Somatosensory cortical neurons with an identifiable electrophysiological signature

A Critical Role of Inhibition in Temporal Processing Maturation in the Primary Auditory Cortex

Faithful representation of sound envelopes in primary auditory cortex (A1) is vital for temporal processing and perception of natural sounds. However, the emergence of cortical temporal processing mechanisms during development remains poorly understood. Although cortical inhibition has been proposed to play an important role in this process, direct in-vivo evidence has been lacking. Using loose-patch recordings in rat A1 immediately after hearing onset, we found that stimulus-following ability in fast-spiking neurons was significantly better than in regular-spiking (RS) neurons. In-vivo whole-cell recordings of RS neurons revealed that inhibition in the developing A1 demonstrated much weaker adaptation to repetitive stimuli than in adult A1. Furthermore, inhibitory synaptic inputs were of longer duration than observed in vitro and in adults. Early in development, overlap of the prolonged inhibition evoked by 2 closely following stimuli disrupted the classical temporal sequence between excitation and inhibition, resulting in slower following capacity. During maturation, inhibitory duration gradually shortened accompanied by an improving temporal following ability of RS neurons. Both inhibitory duration and stimulus-following ability demonstrated exposure-based plasticity. These results demonstrate the role of inhibition in setting the pace for experience-dependent maturation of temporal processing in the auditory cortex.

Fast Inhibitory Decay Facilitates Adult-like Temporal Processing in Layer 5 of Developing Primary Auditory Cortex

The protracted maturational process of temporal processing in layer 4 (L4) of primary auditory cortex (A1) has been extensively studied. Accumulating evidences show that layer 5 (L5) receives direct thalamic inputs as well. How the temporal responses in L5 may developmentally emerge remains unclear. Using in vivo loose-patch recordings in rat A1, we found that putative pyramidal (Pyr) neurons in developing L5 exhibited adult-like stimulus-following ability but less bursting shortly after hearing onset. L5 Pyr neurons in adult A1 exhibited phase-locking similar to L4 neurons, while L5 fast-spiking (FS) neurons showed greater phase-locking at 7 and 12.5 pps. In developing L5, whole-cell recordings revealed inhibition with decay constant comparable to that in adult L5, thereby avoiding the summation of inhibition that contributed to the strong adaptation in L4. Given the targets of L5 outputs, the relatively precocious temporal processing in L5 might contribute to temporal response maturation in connected cortical and subcortical areas. Our findings were in agreement with the idea that L5 may be a "hub" for processing cortical inputs and outputs that can operate independently of L4.

Categorically distinct types of receptive fields in early visual cortex

In the visual cortex, distinct types of neurons have been identified based on cellular morphology, response to injected current, or expression of specific markers, but neurophysiological studies have revealed visual receptive field (RF) properties that appear to be on a continuum, with only two generally recognized classes: simple and complex. Most previous studies have characterized visual responses of neurons using stereotyped stimuli such as bars, gratings, or white noise and simple system identification approaches (e.g., reverse correlation). Here we estimate visual RF models of cortical neurons using visually rich natural image stimuli and regularized regression system identification methods and characterize their spatial tuning, temporal dynamics, spatiotemporal behavior, and spiking properties. We quantitatively demonstrate the existence of three functionally distinct categories of simple cells, distinguished by their degree of orientation selectivity (isotropic or oriented) and the nature of their output nonlinearity (expansive or compressive). In addition, these three types have differing average values of several other properties. Cells with nonoriented RFs tend to have smaller RFs, shorter response durations, no direction selectivity, and high reliability. Orientation-selective neurons with an expansive output nonlinearity have Gabor-like RFs, lower spontaneous activity and responsivity, and spiking responses with higher sparseness. Oriented RFs with a compressive nonlinearity are spatially nondescript and tend to show longer response latency. Our findings indicate multiple physiologically defined types of RFs beyond the simple/complex dichotomy, suggesting that cortical neurons may have more specialized functional roles rather than lying on a multidimensional continuum.

