Developmental transformation of dendritic arbors in mouse whisker thalamus

Thalamo-cortical and cerebello-cortical functional connectivity in development

The thalamus is a critical relay center for neural pathways involving sensory, motor, and cognitive functions, including cortico-striato-thalamo-cortical and cortico-ponto-cerebello-thalamo-cortical loops. Despite the importance of these circuits, their development has been understudied. One way to investigate these pathways in human development in vivo is with functional connectivity MRI, yet few studies have examined thalamo-cortical and cerebello-cortical functional connectivity in development. Here, we used resting-state functional connectivity to measure functional connectivity in the thalamus and cerebellum with previously defined cortical functional networks in 2 separate data sets of children (7-12 years old) and adults (19-40 years old). In both data sets, we found stronger functional connectivity between the ventral thalamus and the somatomotor face cortical functional network in children compared with adults, extending previous cortico-striatal functional connectivity findings. In addition, there was more cortical network integration (i.e. strongest functional connectivity with multiple networks) in the thalamus in children than in adults. We found no developmental differences in cerebello-cortical functional connectivity. Together, these results suggest different maturation patterns in cortico-striato-thalamo-cortical and cortico-ponto-cerebellar-thalamo-cortical pathways.

Neuronal circuits with whisker-related patterns

Neuronal circuits with whisker-related patterns, such as those observed in the rodent somatosensory barrel cortex, have been widely used as a model system for investigating the anatomical organization, development and physiological roles of functional neuronal circuits. Whisker-related patterns exist not only in the barrel cortex but also in subcortical structures along the trigeminal neuraxis from the brainstem to the cortex. Here, we briefly summarize the organization, formation, and function of each neuronal circuit with whisker-related patterns, including the novel axonal trajectories that we recently found with the aid of in utero electroporation. We also discuss their biological implications as model systems for the studies of functional neuronal circuits.

Maturing thalamocortical functional connectivity across development

Recent years have witnessed a surge of investigations examining functional brain organization using resting-state functional connectivity MRI (rs-fcMRI). To date, this method has been used to examine systems organization in typical and atypical developing populations. While the majority of these investigations have focused on cortical–cortical interactions, cortical–subcortical interactions also mature into adulthood. Innovative work by Zhang et al. (2008) in adults have identified methods that utilize rs-fcMRI and known thalamo-cortical topographic segregation to identify functional boundaries in the thalamus that are remarkably similar to known thalamic nuclear grouping. However, despite thalamic nuclei being well formed early in development, the developmental trajectory of functional thalamo-cortical relations remains unexplored. Thalamic maps generated by rs-fcMRI are based on functional relationships, and should modify with the dynamic thalamo-cortical changes that occur throughout maturation. To examine this possibility, we employed a strategy as previously described by Zhang et al. to a sample of healthy children, adolescents, and adults. We found strengthening functional connectivity of the cortex with dorsal/anterior subdivisions of the thalamus, with greater connectivity observed in adults versus children. Temporal lobe connectivity with ventral/midline/posterior subdivisions of the thalamus weakened with age. Changes in sensory and motor thalamo-cortical interactions were also identified but were limited. These findings are consistent with known anatomical and physiological cortical–subcortical changes over development. The methods and developmental context provided here will be important for understanding how cortical–subcortical interactions relate to models of typically developing behavior and developmental neuropsychiatric disorders.

Postsynaptic deregulation in GAP-43 heterozygous mouse barrel cortex

Formation of whisker-related barrels in primary somatosensory cortex (S1) requires communication between presynaptic thalamocortical afferents (TCAs) and postsynaptic cortical neurons. GAP-43 is crucially involved in targeting TCAs to postsynaptic S1 neurons but its influence on the interactions between these 2 elements has not been explored. Here, we tested the hypothesis that reduced early expression of presynaptic GAP-43 (GAP-43 heterozygous [HZ] mice) alters postsynaptic differentiation of barrel cells. We found a transient increase in cytochrome oxidase staining between P6 and P14 in HZ animals, indicative of increased metabolic activity in barrel cortex during this time. Golgi impregnation and microtubule-associated protein 2 immunohistochemistry showed anomalous dendritic patterning in GAP-43 HZ cortex at P5, with altered dendritic length and branching and abnormal retention of dendrites that extend into developing septa. This deficiency was no longer apparent at P7, suggesting partial recovery of dendritic pruning processes. Finally, we showed early defects in synaptogenesis from P4 to P5 with increased colocalization of NR1 and GluR1 staining in HZ mice. By P7, this colocalization had normalized to wild type levels. Taken together, our findings suggest abnormal postsynaptic differentiation in GAP-43 HZ cortex during early barrel development, followed by adaptive compensation and partial phenotypic rescue.

