a given synapse varies upon the arrival of each action potential invading the terminal region. From the perspective of neural information processing, the capability of dynamically tuning the synaptic strength as a function of the level of neuronal activation gives rise to a significant representational and processing power of temporal spike patterns at the synaptic level. Furthermore, there is an exponential growth in such computational power when the specific dynamics of presynaptic mechanisms varies quantitatively across axon terminals of a single neuron, a recently established characteristic of hippocampal synapses. During learning, alterations in the presynaptic mechanisms lead to different pattern transformation functions, whereas changes in the postsynaptic mechanisms determine how the synaptic signals are to be combined. We demonstrate the computational capability of dynamic synapses by performing speech recognition from unprocessed, noisy raw waveforms of words spoken by multiple speakers with a simple neural network consisting of a small number of neurons connected with synapses incorporating dynamically determined probability of release. The dynamics included in the model are consistent with available experimental data on hippocampal neurons in that parameter values were chosen so as to be consistent with time constants of facilitative and inhibitory processes governing the dynamics of hippocampal synaptic transmission studied using nonlinear systems analytic procedures.

NMDA receptor-dependent LTD in different subfields of hippocampus in vivo and in vitro

In simulations with artificial neural networks, efficient information processing and storage has been shown to require that the strength of connections between network elements has the capacity to both increase and decrease in a use-dependent manner. In contrast to long-term potentiation (LTP) of excitatory synaptic transmission, activity-dependent long-term depression (LTD) has been difficult to demonstrate in forebrain in vivo. Theoretical arguments indicate that coincidence of presynaptic excitation and low-magnitude postsynaptic activation are the necessary prerequisites for LTD induction. Here we report that stimulation paradigms which cause 1) sufficient excitation to result in NMDA receptor activation and simultaneously 2) attenuate the level of postsynaptic activation by recruitment of GABAA receptor-mediated inhibition consistently produce LTD of commissural input to area CA1 in the hippocampus of anesthetized adult rats, and of the perforant path input to the dentate gyrus in the hippocampus of anesthetized and unanesthetized adult rabbits. A functionally similar pre- and postsynaptic activation pattern applied to the hippocampal slice preparation by injecting hyperpolarizing current into the postsynaptic cell during NMDA receptor-mediated excitation also was effective in consistently inducing LTD. Results of studies in vitro show that Ca2+ influx through the NMDA channel is necessary for the induction of LTD, and moreover, that NMDA receptors also participate in the expression of LTD. Our findings demonstrate a general mechanism for the implementation of a theoretically derived learning rule in adult forebrain in vivo and in vitro and provide justification for the inclusion of use-dependent decreases of connection weights in formal models of cognitive processing.

Excitatory stimulation during postsynaptic inhibition induces long-term depression in hippocampus in vivo

1. As part of an effort to evaluate the biological plausibility of theoretically derived principles of synaptic modification, we studied activity-dependent long-term depression (LTD) of glutamatergic transmission in the hippocampus of anesthetized adult rats. Field potentials of CA1 pyramidal cells evoked by single-pulse stimulation (0.1 Hz) of the commissural afferents were recorded before and after paired-pulse stimulation (0.5 Hz) of the same pathway. A train of 150 or 200 paired pulses produced robust LTD of the commissural input to the CA1 pyramidal neurons when the interstimulus interval (ISI) of the pairs was short (25 ms) but not when the ISI was long (1,000 ms). 2. Paired-pulse stimulation with the short but not with the long ISI also was associated with pronounced inhibition of pyramidal cell firing upon the second pulse of a pair, despite the fact that the excitatory input was facilitated with the short-ISI paradigm. The inhibition of pyramidal cell activity was mediated by input to the pyramidal cells from local gamma-aminobutyric acid (GABA)-releasing interneurons activated by commissural fibers and/or CA1 recurrent collaterals, because the inhibition was eliminated by local administration of the selective GABAA receptor antagonist, bicuculline (50 microM), near the recording site. 3. Postsynaptic input from GABAergic interneurons was necessary for the induction of LTD, because short-ISI paired-pulse stimulation failed to produce LTD in the presence of bicuculline. 4. N-methyl-D-aspartate (NMDA) receptor-mediated excitation also was necessary for the induction of LTD, because administration of the selective NMDA receptor antagonist, D-2-amino-5-phosphonvaleric acid (100 microM), near the recording site prevented the development of LTD.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification and characterization of striatal cell subtypes using in vivo intracellular recording and dye-labeling in rats: III. Morphological correlates and compartmental localization

In the first two reports of this series, in vivo intracellular recording techniques were used to characterize the electrophysiological properties of two types of striatal neurons that had been identified by their distinct response patterns to stimulation of corticostriatal afferents. In this paper, we examined whether cells showing Type I or Type II response patterns also differed with respect to their morphology or compartmental localization by combining intracellular recording and Lucifer yellow staining with immunocytochemical localization of calbindin 28 kd immunoreactivity. In the majority of cases, both Type I and Type II neurons exhibited similar morphological characteristics, with 80% of the Type I cells (13/16) and all of the Type II cells (n = 40) being small or medium spiny neurons. In each case where the morphological class of the cell was different than the spiny cell class, the cell exhibited a Type I response pattern. These spiny neurons had somata that averaged 17.1 +/- 1.3 microns in diameter and gave rise to between four and eight primary dendrites. The axons typically arose from cell bodies (7/13 for Type I and 25/40 for Type II cells) and emitted extensive local axonal collaterals. However, the axons of Type I cells more frequently originated from the dorsal surface of the somata (9/13; 69%), whereas Type II axons more frequently arose from the ventral surface of the somata (25/35; 71%), which may account for their different extracellular waveforms. In coronally sectioned tissue (n = 18), the axons always projected laterally when the somata were located in the medial striatum and projected medially when the somata were in the lateral striatal region. In a subset of experiments (N = 22), Lucifer yellow-stained neurons were localized with respect to their position within the patch and matrix compartments of the striatum using subsequent staining for calbindin 28 kd immunoreactivity. Of the 20 labeled medium spiny neurons examined (Type II: N = 13; Type I: N = 7), 19 were located in the calbindin-positive matrix compartment. The only neuron localized to the patch compartment was a medium spiny cell that exhibited a Type II paired impulse response pattern. In addition, of the two aspiny neurons from this group with beaded dendrites, one was localized to the border between adjacent patch and matrix compartments, whereas the other was located completely within the matrix compartment. Therefore, despite their distinct paired impulse response patterns, the majority of both Type I and Type II neurons were medium spiny cells located in the matrix compartment of the striatum.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification and characterization of striatal cell subtypes using in vivo intracellular recording in rats: I. Basic physiology and response to corticostriatal fiber stimulation

