Altered spatial patterns of functional thalamocortical connections in the barrel cortex after neonatal infraorbital nerve cut revealed by optical recording

Early-life maturation of the somatosensory cortex: sensory experience and beyond

Early life experiences shape physical and behavioral outcomes throughout lifetime. Sensory circuits are especially susceptible to environmental and physiological changes during development. However, the impact of different types of early life experience are often evaluated in isolation. In this mini review, we discuss the specific effects of postnatal sensory experience, sleep, social isolation, and substance exposure on barrel cortex development. Considering these concurrent factors will improve understanding of the etiology of atypical sensory perception in many neuropsychiatric and neurodevelopmental disorders.

A Transient Translaminar GABAergic Interneuron Circuit Connects Thalamocortical Recipient Layers in Neonatal Somatosensory Cortex

GABAergic activity is thought to influence developing neocortical sensory circuits. Yet the late postnatal maturation of local layer (L)4 circuits suggests alternate sources of GABAergic control in nascent thalamocortical networks. We show that a population of L5b, somatostatin (SST)-positive interneuron receives early thalamic synaptic input and, using laser-scanning photostimulation, identify an early transient circuit between these cells and L4 spiny stellates (SSNs) that disappears by the end of the L4 critical period. Sensory perturbation disrupts the transition to a local GABAergic circuit, suggesting a link between translaminar and local control of SSNs. Conditional silencing of SST+ interneurons or conversely biasing the circuit toward local inhibition by overexpression of neuregulin-1 type 1 results in an absence of early L5b GABAergic input in mutants and delayed thalamic innervation of SSNs. These data identify a role for L5b SST+ interneurons in the control of SSNs in the early postnatal neocortex.

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Early postnatal thalamic synaptic input onto L5b somatostatin interneurons

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Transient reciprocal connectivity between L5b INs and L4 spiny stellate cells

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Sensory activity is required for the transition to a local L4 GABAergic circuit

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Molecular bias toward early local IN synapses delays thalamic innervation of SSNs

Characterization of early cortical population response to thalamocortical input in vitro

The in vitro thalamocortical slice preparation of mouse barrel cortex allows for stimulation of the cortex through its natural afferent thalamocortical pathway. This preparation was used here to investigate the first stage of cortical processing in the large postsynaptic dendritic networks as revealed by voltage sensitive dye imaging (VSDI). We identified the precise location and dimensions of two clearly distinguishable dendritic networks, one in the granular layer (GL) IV and one in the infragranular layer (IGL) V and VI and showed that they have different physiological properties. DiI fluorescent staining further revealed that thalamocortical axons project on to these two networks in the typical barrel like form, not only in the granular but also in the IGL. Finally we investigated the short-term dynamics of both the VSDI signal and the local field potential (LFP) in response to a train of eight-pulses at various frequencies in both these layers. We found evidence of differences in the plasticity between the first two response peaks compared to the remaining six peaks as well as differences in short-term plasticity between the VSDI response and the LFP. Our findings suggest, that at least early cortical processing takes place in two separate dendritic networks that may stand at the beginning of further parallel computation. The detailed characterization of the parameters of these networks may provide tools for further research into the complex dynamics of large dendritic networks and their role in cortical computation.

Functional convergence of thalamic and intrinsic projections to cortical layers 4 and 6

Ascending sensory information is conveyed from the thalamus to layers 4 and 6 of sensory cortical areas. Interestingly, receptive field properties of cortical layer 6 neurons are different from those in layer 4. Do such differences reflect distinct inheritance patterns from the thalamus or are they derived instead from local cortical circuits? To distinguish between these possibilities, we utilized in vitro slice preparations containing the thalamocortical pathways in the auditory and somatosensory systems. Responses from neurons in layers 4 and 6 that resided in the same column were recorded using whole-cell patch clamp. Laser-scanning photostimulation via uncaging of glutamate in the thalamus and cortex was used to map the functional topography of thalamocortical and intracortical inputs to each layer. In addition, we assessed the functional divergence of thalamocortical inputs by optical imaging of flavoprotein autofluorescence. We found that the thalamocortical inputs to layers 4 and 6 originated from the same thalamic domain, but the intracortical projections to the same neurons differed dramatically. Our results suggest that the intracortical projections, rather than the thalamic inputs, to each layer contribute more to the differences in their receptive field properties.

