Regular spiking and intrinsic bursting pyramidal cells show orthogonal forms of experience-dependent plasticity in layer V of barrel cortex

Most functional plasticity studies in the cortex have focused on layers (L) II/III and IV, whereas relatively little is known of LV. Structural measurements of dendritic spines in?vivo suggest some specialization among LV cell subtypes. We therefore studied experience-dependent plasticity in the barrel cortex using intracellular recordings to distinguish regular spiking (RS) and intrinsic bursting (IB) subtypes. Postsynaptic potentials and suprathreshold responses in?vivo revealed a remarkable dichotomy in RS and IB cell plasticity; spared whisker potentiation occurred in IB but not RS cells while deprived whisker depression occurred in RS but not IB cells. Similar RS/IB differences were found in the LII/III to V connections in brain slices. Modeling studies showed that subthreshold changes predicted the suprathreshold changes. These studies demonstrate the major functional partition of plasticity within a single cortical layer and reveal the LII/III to LV connection as a major excitatory locus of cortical plasticity.     

Rapid development and plasticity of layer 2/3 maps in rat barrel cortex in vivo

Cortical synaptic circuitry develops rapidly in the second postnatal week, simultaneous with experience-dependent turnover of dendritic spines. To relate the emergence of sensory maps to synaptogenesis, we recorded synaptic potentials evoked by whisker deflection in layer 2/3 neurons from postnatal day (P) 12 to 20. At P12, synaptic responses were undetectable. Only 2 days later in life (P14), receptive fields had mature organization. Sensory deprivation, if initiated before P14, disrupted receptive field structure. In layer 4, responses and maps were already mature by P12 and insensitive to deprivation, implying that barrel cortex develops from layer 4 to layer 2/3. Thus, P12-14 is a critical period shared by layer 2/3 synapses and their spines, suggesting that spine plasticity is involved in the refinement of maps.     

Effect of environmental enrichment on morphology of deep layer III and layer V pyramidal cells of occipital cortex in oldest-old rat - A quantitative golgi cox study

Dendrites and dendritic spine density regress extensively during aging in rats housed under standard conditions (SC), which can be ameliorated by housing in the enriched environment (EE). This event is particularly pronounced on neurons where high rates of plasticity are conceivable, such as on projection neurons of archicortical regions of dentate gyrus'. However, effects of EE on neocortical projection neurons are still poorly understood. Therefore, we investigated the effect of EE housing on a deep layer III (L3) and layer V pyramidal cell (L5) morphology in the associative occipital neocortex of male Sprague-Dawley rats at 24 months of age. Rats were randomly distributed in two groups and reared under either SC (n=5) or EE conditions (n=6) for 26 days. In depth quantitative analysis of dendritic tree morphology and spine density on occipital projection neurons, from Golgi-Cox stained sections, showed similar trend in both EE occipital layers L3 and L5. Significant increase was found in total number of dendritic segments (L3 - 37.5 %, L5 - 33 %) and in dendritic diameter of intermediate segments (for more than 20 %), while increase in total spine number was around the level of significance (p>0.55; L3 - 30 %, L5 - 64 %). These findings suggest an outgrowth of new dendritic segments, When compared to archicortical region of dentate gyrus, effects of aging in the associative occipital cortex were less pronounced. Taken together, these findings suggest that structures being more affected by the aging process are more susceptible to the environmental enrichment in old age.     

Different threshold levels of postsynaptic [Ca2+]i have to be reached to induce LTP and LTD in neocortical pyramidal cells

Changes in [Ca²⁺]_i were measured in layer II–III pyramid cells of the rat visual cortex slices during application of either LTP or LTD inducing stimulation protocols. At dendritic sites activated by the stimulated afferents [Ca²⁺]_i reached higher amplitudes and decayed more slowly with LTP than with LTD inducing stimuli. In the presence of Ca²⁺ chelators, the stimulation protocol that would normally produce LTP induced either LTD or failed to induce synaptic modifications altogether. These results support the hypothesis that the polarity of synaptic gain changes depends on the magnitude of postsynaptic [Ca²⁺]_i reponses, the induction of LTP requiring a more pronounced surge of [Ca²⁺_i than the induction of LTD.     

