Fitting experimental data to models that use morphological data from public databases.

Ideally detailed neuron models should make use of morphological and electrophysiological data from the same cell. However, this rarely happens. Typically a modeler will choose a cell morphology from a public database, assign standard values for Ra, Cm, and other parameters and then do the modeling study. The assumption is that the model will produce results representative of what might be obtained experimentally. To test this assumption we developed models of CA1 hippocampal pyramidal neurons using 4 different morphologies obtained from 3 public databases. The multiple run fitter in NEURON was used to fit parameter values in each of the 4 morphological models to match experimental data recorded from 19 CA1 pyramidal cells. Fits with fixed standard parameter values produced results that were generally not representative of our experimental data. However, when parameter values were allowed to vary, excellent fits were obtained in almost all cases, but the fitted parameter values were very different among the 4 reconstructions and did not match standard values. The differences in fitted values can be explained by very different diameters, total lengths, membrane areas and volumes among the reconstructed cells, reflecting either cell heterogeneity or issues with the reconstruction data. The fitted values compensated for these differences to make the database cells and experimental cells more similar electrotonically. We conclude that models using fully reconstructed morphologies need to be calibrated with experimental data (even when morphological and electrophysiological data come from the same cell), model results should be generated with multiple reconstructions, morphological and experimental cells should come from the same strain of animal at the same age, and blind use of standard parameter values in models that use reconstruction data may not produce representative experimental results.

Effects of aniracetam after LTP induction are suggestive of interactions on the kinetics of the AMPA receptor channel.

The modulatory influence of aniracetam, a drug which reversibly modifies the kinetic properties of AMPA-type glutamate receptors, on synaptic responses is reported to be detectably changed by the induction of long-term potentiation (LTP). The present study used hippocampal slices to examine three issues arising from this result. First, possible contributions of inhibitory currents and postsynaptic spiking to the aniracetam/LTP interaction were investigated with infusions of GABA receptor antagonists and topical applications of tetrodotoxin. Second, tests were carried out to determine if the altered response to aniracetam is sufficiently persistent to be a plausible substrate for the extremely stable LTP effect. Third, the nature of the change responsible for the aniracetam/LTP interaction was explored with waveform analyses and a kinetic model of the AMPA receptor. The following results were obtained. LTP reduced the effect of aniracetam on the amplitude but increased its effect on the decay time constant of field EPSPs recorded under conditions in which local spiking and inhibitory responses were blocked. The LTP-induced change in the effect of aniracetam was extremely stable in that it was still evident 75 min after induction of potentiation. Finally, the waveform distortions introduced by LTP and aniracetam could be corrected by uniform stretching of the responses, suggesting that the changes introduced by each of the manipulations are unitary in nature. These distortions and the interactions between them could be reproduced in the AMPA receptor model by representing LTP as an acceleration of channel gating kinetics.

Triazolam impairs delayed recall but not acquisition of various everyday memory tasks in humans.

A double-blind test battery was administered to 24 human subjects (8 control, 16 drug) to assess the effects of 0.125 mg triazolam (oral) on memory encoding and retention across delay intervals ranging from seconds to 1 week after presentation. Although the drug reduced immediate psychomotor performance, it did not impair recall of previously learned information, nor did it significantly impair encoding of new information. The drug enhanced immediate recall of the location and identity of playing cards, without affecting 4-h delayed recall. The drug treatment impaired correct recall of object names after a delay of 20 min. At 4 h delay, the drug impaired olfactory recognition and free-recall of object names. At both 1 day and 1 week delay, the drug impaired recall of biographical information and correct identification of picture-photographer pair associations. The drug also impaired the daily improvement of the drug group as compared with the control group in a geometric puzzle solving task. The time course of these memory impairments compares well with the known effects of triazolam on long-term potentiation (LTP), a candidate biological mechanism underlying telencephalic memory formation and expression.

Stoichiometries of AMPA receptor subunit mRNAs in rat brain fall into discrete categories.

