Synaptic integration in layer IV of the ferret striate cortex

Energy-efficient information transfer at thalamocortical synapses

We have previously shown that the physiological size of postsynaptic currents maximises energy efficiency rather than information transfer across the retinothalamic relay synapse. Here, we investigate information transmission and postsynaptic energy use at the next synapse along the visual pathway: from relay neurons in the thalamus to spiny stellate cells in layer 4 of the primary visual cortex (L4SS). Using both multicompartment Hodgkin-Huxley-type simulations and electrophysiological recordings in rodent brain slices, we find that increasing or decreasing the postsynaptic conductance of the set of thalamocortical inputs to one L4SS cell decreases the energy efficiency of information transmission from a single thalamocortical input. This result is obtained in the presence of random background input to the L4SS cell from excitatory and inhibitory corticocortical connections, which were simulated (both excitatory and inhibitory) or injected experimentally using dynamic-clamp (excitatory only). Thus, energy efficiency is not a unique property of strong relay synapses: even at the relatively weak thalamocortical synapse, each of which contributes minimally to the output firing of the L4SS cell, evolutionarily-selected postsynaptic properties appear to maximise the information transmitted per energy used.

Compared to other organs, the brain consumes a vast amount of energy for its size. Most of this energy is used to power the electrical and chemical processes that support neural computation. As the energy supply to the brain is limited, it follows that this computation should be energetically efficient. Previously, we showed that this is indeed the case for transmission of information between cells at synapses. Synapses transferring information from the retina to the brain do not maximise information transmission—some information is lost and does not reach the visual cortex. Instead, these synapses maximise the information transmitted per energy used. Here, we demonstrate that this principle of energetic efficiency also holds at the next synapse in the visual pathway, the thalamocortical synapse. This synapse is weaker and competes with hundreds of other inputs to influence the output firing of the next cell. Using detailed simulations of cortical neurons, and electrophysiological recordings in rodent brain slices, we found that this relatively weak synapse also does not maximise information transmission. Instead, it maximises the amount of information transmitted per energy used. This suggests that energy efficiency at synapses could be a common design principle in the brain.

Intrinsic Plasticity for Natural Competition in Koniocortex-Like Neural Networks

In this paper, we use the neural property known as intrinsic plasticity to develop neural network models that resemble the koniocortex, the fourth layer of sensory cortices. These models evolved from a very basic two-layered neural network to a complex associative koniocortex network. In the initial network, intrinsic and synaptic plasticity govern the shifting of the activation function, and the modification of synaptic weights, respectively. In this first version, competition is forced, so that the most activated neuron is arbitrarily set to one and the others to zero, while in the second, competition occurs naturally due to inhibition between second layer neurons. In the third version of the network, whose architecture is similar to the koniocortex, competition also occurs naturally owing to the interplay between inhibitory interneurons and synaptic and intrinsic plasticity. A more complex associative neural network was developed based on this basic koniocortex-like neural network, capable of dealing with incomplete patterns and ideally suited to operating similarly to a learning vector quantization network. We also discuss the biological plausibility of the networks and their role in a more complex thalamocortical model.

Axon topography of layer IV spiny cells to orientation map in the cat primary visual cortex (area 18)

Our aim was to reveal the relationship between layer IV horizontal connections and the functional architecture of the cat primary visual cortex because these connections play important roles in the first cortical stage of visual signals integration. We investigated bouton distribution of spiny neurons over an orientation preference map using in vivo optical imaging, unit recordings, and single neuron reconstructions. The radial extent of reconstructed axons (14 star pyramidal and 9 spiny stellate cells) was ~1.5 mm. In the vicinity of the parent somata (<400 μm), boutons occupied chiefly iso-orientations, however, more distally, 7 cells projected preferentially to non-iso-orientations. Boutons of each cell were partitioned into 1-15 distinct clusters based on the mean-shift algorithm, of which 57 clusters preferred iso-orientations and 43 clusters preferred cross-orientations, each showing sharp orientation preference "tuning." However, unlike layer III/V pyramidal cells preferring chiefly iso-orientations, layer IV cells were engaged with broad orientations because each bouton cluster from the same cell could show different orientation preference. These results indicate that the circuitry of layer IV spiny cells is organized differently from that of iso-orientation dominant layer III/V cells and probably processes visual signals in a different manner from that of the superficial and deeper layers.

Neural activity in frontal cortical cell layers: evidence for columnar sensorimotor processing

The mammalian frontal cortex (FCx) is at the top of the brain's sensorimotor hierarchy and includes cells in the supragranular Layer 2/3, which integrate convergent sensory information for transmission to infragranular Layer 5 cells to formulate motor system outputs that control behavioral responses. Functional interaction between these two layers of FCx was examined using custom-designed ceramic-based microelectrode arrays (MEAs) that allowed simultaneous recording of firing patterns of FCx neurons in Layer 2/3 and Layer 5 in nonhuman primates performing a simple go/no-go discrimination task. This unique recording arrangement showed differential encoding of task-related sensory events by cells in each layer with Layer 2/3 cells exhibiting larger firing peaks during presentation of go target and no-go target task images, whereas Layer 5 cells showed more activity during reward contingent motor responses in the task. Firing specificity to task-related events was further demonstrated by synchronized firing between pairs of cells in different layers that occupied the same vertically oriented "column" on the MEA. Pairs of cells in different layers recorded at adjacent "noncolumnar" orientations on the MEA did not show synchronized firing during the same task-related events. The results provide required evidence in support of previously suggested task-related sensorimotor processing in the FCx via functionally segregated minicolumns.

Inter- and Intralaminar Subcircuits of Excitatory and Inhibitory Neurons in Layer 6a of the Rat Barrel Cortex

Approximately half the excitatory neurons in layer 6 (L6) of the rat barrel cortex project to the thalamus with axon collaterals ramifying in the granular L4; the remaining project within cortex with collaterals restricted to infragranular laminae. In analogy, L6 inhibitory neurons also include locally arborizing and inter-laminar projecting neurons. We examined whether L6 neurons participating in different laminar interactions were also morphologically and electrically distinct. Corticothalamic (CT) neurons were labeled by in vivo injections of a retrogradely transported fluorescent tracer into the primary thalamic nucleus. Whole cell current-clamp recordings were performed from labeled and unlabeled L6 neurons in brain slices of juvenile rats; the morphology of cells was subsequently recovered and reconstructed. Corticocortical (CC) neurons were distinguished from CT cells based on the absence of a subcortical projection and the predominantly infragranular arborization of their axon collaterals. Two morphological CC subtypes could be further distinguished based on the structure of their apical dendrite. Electrically, CT neurons had shorter membrane time-constants and action potential (AP) durations and higher rheobase currents. CC neurons fired high-frequency spike doublets or triplets on sustained depolarization; the burst frequency also distinguished the two morphological CC subtypes. Among inhibitory L6 cells, the L4-projecting (L6iL4) and local (L6iL6) inhibitory neurons also had contrasting firing properties; L6iL4 neurons had broader APs and lower maximal firing rates. We propose that L6 excitatory and inhibitory neurons projecting to L4 constitute specialized subcircuits distinct from the infragranular network in their connectivity and firing patterns.

Top-down laminar organization of the excitatory network in motor cortex

Cortical layering is a hallmark of the mammalian neocortex and a major determinant of local synaptic circuit organization in sensory systems. In motor cortex, the laminar organization of cortical circuits has not been resolved, although their input-output operations are crucial for motor control. Here, we developed a general approach for estimating layer-specific connectivity in cortical circuits and applied it to mouse motor cortex. From these data we computed a laminar presynaptic --> postsynaptic connectivity matrix, W(post,pre), revealing a complement of stereotypic pathways dominated by layer 2 outflow to deeper layers. Network modeling predicted, and experiments with disinhibited slices confirmed, that stimuli targeting upper, but not lower, cortical layers effectively evoked network-wide events. Thus, in motor cortex, descending excitation from a preamplifier-like network of upper-layer neurons drives output neurons in lower layers. Our analysis provides a quantitative wiring-diagram framework for further investigation of the excitatory networks mediating cortical mechanisms of motor control.

