Reticular suppression of flash-evoked IPSPs in visual cortex neurons

Quantification of mid and late evoked sinks in laminar current source density profiles of columns in the primary auditory cortex

Current source density (CSD) analysis assesses spatiotemporal synaptic activations at somatic and/or dendritic levels in the form of depolarizing current sinks. Whereas many studies have focused on the short (<50 ms) latency sinks, associated with thalamocortical projections, sinks with longer latencies have received less attention. Here, we analyzed laminar CSD patterns for the first 600 ms after stimulus onset in the primary auditory cortex of Mongolian gerbils. By applying an algorithm for contour calculation, three distinct mid and four late evoked sinks were identified in layers I, III, Va, VIa, and VIb. Our results further showed that the patterns of intracortical information-flow remained qualitatively similar for low and for high sound pressure level stimuli at the characteristic frequency (CF) as well as for stimuli ± 1 octave from CF. There were, however, differences associated with the strength, vertical extent, onset latency, and duration of the sinks for the four stimulation paradigms used. Stimuli one octave above the most sensitive frequency evoked a new, and quite reliable, sink in layer Va whereas low level stimulation led to the disappearance of the layer VIb sink. These data indicate the presence of input sources specifically activated in response to level and/or frequency parameters. Furthermore, spectral integration above vs. below the CF of neurons is asymmetric as illustrated by CSD profiles. These results are important because synaptic feedback associated with mid and late sinks—beginning at 50 ms post stimulus latency—is likely crucial for response modulation resulting from higher order processes like memory, learning or cognitive control.

Cortical long-axoned cells and putative interneurons during the sleep-waking cycle

Knowledge of the input-output characteristics of various neuronal types is a necessary first step toward an understanding of cellular events related to waking and sleep. In spite of the oversimplification involved, the dichotomy in terms of type I (long-axoned, output) neurons and type II (short-axoned, local) interneurons is helpful in functionally delineating the neuronal circuits involved in the genesis and epiphenomena of waking and sleep states. The possibility is envisaged that cortical interneurons, which are particularly related to higher neuronal activity and have been found in previous experiments to be more active during sleep than during wakefulness, might be involved in complex integrative processes occurring during certain sleep stages. Electrophysiological criteria for the identification of output cells and interneurons are developed, with emphasis on various possibilities and difficulties involved in recognizing interneurons of the mammalian brain. The high-frequency repetitive activity of interneurons is discussed, together with various possibilities of error to be avoided when interpreting data from bursting cells. Data first show opposite changes in spontaneous and evoked discharges of identified output cells versus putative interneurons recorded from motor and parietal association cortical areas in behaving monkeys and cats during wakefulness (W) compared to sleep with synchronized EEG activity (S): significantly increased rates of spontaneous firing, enhanced antidromic or synaptic responsiveness, associated with shorter periods of inhibition in type I (pyramidal tract, cortico-thalamic and cortico-pontine) cells during W versus significantly decreased frequencies of spontaneous discharge and depression of synaptically elicited reponses of type II cells during W compared to S. These findings are partly explained on the basis of recent iontophoretic studies showing that acetylcholine, viewed as a synaptic transmitter of the arousal system, excites output-type neurons and inhibits high-frequency bursting cells. Comparing W and S to the deepest stage of sleep with desynchronized EEG activity (D) in type I and type II cells revealed that: (a) the increased firing rates of output cells in D, over those in W and S, is substantially due to a tonic excitation during this state, and rapid eye movements (REMs) only contribute to the further increase of discharge frequencies; (b) in contrast, the increased rates of discharge in interneurons during D is entirely ascribable to REM-related firing. On the basis of experiments reporting that increased duration of D has beneficial effects upon retention of information acquired during W, the suggestion is made that increased firing rates of association cortical interneurons during REM epochs of D sleep are an important factor in maintaining the soundness of a memory trace.