Activity-dependent expression of Egr1 mRNA in somatosensory cortex of developing rats

The rat barrel field in somatosensory cortex is a well-characterized model of neocortical development, with activity-dependent and activity-independent components. Egr1 encodes an inducible transcription factor that is required for certain forms activity-dependent plasticity. This study examines Egr1 mRNA expression in the developing barrel field under basal conditions and after short-term deprivation or stimulation of whiskers. Egr1 mRNA was measured with in situ hybridization at postnatal Day (P) 6, P9, P12, P15, and P21. For short-term deprivation, whiskers were trimmed close to the skin and Egr1 mRNA was examined 3 hr later. For controlled stimulation of a single whisker, surrounding whiskers were trimmed, a wire was glued to the designated whisker, and animals were placed in an AC magnetic field pulsed at 2 Hz, 10 mT rms for 15 min. Egr1 mRNA was examined 30 min later. At P6, basal Egr1 mRNA in the barrel field was very low and was increased only slightly by stimulation (P < 0.05). At each of the later ages, there was a large increase in Egr1 mRNA in stimulated versus deprived barrels (P < 0.001). Egr1 mRNA expression after whisker stimulation increased exponentially with age through P15 (P < 0.001) and then declined between P15 and P21. The onset of Egr1 responses to whisker stimulation at P9 and the striking increase in activity-dependent Egr1 mRNA expression in the second postnatal week suggest that this transcription factor may play a role in activity-dependent processes that occur in this developmental period, such as maturation of barrel cortex circuitry.

Synaptic patterns of thalamocortical afferents in mouse barrels at postnatal day 11

This study focuses on the synaptic output patterns of thalamocortical axons in mouse barrel cortex at postnatal day (P) 11. Axons were labeled by biotinylated dextran amine transported anterogradely following injection in vivo into the ventrobasal thalamus. Labeled axons in the posteromedial barrel subfield were examined by light and electron microscopy and then reconstructed in three dimensions to assess the spatial distribution of their synapses. Thalamocortical axons form asymmetrical synapses, both at varicosities and along cylindrical portions of the axons; usually, only one synapse occurs per site, contrasting with the case in the adult, in which multiple synapses are typical. At P11, varicosities without synapses are common. As in adult barrels, approximately 80% of synapses formed by thalamocortical axons are with dendritic spines; 20% are with dendritic shafts. The similarity in the distribution of thalamocortical synapses onto spines vs. dendrites in developing and mature barrels indicates that adult synaptic patterns already are specified at a very early stage of thalamocortical synaptogenesis.

Compartmentalisation of the developing trigeminal ganglion into maxillary and mandibular divisions does not depend on target contact

During development axons contact their target tissues with phenomenal accuracy but the mechanisms that control this homing behaviour remain largely elusive. A prerequisite to the study of the factors involved in hard-wiring the nervous system during neurogenesis is an accurate calendar of developmental events. We have studied the maxillary and mandibular components of the trigeminal system to determine the stages during embryogenesis when a gross somatotopic order is first established within the trigeminal ganglion and the axons projecting to the brainstem. The retrograde transganglionic fluorescent tracers DiO and DiI were injected into the maxillary and mandibular arches or their derivatives in fixed mouse embryos staged between 13 and 40 somites (E9-E11). After 1-4 wk, the distribution of the 2 tracers was determined using confocal laser scanning microscopy. The first maxillary nerve cell bodies and their developing axons were labelled at the 30 somite stage (E10). This was 2 somite stages earlier than the mesencephalic nucleus and the ganglion cell bodies of the mandibular nerve. The gross somatotopic division of cells within the trigeminal ganglion projecting to the maxillary and mandibular targets was established by the 32 somite stage (E10). This arrangement was evident as 2 groups of cell bodies occupying adjacent but separate regions of the trigeminal ganglion. The central branches of the maxillary and mandibular cell bodies entered the metencephalon as 2 distinct bundles at the same stage. The trigeminal motor nucleus was first detected at the 38 somite stage (E10.5). Gross somatotopy in the major divisions of the trigeminal ganglion is established before outgrowing axons have contacted their peripheral target tissue at E10.5. This suggests that target tissues do not induce somatotopy.