Developmental switch in the contribution of presynaptic and postsynaptic NMDA receptors to long-term depression

NMDA receptor (NMDAR) activation is required for many forms of learning and memory as well as sensory system receptive field plasticity, yet the relative contribution of presynaptic and postsynaptic NMDARs over cortical development remains unknown. Here we demonstrate a rapid developmental loss of functional presynaptic NMDARs in the neocortex. Presynaptic NMDARs enhance neurotransmitter release at synapses onto visual cortex pyramidal cells in young mice [before postnatal day 20 (P20)], but they have no apparent effect after the onset of the critical period for receptive field plasticity (>P23). Immunoelectron microscopy revealed that the loss of presynaptic NMDAR function is likely attributable in part to a 50% reduction in the prevalence of presynaptic NMDARs. Coincident with the observed loss of presynaptic NMDAR function, there is an abrupt change in the mechanisms of timing-dependent long-term depression (tLTD). Induction of tLTD before the onset of the critical period requires activation of presynaptic but not postsynaptic NMDARs, whereas the induction of tLTD in older mice requires activation of postsynaptic NMDARs. By demonstrating that both presynaptic and postsynaptic NMDARs contribute to the induction of synaptic plasticity and that their relative roles shift over development, our findings define a novel, and perhaps general, property of synaptic plasticity in emerging cortical circuits.

Lack of orientation and direction selectivity in a subgroup of fast-spiking inhibitory interneurons: cellular and synaptic mechanisms and comparison with other electrophysiological cell types

Neurons in cat area 17 can be grouped in 4 different electrophysiological cell classes (regular spiking, intrinsically bursting, chattering, and fast spiking [FS]). However, little is known of the functional properties of these different cell classes. Here we compared orientation and direction selectivity between these cell classes in cat area 17 and found that a subset of FS inhibitory neurons, usually with complex receptive fields, exhibited little selectivity in comparison with other cell types. Differences in occurrence and amplitude of gamma-range membrane fluctuations, as well as in numbers of action potentials in response to optimal visual stimuli, did not parallel differences observed for orientation and direction selectivity. Instead, differences in selectivity resulted mostly from differences in tuning of the membrane potential responses, although variations in spike threshold also contributed: weakly selective FS neurons exhibited both a lower spike threshold and more broadly tuned membrane potential responses in comparison with the other cell classes. Our results are consistent with the hypothesis that a subgroup of FS neurons receives connections and possesses intrinsic properties allowing the generation of weakly selective responses. The existence of weakly selective inhibitory neurons is consistent with orientation selectivity models that rely on broadly tuned inhibition.

Subcolumnar Dendritic and Axonal Organization of Spiny Stellate and Star Pyramid Neurons within a Barrel in Rat Somatosensory Cortex

Excitatory neurons at the level of cortical layer 4 in the rodent somatosensory barrel field often display a strong eccentricity in comparison with layer 4 neurons in other cortical regions. In rat, dendritic symmetry of the 2 main excitatory neuronal classes, spiny stellate and star pyramid neurons (SSNs and SPNs), was quantified by an asymmetry index, the dendrite-free angle. We carefully measured shrinkage and analyzed its influence on morphological parameters. SSNs had mostly eccentric morphology, whereas SPNs were nearly radially symmetric. Most asymmetric neurons were located near the barrel border. The axonal projections, analyzed at the level of layer 4, were mostly restricted to a single barrel except for those of 3 interbarrel projection neurons. Comparing voxel representations of dendrites and axon collaterals of the same neuron revealed a close overlap of dendritic and axonal fields, more pronounced in SSNs versus SPNs and considerably stronger in spiny L4 neurons versus extragranular pyramidal cells. These observations suggest that within a barrel dendrites and axons of individual excitatory cells are organized in subcolumns that may confer receptive field properties such as directional selectivity to higher layers, whereas the interbarrel projections challenge our view of barrels as completely independent processors of thalamic input.

The sensorimotor slice

The reciprocal connections between primary motor cortex (M1) and primary somatosensory cortex (S1) are hypothesized to play a role in an animal's ability to update its motor plan in response to changes in the sensory periphery. These interactions provide the sensory cortex with anticipatory knowledge of motor plans. In the mouse neocortex there are representations of the body surface within both M1 and S1. Utilizing physiologically targeted micro injections of biotinylated dextran amine into either the whisker representation of M1 (wM1) or S1 (wS1) we characterized the axonal pathways connecting these two areas. We then used this data to determine a plane of section that contained both whisker M1 and whisker S1 and maintained the axonal pathway between these two areas. In vitro physiological studies demonstrated that excitatory synaptic connections are maintained in this novel plane of section. The sensorimotor slice is an ideal preparation to study inter-areal cortical connectivity.

Circuits that build visual cortical receptive fields

Neural sensitivity to basic elements of the visual scene changes dramatically as information is handed from the thalamus to the primary visual cortex in cats. Famously, thalamic neurons are insensitive to stimulus orientation whereas their cortical targets easily resolve small changes in stimulus angle. There are two main types of cells in the visual cortex, simple and complex, defined by the structure of their receptive fields. Simple cells are thought to lay the groundwork for orientation selectivity. This review focuses on approaches that combine anatomy with physiology at the intracellular level, to explore the circuits that build simple receptive fields and that help to maintain neural sensitivity to stimulus features even when luminance contrast changes.

Intracellularly labeled pyramidal neurons in the cortical areas projecting to the spinal cord. I. Electrophysiological properties of pyramidal neurons

To study cortical motor control, we examined electrical characteristics of pyramidal neurons in the present report, and intra- or juxta-columnar connections of the pyramidal neurons to corticospinal neurons in the accompanying report. Pyramidal neurons were intracellularly recorded and stained in slices of rat motorsensory cortices (areas FL, HL and M1) where many corticospinal neurons were labeled retrogradely. They were morphologically classified into classical, star and other modified pyramidal neurons, and electrophysiologically into regular-spiking (RS), intrinsic bursting (IB) and irregular-spiking (IS) neurons on the basis of spiking pattern in response to 500 ms depolarizing current pulses. RS responses were further divided into RS with slow adaptation (RS-SA) and RS with fast adaptation (RS-FA). The electrical properties were associated with the laminar location of the neurons; RS-SA responses were observed frequently in layer II/III and less frequently in layers IV-VI, and IB and IS responses were exclusively found in layers V and VI, respectively. Interestingly, all layer IV neurons in area FL/HL were RS-FA star-pyramidal neurons, whereas layer IV neurons in area M1 were RS-SA classical pyramidal neurons. Although weak stimulation of areas FL/HL and M1 is known to elicit movement, these results suggest different information processings between the two areas.

Single-column thalamocortical network model exhibiting gamma oscillations, sleep spindles, and epileptogenic bursts

To better understand population phenomena in thalamocortical neuronal ensembles, we have constructed a preliminary network model with 3,560 multicompartment neurons (containing soma, branching dendrites, and a portion of axon). Types of neurons included superficial pyramids (with regular spiking [RS] and fast rhythmic bursting [FRB] firing behaviors); RS spiny stellates; fast spiking (FS) interneurons, with basket-type and axoaxonic types of connectivity, and located in superficial and deep cortical layers; low threshold spiking (LTS) interneurons, which contacted principal cell dendrites; deep pyramids, which could have RS or intrinsic bursting (IB) firing behaviors, and endowed either with nontufted apical dendrites or with long tufted apical dendrites; thalamocortical relay (TCR) cells; and nucleus reticularis (nRT) cells. To the extent possible, both electrophysiology and synaptic connectivity were based on published data, although many arbitrary choices were necessary. In addition to synaptic connectivity (by AMPA/kainate, NMDA, and GABA(A) receptors), we also included electrical coupling between dendrites of interneurons, nRT cells, and TCR cells, and--in various combinations--electrical coupling between the proximal axons of certain cortical principal neurons. Our network model replicates several observed population phenomena, including 1) persistent gamma oscillations; 2) thalamocortical sleep spindles; 3) series of synchronized population bursts, resembling electrographic seizures; 4) isolated double population bursts with superimposed very fast oscillations (>100 Hz, "VFO"); 5) spike-wave, polyspike-wave, and fast runs (about 10 Hz). We show that epileptiform bursts, including double and multiple bursts, containing VFO occur in rat auditory cortex in vitro, in the presence of kainate, when both GABA(A) and GABA(B) receptors are blocked. Electrical coupling between axons appears necessary (as reported previously) for persistent gamma and additionally plays a role in the detailed shaping of epileptogenic events. The degree of recurrent synaptic excitation between spiny stellate cells, and their tendency to fire throughout multiple bursts, also appears critical in shaping epileptogenic events.