Synaptic properties of layer VI inverted pyramidal cells in the rodent somatosensory cortex

The properties of specific cortical cell types enable greater understanding of how cortical microcircuits process and transmit sensory, motor, and cognitive information. Previous reports have characterized the intrinsic properties of the inverted pyramidal cell (IPC) where the most prominent dendrite is orientated towards the cortical white matter. Using whole cell patch clamp recordings from rat and mouse somatosensory cortex in conjunction with electric microstimulation of the white matter we characterized the synaptic inputs onto IPCs and the more common upright pyramidal cell (UPC) in the infragranular layers. Both classes of pyramidal cells received monosynaptic glutamatergic input following white matter stimulation, but varied on a number of parameters. Most prominently, UPCs displayed higher amplitude responses and showed greater rates of depression compared to IPCs. These data reinforce the view that IPCs are a separate functional class of cortical neuron.     

Pull-push neuromodulation of LTP and LTD enables bidirectional experience-induced synaptic scaling in visual cortex

Neuromodulatory input, acting on G protein-coupled receptors, is essential for the induction of experience-dependent cortical plasticity. Here we report that G-coupled receptors in layer II/III of visual cortex control the polarity of synaptic plasticity through a pull-push regulation of LTP and LTD. In slices, receptors coupled to Gs promote LTP while suppressing LTD; conversely, receptors coupled to Gq11 promote LTD and suppress LTP. In vivo, the selective stimulation of Gs- or Gq11-coupled receptors brings the cortex into LTP-only or LTD-only states, which allows the potentiation or depression of targeted synapses with visual stimulation. The pull-push regulation of LTP/LTD occurs via direct control of the synaptic plasticity machinery and it is independent of changes in NMDAR activation or neuronal excitability. We propose these simple rules governing the pull-push control of LTP/LTD form a general metaplasticity mechanism that may contribute to neuromodulation of plasticity in other cortical circuits.     

Ultrastructural characteristics of layer III pyramidal neurons in the cat primary auditory cortex (Al)

Electron microscope studies were made of retrogradely horseradish peroxidase-labeled pyramidal neurons forming transcallosal projections in layer III of the cat primary auditory cortex (Al). These showed a significant proportion of the somatic membrane to be covered with processes of astroglia, while synapses occupy 20% of the synaptic surface on average. Between 4 and 10 axosomatic synapses were identified on the profiles of callosal cell somata. All these were formed by axonal terminals containing small, flattened synaptic vesicles and had symmetrical contacts. Average length of these synaptic contacts equaled 1.6 µm. Numerous anterogradely horseradish peroxidase-labeled axonal terminals of callosal fibers were found in cortical area Al in amongst retrogradely HP-labeled neurons. The ultrastructural pattern of these is described.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 22, No. 4, pp. 520–526, July–August, 1990.     

Columnar organization of pyramidal cells of the visual cortex in the albino rat]

1. The arrangement of the pyramidal cells of the visual cortex has been investigated by light and electron microscopy in 36 albino rats. 2. It was shown, that the apical dendrites form bundles. The number of dendrites per bundle is about 5 in the lower lamina Vb and 7.5 in the upper lamina Va, the diameter of bundles is about 25.7 mum. The average distance between the centers of bundles is 76,4 mum, and between the peripheries 53.1 mum. 3. The results are compared with physiological and morphological findings. It was shown, that there is an agreement of diameter of bundles und their lateral branching dendrites with diameters of columns of cells and of terminal branchings of specific afferents. 4. The morphological results are graphically reconstructed.     

Quantitative analysis of basal dendritic tree of layer III pyramidal neurons in different areas of adult human frontal cortex

Large long projecting (cortico-cortical) layer IIIc pyramidal neurons were recently disclosed to be in the basis of cognitive processing in primates. Therefore, we quantitatively examined the basal dendritic morphology of these neurons by using rapid Golgi and Golgi Cox impregnation methods among three distinct Brodmann areas (BA) of an adult human frontal cortex: the primary motor BA4 and the associative magnopyramidal BA9 from left hemisphere and the Broca's speech BA45 from both hemispheres. There was no statistically significant difference in basal dendritic length or complexity, as dendritic spine number or their density between analyzed BA's. In addition, we analyzed each of these BA's immunocytochemically for distribution of SMI-32, a marker of largest long distance projecting neurons. Within layer IIIc, the highest density of SMI-32 immunopositive pyramidal neurons was observed in associative BA9, while in primary BA4 they were sparse. Taken together, these data suggest that an increase in the complexity of cortico-cortical network within human frontal areas of different functional order may be principally based on the increase in density of large, SMI-32 immunopositive layer IIIc neurons, rather than by further increase in complexity of their dendritic tree and synaptic network.     

