Faculty Bibliography

The effect of synaptic inputs on somatodendritic interactions during action potentials was investigated, in the cat, using in vivo intracellular recording and computational models of neocortical pyramidal cells. An array of 10 microelectrodes, each ending at a different cortical depth, was used to preferentially evoke synaptic inputs to different somatodendritic regions. Relative to action potentials evoked by current injection, spikes elicited by cortical microstimuli were reduced in amplitude and duration, with stimuli delivered at proximal (somatic) and distal (dendritic) levels evoking the largest and smallest decrements, respectively. When the inhibitory postsynaptic potential reversal was shifted to around -50 mV by recording with KCl pipettes, synaptically-evoked spikes were significantly less reduced than with potassium acetate or cesium acetate pipettes, suggesting that spike decrements are not only due to a shunt, but also to voltage-dependent effects. Computational models of neocortical pyramidal cells were built based on available data on the distribution of active currents and synaptic inputs in the soma and dendrites. The distribution of synapses activated by extracellular stimulation was estimated by matching the model to experimental recordings of postsynaptic potentials evoked at different depths. The model successfully reproduced the progressive spike amplitude reduction as a function of stimulation depth, as well as the effects of chloride and cesium. The model revealed that somatic spikes contain an important contribution from proximal dendritic sodium currents up to approximately 100 microm and approximately 300 microm from the soma under control and cesium conditions, respectively. Proximal inhibitory postsynaptic potentials can present this dendritic participation thus reducing the spike amplitude at the soma. The model suggests that the somatic spike amplitude and shape can be used as a 'window' to infer the electrical participation of proximal dendrites. Thus, our results suggest that inhibitory postsynaptic potentials can control the participation of proximal dendrites in somatic sodium spikes

Previous work in our laboratory has revealed that the excitability of lateral amygdaloid projection neurons is tightly regulated by GABA-mediated inhibitory postsynaptic potentials and intrinsic conductances that can be activated by synaptic inputs. Here, we studied the synaptic responsiveness of lateral amygdaloid interneurons recorded intracellularly in vivo, in the cat, to investigate their role in regulating the activity of projection cells. Interneurons were identified morphologically by their aspiny dendritic trees and physiologically by their ability to generate high frequency, non-adapting spike trains in response to depolarizing current pulses. Cortical shocks of increasing intensity generated opposite response profiles in interneurons and projection cells, with interneurons becoming progressively more excited and projection cells more inhibited. These cortically-evoked response profiles paralleled the activity of interneurons and projection cells in relation to spontaneous electroencephalographic events of differing amplitudes. Only at the lowest intensities were predominantly excitatory responses elicited in both cell types. As a result, only a narrow range of low stimulus intensities could trigger spikes in projection cells. In both cell types, the initial cortically-evoked excitatory postsynaptic potential was followed by a hyperpolarization, which was of markedly lower amplitude and duration in interneurons. In interneurons, the hyperpolarization reversed at approximately -72 mV with potassium acetate pipettes and approximately -55 mV with potassium chloride pipettes, suggesting that this inhibitory postsynaptic potential is primarily mediated by a chloride conductance. In light of previous findings indicating that inhibition in the lateral amygdaloid nucleus arises mostly from local inhibitory neurons, these results suggest that interneurons are synaptically coupled via GABAA receptors. Moreover, the opposite response profiles of interneurons and projection cells to cortical shocks indicate that interneurons play a critical role in regulating the activity of projection cells. The cellular interactions evidenced in the present study suggest that the lateral amygdaloid nucleus is endowed with an inhibitory gating mechanism that regulates information flow through the amygdala

The frequency of spontaneous synaptic events in vitro is probably lower than in vivo because of the reduced synaptic connectivity present in cortical slices and the lower temperature used during in vitro experiments. Because this reduction in background synaptic activity could modify the integrative properties of cortical neurons, we compared the impact of spontaneous synaptic events on the resting properties of intracellularly recorded pyramidal neurons in vivo and in vitro by blocking synaptic transmission with tetrodotoxin (TTX). The amount of synaptic activity was much lower in brain slices (at 34 degrees C), as the standard deviation of the intracellular signal was 10-17 times lower in vitro than in vivo. Input resistances (Rins) measured in vivo during relatively quiescent epochs ('control Rins') could be reduced by up to 70% during periods of intense spontaneous activity. Further, the control Rins were increased by approximately 30-70% after TTX application in vivo, approaching in vitro values. In contrast, TTX produced negligible Rin changes in vitro (approximately 4%). These results indicate that, compared with the in vitro situation, the background synaptic activity present in intact networks dramatically reduces the electrical compactness of cortical neurons and modifies their integrative properties. The impact of the spontaneous synaptic bombardment should be taken into account when extrapolating in vitro findings to the intact brain