Spatiotemporal evolution of excitation and inhibition in the rat barrel cortex investigated with multielectrode arrays

We investigated the spatiotemporal evolution of activity in the rat barrel cortex using multielectrode arrays (MEAs). In acute brain slices, field potentials were recorded simultaneously from 60 electrodes with high spatial and temporal resolution. This new technique allowed us to map functionally discrete barrels and to observe the interplay between the excitatory and inhibitory network. The local field potentials (LFPs) were elicited by focal electrical stimulation in layer 4 (L4). Excitation recorded in a single barrel was first confined to the stimulated barrel and subsequently spread in a columnar manner to layer 2/3 (L2/3). This excitation in L4 and lower L2/3 was followed by inhibition curtailing excitation to a short period lasting only approximately 2 ms. In the uppermost layer, a long-lasting (approximately 10 ms), laterally spreading band of excitation remained active. Blockade of GABAA-receptors resulted in a long-lasting and diffuse activation of L4 and lower L2/3 and abolition of activation of the upper L2/3. Thus inhibition not only shaped the spatial-temporal map of excitation in L4 and lower L2/3 but also resulted indirectly in an excitatory action in the superficial layers. Stimulation in L6 revealed a feedforward inhibition to L4 and subsequently an excitatory L6-L4-L6 loop. The complex interplay between excitation and inhibition opens two spatial windows of excitation in the infra- and supragranular layers. They may prepare the L5 pyramidal neuron for associating top-down input from other cortical regions with bottom-up input from the whisker pad to generate behaviorally relevant output.

Functionally distinct inhibitory neurons at the first stage of visual cortical processing

Here we explore inhibitory circuits at the thalamocortical stage of processing in layer 4 of the cat's visual cortex, focusing on the anatomy and physiology of the interneurons themselves. Our immediate aim was to explore the inhibitory mechanisms that contribute to orientation selectivity, perhaps the most dramatic response property to emerge across the thalamocortical synapse. The broader goal was to understand how inhibitory circuits operate. Using whole-cell recording in cats in vivo, we found that layer 4 contains two populations of inhibitory cells defined by receptive field class--simple and complex. The simple cells were selective for stimulus orientation, whereas the complex cells were not. Our observations help to explain how neurons become sensitive to stimulus orientation and maintain that selectivity as stimulus contrast changes. Overall, the work suggests that different sources of inhibition, either selective for specific features or broadly tuned, interact to provide appropriate representations of elements within the environment.

Electrophysiological classes of cat primary visual cortical neurons in vivo as revealed by quantitative analyses

To facilitate the characterization of cortical neuronal function, the responses of cells in cat area 17 to intracellular injection of current pulses were quantitatively analyzed. A variety of response variables were used to separate the cells into subtypes using cluster analysis. Four main classes of neurons could be clearly distinguished: regular spiking (RS), fast spiking (FS), intrinsic bursting (IB), and chattering (CH). Each of these contained significant subclasses. RS neurons were characterized by trains of action potentials that exhibited spike frequency adaptation. Morphologically, these cells were spiny stellate cells in layer 4 and pyramidal cells in layers 2, 3, 5, and 6. FS neurons had short-duration action potentials (<0.5 ms at half height), little or no spike frequency adaptation, and a steep relationship between injected current intensity and spike discharge frequency. Morphologically, these cells were sparsely spiny or aspiny nonpyramidal cells. IB neurons typically generated a low frequency (<425 Hz) burst of spikes at the beginning of a depolarizing current pulse followed by a tonic train of action potentials for the remainder of the pulse. These cells were observed in all cortical layers, but were most abundant in layer 5. Finally, CH neurons generated repetitive, high-frequency (350-700 Hz) bursts of short-duration (<0.55 ms) action potentials. Morphologically, these cells were layer 2-4 (mainly layer 3) pyramidal or spiny stellate neurons. These results indicate that firing properties do not form a continuum and that cortical neurons are members of distinct electrophysiological classes and subclasses.

Functional synaptic projections onto subplate neurons in neonatal rat somatosensory cortex

Subplate neurons (SPn) play an important role in the formation of thalamocortical connections during early development and show glutamatergic and GABAergic spontaneous synaptic activity. We characterized these synaptic inputs by performing whole-cell recordings from SPn in somatosensory cortical slices of postnatal day 0-3 rats. At -70 mV, electrical stimulation of the thalamocortical afferents elicited in 68% of the SPn a monosynaptic CNQX-sensitive postsynaptic current (PSC). These fast PSCs were mediated by AMPA receptors, because they were prolonged by cyclothiazide and blocked by GYKI 52466. On membrane depolarization, thalamocortical stimulation elicited in 50% of the cells an additional slow monosynaptic component mediated by NMDA receptors. Stimulation of the cortical plate evoked in 72% of SPn a monosynaptic AMPA receptor-mediated PSC with an additional NMDA component at depolarized membrane potentials and in 40% of the investigated cells polysynaptic responses, depending on GABA(A) and NMDA receptors. Stimulation of the subplate elicited in 67% of SPn a monosynaptic dual-component PSC mediated by AMPA and NMDA receptors activated at -70 mV and in 12% of SPn a monosynaptic single-component PSC mediated by AMPA receptors with an additional NMDA component activated at depolarized membrane potentials. A monosynaptic GABAergic response could be observed in 68% of SPn after stimulation of the subplate. In gramicidin-perforated patch recordings, bath application of GABA caused membrane depolarization to -40 mV and elicited action potentials. These results demonstrate that SPn receive distinct functional synaptic inputs arising from the thalamus, cortical plate, and subplate, indicating that SPn are capable of integrating and processing information from cortical and subcortical regions.

Laminar processing of stimulus orientation in cat visual cortex

One of the most salient features to emerge in visual cortex is sensitivity to stimulus orientation. Here we asked if orientation selectivity, once established, is altered by successive stages of cortical processing. We measured patterns of orientation selectivity at all depths of the cat's visual cortex by making whole-cell recordings with dye-filled electrodes. Our results show that the synaptic representation of orientation indeed changes with position in the microcircuit, as information passes from layer 4 to layer 2+3 to layer 5. At the earliest cortical stage, for simple cells in layer 4, orientation tuning curves for excitation (depolarization) and inhibition (hyperpolarization) had similar peaks (within 0-7 deg, n = 11) and bandwidths. Further, the sharpness of orientation selectivity covaried with receptive field geometry (r = 0.74) - the more elongated the strongest subregion, the shaper the tuning. Tuning curves for complex cells in layer 2+3 also had similar peaks (within 0-4 deg, n = 7) and bandwidths. By contrast, at a later station, layer 5, the preferred orientation for excitation and inhibition diverged such that the peaks of the tuning curves could be as far as 90 deg apart (average separation, 54 deg; n = 6). Our results support the growing consensus that orientation selectivity is generated at the earliest cortical level and structured similarly for excitation and inhibition. Moreover, our novel finding that the relative tuning of excitation and inhibition changes with laminar position helps resolve prior controversy about orientation selectivity at later phases of processing and gives a mechanistic view of how the cortical circuitry recodes orientation.