The electrophysiological characteristics of two subtypes of striatal neurons, identified by their distinct patterns of response to paired impulse stimulation of corticostriatal afferents, were compared using in vivo intracellular recordings in rats. As observed in previous extracellular recording studies, the majority of neurons (73%) were found to be of the Type II class, with the remaining cells exhibiting the Type I response pattern. For all cells, cortical stimulation elicited 5-30 mV EPSPs at latencies ranging from 2.0-5.3 msec. Increasing the stimulating current intensity caused a progressive increase in the amplitude of the evoked EPSPs without altering their latencies, suggesting that the EPSPs are monosynaptically mediated. Both the average amplitude and duration of the evoked EPSPs at spike threshold in Type I neurons (9.8 +/- 1.7 mV, 11.8 +/- 2.8 msec; mean +/- SEM) were significantly smaller than those of Type II cells (20.3 +/- 1.4 mV, 22.7 +/- 2.1 msec). Although the average latency to the onset of the EPSP was similar for both cell classes (Type I cells: 2.3 +/- 0.3 msec; Type II cells: 2.2 +/- 0.2 msec), the EPSPs in Type I cells reached peak amplitude and the spikes were triggered at significantly longer latencies than in the Type II cells (peak I: 11.2 +/- 2.5 msec vs. II: 7.6 +/- 0.7 msec; spike I: 8.0 +/- 1.2 msec vs. II: 5.7 +/- 0.4 msec). Striatal neurons had a comparatively hyperpolarized resting membrane potential (-70.2 +/- 2.1 mV) and had an average input resistance of 35.4 +/- 7.6 M omega. Overall, striatal neurons exhibited low levels of spontaneous activity (0.6 +/- 0.7 Hz) with 50% of the neurons being quiescent. Type I cells exhibited significantly higher firing rates (3.2 +/- 0.8 Hz) than Type II cells (0.8 +/- 0.3 Hz), although their resting membrane potentials were not significantly different. Spontaneously occurring spikes had an average amplitude of 72.7 +/- 3.4 mV and spike thresholds of -50.1 +/- 1.5 mV. Irregularly occurring depolarizing plateau potentials, which typically gave rise to spike discharge, were frequently observed in both spontaneously firing and quiescent neurons. A small proportion of the cells recorded (3/55) exhibited a Type I response pattern but had unique physiological characteristics that were similar to those described by others as arising from large, aspiny striatal neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification and characterization of striatal cell subtypes using in vivo intracellular recording in rats: II. Membrane factors underlying paired-pulse response profiles

Two subtypes of neurons in the striatum have been defined on the basis of their different response patterns to paired-impulse stimulation of corticostriatal afferents, with type I cells showing a longer spike latency, facilitation at short intervals, and inhibition at long intervals, and type II cells defined by the facilitation occurring at long interstimulus intervals. Nevertheless, the companion report has shown that this distinction of cell types cannot be accounted for by differences in the basic physiological properties of these cells, but instead is likely to be due to differences in their synaptic connectivity. The experiments performed in this study were directed at examining in detail the membrane factors and synaptic responses that may contribute to these distinct response patterns. When pairs of stimuli were delivered to the corticostriatal fibers at 10-30 ms interstimulus intervals, the EPSPs elicited in type I neurons exhibited a temporal summation, resulting in a facilitation of spike firing to the second stimulus relative to the first. In contrast, type II cells showed decreased EPSP amplitude at short intervals, and in cells showing a short-interval inhibition, there was a significant increase in spike threshold (+5.3 +/- 1.4 mV) during the second response. All type II neurons recorded with KCl-filled microelectrodes showed short-interval facilitation with little or no change in spike threshold. Although the use of KCl electrodes did not alter the facilitation at short intervals in type I neurons it did increase the rate of rise of the EPSP, causing spikes to be triggered at a latency similar to that of type II cells. Paired stimuli delivered at 75-150 ms interstimulus intervals showed inverse effects on type I and type II cells with respect to the probability of spike firing. In type I cells, the evoked EPSP was followed by a long-latency membrane hyperpolarization that prevented the second EPSP from reaching spike threshold. In contrast, the smaller-amplitude hyperpolarization evoked in type II cells enabled the second stimulus to activate an EPSP at the same membrane potential as the first stimulus, resulting in a facilitation of spiking.(ABSTRACT TRUNCATED AT 400 WORDS)

Na+ influx through Ca2+ channels can promote striatal GABA efflux in Ca(2+)-deficient conditions in response to electrical field depolarization

