Spatiotemporal patterning of glutamate receptors in developing ferret striate cortex

Ontogeny of white matter, toll-like receptor expression, and motor skills in the neonatal ferret

Inflammation caused by perinatal infection, superimposed with hypoxia and/or hyperoxia, appears to be important in the pathogenesis of preterm neonatal encephalopathy, with white matter particularly vulnerable during the third trimester. The associated inflammatory response is at least partly mediated through Toll-like receptor (TLR)-dependent mechanisms. Immunohistochemistry, gene expression, and behavioral studies were used to characterize white matter development and determine TLR3 and TLR4 expression and accumulation in the neonatal ferret brain. Expression of markers of white matter development increased significantly between postnatal day (P)1 and P10 (NG2, PDGFRα) or P15 (Olig2), and either remained elevated (NG2), or decreased again at P40 (PDGFRα, Olig2). Olig2 immunostaining within the internal capsule was also greatest at P15. Myelin basic protein (MBP) immunostaining and mRNA expression increased markedly from P15 to P40 and into adulthood, which correlated with increasing performance on behavioral tests (negative geotaxis, cliff aversion, righting reflex, and catwalk gait analysis). TLR4 and TLR3 positive staining was low at all ages, but TLR3 and TLR4 mRNA expression both increased significantly from P1 to P40. Following lipopolysaccharide (LPS) and hypoxia/hyperoxia exposure at P10, meningeal and parenchymal inflammation was seen, including an increase in TLR4 positive cells. These data suggest that the neuroinflammation associated with prematurity could be modeled in the newborn ferret.

Neocortical Layer 6B as a Remnant of the Subplate - A Morphological Comparison

The fate of the subplate (SP) is still a matter of debate. The SP and layer 6 (which is ontogenetically the oldest and innermost neocortical lamina) develop coincidentally. Yet, the function of sublamina 6B is largely unknown. It has been suggested that it consists partly of neurons from the transient SP, however, experimental evidence for this hypothesis is still missing. To obtain first insights into the neuronal complement of layer 6B in the somatosensory rat barrel cortex, we used biocytin stainings of SP neurons (aged 0-4 postnatal days, PND) and layer 6B neurons (PND 11-35) obtained during in vitro whole-cell patch-clamp recordings. Neurons were reconstructed for a quantitative characterization of their axonal and dendritic morphology. An unsupervised cluster analysis revealed that the SP and layer 6B consist of heterogeneous but comparable neuronal cell populations. Both contain 5 distinct spine-bearing cell types whose relative fractions change with increasing age. Pyramidal cells were more prominent in layer 6B, whereas non-pyramidal neurons were less frequent. Because of the high morphological similarity of SP and layer 6B neurons, we suggest that layer 6B consists of persistent non-pyramidal neurons from the SP and cortical L6B pyramidal neurons.

Postnatal refinement of interareal feedforward projections in ferret visual cortex

We studied the postnatal refinement of feedforward (FF) projections from ferret V1 to multiple cortical targets during the period around eye opening. Our goal was to establish (a) whether the developmental refinement of FF projections parallels that of feedback (FB) cortical circuits, and (b) whether FF pathways from V1 to different target areas refine with a similar rate. We injected the tracer CTb into V1 of juvenile ferrets, and visualized the pattern of labeled axon terminals in extrastriate cortex. Bouton density of FF projections to target areas 18, 19, and 21 declined steadily from 4 to 8 weeks postnatal. However, in area Ssy this decline was delayed somewhat, not occurring until after 6 weeks. During this postnatal period, mean interbouton intervals along individual FF axons to all visual areas increased, and we observed a concomitant moderate decrease in axon density in areas 18, 21, and Ssy. These data suggest that FF circuits linking V1 to its main extrastriate targets remodel largely synchronously in the weeks following eye opening, that FF and FB cortical circuits share a broadly similar developmental timecourse, and that postnatal visual experience is critical for the refinement of both FF and FB cortical circuits.

Early Development of Network Oscillations in the Ferret Visual Cortex

Although oscillations during development have been characterized in a wide range of neural systems, little is known about the interaction between these network oscillations and neuronal spiking, and the interactions among different oscillation frequencies. Here we recorded the spontaneous and visual-elicited local field potential (LFP) and multi-unit activity (MUA) in the visual cortex of freely-moving juvenile ferrets before and after eye-opening. We found that both the spontaneous and visually-elicited LFP power was increased after eye-opening, especially in higher frequency bands (>30 Hz). Spike LFP phase coupling was decreased for lower frequency bands (theta and alpha) but slightly increased for higher frequencies (high-gamma band). A similar shift towards faster frequencies also occurred for phase-amplitude coupling; with maturation, the coupling of the theta/alpha/beta band amplitude to the delta phase was decreased and the high-gamma amplitude coupling to theta/alpha phase was increased. This shift towards higher frequencies was also reflected in the visual responses; the LFP oscillation became more entrained by visual stimulation with higher frequencies (>10 Hz). Taken together, these results suggest gamma oscillation as a signature of the maturation of cortical circuitry.