Synaptic physiology of the flow of information in the cat's visual cortex in vivo

Each stage of the striate cortical circuit extracts novel information about the visual environment. We asked if this analytic process reflected laminar variations in synaptic physiology by making whole-cell recording with dye-filled electrodes from the cat's visual cortex and thalamus; the stimuli were flashed spots. Thalamic afferents terminate in layer 4, which contains two types of cell, simple and complex, distinguished by the spatial structure of the receptive field. Previously, we had found that the postsynaptic and spike responses of simple cells reliably followed the time course of flash-evoked thalamic activity. Here we report that complex cells in layer 4 (or cells intermediate between simple and complex) similarly reprised thalamic activity (response/trial, 99 +/- 1.9 %; response duration 159 +/- 57 ms; latency 25 +/- 4 ms; average +/- standard deviation; n = 7). Thus, all cells in layer 4 share a common synaptic physiology that allows secure integration of thalamic input. By contrast, at the second cortical stage (layer 2+3), where layer 4 directs its output, postsynaptic responses did not track simple patterns of antecedent activity. Typical responses to the static stimulus were intermittent and brief (response/trial, 31 +/- 40 %; response duration 72 +/- 60 ms, latency 39 +/- 7 ms; n = 11). Only richer stimuli like those including motion evoked reliable responses. All told, the second level of cortical processing differs markedly from the first. At that later stage, ascending information seems strongly gated by connections between cortical neurons. Inputs must be combined in newly specified patterns to influence intracortical stages of processing.

Synaptic connections between layer 4 spiny neurone-layer 2/3 pyramidal cell pairs in juvenile rat barrel cortex: physiology and anatomy of interlaminar signalling within a cortical column

Whole-cell voltage recordings were obtained from 64 synaptically coupled excitatory layer 4 (L4) spiny neurones and L2/3 pyramidal cells in acute slices of the somatosensory cortex ('barrel' cortex) of 17- to 23-days-old rats. Single action potentials (APs) in the L4 spiny neurone evoked single unitary EPSPs in the L2/3 pyramidal cell with a peak amplitude of 0.7 +/- 0.6 mV. The average latency was 2.1 +/- 0.6 ms, the rise time was 0.8 +/- 0.3 ms and the decay time constant was 12.7 +/- 3.5 ms. The percentage of failures of an AP in a L4 spiny neurone to evoke a unitary EPSP in the L2/3 pyramidal cell was 4.9 +/- 8.8 % and the coefficient of variation (c.v.) of the unitary EPSP amplitude was 0.27 +/- 0.13. Both c.v. and percentage of failures decreased with increased average EPSP amplitude. Postsynaptic glutamate receptors (GluRs) in L2/3 pyramidal cells were of the N-methyl-D-aspartate (NMDA) receptor (NMDAR) and the non-NMDAR type. At -60 mV in the presence of extracellular Mg2+ (1 mM), 29 +/- 15 % of the EPSP voltage-time integral was blocked by NMDAR antagonists. In 0 Mg2+, the NMDAR/AMPAR ratio of the EPSC was 0.50 +/- 0.29, about half the value obtained for L4 spiny neurone connections. Burst stimulation of L4 spiny neurones showed that EPSPs in L2/3 pyramidal cells depressed over a wide range of frequencies (1-100 s(-1) ). However, at higher frequencies (30 s(-1)) EPSP summation overcame synaptic depression so that the summed EPSP was larger than the first EPSP amplitude in the train. The number of putative synaptic contacts established by the axonal collaterals of the L4 projection neurone with the target neurone in layer 2/3 varied between 4 and 5, with an average of 4.5 +/- 0.5 (n = 13 pairs). Synapses were established on basal dendrites of the pyramidal cell. Their mean geometric distance from the pyramidal cell soma was 67 +/- 34 microm (range, 16-196 microm). The results suggest that each connected L4 spiny neurone produces a weak but reliable EPSP in the pyramidal cell. Therefore transmission of signals to layer 2/3 is likely to have a high threshold requiring simultaneous activation of many L4 neurons, implying that L4 spiny neurone to L2/3 pyramidal cell synapses act as a gate for the lateral spread of excitation in layer 2/3.

Fundamental differences between the thalamocortical recipient layers of the cat auditory and visual cortices

In visual and somatosensory cortices of several species, spiny stellate cells in layer 4 are the first elements in signal processing where thalamic information is integrated and emergent receptive field properties are generated and sent on to more superficial cortical layers. In vivo and in vitro experiments have provided important information about how the anatomy and physiology of these cells and this layer fit into the functional cortical circuitry. No such data exist for the auditory cortex but are requisite if we are to understand whether ideas about information processing in one sensory cortical area can be generalized to another. Accordingly, we used in vitro slices from which to record and labeled cells in the middle layers of the cat auditory and visual cortices to compare basic anatomical and physiological features of cells recovered in similar layers using the same methods. Our results demonstrate a striking difference in a basic characteristic of two primary sensory cortical areas. In the visual cortex, spiny stellate cells predominate, receive short-latency synaptic inputs, and project to supergranular layers. No such spiny stellate population is encountered in the middle layers of the auditory cortex. Spiny cells that are not stellate or pyramidal are occasionally encountered but, as a group, do not display consistent anatomical or physiological features that might allow them to function as auditory cortical versions of the visual spiny stellates. Rather, pyramidal cells in the lower half of layer 3 and layer 4 appear to have assumed this role.

Diverse types of interneurons generate thalamus-evoked feedforward inhibition in the mouse barrel cortex

Sensory information, relayed through the thalamus, arrives in the neocortex as excitatory input, but rapidly induces strong disynaptic inhibition that constrains the cortical flow of excitation both spatially and temporally. This feedforward inhibition is generated by intracortical interneurons whose precise identity and properties were not known. To characterize interneurons generating feedforward inhibition, neurons in layers IV and V of mouse somatosensory ("barrel") cortex in vitro were tested in the cell-attached configuration for thalamocortically induced firing and in the whole-cell mode for synaptic responses. Identification as inhibitory or excitatory neurons was based on intrinsic firing patterns and on morphology revealed by intracellular staining. Thalamocortical stimulation evoked action potentials in approximately 60% of inhibitory interneurons but in <5% of excitatory neurons. The inhibitory interneurons that fired received fivefold larger thalamocortical inputs compared with nonfiring inhibitory or excitatory neurons. Thalamocortically evoked spikes in inhibitory interneurons followed at short latency the onset of excitatory monosynaptic responses in the same cells and slightly preceded the onset of inhibitory responses in nearby neurons, indicating their involvement in disynaptic inhibition. Both nonadapting (fast-spiking) and adapting (regular-spiking) inhibitory interneurons fired on thalamocortical stimulation, as did interneurons expressing parvalbumin, calbindin, or neither calcium-binding protein. Morphological analysis revealed that some interneurons might generate feedforward inhibition within their own layer IV barrel, whereas others may convey inhibition to upper layers, within their own or in adjacent columns. We conclude that feedforward inhibition is generated by diverse classes of interneurons, possibly serving different roles in the processing of incoming sensory information.

Diverse types of interneurons generate thalamus-evoked feedforward inhibition in the mouse barrel cortex

Sensory information, relayed through the thalamus, arrives in the neocortex as excitatory input, but rapidly induces strong disynaptic inhibition that constrains the cortical flow of excitation both spatially and temporally. This feedforward inhibition is generated by intracortical interneurons whose precise identity and properties were not known. To characterize interneurons generating feedforward inhibition, neurons in layers IV and V of mouse somatosensory ("barrel") cortex in vitro were tested in the cell-attached configuration for thalamocortically induced firing and in the whole-cell mode for synaptic responses. Identification as inhibitory or excitatory neurons was based on intrinsic firing patterns and on morphology revealed by intracellular staining. Thalamocortical stimulation evoked action potentials in approximately 60% of inhibitory interneurons but in <5% of excitatory neurons. The inhibitory interneurons that fired received fivefold larger thalamocortical inputs compared with nonfiring inhibitory or excitatory neurons. Thalamocortically evoked spikes in inhibitory interneurons followed at short latency the onset of excitatory monosynaptic responses in the same cells and slightly preceded the onset of inhibitory responses in nearby neurons, indicating their involvement in disynaptic inhibition. Both nonadapting (fast-spiking) and adapting (regular-spiking) inhibitory interneurons fired on thalamocortical stimulation, as did interneurons expressing parvalbumin, calbindin, or neither calcium-binding protein. Morphological analysis revealed that some interneurons might generate feedforward inhibition within their own layer IV barrel, whereas others may convey inhibition to upper layers, within their own or in adjacent columns. We conclude that feedforward inhibition is generated by diverse classes of interneurons, possibly serving different roles in the processing of incoming sensory information.