Changes in dendritic spine density on layer 2/3 pyramidal cells within the cingulate cortex of late pregnant and postpartum rats

A rapid upregulation of astrocytic protein expression within area 2 of the cingulate cortex (Cg2) of the maternal rat occurs within 3 h postpartum and persists throughout lactation. Previous studies have shown that similar changes in astrocytic proteins can signal changes in local synapses and dendritic spines. Thus, here we used the Golgi-Cox impregnation technique to compare spine density in layer 2 and 3 pyramidal cells of Cg2, the CA1 region of the hippocampus and the parietal cortex (ParCx) among metestrus, late pregnant (LP), 3-hour postpartum (3H PP) and 16-day postpartum rats (D16 PP). Rats in the 3H PP group had higher numbers of dendritic spines/10 μm on the apical dendrites of pyramidal neurons in both Cg2 and CA1 than the other groups, which did not differ. A similar pattern was observed in basilar dendrites but this failed to reach significance. In Cg2, Sholl analysis revealed that rats in the D16 PP group had a significantly greater extent of dendritic arborization in the basilar region than any other group. These data suggest that the changes in astrocytic proteins that occur in Cg2 in the postpartum period are associated with neuronal plasticity in pyramidal layers 2 and 3.     

M-channels modulate the intrinsic excitability and synaptic responses of layer 2/3 pyramidal neurons in auditory cortex

Neurons in the auditory cortex are believed to utilize temporal patterns of neural activity to accurately process auditory information but the intrinsic neuronal mechanism underlying the control of auditory neural activity is not known. The slowly activating, persistent K⁺ channel, also called M-channel that belongs to the Kv7 family, is already known to be important in regulating subthreshold neural excitability and synaptic summation in neocortical and hippocampal pyramidal neurons. However, its functional role in the primary auditory cortex (A1) has never been characterized. In this study, we investigated the roles of M-channels on neuronal excitability, short-term plasticity, and synaptic summation of A1 layer 2/3 regular spiking pyramidal neurons with whole-cell current-clamp recordings in vitro. We found that blocking M-channels with a selective M-channel blocker, XE991, significantly increased neural excitability of A1 layer 2/3 pyramidal neurons. Furthermore, M-channels controled synaptic responses of intralaminar-evoked excitatory postsynaptic potentials (EPSPs); XE991 significantly increased EPSP amplitude, decreased the rate of short-term depression, and increased the synaptic summation. These results suggest that M-channels are involved in controlling spike output patterns and synaptic responses of A1 layer 2/3 pyramidal neurons, which would have important implications in auditory information processing.     

Distribution of mitochondria in pyramidal cells and boutons in hippocampal cortex

Summary The amount of mitochondria has been recorded in various parts of neurons. This was done in electron micrographs of cerebral cortex from the hippocampal region. The outlines of boutons, somata and dendrites of varying diameters were transferred to tracing paper together with the outlines of the contained mitochondria. The same was done for whole tissue for comparison. After cutting out and weighing the outlined areas, the fraction of the various tissue constituents, or of whole tissue, occupied by mitochondria was determined. The absolute values are shown in the illustrations (Figs. 4–9). The dendritic shafts of pyramidal cells, coursing through stratum radiatum of regio superior (CA 1), are particularly poor in mitochondria (about 2%). In the branches, the amount as a rule increases with decreasing diameter (to nearly 13% in stratum moleculare).Boutons were the structures richest in mitochondria, but the amount varied with location.This study was supported in part by Grant NB 02215 from the National Institute of Neurological Diseases and Blindness, U.S. Public Health Service. The authors are indebted to Mrs. J. L. Vaaland, Miss M. Johansen and Mr. B. V. Johansen for valuable technical assistance.Fellow of The Norwegian Cancer Society during part of this study.     