Electrical field depolarization releases gamma-aminobutyric acid (GABA) in rat striatal slices in the absence of external Ca2+. omega-Conotoxin GVIA (omega-CgTx; 1-50 nM), a neuronal Ca2+ channel blocker, inhibits electrically evoked efflux of newly taken up [3H]GABA in a concentration-dependent manner in either normal or Ca(2+)-free medium. This suggests that ion influx occurs through Ca2+ channels in the absence of external Ca2+ and contributes to the efflux of GABA. Reducing external Na+ concentration to 27.25 mM (low [Na+]o medium) by equimolarly substituting choline chloride for sodium chloride has differential effects on electrically evoked GABA efflux depending on the external Ca2+ concentrations. In normal Ca2+ medium, electrically evoked GABA efflux increases whereas, in Ca(2+)-free medium, it is greatly inhibited when [Na+]o is reduced to 27.25 mM. In low [Na+]o medium, GABA efflux is largely tetrodotoxin (TTX)-sensitive, however, spike firing evoked by antidromic stimulation of striatal cells is inhibited. In Na(+)-free medium, resting GABA efflux increases 17-fold whereas evoked GABA efflux diminishes. In Ca(2+)-free medium, 70 min of incubation with 1-2-bis-(1-aminophenoxy)ethane-N,N,N',N' tetraacetoxy methyl ester (BATPA-AM, 1 microM), an intracellular calcium chelator, increases both resting GABA efflux and electrically evoked GABA overflow by approximately 100%. These results suggest that: (1) in Ca(2+)-free conditions, Na+ permeability of cells increases via Ca2+ channels and this profoundly affects GABA efflux. (2) Electrical field depolarization is likely to release GABA by directly depolarizing axon terminals. (3) Ca(2+)-independent GABA efflux is not promoted by an increase in intracellular free Ca2+ concentration via Na+/Ca2+ exchange processes from internal pools.

Depression of glutamatergic and GABAergic synaptic responses in striatal spiny neurons by stimulation of presynaptic GABAB receptors

The influence of gamma-aminobutyric acidB (GABAB) receptor stimulation on the excitatory and inhibitory synaptic potentials and membrane properties of identified striatal spiny neurons was examined in a corticostriatal slice preparation. Stimulation of the subcortical white matter evoked a monosynaptic, excitatory postsynaptic potential (EPSP) and a polysynaptic, inhibitory postsynaptic potential (IPSP) in spiny neurons. The EPSP had two components: a large amplitude response which could be blocked by the kainate/quisqualate receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM), and a small amplitude, long-duration depolarization which could be blocked by the N-methyl-D-aspartate receptor antagonist, d-(-)-2-amino-5-phosphonovaleric acid (APV, 100 microM). The IPSP was observed as a membrane depolarization when recorded from neurons at resting membrane potential. However, when neurons were injected with the Na(+)-channel blocker, QX-314, allowing cells to be depolarized above their spike thresholds, a prominent hyperpolarizing IPSP was readily observed which could be blocked by the GABAA antagonist, bicuculline (10-50 microM). This bicuculline-sensitive IPSP was responsible for the inhibition of EPSP amplitude and probability of spike discharge revealed using paired stimulation of the subcortical white matter. The amplitude of both the EPSP and the IPSP were depressed by the GABAB receptor agonist, p-chlophenyl-GABA (baclofen, 0.5-100 microM) in a concentration-dependent manner. Baclofen also blocked paired stimulus inhibition of spike discharge. These effects of baclofen persisted in slices in which the cortex was removed and were reversed by the GABAB receptor antagonist, 3-amino-3-hydroxy-2-(4-chlorophenyl)-propanesulphonic acid (saclofen, 100-500 microM). In contrast to its profound influence on synaptic input, baclofen did not alter resting membrane potential, input resistance, membrane current-voltage relationship, or spike threshold of the cells recorded, and therefore did not appear to exert direct postsynaptic effects on the striatal spiny neurons. Taken together, these data indicate that the depressant effects of baclofen on EPSPs are mediated through GABAB receptors located on the terminals of glutamatergic afferents within the striatum. Moreover, the results suggest that the actions of baclofen on IPSPs and paired stimulus inhibition are produced by activation of GABAB receptors within the striatum at a site presynaptic to spiny neurons, either on the terminals of GABAergic afferents or on an interposed non-spiny GABAergic cell. Thus, GABAB hetero- and auto-receptors have the capacity to provide a negative feedback mechanism through which the major excitatory and inhibitory inputs to striatal spiny neurons are regulated.

Recovery of hippocampal dentate granule cell responsiveness to entorhinal cortical input following norepinephrine depletion

Hippocampal dentate granule cell responsivity to excitatory input from entorhinal perforant path fibers was examined in the chronic rabbit preparation following norepinephrine (NE) depletion induced with the neurotoxin DSP4. To examine granule cell responsivity as a function of perforant path activation, constant low frequency stimulation (0.1 Hz) was applied to the perforant path using an ascending intensity series. To examine granule cell responsivity to more complex patterns of stimulation, a train of impulses, with a random interstimulus interval (Poisson distribution; mean frequency of 2 Hz), was applied to the perforant path. Both single impulse and random interval impulse stimulation revealed that NE depletion increased the average amplitude of the perforant path-granule cell population spike. The random interval impulse stimulation revealed that NE depletion also increased the magnitude and duration of second order inhibitory interactions. These changes were transient, however, and recovered over the 21 day test period. Hippocampal NE levels were reduced an average of 80% between 23 and 38 days post-DSP4. The activity of the rate-limiting enzyme for NE synthesis, tyrosine hydroxylase (TH), was reduced an average of 60%. That NE levels were reduced to a greater extent than was TH activity is suggestive of increased NE synthesis within the remaining nerve terminals. Such an increase in NE synthesis may reflect a compensatory response underlying the functional recovery of electrophysiological responsiveness following partial NE depletion.