Tuning to Y-like figures in the cat striate neurons

Sensitivity to symmetric or asymmetric Y-like figures and crosses of different shapes and orientations flashed in the receptive field was studied in 101 neurons of the cat striate cortex (area 17) and compared with their orientation tuning to a single light bar. Selective sensitivity to the Y-like figure (figure/bar response ratio more than 1.25) was found in 78/101 neurons (77.2% of cases) and to the cross-in 54/101 units (53.4%). In 62.5% of neurons with sensitivity to both figures, sensitivity to the Y-like figure was higher than to a cross. Tuning to Y-like figure was typically (60%) selective to both its shape and orientation. The remaining Y-like selective neurons exhibited invariant tuning to orientation and/or shape of the figure. The preferred angles between two lines of Y-like figures were distributed in the range of 22.5-157.5 degrees with slight preference to 90 degrees, while crosses of 45 degrees and 90 degrees angles were preferable. Response magnitudes to a single bar, a Y-like figure and a cross were positively correlated. Possible mechanisms and functional implication of the striate sensitivity to Y-like figures are discussed.

Columnar organization of dendrites and axons of single and synaptically coupled excitatory spiny neurons in layer 4 of the rat barrel cortex

Cortical columns are the functional units of the neocortex that are particularly prominent in the "barrel" field of the somatosensory cortex. Here we describe the morphology of two classes of synaptically coupled excitatory neurons in layer 4 of the barrel cortex, spiny stellate, and star pyramidal cells, respectively. Within a single barrel, their somata tend to be organized in clusters. The dendritic arbors are largely confined to layer 4, except for the distal part of the apical dendrite of star pyramidal neurons that extends into layer 2/3. In contrast, the axon of both types of neurons spans the cortex from layer 1 to layer 6. The most prominent axonal projections are those to layers 4 and 2/3 where they are largely restricted to a single cortical column. In layers 5 and 6, a small fraction of axon collaterals projects also across cortical columns. Consistent with the dense axonal projection to layers 4 and 2/3, the total number and density of boutons per unit axonal length was also highest there. Electron microscopy combined with GABA postimmunogold labeling revealed that most (>90%) of the synaptic contacts were established on dendritic spines and shafts of excitatory neurons in layers 4 and 2/3. The largely columnar organization of dendrites and axons of both cell types, combined with the preferential and dense projections within cortical layers 4 and 2/3, suggests that spiny stellate and star pyramidal neurons of layer 4 serve to amplify thalamic input and relay excitation vertically within a single cortical column.

Columnar organization of dendrites and axons of single and synaptically coupled excitatory spiny neurons in layer 4 of the rat barrel cortex

Cortical columns are the functional units of the neocortex that are particularly prominent in the "barrel" field of the somatosensory cortex. Here we describe the morphology of two classes of synaptically coupled excitatory neurons in layer 4 of the barrel cortex, spiny stellate, and star pyramidal cells, respectively. Within a single barrel, their somata tend to be organized in clusters. The dendritic arbors are largely confined to layer 4, except for the distal part of the apical dendrite of star pyramidal neurons that extends into layer 2/3. In contrast, the axon of both types of neurons spans the cortex from layer 1 to layer 6. The most prominent axonal projections are those to layers 4 and 2/3 where they are largely restricted to a single cortical column. In layers 5 and 6, a small fraction of axon collaterals projects also across cortical columns. Consistent with the dense axonal projection to layers 4 and 2/3, the total number and density of boutons per unit axonal length was also highest there. Electron microscopy combined with GABA postimmunogold labeling revealed that most (>90%) of the synaptic contacts were established on dendritic spines and shafts of excitatory neurons in layers 4 and 2/3. The largely columnar organization of dendrites and axons of both cell types, combined with the preferential and dense projections within cortical layers 4 and 2/3, suggests that spiny stellate and star pyramidal neurons of layer 4 serve to amplify thalamic input and relay excitation vertically within a single cortical column.

Synaptic efficacy and reliability of excitatory connections between the principal neurones of the input (layer 4) and output layer (layer 5) of the neocortex

A prerequisite for the understanding of how a cortical column functions is a description of small and defined neuronal circuits consisting of only a few identified neurones. Here we summarise, with particular reference to the barrel cortex, the morphological and physiological properties of two synaptic connections, namely those between pairs of spiny neurones in layer 4 and pairs of pyramidal cells in layer 5. While layer 4 spiny neurones are the cortical input neurones that amplify and relay incoming excitation from the periphery, layer 5 pyramidal cells integrate neuronal activity both within and across cortical columns and subsequently distribute it to both cortical and subcortical brain regions.

Anatomy, physiology, and synaptic responses of rat layer V auditory cortical cells and effects of intracellular GABA(A) blockade

The varied extracortical targets of layer V make it an important site for cortical processing and output, which may be regulated by differences in the pyramidal neurons found there. Two populations of projection neurons, regular spiking (RS) and intrinsic bursting (IB), have been identified in layer V of some sensory cortices, and differences in their inhibitory inputs have been indirectly demonstrated. In this report, IB and RS cells were identified in rat auditory cortical slices, and differences in thalamocortical inhibition reaching RS and IB cells were demonstrated directly using intracellular GABA(A) blockers. Thalamocortical synaptic input to RS cells was always a combination of excitation and both GABA(A) and GABA(B) inhibition. Stimulation seldom triggered a suprathreshold response. IB cell synaptic responses were mostly excitatory, and stimulation usually triggered action potentials. This apparent difference was confirmed directly using intracellular chloride channel blockers. Before intracellular diffusion, synaptic responses were stable and similar to control conditions. Subsequently, GABA(A) was blocked, revealing a cell's total excitatory input. On GABA(A) blockade, RS cells responded to synaptic stimulation with large, suprathreshold excitatory events, indicating that excitation, while always present in these cells, is masked by GABA(A). In IB cells that had visible GABA(A) input, it often masked an excitatory postsynaptic potential (EPSP) that could lead to additional suprathreshold events. These findings indicate that IB cells receive less GABA(A)-mediated inhibitory input and are able to spike or burst in response to thalamocortical synaptic stimulation far more readily than RS cells. Such differences may have implications for the influence each cell type exerts on its postsynaptic targets.

Reliable synaptic connections between pairs of excitatory layer 4 neurones within a single 'barrel' of developing rat somatosensory cortex

1. Dual whole-cell recordings were made from pairs of synaptically coupled excitatory neurones in the 'barrel field' in layer (L) 4 in slices of young (postnatal day 12-15) rat somatosensory cortex. The majority of interconnected excitatory neurones were spiny stellate cells with an asymmetrical dendritic arborisation largely confined to a single barrel. The remainder were star pyramidal cells with a prominent apical dendrite terminating in L2/3 without forming a tuft. 2. Excitatory synaptic connections were examined between 131 pairs of spiny L4 neurones. Single presynaptic action potentials evoked unitary EPSPs with a peak amplitude of 1.59 +/- 1.51 mV (mean +/- s. d.), a latency of 0.92 +/- 0.35 ms, a rise time of 1.53 +/- 0.46 ms and a decay time constant of 17.8 +/- 6.3 ms. 3. At 34-36 C, the coefficient of variation (c.v.) of the unitary EPSP amplitude was 0. 37 +/- 0.16 and the percentage of failures to evoke an EPSP was 5.3 +/- 7.8 %. The c.v. and failure rate decreased with increasing amplitude of the unitary EPSP. 4. Postsynaptic glutamate receptors in spiny L4 neurones were of the AMPA and NMDA type. At -60 mV in the presence of 1 mM Mg2+, NMDA receptors contributed 39.3 +/- 12.5 % to the EPSP integral. In Mg2+-free solution, the NMDA receptor/AMPA receptor ratio of the EPSC was 0.86 +/- 0.64. 5. The number of putative synaptic contacts established by the projection neurone with the target neurone varied between two and five with a mean of 3.4 +/- 1.0 (n = 11). Synaptic contacts were exclusively found in the barrel in which the cell pair was located and were preferentially located on secondary to quarternary dendritic branches. Their mean geometric distance from the soma was 68.8 +/- 37.4 microm (range, 33.4-168.0 microm). The number of synaptic contacts and mean EPSP amplitude showed no significant correlation. 6. The results suggest that in L4 of the barrel cortex synaptic transmission between spiny neurones is largely restricted to a single barrel. The connections are very reliable, probably due to a high release probability, and have a high efficacy because of the compact structure of the dendrites and axons of spiny neurones. Intrabarrel connections thus function to amplify and distribute the afferent thalamic activity in the vertical directions of a cortical column.