In situ hybridization was used to estimate the relative concentrations of mRNAs encoding different subunits (GluR1-4) of alpha-amino 3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-type glutamate receptors in rat brain and to test the hypothesis that within-region expression profiles reflect a limited number of recurring patterns. Fractional subunit mRNA concentrations were calculated for 33 brain regions, and cluster analysis methods were applied to test for statistically meaningful groupings in the data. Four relatively homogeneous classes were identified and designated as AMPA receptor (AR) categories, numbered according to dominant subunit mRNAs. The AR-1 class (47% GluR1 mRNA) was expressed by structures near the mesodiencephalic border, including basal ganglia-related areas. The AR-2 class (57% GluR2 mRNA) was expressed in cortex and tectum. The AR-1,2 class (31% GluR1, 45% GluR2) was found in the largest number of regions, including such dissimilar cell fields as hippocampus and substantia nigra pars compacta. The AR-2,3 grouping (33% GluR2, 31% GluR3) was associated with the sensory relay and reticular thalamic nuclei. It is suggested that AR-1,2 and AR-2, the most closely related categories in clustering space, are largely telencephalic receptors with the former predominant in the subcortex and the latter in the cortex. The AR-2,3 class is associated with ascending sensory stations, whereas AR-1 appears to include several smaller categories expressed by specialized systems. If the balance of subunit mRNAs is reflected at the protein level, then the present data suggest that forebrain AMPA-type glutamate receptors can be classified into a limited number of recurring types.

Enhancement by an ampakine of memory encoding in humans.

Acentrally active drug that enhances AMPA receptor-mediated currents was tested for its effects on memory in humans. Evidence for a positive influence on encoding was obtained in four tests: (i) visual associations, (ii) recognition of odors, (iii) acquisition of a visuospatial maze, and (iv) location and identity of playing cards. The drug did not improve scores in a task requiring cued recall of verbal information. The selectivity of drug effects on memory was confirmed using tests of visual recognition, motor performance, and general intellectual functioning. These results suggest that positive modulators of AMPA receptors selectively improve at least some aspects of memory.

Evidence that a positive modulator of AMPA-type glutamate receptors improves delayed recall in aged humans.

Elderly subjects (65-76 years) were tested for recall of nonsense syllables prior to and after oral administration of 1-(quinoxalin-6 ylcarbonyl)piperidine (CX516), a centrally active drug that enhances currents mediated by AMPA-type glutamate receptors. A significant and positive drug effect was found for delayed (5 min) recall at 75 min posttreatment; average scores for the highest dose group were more than twofold greater than for the placebo group. The drug had no evident influence on heart rate or self-assessment of several psychological variables.

Short-term reverberant memory model of hippocampal field CA3.

Synaptic plasticity mechanisms for associative learning require near-simultaneous pairs of inputs to target cells. Sensory cues encountered behaviorally, however, are typically staggered in time, implying the need for active short-term memory traces of antecedent cues. The dense recurrent connectivity within regions of hippocampal field CA3 is suggestive of the kind of re-entrant network that could subserve this kind of "holding" memory. Consequently, we have investigated whether an abstract model of this region incorporating its major anatomical and physiological features could function as a reverberatory memory network. The continuous-time model describes the behavior of highly connected groups of CA3 pyramidal cells, or "patches," in response to brief, rhythmic, sensory stimulation. Time constants for excitatory and inhibitory postsynaptic potentials and axonal transmission delays for local and distal connections were estimated from empirical data. When the inhibitory units in these patches were connected to an oscillator intended to model the theta wave activity of the medial septum, the network entered reverberatory states and maintained second-long memory traces of the cortical input, after which it lost its coherent behavior. Noise analysis indicated that the network's operation was moderately resistant to random fluctuations proportional to patch activity. These results suggest that field CA3 could function as a holding memory that assists the integration of disjoint stimuli found in innumerable associative tasks, and that the duration of its coherent operation might determine the temporal limits in their performance.

Effects of a centrally active benzoylpyrrolidine drug on AMPA receptor kinetics.

A newly developed benzoylpyrrolidine drug (BDP-20) that increases the size of fast, excitatory synaptic responses was examined for its effects on the kinetic properties of alpha-amino-3-hydroxy-5-methyl-4-isoxalepropionic acid (AMPA)-type glutamate receptors. When long pulses of glutamate were applied to excised hippocampal patches of the rat, the compound BDP-20 caused an approximately 15-fold reduction in the rate at which responses desensitized and a similar size increase in steady-state currents. In experiments using 1-ms glutamate pulses, BDP-20 prolonged response deactivation by a factor of about four and greatly reduced the depression in the second response when two consecutive glutamate pulses were given. Two types of equilibrium binding assays indicated that BDP-20 causes a measurable increase in the affinity of AMPA receptors; the EC50 values for this effect were similar to those obtained in excised patch studies. The actions of BDP-20 on physiology and ligand binding could be adequately reproduced in a receptor model by slowing the rate of desensitization and increasing the affinity of the sensitized states. The biochemical and physiological effects of this benzoylpyrrolidine compound were qualitatively different from those obtained with cyclothiazide, although both types of drug increased AMPA receptor-mediated synaptic responses. Moreover, interactions between the drugs were at most only partially competitive; AMPA receptors may thus have multiple modulatory sites with distinct drug preferences and different effects on receptor kinetics.