Characterizationin vivo of the NMDA receptor-mediated component of dentate granule cell population synaptic responses to perforant path input

The NMDA receptor-mediated component of the hippocampal granule cell population excitatory postsynaptic potential response to low frequency (< 0.2 Hz) stimulation of the medial perforant path was characterized in vivo. Extracellular recordings were obtained from the dentate molecular layer in anesthetized rabbits, and glutamatergic and GABAergic antagonists were applied locally by pressure ejection. To measure the NMDA-mediated component, the NMDA receptor antagonist D-5-aminophosphonovalerate (APV) was applied during the constant ejection of physiological saline, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and/or bicuculline methiodide. In general agreement with the results of attempts by other investigators to identify NMDA responses in vivo, APV did not significantly reduce the response to a single stimulus impulse in the presence of saline. However, an NMDA-mediated response was revealed when alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprianate receptor-mediated current flow was eliminated by applying the non-NMDA receptor antagonist CNQX. The NMDA component was negative-going as predicted, but its duration was considerably less than indicated in other studies of the dentate in vitro. The relative magnitudes of the NMDA and non-NMDA components of the EPSP were found to vary as a function of stimulus intensity or frequency. The NMDA receptor-mediated component represented 12% of the control response and increased to over 25% in response to higher stimulus intensities. A brief, high-frequency burst of impulses evoked a larger NMDA component in the presence of CNQX and was able to evoke an NMDA component in the presence of saline. Surprisingly, short trains of stimulation at lower frequencies typically produced suppression of the NMDA component. In a final series of experiments, it was found that many characteristics of the NMDA component were substantially altered by GABAergic inhibition. In the presence of the GABAA antagonist bicuculline, the magnitude of NMDA receptor-mediated responses was increased and their duration was greatly extended. Additionally, in the presence of bicuculline, the NMDA component facilitated markedly in response to frequencies of stimulus input > 20 Hz. These results indicate in vivo that the initiation and duration of NMDA current flow depend strongly upon the intensity and frequency of perforant path stimulation. In addition, the NMDA response to a single impulse appears to be reduced and truncated by input from GABAA receptor-mediated feedback and/or feedforward inhibition, and this inhibition affects temporal summation of NMDA receptor-mediated responses over a wide range of input frequencies. It is suggested that such inhibition results from the activation of GABAA receptors located on granule cell dendritic shafts.(ABSTRACT TRUNCATED AT 400 WORDS)

Functionally distinct subpopulations of striatal neurons are differentially regulated by GABAergic and dopaminergic inputs--II. In vitro analysis

In the companion report [Nisenbaum and Berger (1992) Neuroscience 48, 561-578] the contrasting paired impulse responses to stimulation of the corticostriatal pathway which define the Type I and Type II subpopulations of striatal neurons were shown to reflect differential regulation by GABAergic and dopaminergic inputs. More specifically, the decreased probability of spike discharge (inhibition) to long interstimulus intervals (60-260 ms) characteristic of Type I neurons was found to be dependent on dopaminergic input via D1 receptor activation, whereas the inhibition to short interstimulus intervals (10-20 ms) distinctive of Type II neurons was found to be mediated by GABAergic input acting through GABAA receptor stimulation. The present experiments have further investigated the contribution of GABAergic and dopaminergic feedforward and/or feedback circuits to the functional identities of Type I and Type II neurons using an in vitro corticostriatal slice preparation. In this preparation, the cortical afferents to the striatum are preserved, allowing for activation of striatal cells in a manner similar to that used in vivo; however, all axons arising from midbrain and brainstem structures including the substantia nigra are transected, and intrastriatal GABAergic pathways are reduced. Consistent with the predicted effect of disrupting these two neurotransmitter pathways, the paired impulse responses of striatal neurons recorded in vitro were not similar to the responses of either Type I or Type II neurons recorded in vivo. Indeed, the paired impulse profiles of striatal neurons recorded in vitro were relatively homogeneous in that virtually all cells displayed an increased probability of spike discharge (facilitation) to the second impulse of all interstimulus intervals (10-500ms) tested. Low concentrations of allosteric agonists for the GABAA receptor, pregnanolone (5 microM) and pentobarbital (50 microM), selectively inhibited spike discharge in response to short interstimulus intervals (10-20 ms) for approximately 40% of the neurons sampled, but produced no change in facilitation to longer interstimulus intervals (30-500 ms). The agonist-induced inhibition to short interstimulus intervals was blocked by bicuculline (10-20 microM), and was not mimicked by the GABAB receptor agonist, baclofen (1-5 microM). In addition, application of dopamine (5-10 microM) or the D1 receptor agonist, SKF38393 (2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine; 5 microM), inhibited spike discharge to longer interstimulus intervals (40-500 ms) for approximately 10% of striatal cells recorded. The inhibition to longer interstimulus intervals was blocked by the D1 receptor antagonist, SCH23390 [R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin+ ++-7-ol], but not the D2 antagonist, sulpiride.(ABSTRACT TRUNCATED AT 400 WORDS)

