Reha S. Erzurumlu

Whisker-related neuronal patterns fail to develop in the trigeminal brainstem nuclei of NMDAR1 knockout mice

Sensory pathways of the brain generally develop from crudely wired networks to precisely organized systems. Several studies have implicated neural activity-dependent mechanisms, including N-methyl-D-aspartate (NMDA) receptors, in this refinement process. We applied the gene targeting to the NMDAR1 gene and created a mutant mouse that lacks functional NMDA receptors. The development of whisker-related patterns in the trigeminal nuclei of the mutant mice and their normal littermates was compared. We show that in the mutant mice pathfinding, initial targeting, and crude topographic projection of trigeminal axons in the brainstem are unaffected, but that whisker-specific patches fail to form. Our results provide a direct demonstration of the involvement of the NMDA receptor in the formation of periphery-related neural patterns in the mammalian brain.

Thalamic axons confer a blueprint of the sensory periphery onto the developing rat somatosensory cortex

In order to study the role of afferents in the maturation of cortical axons projecting from the ventrobasal thalamic complex (VB) to the barrel field (SI) cortex were labeled with the carbocyanine dye DiI, in aldehyde-fixed embryonic and newborn rat brains. Our results reveal that the first few thalamic axons are in the cortical plate by embryonic day (E) 19. Between E19 and the day of birth (E21 = PND 0), layers V and VI differentiate from the lower part of the cortical plate. On PND 0, a plexus of growth-cone tipped thalamic axons is present within the cortical plate and a few VB fibers have reached the marginal zone. Increasing numbers of thalamic afferents invade and ramify within the cortical plate on PND 1 and, over the course of the next 24 h, form a vibrissa-specific pattern in the lower part of this zone, prior to the differentiation of layer IV into a distinct lamina. This periphery-related organization is exhibited by VB afferents earlier than reported for other afferents to the cortex, by glia- or neuron-associated extracellular elements or by the cytoarchitectonic specializations (barrels) of stellate cells. Our observations, in conjunction with the previous studies, demonstrate that thalamic afferents may have a pivotal role in determining the morphological specification of the primary somatosensory cortex.

Target Influences on the Morphology of Trigeminal Axons

Axons grow in two stages: First, they exhibit rapid, target-directed extension; then they begin to collateralize and elaborate terminal arbors in their targets. To investigate possible regulatory influences on these phases of axon growth, we have used an in vitro paradigm in which we cocultured embryonic or postnatal rat trigeminal ganglion explants with isochronic, heterochronic, and/or heterotypic targets. Cultures were fixed after 5 days and ganglion cell processes were labeled with DiI. Trigeminal processes were able to regenerate into several peripheral targets as well as into CNS explants from trigeminal or nontrigeminal regions of the brain. In peripheral tissues, the processes showed target-specific growth patterns. In CNS tissue, the type of growth (unbranched extension versus collateralization/arbor formation) varied markedly with the explant age: If trigeminal ganglia were harvested at a time (E15) when their axons would be elongating in the embryo and cocultured with isochronic tissues, their processes had a simple morphology, were loosely bundled, and reconstituted a distinct fiber tract, mimicking their in vivo growth pattern. If, challenged by more mature tissue, axons of E15 ganglion cells formed discrete arbors. Finally, if trigeminal ganglia were harvested at an age (E20, PND 5) when their axons had already formed arbors in the brain and induced to innervate younger (E15) targets, their axons reverted back to the elongation stage. These results demonstrate that the target environment sets specific, developmentally regulated constraints on the patterns of growth manifested by primary sensory axons.

Peripheral nerve regeneration induces elevated expression of GAP-43 in the brainstem trigeminal complex of adult hamsters

A polyclonal antibody was used to delineate the pattern of GAP-43 expression in central processes of trigeminal ganglion cells while their peripheral processes were undergoing regeneration. Two weeks after transection of the infraorbital nerve, levels of GAP-43 ipsilateral to the transection were greatly increased along the trajectory of infraorbital axons in the central trigeminal tract and also within the target neuropil. We conclude that elevated levels of GAP-43 in central processes of injured trigeminal ganglion cells occur in direct response to the regenerative response of the cell body and may have important implications for plasticity-related changes seen in the adult trigeminal system.

The gene knockout technology for the analysis of learning and memory, and neural development

Publisher Summary Using the gene knockout technology, this chapter presents an analysis of learning and memory, and neural development. It investigates the molecular substrates of synaptic plasticity by producing mice that lack the γ isoform of Ca 2+ /phospholipid-dependent-protein kinase (PKCγ) using the embryonic stem (ES) cell gene targeting technology. PKC constitutes a family of isoenzymes involved in signal transduction pathways in diverse systems. This enzyme was chosen for the study reviewed in the chapter because pharmacological studies have repeatedly implicated PKC as playing a role in long-term potentiation (LTP). To re-examine the role of N -methyl-D-aspartate (NMDA) receptor (NMDAR)-mediated activity in the establishment of neural patterns in the whisker-to-barrel system, reverse genetics is used to selectively “knock out” the NMDAR1 subunit of the NMDA receptor. The results from this examination show that in the knockout animals, although central targeting and topographic projection of trigeminal afferent appear to be normal and postsynaptic neurons are responsive to the stimulation of primary trigeminal afferent, whisker-specific neural patterns fail to develop in the absence of the NMDA receptor.