Excitatory inputs to spiny cells in layers 4 and 6 of cat striate cortex

The principal target of lateral geniculate nucleus in the cat visual cortex is the stellate neurons of layer 4. In previously reported work with intracellular recording and extracellular stimulation in slices of visual cortex, three general classes of fast excitatory synaptic potentials (EPSPs) in layer 4a spiny stellate neurons were identified. One of these classes, characterized by large and relatively invariant amplitudes (mean 1.7 mV, average coefficient of variation (CV) 0.083) were attributed to the action of geniculate axons because, unlike the other two classes, they could not be matched by intracortical inputs, using paired recording. We have examined in detail the properties of this synaptic input in twelve examples, selecting for study those EPSPs where there was secure extracellular stimulation of the single fibre input to a pair of stimuli 50 ms apart. In our analysis, we conclude that the depression that these inputs show to the second stimulus is entirely postsynaptic, since the evidence strongly suggests that the probability of transmitter release at the synaptic site(s) remains 1.0 for both stimuli. We argue that the most plausible explanation for this postsynaptic depression is a reduction in the average probability of opening the synaptic channels. Using a simple biochemical analysis (c.f. Sigworth plot), it is then possible to calculate the number of synaptic channels and their probability of opening, for each of the 12 connections. The EPSPs had a mean amplitude of 1.91 mV (+/- 1.3 mV SD) and a mean CV of 0.067 (+/- 0.022). The calculated number of channels ranged from 20 to 158 (59.4 +/- 48.7) and their probability of opening to the first EPSP had an average of 0.83 (+/- 0.09), with an average depression of the probability to 0.60 for the second EPSP. Geniculate afferents also terminate in layer 6. Intracellular recordings were also made in the upper part of this layer and a total of 51 EPSPs were recorded from pyramidal cells of three principal types. Amongst this dataset we sought EPSPs with similar properties to those characterized in layer 4a. Three examples were found, which is a much lower percentage (6%) than the incidence of putative geniculate EPSPs found in layer 4a (42%).     

Cellular and network contributions to excitability of layer 5 neocortical pyramidal neurons in the rat

There is a considerable gap between investigating the dynamics of single neurons and the computational aspects of neural networks. A growing number of studies have attempted to overcome this gap using the excitation in brain slices elicited by various chemical manipulations of the bath solution. However, there has been no quantitative study on the effects of these manipulations on the cellular and network factors controlling excitability. Using the whole-cell configuration of the patch-clamp technique we recorded the membrane potential from the soma of layer 5 pyramidal neurons in acute brain slices from the somatosensory cortex of young rats at 22 degrees C and 35 degrees C. Using blockers of synaptic transmission, we show distinct changes in cellular properties following modification of the ionic composition of the artificial cerebrospinal fluid (ACSF). Thus both cellular and network changes may contribute to the observed effects of slice excitation solutions on the physiology of single neurons. Furthermore, our data suggest that the difference in the ionic composition of current standard ACSF from that of CSF measured in vivo cause ACSF to depress network activity in acute brain slices. This may affect outcomes of experiments investigating biophysical and physiological properties of neurons in such preparations. Our results strongly advocate the necessity of redesigning experiments routinely carried out in the quiescent acute brain slice preparation.     

Functional organization and plasticity in the adult rat barrel cortex: moving out-of-the-box

Recent advances in functional imaging and neuronal recording techniques demonstrate that the spatial spread and amplitude of whisker functional representation in the somatosensory cortex of the adult rodent is extensive, but subject to modulations. One of the strongest modulators is naturalistic whisker use. In the cortices of rodents that have been transferred from their home cage to live for an extensive period in a naturalistic habitat, there is suppression of evoked neuronal responses accompanied by contraction and sharpening of receptive fields, and contraction and weakening of whisker functional representations. These unexpected characteristics also describe modulations of whisker functional representations in the cortex of a freely exploring rodent during short whisker-based explorations. These and related findings suggest that cortical modulations and plasticity could follow a 'less is more' strategy and, therefore, highlight how different cortical strategies could be utilized for different behavioral demands.     

Clenbuterol reduces GABAergic transmission in prefrontal cortex layer 5/6 pyramidal neurons of juvenile rat via reducing action potentials firing frequency of GABAergic interneurons

Beta‐adrenoceptors (β₂‐AR s) have beneficial effects on prefrontal cortex (PFC ) working memory, however, the cellular and molecular mechanisms are unclear yet. In this study, we probed the effect of β₂‐AR ‐selective agonist clenbuterol (Clen) on synaptic transmission in layer 5/6 pyramidal neurons of PFC . Bath application of Clen reduced spontaneous IPSC (sIPSC ) frequency without effects on sEPSC s. Clen did not alter the frequency and amplitude of miniature IPSC s (mIPSC s), but exerted heterogeneous effects on evoked IPSC s (eIPSC s) recorded from PFC layer 5/6 pyramidal neurons. Clen decreased the firing rate of action potentials of fast‐spiking GABA ergic interneurons. Clen‐induced hyperpolarization of fast‐spiking GABA ergic interneurons required potentiation of an inward rectifier K⁺ channels. Clen‐induced hyperpolarization of fast‐spiking interneurons was dependent on Gs protein rather than cAMP and protein kinase A. Our findings demonstrate that Clen (10 μM) enhances inward rectifier K⁺ channels via Gs protein to cause membrane hyperpolarization of fast‐spiking GABA ergic interneurons resulting in reduction of action potentials firing rate to reduce GABA ergic transmission.