Effect of Contrast on Visual Spatial Summation in Different Cell Categories in Cat Primary Visual Cortex

Multiple cell classes have been found in the primary visual cortex, but the relationship between cell types and spatial summation has seldom been studied. Parvalbumin-expressing inhibitory interneurons can be distinguished from pyramidal neurons based on their briefer action potential durations. In this study, we classified V1 cells into fast-spiking units (FSUs) and regular-spiking units (RSUs) and then examined spatial summation at high and low contrast. Our results revealed that the excitatory classical receptive field and the suppressive non-classical receptive field expanded at low contrast for both FSUs and RSUs, but the expansion was more marked for the RSUs than for the FSUs. For most V1 neurons, surround suppression varied as the contrast changed from high to low. However, FSUs exhibited no significant difference in the strength of suppression between high and low contrast, although the overall suppression decreased significantly at low contrast for the RSUs. Our results suggest that the modulation of spatial summation by stimulus contrast differs across populations of neurons in the cat primary visual cortex.

Parvalbumin-expressing inhibitory interneurons in auditory cortex are well-tuned for frequency

In the auditory cortex, synaptic inhibition is known to be involved in shaping receptive fields, enhancing temporal precision, and regulating gain. Cortical inhibition is provided by local GABAergic interneurons, which comprise 10-20% of the cortical population and can be separated into numerous subclasses. The morphological and physiological diversity of interneurons suggests that these different subclasses have unique roles in sound processing; however, these roles are yet unknown. Understanding the receptive field properties of distinct inhibitory cell types will be critical to elucidating their computational function in cortical circuits. Here we characterized the tuning and response properties of parvalbumin-positive (PV+) interneurons, the largest inhibitory subclass. We used channelrhodopsin-2 (ChR2) as an optogenetic tag to identify PV+ and PV- neurons in vivo in transgenic mice. In contrast to PV+ neurons in mouse visual cortex, which are broadly tuned for orientation, we found that auditory cortical PV+ neurons were well tuned for frequency, although very tightly tuned PV+ cells were uncommon. This suggests that PV+ neurons play a minor role in shaping frequency tuning, and is consistent with the idea that PV+ neurons nonselectively pool input from the local network. PV+ interneurons had shallower response gain and were less intensity-tuned than PV- neurons, suggesting that PV+ neurons provide dynamic gain control and shape intensity tuning in auditory cortex. PV+ neurons also had markedly faster response latencies than PV- neurons, consistent with a computational role in enhancing the temporal precision of cortical responses.

Layer 4 in primary visual cortex of the awake rabbit: contrasting properties of simple cells and putative feedforward inhibitory interneurons

Extracellular recordings were obtained from two cell classes in layer 4 of the awake rabbit primary visual cortex (V1): putative inhibitory interneurons [suspected inhibitory interneurons (SINs)] and putative excitatory cells with simple receptive fields. SINs were identified solely by their characteristic response to electrical stimulation of the lateral geniculate nucleus (LGN, 3+ spikes at >600 Hz), and simple cells were identified solely by receptive field structure, requiring spatially separate ON and/or OFF subfields. Notably, no cells met both criteria, and we studied 62 simple cells and 33 SINs. Fourteen cells met neither criterion. These layer 4 populations were markedly distinct. Thus, SINs were far less linear (F1/F0 < 1), more broadly tuned to stimulus orientation, direction, spatial and temporal frequency, more sensitive to contrast, had much higher spontaneous and stimulus-driven activity, and always had spatially overlapping ON/OFF receptive subfields. SINs responded to drifting gratings with increased firing rates (F0) for all orientations and directions. However, some SINs showed a weaker modulated (F1) response sharply tuned to orientation and/or direction. SINs responded at shorter latencies than simple cells to stationary stimuli, and the responses of both populations could be sustained or transient. Transient simple cells were more sensitive to contrast than sustained simple cells and their visual responses were more frequently suppressed by high contrasts. Finally, cross-correlation between LGN and SIN spike trains confirmed a fast and precisely timed monosynaptic connectivity, supporting the notion that SINs are well suited to provide a fast feedforward inhibition onto targeted cortical populations.