Hebbian synapses in cortical and hippocampal pathways

Use-dependent alterations in synaptic efficacy are believed to form the basis for such complex brain functions as learning and memory and significantly contribute to the development of neuronal networks. The algorithm of synapse modification proposed by Hebb as early as 1949 is the coincident activation of pre- and postsynaptic neurons. The present review considers the evolution of experimental protocols in which postsynaptic cell depolarization through the recording microelectrode was used to reveal the manifestation of Hebb-type plasticity in the synaptic inputs of the neocortex and hippocampus. Special attention is focused on the inhibitory control of the Hebb-type plasticity. Disinhibition within the local neuronal circuits is considered to be an important factor in Hebbian plasticity, contributing to such phenomena as priming, primed burst potentiation, hippocampal theta-rhythm and cortical arousal. The role of various transmitters (acetylcholine, norepinephrine, gamma-amino-butyric acid) in disinhibition is discussed with a special emphasis on the brain noradrenergic system. Possible mechanisms of Hebbian synapse modification and their modulation by memory enhancing substances are considered. It is suggested that along with their involvement in disinhibition processes these substances may control Hebb-type plasticity through intracellular second messenger systems.

Human flash-VEP and quantitative EEG are independently affected by acute scopolamine

Scopolamine in acute intramuscular doses of 0.25-0.75 mg reduced the P2-N3 flash-VEP amplitude and, in the quantitative EEG, the 8.5-12.0 Hz power and total power in 8 healthy young male volunteers. The effects on flash-VEP and EEG total power were dose dependent and were evident 30 min and 90 min respectively after drug administration, regardless of dose. The reduction in 8.5-12.0 Hz power was limited to the 0.50 and 0.75 mg doses. No systematic effects on the pattern-VEP were observed. Possible interferences with flash- or pattern-VEP amplitude of the scopolamine-induced EEG changes were identified and removed by regression analysis and computation of VEP residuals from the regression function. The P2-N3 flash-VEP residuals proved EEG independent and showed relationships with dose and time after drug administration that were superimposable on those of the original data, with comparable significance levels at the drug/placebo and pre/postdrug statistical comparisons. The results indicate that VEP estimates of drug effects which are independent from EEG changes can be identified in human studies and allow some inference on the cholinergic specificity of the systems affecting late flash-VEP components. The statistical approach used in this study is suitable for application in VEP studies when effects of interacting factors are to be expected.

Electrical activity of the visual cortex under conditions of change in the levels of monoamines in the brain of animals

The changes in the electrograms of the visual cortex of awake animals under the influence of light stimulation in conditions of a pharmacological effect on the monoamine (MA) systems of the brain were investigated in experiments on 8 rabbits and 12 rats. The following was found following the administration of MA precursors (5-hydroxytryptophan and d,l-dihydroxyphenylalanine): a) a decrease in the amplitude of the averaged evoked potentials in response to rhythmical light stimuli (1-20 pulses/sec); b) intensification of rapid (15-25 Hz) oscillations in the spontaneous electrical activity of the cortex, as well as attenuation and modification of the effects of the blocker of MA synthesis, a-methyl-dihydroxyphenylalanine. A potentiation of the MA precursors was observed with light stimulation in the frequency spectra of the electrocorticograms. The specific characteristics of the action of the catecholamine precursor were manifested in the same conditions in the form of an intensification of the power of the 5-7 Hz rhythms, and an attenuation of the power of the 2-3 Hz rhythms.

Intracellular synaptic potentials of primate motor cortex neurons during voluntary movement

An intracellular recording technique was applied to the precentral motor cortex of the unanesthetized, chronically behaving monkey. Postsynaptic potentials, responsible for an initiation of the voluntary movement, were recorded. In total, 22 pyramidal tract neurons (PTNs) and 40 non-pyramidal tract neurons (non-PTNs) were successfully penetrated in 5 monkeys while the monkey was performing a flexion-extension wrist movement after a visual cue (reaction time, 200--350 msec). The neurons showed a negative membrane potential shift of at least 30 mV for more than 30 sec. A slowly rising PSP appeared 80--180 msec after the visual cue, and was 70--180 msec prior to an onset of the movement. Spike activities were superimposed upon this slow PSP with 20--80 msec rise time and 2--6 mV depolarization (8 PTNs and 6 non-PTNs). Since these depolarizations were variable in magnitude and latency, these were considered to be summated potentials of small EPSPs and hidden IPSPs. Membrane resistance was measured from an IR drop by a hyperpolarizing current (1.2 X 10(-9) A) passed through a recording electrode, and was 3.5 +/- 1.7 Momega in 18 PTNs and 4.5 +/- 2.5 Momega in 28 non-PTNs. There was a linear relationship in PTNs between membrane resistance and antidromic latency from the pontine pyramid. Because of the time course of PSPs, their possible dendritic origins were discussed.