Isolated NMDA receptor-mediated synaptic responses express both LTP and LTD

1. The possibility of use-dependent, long-lasting modifications of pharmacologically isolated N-methyl-D-aspartate (NMDA) receptor-mediated synaptic transmission was examined by intracellular recordings from granule cells of the hippocampal dentate gyrus in vitro. In the presence of the non-NMDA receptor antagonist 6-cyano-7-nitroquinaxaline-2,3-dione (CNQX, 10 microM) robust, long-term potentiation (LTP) of NMDA receptor-mediated synaptic potentials was induced by brief, high (50 Hz) and lower (10 Hz) frequency tetanic stimuli of glutamatergic afferents (60 +/- 6%, n = 8, P less than 0.001 and 43 +/- 12%, n = 3, P less than 0.05, respectively). 2. Hyperpolarization of granule cell membrane potential to -100 mV during 50-Hz tetanic stimuli reversibly blocked the induction of LTP (-6 +/- 2%, n = 6, P greater than 0.05) indicating that simultaneous activation of pre- and postsynaptic elements is a prerequisite for potentiation of NMDA receptor-mediated synaptic transmission. In contrast, hyperpolarization of the granule cell membrane potential to -100 mV during 10-Hz tetanic stimuli resulted in long-term depression (LTD) of NMDA receptor-mediated synaptic potentials (-34 +/- 8%, n = 8, P less than 0.01). 3. We also studied the role of [Ca2+]i in the induction of LTP and LTD of NMDA receptor-mediated synaptic responses. Before tetanization, [Ca2+]i was buffered by iontophoretic injections of bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA). BAPTA completely blocked the induction of LTP (3 +/- 5%, n = 13) and partially blocked LTD (-14.8 +/- 6%, n = 10).(ABSTRACT TRUNCATED AT 250 WORDS)

Functionally distinct subpopulations of striatal neurons are differentially regulated by GABAergic and dopaminergic inputs--I. In vivo analysis

Two subpopulations of striatal neurons, Type I and Type II, are distinguished by their contrasting electrophysiological responses to paired impulse stimulation of cortical afferents. Although both Type I and Type II striatal neurons are excited by the first impulse of any pair of impulses, in response to short interstimulus intervals (10-30 ms) Type I neurons display an increase in probability of spike discharge to the second impulse (facilitation), whereas Type II neurons exhibit a decrease in probability of discharge (inhibition); in response to longer interstimulus intervals (50-250 ms) Type I cells display inhibition, whereas Type II cells show facilitation. The present experiments investigated the possibility that the unique paired impulse responses of Type I and Type II neurons reflect differential regulation by GABAergic and dopaminergic afferents. Extracellular recording techniques were combined with micropressure ejection of specific antagonists for GABAA (bicuculline), GABAB (phaclofen), D1 (R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-IH-3-benzazepin+ ++-7-ol; SCH23390) or D2 (sulpiride) receptors; the role of dopamine was also examined using the specific neurotoxin, 6-hydroxydopamine. Results showed that bicuculline (250-500 microM) reduced stimulation threshold for spike discharge of both Type I and Type II neurons and completely antagonized the paired impulse inhibition in response to short interstimulus intervals characteristic of Type II neurons. In contrast, phaclofen (2-30 mM) had only a variable influence on spike threshold for Type II cells and no effect on the paired impulse responses of either Type I or Type II neurons. Micropressure ejection of SCH23390 (1 mM) decreased spike thresholds for both cell types and attenuated the inhibition of spike discharge to long interstimulus intervals distinctive of Type I neurons, an effect which was mimicked by dopaminergic denervation. In contrast, sulpiride (1 mM) had little effect on spike thresholds, and no influence on the paired impulse responses of either cell type. These results indicate that the excitability of both Type I and Type II neurons is tonically inhibited by GABAergic and dopaminergic input via stimulation of GABAA and D1 receptors, respectively. Moreover, the bicuculline sensitivity of Type II neurons suggests that GABAergic input to this cell class arises from neurons within a cortically driven feedforward and/or feedback loop, whereas Type I cells receive input from neurons which lie outside of such a loop. In addition, the inhibition to longer interstimulus intervals characteristic of Type I cells is, at least in part, dependent on dopaminergic input through D1 receptor stimulation.

Presynaptic modulation by GABAB receptors of glutamatergic excitation and GABAergic inhibition of neostriatal neurons

1. The present experiments investigated the effects of gamma-amino-butyric acidB (GABAB) receptor stimulation on the excitatory and inhibitory responses of neostriatal neurons evoked by stimulation of the subcortical white matter in a rat neostriatal slice preparation. 2. Intracellular recordings showed that single-impulse stimulation of the corpus callosum evoked monosynaptic, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)-sensitive excitatory postsynaptic potentials (EPSPs) that were attenuated by the GABAB receptor agonist, p-chlorophenyl-GABA (baclofen, 0.5-10 microM) in a concentration-dependent manner. Baclofen also blocked the GABAA-mediated inhibition of neostriatal cell responses, which were revealed by paired-impulse stimulation of the subcortical white matter. Both of these effects persisted in slices in which the anterior cortex was removed, indicating that the site of action for baclofen was intrinsic to the neostriatum. The GABAB antagonist 3-amino-2-hydroxy-2-(4-chlorophenyl)-propanesulfonic acid (saclofen, 250-500 microM) reversed the depressant actions of baclofen on both the excitatory and inhibitory responses of neostriatal cells. 3. Concentrations of baclofen as high as 100 microM, which markedly attenuated EPSP amplitude, did not exert direct effects on resting membrane potential, current-voltage relationship, input resistance, or spike threshold and thus appeared to have no postsynaptic effect on the population of neurons recorded. 4. These results indicate that, in contrast to other regions of the CNS, the depressant effects of baclofen on glutamate-dependent EPSPs are mediated exclusively through GABAB receptors located presynaptically on the terminals of glutamatergic afferents.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo modulation of N-methyl-D-aspartate receptor-dependent long-term potentiation by the glycine modulatory site