Intrinsic electrophysiology of mouse corticospinal neurons: a class-specific triad of spike-related properties

Corticospinal pyramidal neurons mediate diverse aspects of motor behavior. We measured spike-related electrophysiological properties of identified corticospinal neurons in primary motor cortex slices from young adult mice. Several consistent features were observed in the suprathreshold responses to current steps: 1) Corticospinal neurons fired relatively fast action potentials (APs; width at half-maximum 0.65 ± 0.13 ms, mean ± standard deviation [SD]) compared with neighboring callosally projecting corticostriatal neurons. Corticospinal AP width was intermediate between 2 classes of inhibitory interneuron in layer 5B. Spike-to-spike variability in AP width and other spike waveform parameters was low, even during repetitive firing up to 20 Hz, that is, the relative narrowness of corticospinal APs was essentially frequency independent. 2) Frequency-current (f-I) relationships were nearly linear. 3) Trains of APs displayed regular firing, with rates typically staying constant or accelerating over time. Corticospinal neurons recorded from older mice (up to 4 months) or from a separate lateral cortical area (Region B; corresponding to secondary somatosensory cortex) showed generally similar intrinsic properties. Our findings have implications for interpreting spike waveforms of in vivo recorded neurons in the motor cortex. This analysis provides a framework for further biophysical and computational investigations of corticospinal neurons and their roles in motor cortical function.

Response features of parvalbumin-expressing interneurons suggest precise roles for subtypes of inhibition in visual cortex

Inhibitory interneurons in the cerebral cortex include a vast array of subtypes, varying in their molecular signatures, electrophysiological properties, and connectivity patterns. This diversity suggests that individual inhibitory classes have unique roles in cortical circuits; however, their characterization to date has been limited to broad classifications including many subtypes. We used the Cre/LoxP system, specifically labeling parvalbumin(PV)-expressing interneurons in visual cortex of PV-Cre mice with red fluorescent protein (RFP), followed by targeted loose-patch recordings and two-photon imaging of calcium responses in vivo to characterize the visual receptive field properties of these cells. Despite their relative molecular and morphological homogeneity, we find that PV+ neurons have a diversity of feature-specific visual responses that include sharp orientation and direction-selectivity, small receptive fields, and band-pass spatial frequency tuning. These results suggest that subsets of parvalbumin interneurons are components of specific cortical networks and that perisomatic inhibition contributes to the generation of precise response properties.

Visual receptive field structure of cortical inhibitory neurons revealed by two-photon imaging guided recording

Synaptic inhibition plays an important role in shaping receptive field (RF) properties in the visual cortex. However, the underlying mechanisms remain not well understood, partly because of difficulties in systematically studying functional properties of cortical inhibitory neurons in vivo. Here, we established two-photon imaging guided cell-attached recordings from genetically labeled inhibitory neurons and nearby "shadowed" excitatory neurons in the primary visual cortex of adult mice. Our results revealed that in layer 2/3, the majority of excitatory neurons exhibited both On and Off spike subfields, with their spatial arrangement varying from being completely segregated to overlapped. In contrast, most layer 4 excitatory neurons exhibited only one discernable subfield. Interestingly, no RF structure with significantly segregated On and Off subfields was observed for layer 2/3 inhibitory neurons of either the fast-spike or regular-spike type. They predominantly possessed overlapped On and Off subfields with a significantly larger size than the excitatory neurons and exhibited much weaker orientation tuning. These results from the mouse visual cortex suggest that different from the push-pull model proposed for simple cells, layer 2/3 simple-type neurons with segregated spike On and Off subfields likely receive spatially overlapped inhibitory On and Off inputs. We propose that the phase-insensitive inhibition can enhance the spatial distinctiveness of On and Off subfields through a gain control mechanism.

