Faculty Bibliography

1. The integration of synaptic inputs to the apical dendrite of layer 5 neocortical pyramidal cells was studied using compartment model simulations. The goal was to characterize the generation of regenerative responses to synaptic inputs under two conditions: (a) where there was an absence of background synaptic input, and (b) when the entire cell surface was subjected to a uniform blanket of synaptic background conductance such that somatic input resistance was reduced 5-fold. 2. Dendritic morphology corresponded to a layer 5 thick-trunked pyramidal cell from rat primary visual cortex at postnatal day 28 (P28), with distribution of dendritic active currents guided by the electrophysiological characteristics of the apical trunk reported in this cell type. Response characteristics for two dendritic channel distributions were compared, one of which supported Ca(2+) spikes in the apical dendrite. 3. In the absence of background, synaptic input to the apical tuft was surprisingly effective in eliciting somatic firing when compared with input to apical oblique branches. This result obtained even when the tuft membrane was the least excitable in the dendritic tree. 4. The special efficacy of tuft input arose because its electrotonic characteristics favour development of a sustained depolarization which charged the apex of the apical trunk to its firing threshold; once initiated in the distal trunk, firing propagated inward to the soma. This mechanism did not depend upon the presence of depolarizing channels in tuft membrane, but did require an excitable apical trunk. 5. Rather than disconnect the tuft, background synaptic conductance enhanced the efficacy advantage enjoyed by input arriving there. This counterintuitive result arose because background reduced the subthreshold spread of voltage, and so diminished the ability of the excitation of various individual oblique branches to combine to charge the relatively thick adjacent trunk. In contrast, drive from the depolarized tuft is exerted at a single critical point, the apex of the distal trunk, and so was relatively undiminished by the background. Further, once initiation at the apex occurred, background had little effect on inward propagation along the trunk. 6. We conclude that synaptic input to the apical tuft of layer 5 cells may be unexpectedly effective in triggering cell firing in vivo. The advantage in efficacy was not dependent upon the characteristics of tuft membrane excitability, but rather stemmed from the geometry of the tuft and its junction with the distal apical trunk. The efficacy of tuft input was, however, critically dependent upon inward propagation, suggesting that modulation of membrane currents which affect propagation in the apical trunk might sensitively control the efficacy of tuft input

The goal of this paper is to explore the basic assumption that largescale, temporal coincidence of specific and nonspecific thalamic activity generates the functional states that characterize human cognition

The interaction between cortical input frequency and intrinsic thalamic neuron (TN) properties were investigated using intracellular recordings from mice TNs in thalamocortical (TC) slices. Excitatory postsynaptic potentials (EPSPs) of corticothalamic (CT) origin were recorded at TN membrane potentials (V-m) held, by current clamp means, between -59 and -55 mV to avoid low-threshold calcium currents (I-T) activation. EPSPs elicited in ventrobasal neurons (n = 25) by stimulation in the internal capsule showed constant latency, relatively fast rise time (2.9 +/- 0.56 ms) and short duration (26.6 +/- 9.11 ms). EPSPs evoked by threshold stimulation (n = 10) showed similar characteristics (mean rise time, 2.74 +/- 0.42 ms; mean duration, 30 +/- 8.00 ms). The time course of CT synaptic facilitation was determined using pairs of stimuli. Paired-pulse facilitation (PPF) of CT EPSPs peaked at 25-30 ms stimulus intervals and decayed exponentially with an average time constant of 130 ms (n = 50). Application of the NMDA receptor blocker APV (25 mu M, n = 4) did not modify PPF for any interstimulus interval studied but suppressed frequency facilitation evoked by trains of CT stimuli. We compared the number of spikes per stimulus (F-s) evoked in TNs by repetitive CT stimulation over a range of frequencies at different V-m. At hyperpolarized V-m (below -65 mV) and frequencies of stimulation >= 10 Hz, F-s decreased along the train while at depolarized V-m (above -59 mV) F-s increased along the train. Decremental patterns resulted from the activation of I-T while facilitatory patterns emerged from superposition of synaptic and intrinsic mechanisms. At hyperpolarized V-m steady-state F-s was maximal for frequencies <= 2 Hz, intermediate for frequencies between 2 and 10 Hz and zero at >= 10 Hz. At depolarized V-m, steady-state F-s increased with increasing frequencies (from 1 to 40 Hz).We conclude that the CT-TN junctions are tuned to establish stable thalamocortical resonant dynamics