The critical period for ocular dominance plasticity in the Ferret's visual cortex

Microelectrode recordings and optical imaging of intrinsic signals were used to define the critical period for susceptibility to monocular deprivation (MD) in the primary visual cortex of the ferret. Ferrets were monocularly deprived for 2, 7 or >14 d, beginning between postnatal day 19 (P19) and P110. The responses of visual cortical neurons to stimulation of the two eyes were used to gauge the onset, peak, and decline of the critical period. MDs ending before P32 produced little or no loss of response to the deprived eye. MDs of 7 d or more beginning around P42 produced the greatest effects. A rapid decline in cortical susceptibility to MD was observed after the seventh week of life, such that MDs beginning between P50 and P65 were approximately half as effective as those beginning on P42; MDs beginning after P100 did not reduce the response to the deprived eye below that to the nondeprived eye. At all ages, 2 d deprivations were 55-85% as effective as 7 d of MD. Maps of intrinsic optical responses from the deprived eye were weaker and less well tuned for orientation than those from the nondeprived eye, with the weakest maps seen in the hemisphere ipsilateral to the deprived eye. Analysis of the effects of 7 d and longer deprivations revealed a second period of plasticity in cortical responses in which MD induced an effect like that of strabismus. After P70, MD caused a marked loss of binocular responses with little or no overall loss of response to the deprived eye. The critical period measured here is compared to other features of development in ferret and cat.

The critical period for ocular dominance plasticity in the Ferret's visual cortex

Microelectrode recordings and optical imaging of intrinsic signals were used to define the critical period for susceptibility to monocular deprivation (MD) in the primary visual cortex of the ferret. Ferrets were monocularly deprived for 2, 7 or >14 d, beginning between postnatal day 19 (P19) and P110. The responses of visual cortical neurons to stimulation of the two eyes were used to gauge the onset, peak, and decline of the critical period. MDs ending before P32 produced little or no loss of response to the deprived eye. MDs of 7 d or more beginning around P42 produced the greatest effects. A rapid decline in cortical susceptibility to MD was observed after the seventh week of life, such that MDs beginning between P50 and P65 were approximately half as effective as those beginning on P42; MDs beginning after P100 did not reduce the response to the deprived eye below that to the nondeprived eye. At all ages, 2 d deprivations were 55-85% as effective as 7 d of MD. Maps of intrinsic optical responses from the deprived eye were weaker and less well tuned for orientation than those from the nondeprived eye, with the weakest maps seen in the hemisphere ipsilateral to the deprived eye. Analysis of the effects of 7 d and longer deprivations revealed a second period of plasticity in cortical responses in which MD induced an effect like that of strabismus. After P70, MD caused a marked loss of binocular responses with little or no overall loss of response to the deprived eye. The critical period measured here is compared to other features of development in ferret and cat.

Role of GABAB receptor-mediated inhibition in reciprocal interareal pathways of rat visual cortex

In neocortex, synaptic inhibition is mediated by gamma-aminobutyric acid-A (GABAA) and GABAB receptors. By using intracellular and patch-clamp recordings in slices of rat visual cortex we studied the balance of excitation and inhibition in different intracortical pathways. The study was focused on the strength of fast GABAA- and slow GABAB-mediated inhibition in interareal forward and feedback connections between area 17 and the secondary, latero-medial visual area (LM). Our results demonstrate that in most layer 2/3 neurons forward inputs elicited excitatory postsynaptic potentials (EPSPs) that were followed by fast GABAA- and slow GABAB-mediated hyperpolarizing inhibitory postsynaptic potentials (IPSPs). These responses resembled those elicited by horizontal connections within area 17 and those evoked by stimulation of the layer 6/white matter border. In contrast, in the feedback pathway hyperpolarizing fast and slow IPSPs were rare. However weak fast and slow IPSPs were unmasked by bath application of GABAB receptor antagonists. Because in the feedback pathway disynaptic fast and slow IPSPs were rare, polysynaptic EPSPs were more frequent than in forward, horizontal, and interlaminar circuits and were activated over a broader stimulus range. In addition, in the feedback pathway large-amplitude polysynaptic EPSPs were longer lasting and showed a late component whose onset coincided with that of slow IPSPs. In the forward pathway these late EPSPs were only seen with stimulus intensities that were below the activation threshold of slow IPSPs. Unlike strong forward inputs, feedback stimuli of a wide range of intensities increased the rate of ongoing neuronal firing. Thus, when forward and feedback inputs are simultaneously active, feedback inputs may provide late polysynaptic excitation that can offset slow IPSPs evoked by forward inputs and in turn may promote recurrent excitation through local intracolumnar circuits. This may provide a mechanism by which feedback inputs from higher cortical areas can amplify afferent signals in lower areas.

Tonic and bursting activity in the cuneate nucleus of the chloralose-anesthetized cat

Whole-cell recordings were obtained from cuneate neurons in anesthetized, paralysed cats. Stimulation of the contralateral medial lemniscus permitted us to separate projection cells from presumed interneurons. Pericruciate motor cortex electrical stimulation inhibited postsynaptically all the projection cells (n=57) and excited all the presumed interneurons (n=29). The cuneothalamic cells showed an oscillatory and a tonic mode of activity. Membrane depolarization and primary afferent stimulation converted the oscillatory to the tonic mode. Hyperpolarizing current steps applied to projection neurons induced a depolarizing sag and bursts of conventional spikes in current-clamp records. This indicates the probable existence of low-threshold and hyperpolarization-activated inward currents. Also, the hyperpolarization induced on projection cells by motor cortex stimulation deinactivated a low-threshold conductance that led to bursting activity. The presumed cuneate interneurons had larger and more proximally located peripheral receptive fields than the cuneothalamic cells. Finally, experiments specifically designed to test whether motor cortex-induced presynaptic inhibition could be postsynaptically detected gave negative results. These results demonstrate, for the first time, that the cuneothalamic cells possess both bursting and tonic firing modes, and that membrane depolarization, whether produced by injection of positive current or by primary afferent stimulation, converts the oscillatory into the tonic mode.

Synaptic interactions between smooth and spiny neurones in layer 4 of cat visual cortex in vitro

1. Dual intracellular recording was used to examine the interactions between neighbouring spiny (excitatory) and smooth (inhibitory) neurones in layer 4 of cat visual cortex in vitro. Synaptic connections were found in seventeen excitatory-inhibitory neurone pairs, along with one inhibitory-inhibitory connection. 2. Fast excitatory inputs onto smooth neurones (basket cells) from spiny cells (spiny stellate or pyramidal cells) (n = 6) produce large excitatory postsynaptic potentials (EPSPs) of up to 4 mV mean amplitude, whereas basket cells evoke slower inhibitory postsynaptic potentials (IPSPs) in their postsynaptic targets (n = 17), of smaller amplitude (up to 1.6 mV at membrane potentials of -60 mV). 3. Both types of PSP appear to be multiquantal, and both may exhibit depression of up to 60 % during short trains of presynaptic spikes. This depression can involve presynaptic and/or postsynaptic factors. 4. One-third (n = 5) of the spiny cell-smooth cell pairs tested were reciprocally connected, and in the one pair for which the suprathreshold interactions were comprehensively investigated, the pattern of basket cell firing was strongly influenced by the activity in the connected excitatory neurone. The basket cell was only effective in inhibiting spiny cell firing when the excitatory neurone was weakly driven.