Spectrotemporal processing differences between auditory cortical fast-spiking and regular-spiking neurons

Excitatory pyramidal neurons and inhibitory interneurons constitute the main elements of cortical circuitry and have distinctive morphologic and electrophysiological properties. Here, we differentiate them by analyzing the time course of their action potentials (APs) and characterizing their receptive field properties in auditory cortex. Pyramidal neurons have longer APs and discharge as regular-spiking units (RSUs), whereas basket and chandelier cells, which are inhibitory interneurons, have shorter APs and are fast-spiking units (FSUs). To compare these neuronal classes, we stimulated cat primary auditory cortex neurons with a dynamic moving ripple stimulus and constructed single-unit spectrotemporal receptive fields (STRFs) and their associated nonlinearities. FSUs had shorter latencies, broader spectral tuning, greater stimulus specificity, and higher temporal precision than RSUs. The STRF structure of FSUs was more separable, suggesting more independence between spectral and temporal processing regimens. The nonlinearities associated with the two cell classes were indicative of higher feature selectivity for FSUs. These global functional differences between RSUs and FSUs suggest fundamental distinctions between putative excitatory and inhibitory interneurons that shape auditory cortical processing.

Characteristic membrane potential trajectories in primate sensorimotor cortex neurons recorded in vivo

We examined the membrane potentials and firing properties of motor cortical neurons recorded intracellularly in awake, behaving primates. Three classes of neuron were distinguished by 1) the width of their spikes, 2) the shape of the afterhyperpolarization (AHP), and 3) the distribution of interspike intervals. Type I neurons had wide spikes, exhibited scoop-shaped AHPs, and fired irregularly. Type II neurons had narrower spikes, showed brief postspike afterdepolarizations before the AHP, and sometimes fired high-frequency doublets. Type III neurons had the narrowest spikes, showed a distinct post-AHP depolarization, or "rebound AHP" (rAHP), lasting nearly 30 ms, and tended to fire at 25-35 Hz. The evidence suggests that an intrinsic rAHP may confer on these neurons a tendency to fire at a preferred frequency governed by the duration of the rAHP and may contribute to a "pacemaking" role in generating cortical oscillations.

A 14-day period of hindpaw sensory deprivation enhances the responsiveness of rat cortical neurons

Hypodynamia-hypokinesia (HH) is a model of hindpaw sensory deprivation. It is obtained by unloading of the hindquarters during 14 days. In this situation, the feet are not in contact with the ground and as a consequence, the cutaneous receptors are not activated; the sensory input to the primary somatosensory cortex (SmI) is thus reduced. In a previous study, we have shown that HH induced a cortical reorganisation of the hindlimb representation. The understanding of the mechanisms involved in cortical map plasticity requires a close examination of the changes in response properties of cortical neurons during HH. The aim of the present study was thus to study the characteristics of neurons recorded from granular and infragranular layers in hindlimb representation of SmI. A total of 289 cortical neurons were recorded (158 from control rats and 131 from HH rats) in pentobarbital-anaesthetized rats. Cutaneous threshold, cutaneous receptive fields, spontaneous activity (discharge rate and instantaneous frequency) and activity evoked by air-jet stimulation (response latency and duration, amplitude) were analysed. The present study suggests that activity-dependent changes occur in the cortex. The duration of the spike waveform presented two populations of spikes: thin-spike cells (<1 ms, supposed to be inhibitory interneurons) and regular cells (>1 ms). Thin-spike cells were less frequently encountered in HH than in control rats. The analysis of regular cells revealed that after HH (1) spontaneous activity was unchanged and (2) cortical somatosensory neurons were more responsive: the cutaneous threshold was reduced and the response magnitude increased. Taken together, these results suggest a down-regulation of GABAergic function.

Sex and season influence the proportion of thin spike cells in the canary HVc

In songbirds, anatomical attributes of song nuclei exhibit sexual and seasonal differences. To extend these data to physiological correlates, neurons ( n= 374) were recorded in the HVc of male and female canaries during and outside the breeding period. Surprisingly, a particular type of action potential waveforms was observed more frequently in breeding than in non-breeding birds and in males than in females. These neurons showed both a shorter action potential duration (< 0.4 ms) and a higher firing rate (2.5 1.4 spikes/s) than the other neurons. Such characteristics are usually associated with interneurons in the songbird HVc as well as in the mammalian neocortex. Thus, these results provide the first electrophysiological evidence for an alteration of the neuronal network of HVc across sexes and seasons.