The role of the glycine modulatory site in N-methyl-D-aspartate receptor function was examined by determining the effect of the glycine site antagonist, 7-chlorokynurenic acid, on the induction of long-term potentiation at the commissural-CA1 synapse in anesthetized rats. Robust long-term potentiation of population excitatory postsynaptic potentials and population spike responses recorded extracellularly in the stratum pyramidale and in stratum radiatum of CA1 developed after high frequency stimulation (100 Hz for 1 s) of commissural fibers during continuous intrahippocampal administration of vehicle solution (0.15 M NaCl). In contrast, infusion of either 7-chlorokynurenic acid (400 microM) or of the N-methyl-D-aspartate receptor antagonist, D-2-amino-5-phosphonovaleric acid (100 microM), significantly attenuated or completely blocked the development of long-term potentiation. When 7-chlorokynurenic acid was infused together with the glycine analog, D-serine (1 mM), long-term potentiation developed that was comparable to that observed in control animals. Intrahippocampal administration of D-serine alone was associated with slightly greater magnitude of long-term potentiation than observed in control animals. Collectively, these findings establish that in intact hippocampus, activity at the glycine modulatory site is necessary for activation of the N-methyl-D-aspartate receptor complex. Furthermore, these results suggest that the glycine modulatory site may not be fully saturated in vivo, and thus can serve to regulate N-methyl-D-aspartate receptor function.

Instantaneous characterization of time-varying nonlinear systems

A nonlinear system may be characterized by an orthogonal functional power series (FPS) computed from cross correlations between input and output variables. "Is the response changing over the course of the experiment?" is a fundamental question encountered in the analysis of both FPS and evoked potentials (EP's). Regression on closed-form functions of time produces a time-varying FPS or EP. Evaluation of these functions at a specified time point produces a system characterization for that instant.

Kindling-induced potentiation of excitatory and inhibitory inputs to hippocampal dentate granule cells. I. Effects on linear and non-linear response characteristics

Epileptiform activity is known to alter both excitatory and inhibitory circuits within the network of neurons that comprise the hippocampal formation. In the present experiment, kindling-induced alterations in the functional properties of the rabbit perforant path-dentate circuit were analyzed using non-linear system analytic procedures. System input consisted of a random train of impulses applied to the perforant path. System output was the perforant path-granule cell population spike amplitude evoked by each impulse in the train. The results of non-linear systems analysis were compared with the results from twin impulse analysis of kindling-induced alterations within the hippocampal dentate gyrus. Compared to twin impulse procedures, non-linear systems analytic procedures revealed a reduced duration and magnitude of kindling-induced inhibitory interactions to interstimulus intervals of 10-200 ms. The increased magnitude of inhibitory interactions did not decay to prekindled magnitude until 16 weeks postkindling. In contrast, kindling-induced potentiation of the population spike had decayed within 10 weeks of the last stage 5 seizure. Despite the decay of electrophysiological responses to prekindled levels, only a few kindling stimulations were required to evoke fully kindled seizures. Thus, electrophysiological alterations within the first synaptic relay of the hippocampal trisynaptic circuit, the dentate gyrus, cannot explain the long duration of the kindling effect.

Feedforward excitation of the hippocampus by afferents from the entorhinal cortex: redefinition of the role of the trisynaptic pathway

For the past 3 decades, functional characterizations of the hippocampus have emphasized its intrinsic trisynaptic circuitry, which consists of successive excitatory projections from the entorhinal cortex to the dentate gyrus, from granule cells of the dentate to the CA3/4 pyramidal cell region, and from CA3/4 to the CA1/2 pyramidal cell region. Despite unequivocal anatomical evidence for a monosynaptic projection from entorhinal to CA3 and CA1/2, few in vivo electrophysiological studies of the direct pathway have been reported. In the experiments presented here, we stimulated axons of entorhinal cortical neurons in vivo and recorded evoked single unit and population spike responses in the dentate, CA3, and CA1 of hippocampus, to determine if pyramidal cells are driven primarily via the monosynaptic or trisynaptic pathways. Our results show that neurons within the three subfields of the hippocampus discharge simultaneously in response to input from a given subpopulation of entorhinal cortical neurons and that the initial monosynaptic excitation of pyramidal cells then is followed by weaker excitatory volleys transmitted through the trisynaptic pathway. In addition, we found that responses of CA3 pyramidal cells often precede those of dentate granule cells and that excitation of CA3 and CA1 pyramidal cells can occur in the absence of granule cell excitation. In total, these results argue for a different conceptualization of the functional organization of the hippocampus with respect to the propagation of activity through its intrinsic pathways: input from the entorhinal cortex initiates a two-phase feedforward excitation of pyramidal cells, with the dentate gyrus providing feedforward excitation of CA3, and with both the dentate and CA3 providing feedforward excitation of CA1.

Compensations after lesions of central dopaminergic neurons: some clinical and basic implications

Parkinson's disease is associated with degeneration of the dopaminergic component of the nigrostriatal pathway. However, the neurological symptoms of this disorder do not emerge until the degenerative process is almost complete. A comparable phenomenon can be observed in animal models of Parkinson's disease produced by the administration of the selective neurotoxin, 6-hydroxydopamine (6-OHDA). Studies using such models suggest that the extensive loss of dopaminergic neurons is compensated, in large part, by increased synthesis and release of dopamine (DA) from those DA neurons that remain, together with a reduced rate of DA inactivation. These findings may have important implications for the diagnosis and treatment of a variety of neurological and psychiatric diseases, as well as for our understanding of plasticity in monoaminergic systems.