We studied the impact of transmitter release resistant to tetrodotoxin (TTX) in morphologically identified neocortical pyramidal neurons recorded intracellularly in barbiturate-anesthetized cats. It was observed that TTX-resistant release occurs in pyramidal neurons in vivo and at much higher frequencies than was previously reported in vitro. Further, in agreement with previous findings indicating that GABAergic and glutamatergic synapses are differentially distributed in the somata and dendrites of pyramidal cells, we found that most miniature synaptic potentials were sensitive to gamma-aminobutyric acid-A (GABA(A)) or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) antagonists in presumed somatic and dendritic impalements, respectively. Pharmacological blockage of spontaneous synaptic events produced large increases in input resistance that were more important in dendritic (approximately 50%) than somatic (approximately 10%) impalements. These findings imply that in the intact brain, pyramidal neurons are submitted to an intense spike-independent synaptic bombardment that decreases the space constant of the cells. These results should be taken into account when extrapolating in vitro findings to intact brains

How can information hidden in a spatial configuration of neuronal activity be addressed? The Markov Random Field method for the analysis of the spatial component of a multidimensional neuronal process is introduced and after simulations is applied to experimental data on rat at olivocerebellar activity. Using this method it was determined, for the first time, that the activity demonstrates dynamic coupling and may have different fine spatial substructures. The results obtained support the view that the inferior olive serves as a movement organizing centre that controls motor activity by means of spatially as well as temporally organized patterns of coherent activity.

The basic electrical rhythmicity of the olivocerebellar system was investigated in vivo using multiple electrode recordings of Purkinje cell (PC) complex spike (CS) activity. CSs demonstrate a 10 Hz rhythmicity, thought to result from the interaction of Ca2+ and Ca2+-dependent K+ conductances present in inferior olivary (IO) neurons. To assess the roles of different K+ channels in generating this rhythmicity, intraolivary microinjections of charybdotoxin (CTX) and apamin were used. Both K+ channel blockers increased average CS spike-firing rates. However, apamin produced a tonic increase in firing with a decrement in the CS rhythmicity. In contrast, after CTX administration, highly rhythmic CS discharges were interleaved with silent periods, suggesting that apamin- and CTX-sensitive K+ channels have distinct rhythmogenic roles in IO neurons. CTX-sensitive channels seem to be functionally coupled to low threshold Ca2+ channels, whereas the apamin-sensitive channels relate to high threshold Ca2+ channels. Blocking intraolivary GABAA receptors increases IO excitability and the spatial distribution of synchronized CS activity while disrupting its rostrocaudal banding pattern (). The present experiments show that K+ channel blockers increase IO excitability without causing widespread synchronization of CS activity. Thus, changes in the IO excitability have relatively little effect in determining the spatial organization of CS synchrony. In contrast, the degree of CS rhythmicity seemed to influence the patterns of CS synchrony. Thus, after CTX, increased CS rhythmicity was associated with increased intraband synchrony and decreased interband synchrony, whereas apamin had the opposite effects on intra- and interband synchronization