Differences in auditory and physiological properties of HVc neurons between reproductively active male and female canaries (Serinus canaria)

Based on neuronal recordings in the HVc, this study investigated differences between reproductively active male and sexually receptive female canaries. It is the first study to describe auditory responses and cell characteristics of HVc neurons in female songbirds and to compare them with the responses and characteristics obtained in males. Extracellular single unit recordings showed that in males HVc cells exhibited two types of auditory responses to conspecific and heterospecific song playbacks: tonic and phasic responses. The major finding of the present study is the absence of tonic responses in females. Neurons in the HVc of females only responded phasically to song playbacks. In both sexes, neurons exhibiting auditory responses had thinner action potentials than the others. As all the tonic cells recorded in males were thin spike cells (action potential < or = 0.6 ms) [corrected] and had high firing rates (6 Hz in average), they are potentially interneurons. In both sexes, two categories of nonresponsive cells were found: neurons that did not fire at song onset and had the lowest spontaneous firing rate; and neurons that did not exhibit changes in activity in response to song playbacks. Analyses of physiological characteristics of HVc neurons revealed that the rate of spontaneous activity was higher in males than in females. This study is a first step towards identifying [corrected] the cellular bases of the sexual dimorphism in HVc function and highlights the pivotal role of interneurons in HVc auditory processing.

Linking 600-Hz "spikelike" EEG/MEG wavelets ("sigma-bursts") to cellular substrates: concepts and caveats

Somatosensory evoked human EEG and magnetoencephalographic (MEG) responses comprise a brief burst of low-amplitude, high-frequency (approximately 600 Hz) spikelike wavelets ("sigma-bursts") superimposed on the primary cortical response (e.g., the N20 to electrical median nerve stimulation). The recent surge of interest in these macroscopic sigma-burst responses is energized by the prospect of monitoring noninvasively, highly synchronized and rapidly repeating population spikes generated in the human thalamic and cortical somatosensory system. Thus, analyses of spike-related sigma-bursts could uniquely complement conventional low-frequency EEG/MEG, reflecting mass excitatory and inhibitory postsynaptic potentials that potentially also incorporate subthreshold activities of undetermined functional relevance. Recent studies using spatiotemporal source analysis of multichannel recordings identified regional burst sources subcortically (near-thalamic) as well as cortically. At the primary somatosensory cortex, sigma-burst generators showed the well-established homuncular somatotopic ordering. Functionally, the 600-Hz burst appears to comprise multiple subcomponents with differential sensitivity to stimulus rate, intensity, sleep-wake cycle, tactile interference, subject age, and certain movement disorders. A plenitude of cellular candidates contributing to burst generation at different levels can already now be envisaged, including cuneothalamic and thalamocortical relay cells, as well as cortical bursting pyramidal cells and fast-spiking inhibitory interneurons. Although cellular burst coding might serve to relay information with high efficiency, concepts to link macroscopic sigma-bursts and cellular substrates call for additional study.

Intrinsic electrophysiology of neurons in thalamorecipient layers of developing rat auditory cortex