A polysynaptic feedback circuit in rat visual cortex

Feedback connections from extrastriate cortex to primary visual cortex (V1) in the primate may provide "top-down" information that plays a role in visual attention and object recognition. Our work in a rodent model of corticocortical circuitry demonstrates that feedback pathways synapse preferentially with pyramidal cells in V1 () and favor excitation over inhibition in cortical microcircuits (). To investigate the polysynaptic circuits activated by feedback inputs, we studied chains of neurons postsynaptic to feedback connections using a combination of axonal tract tracing and anterograde degeneration. This approach enabled independent labeling of local collaterals of forward-projecting neurons in V1 and feedback connections from extrastriate lateromedial (LM) visual area to V1. Postsynaptic targets were identified in the electron microscope after retrograde transport of biotinylated dextran amine (BDA) to identify dendrites of forward-projecting neurons (i.e., from V1 to LM) and postembedding immunogold labeling to identify GABAergic interneurons. The results show that feedback connections provide strong monosynaptic input to forward-projecting neurons in V1. These neurons in turn make local connections that preferentially form synapses with other pyramidal cells ( approximately 97%), many of which were identified as forward-projecting neurons. This indicates that feedback pathways provide input directly to neurons which make the reciprocal forward connection, and that feedback-recipient forward-projecting neurons are strongly interconnected. The function of these excitatory networks within V1 may be to amplify feedback activity and provide a circuit for modulation of striate cortical activity by top-down influences.

A polysynaptic feedback circuit in rat visual cortex

Feedback connections from extrastriate cortex to primary visual cortex (V1) in the primate may provide "top-down" information that plays a role in visual attention and object recognition. Our work in a rodent model of corticocortical circuitry demonstrates that feedback pathways synapse preferentially with pyramidal cells in V1 () and favor excitation over inhibition in cortical microcircuits (). To investigate the polysynaptic circuits activated by feedback inputs, we studied chains of neurons postsynaptic to feedback connections using a combination of axonal tract tracing and anterograde degeneration. This approach enabled independent labeling of local collaterals of forward-projecting neurons in V1 and feedback connections from extrastriate lateromedial (LM) visual area to V1. Postsynaptic targets were identified in the electron microscope after retrograde transport of biotinylated dextran amine (BDA) to identify dendrites of forward-projecting neurons (i.e., from V1 to LM) and postembedding immunogold labeling to identify GABAergic interneurons. The results show that feedback connections provide strong monosynaptic input to forward-projecting neurons in V1. These neurons in turn make local connections that preferentially form synapses with other pyramidal cells ( approximately 97%), many of which were identified as forward-projecting neurons. This indicates that feedback pathways provide input directly to neurons which make the reciprocal forward connection, and that feedback-recipient forward-projecting neurons are strongly interconnected. The function of these excitatory networks within V1 may be to amplify feedback activity and provide a circuit for modulation of striate cortical activity by top-down influences.

Intracortical axonal projections of lamina VI cells of the primary somatosensory cortex in the rat: a single-cell labeling study

A sample of 84 neurons in lamina VIa of rat somatosensory cortex (S1) was juxtacellularly labeled with biocytin, and the axons of the neurons were traced. Three classes of cells were identified as corticothalamic, corticocortical, and local circuit neurons. Corticothalamic cells (46%) are small, short pyramids projecting either to the ventral posteromedial nucleus alone or to the posterior group as well. The former are in upper lamina VI, have apical dendrites terminating in layer IV, and have intracortical collaterals ascending to layer IV as a narrow column about the size of a barrel. The latter are in the lower half of lamina VI, have apical dendrites terminating in layer V, and have a more extensive network of collaterals terminating in the upper part of lamina V. Corticothalamic cells do not project to distant cortical targets through branching axons. Corticocortical cells (44%) are small, short pyramids, inverted or modified pyramids, or bipolar spiny neurons. They send collaterals principally to infragranular layers of S1 and branches to the second somatosensory cortex, the motor cortex, or the corpus callosum. Local circuit neurons (10%) are basket cells, concentrated in upper lamina VI, having smooth, beaded dendrites and a rich collateral network densely covered with varicosities in layers V and VI. We conclude that (1) dendritic morphology and axonal arborizations of corticothalamic cells relate to the projection target; (2) many apparently diverse layer VI cells project to other cortical fields; and (3) lamina VI is a network for corticothalamic and corticocortical communication.

Intracortical axonal projections of lamina VI cells of the primary somatosensory cortex in the rat: a single-cell labeling study

A sample of 84 neurons in lamina VIa of rat somatosensory cortex (S1) was juxtacellularly labeled with biocytin, and the axons of the neurons were traced. Three classes of cells were identified as corticothalamic, corticocortical, and local circuit neurons. Corticothalamic cells (46%) are small, short pyramids projecting either to the ventral posteromedial nucleus alone or to the posterior group as well. The former are in upper lamina VI, have apical dendrites terminating in layer IV, and have intracortical collaterals ascending to layer IV as a narrow column about the size of a barrel. The latter are in the lower half of lamina VI, have apical dendrites terminating in layer V, and have a more extensive network of collaterals terminating in the upper part of lamina V. Corticothalamic cells do not project to distant cortical targets through branching axons. Corticocortical cells (44%) are small, short pyramids, inverted or modified pyramids, or bipolar spiny neurons. They send collaterals principally to infragranular layers of S1 and branches to the second somatosensory cortex, the motor cortex, or the corpus callosum. Local circuit neurons (10%) are basket cells, concentrated in upper lamina VI, having smooth, beaded dendrites and a rich collateral network densely covered with varicosities in layers V and VI. We conclude that (1) dendritic morphology and axonal arborizations of corticothalamic cells relate to the projection target; (2) many apparently diverse layer VI cells project to other cortical fields; and (3) lamina VI is a network for corticothalamic and corticocortical communication.

Different balance of excitation and inhibition in forward and feedback circuits of rat visual cortex

Different cortical areas are linked reciprocally via forward and feedback connections. Forward connections are involved in the representation of retinal images, whereas feedback pathways may play a role in the selection and interpretation of visual information. To examine the synaptic mechanisms of forward and feedback connections between primary and secondary visual cortical areas directly, we have performed intracellular recordings in slices of rat visual cortex. Irrespective of stimulus intensity and membrane potential, 78% (45/58) of the cells in striate cortex activated by feedback input showed monosynaptic responses that were depolarizing only, and inhibitory inputs were evident merely as a slight acceleration in the decay of EPSPs. In contrast, in 89% (17/19) of the cells, stimulation of forward input evoked monosynaptic excitatory postsynaptic potentials (EPSPs), followed by disynaptic, hyperpolarizing inhibitory postsynaptic potentials (IPSPs). EPSPs followed by IPSPs also were recorded after stimulation of local connections within primary visual cortex (92%, 12/13) and after activation of thalamocortical input (91%, 10/11). These results suggest that the synaptic organization of feedback connections are distinct from forward, local, and thalamocortical circuits. The findings further indicate that intracortical back projections exert modulatory influences via synaptic mechanisms in which weak inhibitory input is strongly dominated by excitation.