The dynamic organization of the olivocerebellar afferent input to Purkinje cells was examined in rat cerebellar cortex. The distribution of synchronous Purkinje cell complex spike activity was characterized, bilaterally, utilizing multiple electrode recordings in crus IIa folium under ketamine anaesthesia. The results confirmed the existence of rostrocaudal complex spike isochronicity bands with a mediolateral width of 500 microm. For a given band, no finer spatial submicrostructures could be discerned at a first-order approximation (two-dimensional projection). Closer analysis determined that isochronicity between bands is not continuous in space but demonstrates discrete discontinuities at the mediolateral boundaries. Principal component multivariate analysis revealed that the first principal component of the spatio-temporal variance is synchronicity along the rostrocaudal band with a decreased level of coupling in the mediolateral direction at the band boundary. Furthermore, this discrete banding isochronicity is organized by the distribution of feedback inhibition from the cerebellar nuclei on to the inferior olive nucleus. The usual multiple band structure can be dynamically altered to a single wide-band dynamic architecture, or to other patterns of activity, as may be required by movement coordination

Complex spike activity was simultaneously recorded from 96 Purkinje cells in the rat cerebellar cortex. Rostrocaudal complex spike synchronicity bands were studied in crus I, IIa and IIb and in vermal lobule 6c. Detailed analysis in crus IIa revealed that complex spike activity was staggered sequentially with a 20--50 cm/sec 'propagation velocity' in the mediolateral direction, and that such activity was bilaterally synchronous. The 'propagation' of complex spike activity was symmetrical between right and left crus IIa. Temporally, the neurons that aligned in the rostrocaudal direction typically generated complex spikes close to simultaneously. The correlation of complex spike firing was high between crus IIa and crus IIb, moderate between crus IIa and vermis 6c, and relatively low between Purkinje cells in crus I and crus IIa. These results indicate that, whilst discrete boarders exist between different isochronicity bands, these bands do communicate with each other in the mediolateral direction via slow 'propagation waves' that loosely bind their activity. The results indicate that the olivocerebellar system is organized, bilaterally, to take advantage of the timing signals generated at the inferior olive nucleus

The spatial and temporal patterns of neocortex activation are determined not only by the dynamic character of the input but also by the intrinsic dynamics of the cortical circuitry. To study the role of afferent input frequency on cortical activation dynamics, the electrical activity of in vitro neocortex slices was imaged during white-matter electrical stimulation. High-speed optical imaging was implemented using voltage-sensitive dyes in guinea pig visual and somatosensory cortex slices concomitantly with intracellular recordings. Single white-matter electrical stimuli activated well-defined cortical sites with a radially oriented columnar configuration. This configuration was followed, over the next few milliseconds, by a lateral spread of excitation through cortical layers 5 and 6 and layers 2 and 3. Much of the optical response was eliminated in low extracellular calcium, indicating that it was primarily synaptically mediated. Repetitive stimuli at 10 Hz reproduced the spatiotemporal pattern observed for single stimuli. In contrast, repetitive stimulation in the gamma frequency range ( approximately 40 Hz) rapidly restrained the area of excitation to a small columnar site directly above the stimulating electrode. Intracellular recordings from cells lateral to the activated column revealed increased inhibitory synaptic activity and/or decreased excitatory responses during the train at 40 Hz, but not during a 10 Hz stimulation. Localized microinjections of GABA(A) antagonist produced a reorganization of the geometrical activity pattern that was dependent on the position of the microinjection site. These findings indicate that the frequency-dependent spatial organization of neocortex activation is determined by inhibitory sculpting attributable to local network dynamics