During early postnatal life, several critical events contribute to the functional development of rat sensory neocortex. Thalamocortical innervation of sensory cortex is completed during the first postnatal week and extrathalamic innervation develops over the first several weeks. In auditory cortex, acoustic-evoked potentials first occur in week 2 and develop most rapidly over weeks 2-3. Thus, rapid functional maturation of cortical circuits in sensory cortex occurs during the second and third postnatal weeks. The electrophysiological properties of cortical neurons that receive afferent inputs during this time may play an important role in development and function. In this study we examined the intrinsic electrophysiology, including spiking patterns, of neurons in layers II/III and IV of auditory cortex during postnatal weeks 2 and 3. Many neurons displayed characteristics consistent with previous descriptions of response classes (regular spiking, fast spiking, intrinsic bursting). In addition, we identified two groups, Rectifying and On-spiking neurons, that were characterized by (i) brief spike trains in response to maintained intracellular depolarizations, and (ii) striking outward rectification upon depolarization. Unusually brief spike trains (1-2 spikes) and short spike latencies (<10 ms) further distinguished On-spiking from Rectifying cells. Biocytin labeling demonstrated that On-spiking and Rectifying cells could be either pyramidal or nonpyramidal neurons. The intrinsic physiology of these cell groups may play an important role in auditory cortex function.

Effects of noradrenaline on frequency tuning of rat auditory cortex neurons

The selectivity of rat auditory cortex neurons for pure tone frequency was studied during and after ionophoretic application (5-40 nA) of noradrenaline in urethane-anaesthetized rats. The dominant effect induced by noradrenaline was a significant decrease in spontaneous (93/268 cells) and evoked activity (133/268 cells) which outlasted the application. In the whole population of cells (n = 268) the signal-to-noise ratio, computed using as the signal either the mean evoked response or the response at the best frequency, was unchanged during noradrenaline application. It was significantly increased only for cells showing significantly decreased spontaneous activity, and was significantly decreased for cells showing increased spontaneous activity. Frequency selectivity was significantly increased for the whole population during and after noradrenaline application. It was also significantly increased for cells showing significantly decreased evoked activity, and was significantly decreased for cells showing increased evoked activity. The noradrenaline-induced inhibition was not blocked by propranolol (beta antagonist); it was blocked by prazosin (alpha1 antagonist) and partly mimicked by phenylephrine (alpha1 agonist). GABA, which also inhibited spontaneous and evoked activity, slightly increased the signal-to-noise ratio and significant increased frequency selectivity. However, when noradrenaline was ejected in the presence of bicuculline at doses that were able to block GABAergic inhibition, the inhibitory effects of noradrenaline on spontaneous and evoked activity were still observed. The possible function of noradrenaline-induced inhibitions in sensory cortices is briefly discussed.

The effects of peripheral deafferentation on spontaneously bursting neurons in the somatosensory cortex of waking cats

Single neurons (n = 356) were studied in the forelimb representation of awake, quietly resting cats. Thirty-five spontaneously bursting neurons in a sample of 206 cells recorded before forelimb deafferentation were compared to 39 spontaneously bursting neurons in a sample of 127 neurons studied 1-3 weeks after deafferentation. The probability of encountering bursting neurons increased significantly following deafferentation from 17% to 31% of the sample (P < 0.005). The same 5 classes of bursting cells were observed after deafferentation but there were significant changes in the duration of interspike intervals in some classes, in the probability of observing certain classes, and in the proportion of spikes found in bursts. The probability of encountering class III cells, a class thought to consist primarily of non-inactivating pyramidal burst neurons, nearly doubled and the average interspike interval length within the burst increased from 1.9 to 3.0 ms. The burst structure in the other classes did not change but they were found less frequently. These other classes may include inhibitory interneurons which receive less excitatory drive after deafferentation and therefore provide less inhibition to class III cells. The differential behavior of the different classes of bursting cells may be one reason why the overall level of spontaneous activity does not change after deafferentation and it suggests that there are homeostatic mechanisms in primary somatosensory cortex that maintain a certain level of neural activity.

Spontaneous periodic synchronized bursting during formation of mature patterns of connections in cortical cultures

Long-term recording of spontaneous activity in cultured cortical neuronal networks was carried out using substrates containing multi-electrode arrays. Spontaneous uncorrelated firing appeared within the first 3 days and transformed progressively into synchronized bursting within a week. By 30 days from the establishment of the culture, the network exhibited a complicated non-periodic, synchronized activity pattern which showed no changes for more than 2 months and thus represented the mature state of the network. Pharmacological inhibition of activity only during the period when regular synchronized bursting was observed was capable of producing a different mature activity pattern from the control. These results suggest that periodic synchronized bursting plays a critical role in the development of synaptic connections.