Lesions of the dopaminergic nigrostriatal system in neonatal rats: effects on the electrophysiological activity of striatal neurons recorded during adulthood

The spontaneous activity of single striatal neurons was recorded extracellularly from 3-4-month-old adult rats that had been given dopamine (DA)-depleting brain lesions 3 days after birth. Behavioral observations made prior to recording indicated no gross sensorimotor deficits, yet subsequent biochemical analyses revealed that animals had sustained near-total DA depletions (greater than 99%). Electrophysiological results showed that the firing rates of type II striatal cells were greatly increased relative to control levels. This finding contrasts sharply with the effects of DA-depleting brain lesions given to adult animals, in which similarly high levels of striatal cell activity are invariably associated with akinesia.

Compensatory responses to nigrostriatal bundle injury. Studies with 6-hydroxydopamine in an animal model of parkinsonism

Intracerebral injections of the neurotoxin 6-hydroxydopamine (6-HDA) can produce selective, near-total destruction of the dopamine (DA)-containing neurons of the nigrostriatal bundle. The dysfunctions in animals with these lesions show many parallels with those present in Parkinsonian patients. Among these are the extensive loss of DA neurons in the basal ganglia, neurological impairments including akinesia, paradoxical kinesia in response to activating conditions, and improved sensory-motor function after the administration of DOPA. Moreover, as with patients with preclinical Parkinsonism, 6-HDA-treated rats with less extensive lesions show few or no behavioral dysfunctions, but are unusually sensitive to the akinesia-inducing effects of stress and dopaminergic antagonists. In this review, we summarize the behavioral effects of 6-HDA-induced depletion of striatal DA in the rat and then focus on the compensatory changes that may underlie the preclinical stage of the disorder. These compensations appear to include an increase in the number of active DA neurons, an increase in the release of DA per impulse from residual terminals, and a decrease in the amount of DA inactivated by high affinity uptake. Collectively, these alterations permit a few residual DA neurons to maintain a normal level of control over cellular activity in the striatum.

Kindling facilitates acquisition of discriminative responding but disrupts reversal learning of the rabbit nictitating membrane response

The effect of kindling the hippocampal perforant path-dentate projection on subsequent discrimination-reversal conditioning of the rabbit nictitating membrane (NM) response was examined. Kindling facilitated acquisition of the initial discriminative response but severely impaired performance during reversal training. The facilitative effect on initial acquisition is highly similar to previously reported effects of long-term potentiation on NM discrimination learning, and thus may reflect a kindling-induced increase in perforant path-dentate synaptic strength. The learning deficit during reversal training is similar to the effects of hippocampal ablation; i.e. characterized by a continued high response rate to the CS- rather than an inability to respond to the CS +. These findings demonstrate that kindling-induced seizures can have profound effects on associative learning. The effects are different for the discrimination and reversal phases of the task, however, which may reflect the multi-dimensional effects of kindling at the cellular level.

Spontaneous activity of type II but not type I striatal neurons is correlated with recovery of behavioral function after dopamine-depleting brain lesions

The relation between the spontaneous firing of Type I striatal neurons and recovery of behavioral function after near-total dopamine depletions of the rat striatum was investigated. The results demonstrate that the activity of Type I neurons remains elevated in recovered animals, which contrasts with our previous finding that the firing rates of Type II striatal neurons return to normal levels in association with behavioral recovery.

Evidence for two functionally distinct subpopulations of neurons within the rat striatum

Type I and Type II extracellular action potential waveforms were recorded from the rat striatum and studied with respect to their dependence on recording conditions, response to paired impulse stimulation of the corticostriatal pathway, and iontophoretic application of dopamine (DA). Results showed that the distinguishing characteristics of Type I and Type II waveforms are relatively independent of the degree of filtering, distance of the electrode tip from the target neuron, type of recording electrode, and firing rate of the neuron. Very low impedance electrodes, however, were found to mask the difference in spike shape. Electrical stimulation of cortical afferents results in excitation of both action potential waveforms, though the Type II class exhibits a significantly shorter latency than the Type I class. Paired impulse analyses revealed that both waveforms exhibit variation in the probability of discharge (facilitation or inhibition) to the second impulse of each impulse pair that are a function of the interimpulse interval. Most importantly, however, the probabilities of discharge of Type I and Type II neurons to the second impulse are inversely related, i.e., when one cell type exhibits facilitation, the other displays inhibition. These data demonstrate that Type I and Type II waveforms represent the activity of functionally different subpopulations of striatal neurons. Moreover, Type II neurons are found much more often than Type I cells, suggesting that the 2 cell classes may be represented with different frequencies within striatum. Finally, Type II neurons display at least a 5 times greater sensitivity to iontophoretically applied DA than Type I cells, suggesting that the 2 cell populations also are affected differentially by dopaminergic input from the substantia nigra.

Nonlinear systems analysis of the hippocampal perforant path-dentate projection. III. Comparison of random train and paired impulse stimulation