Early and late components of AMPA-receptor mediated field potentials in hippocampal slices.

Field EPSPs were recorded from the CA1 region of hippocampal slices under conditions in which components of the responses other than those generated by AMPA-type glutamate receptors were blocked. Laminar profile analysis indicated that the resultant potentials had separable phases: an early and fast stage followed by a late and slow stage. The location of the fast response was sensitive to stimulation position in the stratum radiatum; i.e., distal stimulation elicited maximum, negative going potentials in the distal stratum radiatum while proximal stimulation recordings were maximal in the proximal segment. The distribution of the late component was largely independent of stimulation electrode position in stratum radiatum. Current source density analysis revealed that the late response had a source in the most distal dendrites (stratum moleculare) and a sink near the cell body layer. It was not accompanied by evident changes in membrane conductance and had a decay time constant similar to the membrane time constant. Stimulation of the afferents to the basal dendrites of the pyramidal cells also resulted in a late response which again had a source in stratum moleculare. These results strongly suggest that the magnitude and waveform of the late component of the AMPA receptor mediated field potential reflects the biophysical properties of the most distal branches of the dendritic arborization. The laminar analyses also show that the late potential is minimal in the mid-stratum radiatum and thus suggest that this site is most appropriate for investigations concerned with the waveform of the fast component of AMPA receptor mediated synaptic response.

Psychological effects of a drug that facilitates brain AMPA receptors.

The effects of 1-(quinoxalin-6-ylcarbonyl)piperidine (CX516), a centrally active compound that facilitates AMPA receptor-mediated synaptic responses, were tested in human subjects. Separate tests of delayed recall were given prior to and nearly 3 h after administration of placebo (n = 12) or drug (n = 36). Control subjects exhibited poorer performance in the second session than in the first while subjects given 600-1200 mg of the drug did not. There were no pre- vs post-treatment differences in immediate recall in either group. The drug did not reliably affect self-assessment scores for any of several psychological variables but did disrupt the normally present correlations for within-subject changes in the variables. These results suggest that AMPA receptor modulators may (1) improve memory under some circumstances and (2) produce psychological effects that are subtle or not related to specific mood states.

A centrally active drug that modulates AMPA receptor gated currents.

Systemic administration of the drug 1-(1,3-benzodioxol-5-ylcarbonyl)-piperidine (1-BCP) has been reported to enhance monosynaptic responses in the hippocampus in vivo and to improve spatial and olfactory memory in rats. The drug's mechanism of action was investigated in the present study using membrane patches excised from cultured hippocampal slices. The decay time of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor mediated inward currents was greatly increased by 1-BCP in a concentration dependent and reversible fashion; peak current was also enhanced but to a lesser degree. In vitro slice experiments indicated that the drug has parallel effects on the field EPSP. It is concluded that 1-BCP is a centrally active modulator of the AMPA receptor.

Cyclothiazide decreases [3H]AMPA binding to rat brain membranes: evidence that AMPA receptor desensitization increases agonist affinity.

The effects of cyclothiazide, a drug which blocks AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor desensitization, were tested on binding of [3H]AMPA to rat brain membranes. Cyclothiazide reduced [3H]AMPA binding by lowering the apparent affinity of the AMPA receptor. The magnitude of the decrease was temperature dependent and greater for membrane-bound than for solubilized receptors. These data provide evidence that desensitization increases the affinity of the AMPA receptor for agonists and indicate that a significant percentage of AMPA receptors in conventional equilibrium binding assays are in a desensitized state.

Waveform analysis suggests that LTP alters the kinetics of synaptic receptor channels.

The waveform of an isolated excitatory monosynaptic response reflects the kinetics of transmitter release, the kinetics of synaptic receptor channels and the filtering properties of neurons. Results reported here indicate that long-term potentiation (LTP) causes correlated decreases in the rise time and decay time constant of synaptic potentials recorded in hippocampal slices in which inhibitory currents and post-synaptic spiking were suppressed. Statistical comparisons of waveforms revealed that the distortions introduced by LTP could be corrected by stretching the time-scale of potentiated responses according to the percent change in the decay time constant. The LTP associated decrease in the decay time constant also obtained in slices from immature hippocampus which contain spines and dendrites greatly simplified from those of the adult. Hence, filtering properties of spines are not likely involved in the effect. Paired-pulse facilitation (PPF), a transient increase in transmitter release, did not reproduce the waveform effects of LTP but did cause a slight leftward shift of the response. These results suggest that LTP modifies the kinetics of receptor channels, and that PPF accelerates release.