Neuronal activity in normal and deafferented forelimb somatosensory cortex of the awake cat

Three hundred and seventy-three neurons were recorded from the forelimb representation in the primary somatosensory cortex of unanesthetized, quietly resting adult cats. Of these, 177 were studied from 2 days to 3 weeks after transection of the radial, median and ulnar nerves. Following deafferentation the proportion of cells without receptive fields increased from 24 to 82%, however, the average rate of spontaneous activity did not change nor did the probability of encountering a neuron with a receptive field as a function of depth. Receptive field sizes increased dramatically following deafferentation and the response changed from a reliable short-latency, brisk discharge to one that did not occur on every stimulus. After deafferentation the edges of the receptive field often could not be defined accurately. Spontaneous activity in 31% (n = 47) of the neurons from deprived cortex could be modulated by manipulations of the body but these changes were sufficiently slow and ill-defined that they were not classified as a receptive field. In some cases, manipulation of the body gradually reduced the discharge rate. This slow decline in activity was different from the abrupt inhibition of spontaneous activity elicited by somatic stimuli in another class of cells (n = 18). In other cases the manipulation produced a gradual increase in the discharge rate. After deafferentation antidromically identified corticothalamic and pyramidal tract neurons did not display behaviors different from their counterparts in normal cortex. However, the mean latency for synaptic activation from the ventroposterior thalamus increased from 2.7 ms to 4.6 ms. The lost forelimb receptive fields were rarely replaced by inputs from adjacent body parts over the two-week duration of this study. Most responses to somatic stimuli obtained from cortical neurons in the deafferented cortex were clearly abnormal.

The mode of activation of a barrel column: response properties of single units in the somatosensory cortex of the mouse upon whisker deflection

Response properties of single units in the mouse barrel cortex were studied to determine the sequence in which the neurons that form a cortical column become activated by a single 'natural' stimulus. Mice (n = 11) were anaesthetized with urethane. For a total of 153 cells, grouped by cortical layer, responses to a standardized deflection of a single whisker were characterized using poststimulus time and latency histograms. Usually, for each unit, data were collected for stimulation of its principal whisker (PW; the whiskers corresponding to the barrel column in which the cell was located) and of the four whiskers surrounding the PW. In all layers, PW stimulation evoked responses at shorter latency than surround whisker stimulation. In layers II-III and IV a bimodal distribution of cells according to latency to PW stimulation was found. Statistical analysis indicated the presence of two classes of cells in each of these layers: 'fast' units (latency < 15 ms) and 'slow' units (latency > or = 15 ms). The great majority of cells in layers I, V and VI fired at latencies of > 20 ms to PW stimulation. In general, stimulation of surround whiskers evoked a smaller response than PW stimulation. The fast cells of layer IV showed the greatest response to PW stimulation (mean = 1.78 spikes/100 ms poststimulus). Their firing was maximal during the 10-20 ms poststimulus epoch, while the slow layer IV cells fired maximally during the 20-30 ms poststimulus epoch. Surround inhibition occurred in all layers within the first 10 ms after stimulus onset, during which period the fast cells are the most active ones, and are thus likely to be responsible for the surround inhibition. This notion is supported by an analysis of spike duration that showed that eight of the ten cells with a thin spike (supposed to be GABAergic; McCormick et al., J. Neurophysiol., 54, 782-806, 1985), had PW latencies of < 15 ms. We conclude that the activation of a barrel column is initially inhibitory in nature.