1. The transformational properties of the network of hippocampal neurons activated monosynaptically and polysynaptically by electrical stimulation of the perforant path were analyzed using random impulse train and paired impulse stimuli. In response to both types of input, the amplitudes of granule cell population spikes evoked in the dentate gyrus were used as the measure of network output. The random stimulus train consisted of a series of 4,064 electrical impulses, with interimpulse intervals determined by a Poisson distribution; the mean interimpulse interval of the train was 500 ms, and the range was 1-5,000 ms. Paired impulse stimuli consisted of pairs of impulses separated by 10-1,200 ms; impulses pairs were delivered once every 20 s. The procedures were applied to both anesthetized and chronically implanted, unanesthetized preparations. 2. Nonlinear systems analysis of population spike responses evoked during random train stimulation revealed that dentate granule cell output to any impulse was highly dependent on the interval since a prior impulse. Data from anesthetized animals showed that population spike amplitudes were markedly suppressed in response to intervals less than 50 ms, facilitated in response to intervals of approximately 100 ms, suppressed slightly in response to intervals of 300-700 ms, and unaffected by intervals greater than 700 ms. Data from unanesthetized animals showed similar results except that facilitation rather than suppression of spike amplitude was observed in response to intervals of 300-700 ms, and could extend to intervals as great as 1,000 ms. 3. The results of paired impulse stimulation applied to the same preparations also showed that granule cell response was highly dependent on interimpulse interval. However, nonlinearities observed with paired impulse stimulation differed from those revealed by a random impulse signal. Compared to results of random train stimulation, a paired impulse format produced greater magnitude spike suppression in response to short interimpulse intervals (e.g., 10-20 ms), maximum facilitation in response to shorter interstimulus intervals (50 ms rather than 100 ms), greater magnitude spike facilitation, and greater suppression in response to intervals greater than or equal to 300 ms. Furthermore, there were virtually no differences in the nonlinearities of granule cell response recorded from anesthetized and unanesthetized animals when a paired impulse format was used, whereas several differences were observed with random train stimuli. 4.(ABSTRACT TRUNCATED AT 400 WORDS)

Nonlinear systems analysis of the hippocampal perforant path-dentate projection. I. Theoretical and interpretational considerations

1. Nonlinear systems analytic procedures, based on an orthogonalized functional power series approach, were developed for study of the transformational properties of the hippocampal formation. As a testing stimulus, the procedures utilize a train of electrical impulses with randomly varying interimpulse intervals. The specific case was considered of applying such a stimulus to the perforant path, a major afferent to the hippocampal dentate gyrus that arises from the entorhinal cortex. Resulting field potentials evoked within the dentate gyrus are recorded to all impulses in the train. Computational algorithms based on cross-correlations determine the relationship between the interimpulse interval within the random train and amplitude of the evoked dentate potentials. The calculations, which reduce to averaging procedures, were derived for first- and second-order terms, or kernels, of the orthogonalized functional power series. 2. It is proposed that such an approach can be applied to a single component of the complex field potential evoked in the dentate gyrus. This component, the population spike, reflects the action potential discharge of dentate granule cells. Thus, a field potential component for which the underlying neuronal generator is well-known can be analyzed with respect to the transformational characteristics of the network of neurons that influence that generator. Other components of the complex field potential produced by other generators can be ignored. It is shown that this adaptation has the effect of greatly simplifying both the computation and presentation of kernels. 3. As a further consequence of this adaptation, the resulting first- and second-order kernels were shown to have specific interpretations. The first-order kernel represents the average response of the orthodromically driven granule cells to the set of stimuli comprising the random impulse train. The second-order kernel quantitatively characterizes the nonlinearity of the granule cell response, and may be interpreted as a generalized recovery function; i.e., the first input of any pair of stimuli in the train activates the newtork, and the second input tests the modulatory influence of the network excited by the initial input. 4. Most past investigations of nonlinearities of the perforant path-dentate projection have utilized pairs of stimulus impulses. We show here that, for a second-order system, the expected results from paired impulse experiments may be predicted from second-order kernels. Disagreement between the measured and predicted results reflects interactions of a higher order, and thus, greater system complexity.(ABSTRACT TRUNCATED AT 400 WORDS)

Nonlinear systems analysis of the hippocampal perforant path-dentate projection. II. Effects of random impulse train stimulation

1. Nonlinear systems analytic techniques were used to characterize transformational properties of the network of neurons activated by perforant path input to the rabbit hippocampus. Trains of 4,064 impulses with randomly varying interimpulse intervals were used to stimulate perforant path fibers, and amplitudes of evoked dentate granule cell population spikes were measured. Interimpulse intervals of the random stimulus train were determined by a Poisson distribution with a mean interimpulse interval of 500 ms, and with intervals ranging from 1 to 5,000 ms. The response of dentate granule cells to this stimulation was assumed to reflect activity in the larger hippocampal network, because other subpopulations of neurons activated monosynaptically and polysynaptically within the hippocampal formation contribute to granule cell excitability through multiple feedforward and feedback pathways. System properties were characterized both for halothane anesthetized and chronically implanted, unanesthetized preparations. 2. Second-order kernel analysis showed that population spike amplitude was highly dependent on interimpulse interval. When population spikes of all latencies were included in the same analysis, stimulation impulses produced near-total suppression of spike amplitude when they were preceded 10-20 ms by another impulse in the train. Spike suppression extended to approximately 50 ms and was inversely related to length of the interimpulse interval. Suppression of granule cell response to intervals within the range of 10-50 ms was not influenced by halothane anesthesia. 3. Interstimulus intervals greater than approximately 50 ms resulted in a facilitation of population spike amplitude, with maximum facilitation occurring in response to intervals of 90-100 ms. The magnitude of maximum facilitation was significantly greater for anesthetized (129%) than for unanesthetized (74%) preparations. The range of intervals resulting in facilitation for unanesthetized animals could extend to 1,000-1,100 ms (average range, 61-714 ms). This was much greater than observed for population spikes recorded from anesthetized animals (50-364 ms), which exhibited suppression in response to intervals of approximately 300-700 ms. 4. Further analysis revealed that the nature of nonlinearities in population spike amplitude may depend on spike latency. For example, population spikes of "short" latency (3-4 or 4-5 ms, depending on the animal) exhibited only facilitation in response to interstimulus intervals of 1-4 ms.(ABSTRACT TRUNCATED AT 400 WORDS)