Recurrent axon collaterals of corticothalamic projection neurons in rat primary somatosensory cortex contribute to excitatory and inhibitory feedback-loops

Intrinsic circuitry within the primary somatosensory cortex of the rat was examined in a combined light and electron microscope study. Corticothalamic projection neurons were retrogradely labeled by applying Phaseolus vulgaris leucoagglutinin (PHA-L) into the ventro-posteromedial thalamic nucleus (VPM). Most labeled neurons were pyramidal cells of layer VI. Postsynaptic targets of recurrent axon collaterals originating from these neurons were assessed in layers IV and V. Single labeled cells, complete with recurrent collaterals, could be isolated in "barrels" in which no anterograde transport had taken place. These findings were confirmed by first eliminating thalamocortical projections from the VPM with kainic acid and then applying PHA-L into the same nucleus. This procedure led to selective retrograde accumulation of tracer in layer VI pyramidal cells. Reconstructed portions of labeled axonal trees reached layer IV, bringing numerous boutons to layers IV, V and VI. The boutons had characteristic drumstick-like shapes. In order to identify postsynaptic targets, 4 sections of axons stemming from 3 neurons were reembedded and serially sectioned for electron microscopy. The ultrastructure of 72 asymmetric synapses, all belonging to identified collaterals, was analysed. Of the 72 terminals, 44 (59.5%) ended on dendritic spines and 30 on shafts of dendrites (40.5%). Perikarya were not among the targets. In a subset of the sample, the nature of the target neurons was examined by postembedding immunohistochemistry for gamma-amino butyric acid (GABA) after staining for PHA-L. A total of 42 labeled terminals was found in layers IV and V; 23 (55%) were located on GABA-negative spines and 19 (45%) on dendritic shafts. Only 6 (32%) of the shafts were GABA-positive. The remaining ones were either clearly GABA-negative, or labeled only at background levels (n = 13; 68%). The results show that most synapses of corticothalamic projection neurons found in layers IV and V terminate on spines and shafts of GABA-negative dendrites. This finding suggests that such recurrent collaterals are involved in both excitatory and inhibitory feedback mechanisms.

Different balance of excitation and inhibition in forward and feedback circuits of rat visual cortex

Different cortical areas are linked reciprocally via forward and feedback connections. Forward connections are involved in the representation of retinal images, whereas feedback pathways may play a role in the selection and interpretation of visual information. To examine the synaptic mechanisms of forward and feedback connections between primary and secondary visual cortical areas directly, we have performed intracellular recordings in slices of rat visual cortex. Irrespective of stimulus intensity and membrane potential, 78% (45/58) of the cells in striate cortex activated by feedback input showed monosynaptic responses that were depolarizing only, and inhibitory inputs were evident merely as a slight acceleration in the decay of EPSPs. In contrast, in 89% (17/19) of the cells, stimulation of forward input evoked monosynaptic excitatory postsynaptic potentials (EPSPs), followed by disynaptic, hyperpolarizing inhibitory postsynaptic potentials (IPSPs). EPSPs followed by IPSPs also were recorded after stimulation of local connections within primary visual cortex (92%, 12/13) and after activation of thalamocortical input (91%, 10/11). These results suggest that the synaptic organization of feedback connections are distinct from forward, local, and thalamocortical circuits. The findings further indicate that intracortical back projections exert modulatory influences via synaptic mechanisms in which weak inhibitory input is strongly dominated by excitation.

A linear model fails to predict orientation selectivity of cells in the cat visual cortex

1. Postsynaptic potentials (PSPs) evoked by visual stimulation in simple cells in the cat visual cortex were recorded using in vivo whole-cell technique. Responses to small spots of light presented at different positions over the receptive field and responses to elongated bars of different orientations centred on the receptive field were recorded. 2. To test whether a linear model can account for orientation selectivity of cortical neurones, responses to elongated bars were compared with responses predicted by a linear model from the receptive field map obtained from flashing spots. 3. The linear model faithfully predicted the preferred orientation, but not the degree of orientation selectivity or the sharpness of orientation tuning. The ratio of optimal to non-optimal responses was always underestimated by the model. 4. Thus non-linear mechanisms, which can include suppression of non-optimal responses and/or amplification of optimal responses, are involved in the generation of orientation selectivity in the primary visual cortex.

Excitatory synaptic inputs to spiny stellate cells in cat visual cortex

In layer 4 of cat visual cortex, the monocular, concentric receptive fields of thalamic neurons, which relay retinal input to the cortex, are transformed into 'simple' cortical receptive fields that are binocular and selective for the precise orientation, direction of motion, and size of the visual stimulus. These properties are thought to arise from the pattern of connections from thalamic neurons, although anatomical studies show that most excitatory inputs to layer 4 simple cells are from recurrently connected circuits of cortical neurons. We examined single fibre inputs to spiny stellate neurons. We examined single fibre inputs to spiny stellate neurons in slices of cat visual cortex, and conclude that thalamocortical synapses are powerful and the responses they evoke are unusually invariant for central synapses. However, the responses to intracortical inputs, although less invariant, are strong enough to provide most of the excitation to simple cells in vivo. Our results suggest that the recurrent excitatory circuits of cortex may amplify the initial feedforward thalamic signal, subserving dynamic modifications of the functional properties of cortical neurons.

Developmental down-regulation of LTD in cortical layer IV and its independence of modulation by inhibition

For in vitro LTD to remain viable as a model for synaptic weakening in visual cortical plasticity, it is crucial that it display a critical period for its induction within layer IV. A complicating factor, however, is that LTD in layer IV is modulated by inhibitory postsynaptic potentials (IPSPs); postsynaptic responses characterized as containing IPSPs do not depress in response to 1 Hz afferent stimulation. By blocking IPSPs intracellularly, we find that the ability to induce LTD in layer IV neurons is restored in juvenile, but not in mature animals. This developmental down-regulation of LTD induction is specific for layer IV when compared with LTD induction in layers II/III. These data are consistent with the hypothesis that an LTD-like phenomenon is involved in critical period plasticity and is apparently independent of developmental changes in inhibitory circuitry.

Microcircuitry of forward and feedback connections within rat visual cortex

Visual cortex in mammals is composed of many distinct areas that are linked by reciprocal connections to form a multilevel hierarchy. Ascending information is sent via forward connections from lower to higher areas and is thought to contribute to the emergence of increasingly complex receptive field properties at higher levels. Descending signals are transmitted via feedback connections from higher to lower areas and are believed to provide information about the context in which a stimulus appears, to contribute to modulation of visual responses by attention, and to play a role in memory processes. To determine whether forward and feedback pathways in rat visual cortex constitute distinct intracortical circuits, we have studied the distribution of reciprocal corticocortical inputs to pyramidal cells and gamma-aminobutyric acid (GABA)ergic interneurons. For this purpose, we chose forward and feedback connections between primary visual cortex and the secondary extrastriate lateromedial (LM) area as a model system. Pathways were traced with the axonal marker phaseolus vulgaris-leucoagglutinin. Labeled terminals were identified in the electron microscope, and GABA immunocytochemistry was used to identify the postsynaptic dendritic shafts of GABAergic interneurons. In both pathways, inputs to pyramidal cells were directed preferentially to dendritic spines and not to shafts. In the forward pathway, 90% of labeled inputs were distributed to pyramidal cells and 10% to interneurons. This proportion was similar to that of nearby unlabeled connections in the neuropil, indicating that forward connections are not selective for pyramidal cells or interneurons. In sharp contrast, feedback connections were significantly different from the unlabeled connections and supplied almost exclusively pyramidal cells (98%). Feedback inputs to GABAergic neurons were five times weaker (2%) relative to the forward direction. These structural differences suggest that disynaptic GABAergic inhibition is much stronger in forward than in feedback pathways. Recent physiological experiments have confirmed this prediction (Shao et al. [1995] Soc. Neurosci. Abstr., 21:1274) and we, therefore, conclude that relatively small anatomical differences in the microcircuitry can have important functional consequences. It remains an open question whether generally reciprocal interareal circuits at all levels of the cortical hierarchy are organized in similar fashion.

Visually evoked calcium action potentials in cat striate cortex

Early intracellular studies of cerebral cortical neurons indicated that synaptic input evokes dendritic action potentials that convey information towards the soma. Subsequent work in vitro established that neocortical neurons produce dendritic Ca2+ action potentials. To determine whether natural stimuli elicit Ca2+ spikes, we combined the techniques of whole-cell recording, pharmacology and quantitative receptive field mapping. Our findings show that visual stimulation routinely evoked Ca2+ spikes in distinct functional and anatomical classes of cells in different layers of the cat striate cortex. Hence regenerative Ca2+ potentials appear to play a role in both the initial and later stages of cortical sensory processing.