Formation of spike response to sound tones in cat auditory cortex neurons: interaction of excitatory and inhibitory effects

Responses of the auditory cortical neurons to sound tones were studied extra- and intracellularly in anaesthetized cats. The pattern of response to tone stimuli could most differ in neurons tuned to the same sound frequency and forming a vertical cortical column. Phasic reactions were found in 69% of the neurons studied. Such neurons were encountered in all cortical layers but about 50% of them were localized at a depth of 0.4-1.0 mm, which corresponds to layers III and IV of the auditory cortex. Neurons with phasic reactions were able to respond to a relatively narrow frequency band that demonstrates high discriminative ability of these cells to the frequency analysis of sound signals. Inhibitory processes realized via both forward afferent and recurrent intracortical inhibition mechanisms play particular roles in the formation of phasic reaction of such neurons to different frequency tones. Twenty-six per cent of neurons generated tonic responses to the sound. The majority of such cells (94%) were localized at a depth of 1.0-2.2 mm, which corresponds to cortical layers V and VI. Inhibitory processes exert a much lesser influence on formation of tonic responses in comparison with phasic ones. Neurons of the tonic type, in contrast to phasic neurons, respond to a wider frequency band; their lower ability to discriminate sound frequency is obvious. Parameters of the responses of tonic neurons strictly correlated with the duration and intensity of the acoustic signal. The possibility of some tonic neurons playing an inhibitory role in auditory cortex is discussed [Volkov I. O. et al. (1989) Neurophysiology, Kiev 21, 498-506, 613-620 (in Russian)]. A small portion of the auditory area AI neurons (2%) demonstrated the suppression of background activity during tone stimulation. They were localized mainly in deep cortical layers (V and VI). Intracortical inhibition is supposed to play a dominant role in the formation of this type of response. About 3% of the studied auditory cortex neurons with background activity generated no response to tonic stimuli. Such cells were usually encountered in the superficial auditory cortex layers (I and II).

Identification of neurons producing long-term potentiation in the cat motor cortex: intracellular recordings and labeling

Intracellular, in vivo recordings were used to identify and subsequently to label neurons in the cat motor cortex in which long-term potentiation (LTP) was induced. Thirty-nine motor cortical neurons that produced excitatory postsynaptic potentials (EPSPs) in response to microstimulation in areas 1-2 (SI) or in area 5a (SIII) were studied. Amplitudes of EPSPs produced in response to test stimulation (1 Hz) were recorded before and after tetanic stimulation (200 Hz, 20 seconds). In 25/39 cells (64%), EPSP amplitudes were significantly increased following the tetanic stimulation (65 +/- 51% average increase), and remained at the potentiated level as long as stable recordings could be maintained (20 +/- 18 minutes, maximum = 90 minutes). LTP was induced exclusively in cells that produced monosynaptic EPSPs in response to area 1-2 or area 5a stimulation. Of the 39 analyzed cells, 13 were labeled by intracellular injections of 5% biocytin. Neurons in which LTP was induced included both pyramidal and nonpyramidal cells and were located exclusively in layers II or III of the motor cortex; cells in deeper cortical layers were not potentiated. These findings indicate that various corticocortical inputs can increase the efficacy of synaptic transmission in a subset of motor cortical neurons. We propose that this plasticity in synaptic transmission constitutes one of the bases of motor learning and memory.

Evidence for a cholinergic mechanism of "learned" changes in the responses of barrel field neurons of the awake and undrugged rat

Due to its functional importance and its large and highly differentiated central projections, the vibrissal system of rodents is a prime object for the study of sensory plasticity, especially at the cortical level: the representation of vibrissae in the "barrel field", a part of the somatic cortex, is exceptionally precise and is susceptible to experience-induced changes. In a previous series of experiments, we found that a sensory-sensory conditioning procedure, pairing two vibrissal stimulations, produces significant changes in responses of single neurons of the barrel field in the chronic awake and undrugged rat: (1) the appearance of an excitatory response to a stimulus that was ineffective before pairing ("conditioned response"); (2) the modifications of pre-existing responses consisting of the suppression of afferent inhibition and the appearance of long-latency excitatory components. We report here that the micro-iontophoretic application of atropine abolishes "conditioned responses" and restores afferent inhibition. Acetylcholine facilitates an enlargement of the receptive field and induces a sustained mode of discharge to stimuli. These data provide a new and direct support to the hypothesis that cholinergic mechanisms are involved in the sensory cortex plasticity.