The companion paper demonstrated that the responses of lateral amygdaloid (LAT) projection neurons to the stimulation of major input and output structures are dominated by monophasic hyperpolarizing potentials of large amplitude. To characterize the mechanisms underlying these inhibitory potentials, intracellular recordings of cortically evoked responses were obtained from morphologically and/or physiologically identified LAT projection neurons in barbiturate anesthetized cats. The reversal potential of the cortically evoked hyperpolarization was measured at its peak, and 115 ms later (tail), an interval corresponding to the peak latency of the gamma-aminobuturic acid-B (GABAB) response previously recorded in vitro. When recorded with K-acetate (KAc) pipettes, these reversal potentials were -86.9 +/- 1.6 mV (peak; mean +/- SE) and -90.7 +/- 1.7 mV (tail), suggesting that both Cl- and K+ conductances contribute throughout the cortically evoked hyperpolarization. The small, but consistent, difference between the two reversal potentials suggested that an additional slowly activating K(+)-mediated component contributed to the inhibitory postsynaptic potential (IPSP) tail. To determine whether Cl- conductances contributed to the evoked hyperpolarization, recordings were performed with KCl; the peak (-57.8 +/- 2.2 mV) and tail (-61.3 +/- 2.1 mV) reversal potentials were approximately 15-20 mV more depolarized than those recorded with KAc pipettes. However, the difference between the peak and tail reversals remained. In an attempt to block the Cl- conductance, recordings were obtained with pipettes filled with KAc or KCl and 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS), a Cl- pump blocker that also was reported to block GABAA responses. With KAc and DIDS, the initial depolarization was prolonged and the amplitude of the hyperpolarization decreased relative to that seen with KAc alone. However, with KCl and DIDS, the reversal potential was shifted to an even greater extent than with KCl pipettes with the evoked response consisting entirely of a large depolarization, which produced a spike burst. These results suggest that LAT neurons have a Cl- pump that is blocked by DIDS, but that their Cl- channels are not blocked by DIDS. To assess the contribution of K+ conductances to cortically evoked hyperpolarizing potentials, recordings were obtained with Cs-acetate pipettes. Under these conditions, the response reversed at more depolarized potentials (peak, -71.9 +/- 1.0 mV; tail, -72.0 +/- 0.9 mV) compared with KAc recordings, with no difference between the peak and tail reversal potentials. These cells also had depolarized resting potentials (-66.2 +/- 1.8 mV) compared with those of cells recorded with KAc pipettes (-73.6 +/- 1.8 mV); however, this difference was too small to attribute the shift in reversals to a redistribution of Cl- ions across the membrane. The action potentials generated by LAT neurons under Cs+ had a shoulder that prolonged their falling phase. The increased duration of the spikes was presumably due to a dendritic Ca2+ conductance because LAT amygdaloid neurons are known to possess such conductances and Cs+ blocks the delayed rectifier and some Ca(2+)-dependent K+ currents. The dramatic reduction of this shoulder by spontaneous and evoked IPSPs suggests that the activation of dendritic conductances by back-propagating somatic action potentials is regulated tightly by synaptic events. Intracellular injection of the Ca2+ chelating agent, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (100 mM) caused a depolarization of the peak (-75.3 +/- 1.3 mV) and tail (-77.7 +/- 1.7 mV) reversal potentials during a time course of 15-45 min. Concurrently, the amplitude of the excitatory postsynaptic potential increased whereas that of the hyperpolarization decreased, suggesting that a Ca(2+)-dependent K+ conductance contributes significantly to the evoked hyperpolarization. (ABSTRACT TRUNCATED)

To investigate the impact of inhibitory processes on responses of lateral amygdaloid (LAT) neurons, intracellular recordings were obtained from identified LAT projection neurons in barbiturate-anesthetized cats. Synaptic responses evoked by perirhinal (PRH), entorhinal (ENT), basomedial, and LAT stimulation were investigated. Regardless of stimulation site, responses consisted of an excitatory postsynaptic potential (EPSP) that either preceded and was truncated by an inhibitory postsynaptic potential (IPSP) or occurred just after the IPSP onset. IPSPs were monophasic, lasted hundreds of milliseconds, and were of such large amplitude and rapid onset that they effectively opposed the EPSPs, generally preventing orthodromic spikes. All sites elicited IPSPs with relatively negative reversal potentials around -85 mV. Experiments analyzing the underlying ionic mechanisms are presented in the companion paper. Evoked responses were similar to synaptic potentials associated with spontaneous EEG events, known as simple (small, monophasic) and complex (large, triphasic) ENT sharp potentials (SPs), with no difference between the reversals of evoked and SP-related IPSPs (-83.2 +/- 2.7 mV). IPSPs coinciding with complex SPs truncated SP-related EPSPs more rapidly and had larger amplitudes and longer durations than those related to simple SPs. These differences reflected the fact that the amplitude and duration of SP-related IPSPs were correlated with SP amplitude. Similar variations were reproduced in evoked IPSPs by varying the stimulus intensity. Low intensities generated predominantly excitatory responses consisting of EPSPs sometimes followed by small IPSPs, whereas high intensities evoked predominantly inhibitory responses comprised of a large IPSP that truncated or occluded the EPSPs. Orthodromic spikes were elicited only in a narrow range of intermediate intensities. These changes in the evoked response primarily reflected increases in the IPSP evoked at high intensities. PRH stimulation at different rostro-caudal levels demonstrated that rostral sites elicited larger EPSPs and IPSPs with shorter latencies and longer durations than caudal sites. These differences probably reflect contrasting patterns of activity spread through the PRH cortex, suggesting that the intact cortical circuitry allowed a temporally distributed activation of inhibitory interneurons and thereby partly explains the long duration and monophasic nature of the IPSPs. Inhibition, thus, plays a primary role in shaping LAT neuronal responses. The profuse intrinsic connectivity of the LAT nucleus and parahippocampal cortices may underlie the relatively invariant response pattern of LAT neurons and suggests a common mode of information processing, based upon quantitative, rather than qualitative, differences in activation of LAT circuitry. Therefore we propose that effective transmission of signals through the LAT nucleus may require activation of specifically sized neuronal ensembles, rather than widespread afferent excitation