Maintenance of discrete somatosensory maps in subcortical relay nuclei is dependent on an intact sensory cortex

Lesions of the rat barrelfield cortex drastically alter the discrete representations of the somatosensory periphery in the central nervous system. We have found that lesions placed in the parietal cortex, after the formation of barrels (postnatal day 5), can irreversibly abolish vibrissae- and extremity-related patterns of cytochrome oxidase activity in the principal sensory nucleus of the trigeminal nerve and in the dorsal column nuclei. Furthermore, abnormal patterns of enzymatic activity occur in the remaining primary somatosensory cortex and the ventrobasal nucleus of the thalamus. We conclude that cortical barrels are essential in maintenance of periphery-related discrete morphological organization in the rodent somatosensory system.

Distribution of morphologically different retinal axon terminals in the hamster dorsal lateral geniculate nucleus

Afferent terminal arbors in the hamster LGBd were labelled with horseradish peroxidase (HRP) implanted into the optic tract. Three morphologically distinct terminal types, each with a different regional distribution, were observed. Type R1 terminals are large, ovoid swellings and are predominantly distributed medially within the nucleus. Type R2 terminals are very small, clustered varicosities and are distributed laterally and ventrally. Type R3 terminals are medium in size and their distribution overlaps with that of Type R1 and R2 terminals.

Synaptic plasticity in the trigeminal principal nucleus during the period of barrelette formation and consolidation

We examined whether the postsynaptic responses of cells in the principal sensory nucleus of the trigeminal nerve (PrV) are subject to long-term changes in synaptic strength, and if such changes were correlated the whisker-specific patterning during and just after the critical period for pattern formation. We used an in vitro brainstem preparation in which the trigeminal ganglion (TG) and PrV remained attached. By electrically activating TG afferents, we evoked large-amplitude extracellular field potentials. These responses were postsynaptic in origin and blocked by the glutamate antagonist, DNQX. At P1, a time when barrelettes are consolidating, high frequency stimulation of their afferents led to an immediate (<1 min) and long-lasting (> or =90 min) reduction (35%) in the amplitude of the evoked response. At P3-7, when the pattern of barrelettes have stabilized, the same form of tetanus led to an immediate and long-lasting increase (40%) in the amplitude of the response. Both forms of synaptic plasticity were mediated by the activation of L-type Ca(2+) channels. Application of the L-type channel blocker, nitrendipine, led to a complete blockade of any the tetanus induced changes. These associative processes may regulate the patterning and maintenance of whisker-specific patterns in the brainstem trigeminal nuclei.

Two Phases of Pattern Formation in the Developing Rodent Trigeminal System

The patterning of neural connections, as reflected by the ordered projections between functionally related groups of cells, is a hallmark of the vertebrate CNS. The precision and specificity of such organization is especially striking in the interconnections within sensory systems wherein topographic representations of the periphery are mapped onto multiple central cell groups. These features are epitomized in the rodent trigeminal system: the array of vibrissae and sinus hairs on the snout is replicated in the punctate arrangement of cells and axons along the entire trigeminal neuraxis (see Woolsey, 1989 for a review). The question of how such ordered patterns come about has been addressed at various levels, including the establishment of regional specificity (Schneider, 1973), the acquisition of positional identity (rev., Gaze, 1970; Jacobson, 1978; Lund 1978), the selection of postsynaptic partners (Purves and Lichtman,1985), and so on. In this chapter we review our studies on pattern formation in the trigeminal system within the context of two particular aspects of this issue: the conferring of axial orientation of the somatotopic map in the CNS and the emergence of modular patterning within this map.

The optic tract in embryonic hamsters : Fasciculation, defasciculation, and other rearrangements of retinal axons

The early development of the optic tract in hamsters was studied by labeling retinal axons with Dil applied to the eye, and then examining the labeled axons in flatmount preparations of the rostral brain stem. This technique permits a panoramic view of the entire retinal projection, from the chiasm to the caudal end of the superior colliculus. In the E11 embryo, retinal axons have reached the chiasm. They defasciculate as they emerge from the nerve, prior to reaching the ventral midline of the diencephalon, then converge again as they pass over to the opposite side. At the midline, many axonal trajectories crisscross, implying some shuffling of relative positions. Retinal axons are tightly bundled within the optic tract. Upon reaching the ventral border of the lateral geniculate body (LGB), they splay out over the nucleus, revealing a wavefront of pioneer axons individually distributed across the rostro-caudal extent of the LGB. Later-emerging retinal axons course over the surface of the thalamus in waves; subsequent waves of axons interdigitate between the lead fibers without fasciculating along them. Past the LGB, the axons undergo a second change in relative positions as the ribbon of fibers swerves caudally, prior to entering the superior colliculus. Retinal axons are tipped with growth cones of varying morphologies. No strong correlation is evident between the structural complexity of the growth cone and its position within the tract. In the majority of cases, ipsilaterally and contralaterally directed axons follow a similar developmental course along the optic tract, without any indication of a temporal lag in the ipsilateral projection as claimed in earlier reports. Understanding the changes in spatial distribution of embryonic retinal axons as they navigate along the optic tract provides a further step towards elucidating how point-to-point projections form in developing sensory systems.