Functional thalamocortical synapse reorganization from subplate to layer IV during postnatal development in the reeler-like mutant rat (shaking rat Kawasaki)

Transient synapse formation between thalamic axons and subplate neurons is thought to be important in thalamocortical targeting. Shaking rat Kawasaki (SRK), having reversed cortical layering similarly observed in reeler mouse, provides an interesting model system to test this idea. The spatial and temporal pattern of excitation was investigated using optical recording with voltage-sensitive dyes in thalamocortical slice preparations from SRK. At postnatal day 0 (P0), a strong optical response was elicited within the superplate of the SRK in the cell layer corresponding to subplate in wild-type (WT) rats. By P3, this response rapidly descended into deep cortical layers comprised of layer IV cells, as identified with 5-bromo-2'-deoxyuridine birthdating at embryonic day 17. During the first 3 postnatal days, both the subplate and cortical plate responses were present, but by P7, the subplate response was abolished. Tracing individual axons in SRK revealed that at P0-P3, a large number of thalamocortical axons reach the superplate, and by P7-P10, the ascending axons develop side branches into the lower or middle cortical layers. Synaptic currents were also demonstrated in WT subplate cells and in SRK superficial cortical cells using whole-cell recording. These currents were elicited monosynaptically, because partial AMPA current blockade did not modify the latencies. These results suggest that the general developmental pattern of synapse formation between thalamic axons and subplate (superplate) neurons in WT and SRK is very similar, and individual thalamic arbors in cortex are considerably remodeled during early postnatal development to find layer IV equivalent neurons.

Bilateral effects of spinal overhemisections on the development of the somatosensory system in rats

Connections of the forepaw regions of somatosensory cortex (S1) were determined in rats reared to maturity after spinal cord overhemisections at cervical level C3 on postnatal day 3. Overhemisections cut all ascending and descending pathways and intervening gray on one side of the spinal cord and the pathways of the dorsal funiculus contralaterally. Bilateral lesions of the dorsal columns reduced the size of the brainstem nuclei by 41%, and the ventroposterior lateral subnucleus (VPL) of the thalamus by 20%. Bilateral lesions also prevented the emergence of the normal cytochrome oxidase barrel pattern in forepaw and hindpaw regions of S1. Injections of wheat germ agglutinin conjugated to horseradish peroxidase were placed in the forepaw region of granular S1 and surrounding dysgranular S1 contralateral to the hemisection. The VPL nucleus was densely labeled, whereas the adjoining ventroposterior medial subnucleus, VPM, representing the head, was unlabeled. Thus, there was no evidence of abnormal connections of VPM to forepaw cortex. Foci of transported label in the ipsilateral hemisphere appeared to be in normal locations and of normal extents, but connections in the opposite hemisphere were broadly and nearly uniformly distributed in sensorimotor cortex in a pattern similar to that in postnatal rats. Rats with incomplete lesions that spared the dorsal column pathway on the left side but not the right demonstrated surprisingly normal distributions of callosal connections in the nondeprived right hemisphere, even though the injected left hemisphere was deprived. Thus, the development of the normal pattern of callosal connections depends on dorsal column input and not on normal interhemsipheric interactions.

Prenatal development of neural excitation in rat thalamocortical projections studied by optical recording

To elucidate the formation of early thalamocortical synapses we recorded optical images with voltage-sensitive dyes from the cerebral cortex of prenatal rats by selective thalamic stimulation of thalamocortical slice preparations. At embryonic day (E) 17, thalamic stimulation elicited excitation that rapidly propagated through the internal capsule to the cortex. These responses lasted less than 15 ms, and were not affected by the application of glutamate receptor antagonists, suggesting that they might reflect presynaptic fiber responses. At E18, long-lasting (more than 300 ms) responses appeared in the internal capsule and in subplate. By E19, long-lasting responses increased in the cortical subplate. By E21, shortly before birth, the deep cortical layers were also activated in addition to the subplate. These long-lasting responses seen in the internal capsule and subplate were blocked by the antagonist perfusion, but the first spike-like responses still remained. The laminar location of the responses was confirmed in the same slices by Nissl staining and subplate cells were labeled by birthdating with bromodeoxyuridine at E13. Our results demonstrate that there is a few days delay between the arrival of thalamocortical axons at the subplate at E16 and the appearance of functional thalamocortical synaptic transmission at E19. Since thalamocortical connections are already functional within the subplate and in the deep cortical plate at embryonic ages, prenatal thalamocortical synaptic connections could influence cortical circuit formation before birth.