Spatiotemporal properties of thalamocortical and corticothalamic loops were studied in vitro using optical imaging of voltage-sensitive dye signals combined with field potentials and intracellular recordings. Optical recordings were implemented in thalamocortical slices (400-450 mum thick) from 10-21 days old mice. The slices were stained with a voltage-sensitive dye RH-795 (Molecular Probes). Fluorescent activity was recorded using a fast CCD camera (Fujix HRDeltaron 1700) with 128X128 pixels of spatial resolution and 0.6 ms of temporal resolution. Specific (Ventrobasal) or unspecific (Medial and intralaminar) thalamic nuclei were stimulated with either single pulses or trains of stimuli using concentric bipolar electrodes. In the presence of dye, both specific and unspecific thalamic nuclei stimulation with low (2-10Hz) and high frequencies (30-50 Hz) trains were able to activate the thalamic reticular nucleus followed by neostriatum and specific cortical layers. Repetitive stimulation at high but not low frequencies was able to facilitate cortical signals. Stimulation of deep cortical layers resulted in the activation of the corresponding thalamic nuclei. The synaptic delays calculated using both electrophysiological and optical responses were similar. Nissl staining of the slices was performed to determine boundaries of the activated cortical layer. These results suggest the usefulness of thalamocortical slices in studying the physiology of specific and unspecific thalamocortical interactions and the dynamic properties of corticothalamic loops

Purkinje cell LTD was evoked in mouse cerebellar slices by pairing either a) direct b) glutamate iontophoresis, or c) climbing fiber activation with parallel fiber stimulation. Patch clamping was implemented at the soma which supported parallel fiber evoked synaptic current measurements as well as direct activation of the Purkinje cell. The stimulus paradigm consisted of low frequency pairing (1Hz) for a period of 5 minutes of the either of the three (a, b or c) activation modes with parallel fiber stimuli. Parallel fiber Purkinje cell LTD was evoked by all three paring paradigms and the depression was followed for periods of over one hour. Slice pre-incubation with the neuroprotection agent T-588 (Toyama Chemical Co., Ltd.), at concentration less than 1 microM, produced a total suppression of LTD under similar conditions as describe above. Two-photon microscopy determination of calcium concentration profiles in Purkinje cells demonstrated a good correlation between I Ca2+ increase and LTD. This calcium increase was prevented in the presence of T-588. The results suggest that LTD is triggered to prevent cellular damage due to excessive intracellular calcium concentration. Thus, the presence of T-588 obviates the necessity of triggering LTD neuroprotection. These results are particularly significant, as T-588 while preventing LTD in vitro, does not impair motor learning in pretreated animals

When brain responses to sensory stimuli are averaged over time, non-timelocked neural activity is canceled out. Because sensory systems fluctuate in their reponsiveness over time, some of this non-timelocked ('induced') activity is likely to be functionally relevant for sensory processing. To analyze non-timelocked activity, we used multitaper spectral analysis and FFT to identify frequency components of single evoked responses. Frequency components of groups of single evoked responses were then analyzed by parametric and nonparametric statistical analyses. When applied to magnetoencephalographic (MEG) recordings, this method identifies which MEG sensors recorded statistically significant differences in specific frequency ranges in different simulus and response conditions. This method was applied to MEG recordings of human cortical magnetic responses evoked by pairs of brief sequential somatosensory stimulation of both index fingers. Intrapair intervals were 2-34ms (around the somatosensory flicker-fusion threshold). Subjects' button-press reports indicated whether tap pairs were perceived as single events (fused) or sequential events (flicker). MEG recordings of individual trials were grouped and analyzed according to (a) subjects' responses and (b) stimulus type. Preliminary results indicate significant differences (P<0.0005) in the amplitude of specific frequency ranges depending on whether the subjects reported that the tap pairs were perceived as flickering vs fused