Channel gating kinetics and synaptic efficacy: a hypothesis for expression of long-term potentiation.

A kinetic model of the glutamate DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor/channel complex was used to test whether changes in the rate constants describing channel behavior could account for various features of long-term potentiation (LTP). Starting values for the kinetic parameters were set to satisfy experimental data (e.g., affinity, mean open time, mean burst length, etc.) and physical constraints (i.e., microreversibility). The resultant model exhibited a variety of dynamic properties known to be associated with the receptor. Increasing the rate constants governing opening/closing of the channel produced an unexpected increase in the probability of the channel being open shortly after transmitter binding. This would account for the enhanced response size with LTP. Increases in rate constants produced two other aspects of LTP: (i) an alteration of the waveform of the synaptic response and (ii) an interaction with changes in desensitization kinetics. The results obtained with the model corresponded closely to those found in LTP experiments. Thus, an increase in opening/closing rates for the postsynaptic receptor channel provides a single explanation for diverse characteristics of LTP. Finally, the kinetic manipulation reduced the coefficient of variation of synaptic currents in a model involving 250 receptors. This calls into question the use of variance measures for distinguishing pre- vs. postsynaptic sites of potentiation.

Receptor changes and LTP: an analysis using aniracetam, a drug that reversibly modifies glutamate (AMPA) receptors.

The hypothesis that long-term potentiation (LTP) involves receptor modifications was tested with aniracetam, a nootropic drug that selectively increases currents mediated by the AMPA subclass of glutamate receptors. Aniracetam had different effects on the waveform of synaptic potentials in hippocampus before and after induction of LTP: (1) the drug caused a slight reduction (or delay) of the initial segment of the response after LTP; and (2) the facilitatory effects of aniracetam occurred at a later time point in the response after LTP than before. The interactions between LTP and aniracetam were still present when synaptic responses were greatly reduced by partial blockade of postsynaptic receptors and were not reproduced by increasing release or the number of stimulated synapses. A mathematical treatment of synaptic currents produced the following results: (1) if aniracetam facilitates AMPA receptor currents simply by reducing desensitization, then its complex interaction with LTP emerges when potentiation changes the kinetic and conductance properties of receptor channels; (2) if aniracetam also significantly increases conductance, then the experimental data can be reproduced by modeling LTP as an increase in channel conductance alone.

Simulation of paleocortex performs hierarchical clustering.

Simulations were performed of layers I and II of olfactory paleocortex, as connected to its primary input structure, olfactory bulb. Induction of synaptic long-term potentiation by means of repetitive sampling of inputs caused the simulation to organize encodings of learned cues into a hierarchical memory that uncovered statistical relationships in the cue environment, corresponding to the performance of hierarchical clustering by the biological network. Simplification led to characterization of those parts of the network responsible for the mechanism, resulting in a novel, efficient algorithm for hierarchical clustering. The hypothesis is put forward that these corticobulbar networks and circuitry of similar design in other brain regions contain computational elements sufficient to construct perceptual hierarchies for use in recognizing environmental cues.

Derivation of Encoding Characteristics of Layer II Cerebral Cortex.

Computer simulations of layers I and II of pirifonn (olfactory) cortex indicate that this biological network can generate a series of distinct output responses to individual stimuli, such that different responses encode different levels of information about a stimulus. In particular, after learning a set of stimuli modeled after distinct groups of odors, the simulated network's initial response to a cue indicates only its group or category, whereas subsequent responses to the same stimulus successively subdivide the group into increasingly specific encoding of the individual cue. These sequences of responses amount to an automated organization of perceptual memories according to both their similarities and differences, facilitating transfer of learned information to novel stimuli without loss of specific information about exceptions. Human recognition performance robustly exhibits such multiple levels: a given object can be identified as a vehicle, as an automobile, or as a Mustang. The findings reported here suggest that a function as apparently complex as hierarchical recognition memory, which seems suggestive of higher 'cognitive' processes, may be a fundamental intrinsic property of the operation of this single cortical cell layer in response to naturally-occurring inputs to the structure. We offer the hypothesis that the network function of superficial cerebral conical layers may simultaneously acquire and hierarchically organize information about the similarities and differences among perceived stimuli. Experimental manipulation of the simulation has generated hypotheses of direct links between the values of specific biological features and particular attributes of behavior, generating testable physiological and behavioral predictions.