Spontaneous development of synchronous oscillatory activity during maturation of cortical networks in vitro

Recent studies have focused attention on mechanisms of spontaneous large-scale wavelike activity during early development of the neocortex. In this study, we describe and characterize synchronous neuronal activity that occurs in cultured cortical networks naturally without pharmacological intervention. The synchronous activity that can be detected by means of Fluo-3 fluorescence imaging starts to develop at the beginning of the second week in culture and eventually includes the entire neuronal population about 1 wk later. A synchronous increase of [Ca(2+)](i) in the neuronal population is associated with a burst of action potentials riding on a long-lasting depolarization recorded in a single cell. It is suggested that this depolarization results directly from synaptic current, which was comprised of at least three different components mediated by AMPA, N-methyl-D-aspartate (NMDA), and GABA(A) receptors. We never observed a gradually depolarizing pacemaker potential and found no evidence for a change of excitability during inter-burst periods. However, we found evidence for a period of synaptic depression after bursts. Network excitability recovers gradually over seconds from this depression that can explain the episodic nature of spontaneous network activity. Using pharmacological manipulation to investigate the propagation of activity in the network, we show that synchronous network activity depends on both glutamatergic and GABA(A)ergic neurotransmission during a brief period. Reversal potential of GABA(A) receptor-mediated current was found to be significantly more positive than resting membrane potential both at 1 and 2 wk in culture, suggesting depolarizing action of GABA. However, in cultures older than 2 wk, inhibition of GABA(A) receptors does not result in block of synchronous network activity but in modulation of burst width and frequency.

Functionally independent columns of rat somatosensory barrel cortex revealed with voltage-sensitive dye imaging

Whisker movement is somatotopically represented in rodent neocortex by electrical activity in clearly defined barrels, which can be visualized in living brain slices. The functional architecture of this part of the cortex can thus be mapped in vitro with respect to its physiological input and compared with its anatomical architecture. The spatial extent of excitation was measured at high temporal resolution by imaging optical signals from voltage-sensitive dye evoked by stimulation of individual barrels in layer 4. The optical signals correlated closely with subthreshold EPSPs recorded simultaneously from excitatory neurons in layer 4 and layer 2/3, respectively. Excitation was initially (<2 msec) limited to the stimulated barrel and subsequently (>3 msec) spread in a columnar manner into layer 2/3 and then subsided in both layers after approximately 50 msec. The lateral extent of the response was limited to the cortical column defined structurally by the barrel in layer 4. Two experimental interventions increased the spread of excitation. First, blocking GABA(A) receptor-mediated synaptic inhibition caused excitation to spread laterally throughout wide regions of layer 2/3 and layer 5 but not into neighboring barrels, suggesting that the local excitatory connections within layer 4 are restricted to single barrels and that inhibitory neurons control spread in supragranular and infragranular layers. Second, NMDA receptor-dependent increase of the spread of excitation was induced by pairing repetitive stimulation of a barrel column with coincident stimulation of layer 2/3 in a neighboring column. Such plasticity in the spatial extent of excitation in a barrel column could underlie changes in cortical map structure induced by alterations of sensory experience.

Optical recording of spatiotemporal activation of rat somatosensory and visual cortex in vitro

A comparative analysis of spatiotemporal activation patterns of somatosensory and visual cortex was carried out in slice preparations using optical recording with voltage-sensitive dyes. Activity propagation velocities were found to be similar in both areas in all layers. Vertical propagation velocity is higher than horizontal propagation velocities. Differences between the two sensory areas exist in terms of horizontal activity spread, that is similar in extragranular layers but smaller in somatosensory than in visual cortex in layer IV. These results imply that despite the extensive similarities in the organization of sensory cortical areas, systematic areal variations in the horizontal cortical plane are present that may reflect adaptations needed for the processing of the corresponding sensory modality.