Subplate neurons are the first cortical neurons to respond to sensory stimuli

In utero experience, such as maternal speech in humans, can shape later perception, although the underlying cortical substrate is unknown. In adult mammals, ascending thalamocortical projections target layer 4, and the onset of sensory responses in the cortex is thought to be dependent on the onset of thalamocortical transmission to layer 4 as well as the ear and eye opening. In developing animals, thalamic fibers do not target layer 4 but instead target subplate neurons deep in the developing white matter. We investigated if subplate neurons respond to sensory stimuli. Using electrophysiological recordings in young ferrets, we show that auditory cortex neurons respond to sound at very young ages, even before the opening of the ears. Single unit recordings showed that auditory responses emerged first in cortical subplate neurons. Subsequently, responses appeared in the future thalamocortical input layer 4, and sound-evoked spike latencies were longer in layer 4 than in subplate, consistent with the known relay of thalamic information to layer 4 by subplate neurons. Electrode array recordings show that early auditory responses demonstrate a nascent topographic organization, suggesting that topographic maps emerge before the onset of spiking responses in layer 4. Together our results show that sound-evoked activity and topographic organization of the cortex emerge earlier and in a different layer than previously thought. Thus, early sound experience can activate and potentially sculpt subplate circuits before permanent thalamocortical circuits to layer 4 are present, and disruption of this early sensory activity could be utilized for early diagnosis of developmental disorders.

Expression of the eight AMPA receptor subunit genes in the developing central nervous system and sensory organs of zebrafish

The AMPA type glutamate receptors mediate the majority of fast synaptic transmission in the vertebrate nervous system. Whereas mammals have four subunit genes, Gria1-4, zebrafish has retained a duplicated set of eight genes named gria1-4a and b. We give here a detailed overview of the expression patterns of all eight zebrafish subunits within the developing central nervous system and sensory organs at 24, 48, and 72 hr after fertilization. Expression domains include distinct neuronal subsets in the developing forebrain, midbrain, hindbrain, and spinal cord, as well as in the ganglion- and inner nuclear layers of the retina. As a general rule, each pair of duplicated gria genes is differentially expressed, indicating subfunctionalization of AMPA receptor subunit expression in the teleost lineage. Our findings suggest that zebrafish can serve as a useful model system to investigate the role of AMPA receptors and their differential expression in the vertebrate nervous system.

Vision and cortical map development

Functional maps arise in developing visual cortex as response selectivities become organized into columnar patterns of population activity. Recent studies of developing orientation and direction maps indicate that both are sensitive to visual experience, but not to the same degree or duration. Direction maps have a greater dependence on early vision, while orientation maps remain sensitive to experience for a longer period of cortical maturation. There is also a darker side to experience: abnormal vision through closed lids produces severe impairments in neuronal selectivity, rendering these maps nearly undetectable. Thus, the rules that govern their formation and the construction of the underlying neural circuits are modulated-for better or worse-by early vision. Direction maps, and possibly maps of other properties that are dependent upon precise conjunctions of spatial and temporal signals, are most susceptible to the potential benefits and maladaptive consequences of early sensory experience.

Perinatal subplate neuron injury: implications for cortical development and plasticity

Perinatal brain injury may result in widespread deficits in visual, motor and cognitive systems suggesting disrupted brain development. Neurosensory and cognitive impairment are observed at increasing frequency with decreasing gestational ages, suggesting a unique vulnerability of the developing brain. The peak of human subplate neuron development coincides with the gestational ages of highest vulnerability to perinatal brain injury in the premature infant. At the same time, human thalamocortical connections are forming and being refined by activity-dependent mechanisms during critical periods. Subplate neurons are the first cortical neurons to mature and are selectively vulnerable to early hypoxic-ischemic brain injury in animal models. Timing of subplate neuron death determines the resulting defect in thalamocortical development: very early excitotoxic subplate neuron death results in failure of thalamocortical innervation, while later subplate neuron death interferes with the refinement of thalamocortical connections into mature circuits. We suggest that subplate neuron injury may be a central component of perinatal brain injury resulting in specific neurodevelopmental consequences.

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.

Taurine-, aspartate- and glutamate-like immunoreactivity identifies chemically distinct subdivisions of Kenyon cells in the cockroach mushroom body

The lobes of the mushroom bodies of the cockroach Periplaneta americana consist of longitudinal modules called laminae. These comprise repeating arrangements of Kenyon cell axons, which like their dendrites and perikarya have an affinity to one of three antisera: to taurine, aspartate, or glutamate. Taurine-immunopositive laminae alternate with immunonegative ones. Aspartate-immunopositive Kenyon cell axons are distributed across the lobes. However, smaller leaf-like ensembles of axons that reveal particularly high affinities to anti-aspartate are embedded within taurine-positive laminae and occur in the immunonegative laminae between them. Together, these arrangements reveal a complex architecture of repeating subunits whose different levels of immunoreactivity correspond to broader immunoreactive layers identified by sera against the neuromodulator FMRFamide. Throughout development and in the adult, the most posterior lamina is glutamate immunopositive. Its axons arise from the most recently born Kenyon cells that in the adult retain their juvenile character, sending a dense system of collaterals to the front of the lobes. Glutamate-positive processes intersect aspartate- and taurine-immunopositive laminae and are disposed such that they might play important roles in synaptogenesis or synapse modification. Glutamate immunoreactivity is not seen in older, mature axons, indicating that Kenyon cells show plasticity of neurotransmitter phenotype during development. Aspartate may be a universal transmitter substance throughout the lobes. High levels of taurine immunoreactivity occur in broad laminae containing the high concentrations of synaptic vesicles.