Development of trigeminal nucleus principalis in the rat: effects of target removal at birth

Little is known about how neurons develop in the trigeminal nucleus principalis (PrV) despite their acknowledged role in establishing whisker-related patterns in the thalamus and cortex. Golgi-impregnated PrV cells were studied in newborn, 4-day-old and adult rats. Adult neurons typically had short dendrites that were confined to a hemisphere around the soma. In contrast, at birth PrV neurons had radial trees and more primary dendrites than did adults, but adult-like numbers of dendritic spines. By day 4, most neurons had eccentric dendritic trees and the numbers of primary dendrites per neuron were adult-like, yet spines were more prevalent than in adults and newborns. Thus, it appears that there is a pruning of the dendritic tree during the first postnatal week. To assess the role of retrograde signals from the thalamus on PrV development, the right thalamus was destroyed at birth. By postnatal day 6, the number of neurons in the left PrV was 59% of that in the right PrV, PrV transverse area was reduced by 21%, cell density was reduced by 48%, and somatic diameter was increased by 36%, relative to the intact right PrV. By contrast, in the left V subnucleus interpolaris, which has only a weak thalamic projection, these measures were unaffected. Thus, neonatal thalamic lesions selectively depopulated the PrV. The morphology of PrV neurons was affected by the thalamic lesions: e.g. the total dendritic length, the number of dendritic branch points and the total number of spines were increased. The number of primary dendrites and the tree's eccentricity, area, and volume of influence were unaffected by the lesion. The structure of neurons in subnucleus interpolaris was unaffected by the lesion. Thus, normal afferent patterning is insufficient for normal development of PrV cells. Interactions among dendrites and retrograde signals from a target are also important.

Postnatal development of mouse "whisker" thalamus: ventroposterior medial nucleus (VPM), barreloids, and their thalamocortical relay neurons

We followed developmental changes in "barreloid" thalamocortical relay cell (TCR) dendritic arbors between postnatal day 5 (P5; birth = P0) and adulthood. Single neurons in 150- to 250-microns coronal or oblique slices through the somatosensory thalamus in mice of different postnatal ages were injected with lucifer yellow (LY) under direct visualization. Filled cells in the ventroposterior medial nucleus (VPM) were imaged with a confocal microscope, and rendered and analyzed on a computer workstation with special-purpose software. The whisker representation in the thalamus, as revealed by the pattern of barreloids, was demonstrated by oblique illumination of the slices and/or later cytochrome oxidase (CO) staining. VPM cross-sectional area trebles from P5 to adulthood. Barreloids (single-whisker representations) are well delineated in unstained sections until P10-P11; thereafter, barreloids can only be recognized with difficulty with the CO stain. Thalamocortical relay cell (TCR) somal volumes increase rapidly in the first 2 weeks. The number of primary dendrites does not change, nor does the length of the primary dendritic segments, from P5 to adulthood; however, distal dendritic segments elongate and increase in number. Dendritic arbors are confined on P5 to single barreloids; in adults they extend to adjacent barreloids. The postnatal transformation of dendritic arbors by process growth to adjacent barreloids is mainly completed by P18. A change in the developmental role of these cells, from instructing whisker pattern formation to integrating sensory information from more than one whisker, thus occurs after the whisker pattern in the barrel cortex is established. It coincides with the age at which animals are known to begin exploratory whisking behaviors. The mechanism appears to be by growth and remodeling of distal dendrites rather than by oriented growth and regression, as has been reported for stellate cells in cortical whisker barrels.

Structure-function relationships in rat brainstem subnucleus interpolaris: XII. neonatal deafferentation effects on cell morphology

In the developing whisker-barrel neuraxis, it is known that pattern formation, receptive fields, axon projections, and even cell survival are under the control of peripheral signals transmitted through the infraorbital nerve. However, afferent influences upon the development of single-cell morphologies have not received thorough study. Intracellular recording, antidromic activation, receptive field mapping, dye injection, and computer-assisted cell reconstruction methods were used to assess the morphology of trigeminal (V) brainstem neurons in adult rats whose infraorbital nerves were transected at birth. Projection and local-circuit neurons in the spinal V subnucleus interpolaris (SpVi; n = 43) and local-circuit neurons in the adjacent subnucleus caudalis (SpVc; n = 11) were compared with similar cell types in normal control rats, as well as with spinal V neurons located outside of the deafferented region in experimental rats. SpVi cells displayed abnormally convergent and discontinuous receptive fields that included greater-than-normal numbers of vibrissae and other receptor organs. However, their morphologies did not differ significantly from normal on any quantitative measure, including soma size, number of proximal dendrites, or dendritic tree area, perimeter, or shape. Moreover, SpVi cells near deafferented brainstem territories did not display dendritic tree polarity toward or away from the deafferented region. In SpVc, laminae I-V cells had responses and morphologies that were indistinguishable from those of controls. Thus, (1) altered receptive fields of neonatally deafferented SpVi neurons are not attributable to changes in their morphology; (2) SpVc cells are resilient following deafferentation; and (3) the development of SpV dendrites and local axon collaterals is controlled by factors other than those directly conveyed by primary afferents.