Carolyn A. Bates

Differential organization of thalamic projection cells in the brain stem trigeminal complex of the rat

Following large injections of horseradish peroxidase into the thalamus of the adult rat, labeled neurons can be detected in the contralateral principal sensory nucleus and the subnucleus interpolaris of the spinal trigeminal nucleus. Thalamic projection neurons in the principal sensory nucleus are arranged in discrete aggregates which replicate the organization of the vibrissae and other parts of the face. This cellular organization corresponds very well with the pattern of vibrissae related afferent termination in this region of the nucleus as seen in SDH preparations. There is no such cellular organization in the subnucleus interpolaris.

The emergence of a discretely distributed pattern of corticospinal projection neurons

The distribution of cortical neurons which project to the spinal cord was investigated in the developing and adult rat using the retrograde transport of horseradish peroxidase (HRP). Following injections of HRP into the cervical spinal cord of a postnatal day 4 rat, retrogradely labeled neurons are located in layer Vb throughout the neocortex. Gradually, over the first two postnatal weeks, labeled cells can no longer be found laterally or caudally in the neocortex. By the end of the second postnatal week, cortical neurons which project to the spinal cord are located in essentially their adult positions. These consist of: (1) a dorsomedial band of cells in layer V of frontal cortex, (2) a dorsomedial band in layer Vb of posterior frontal/anterior parietal cortex, and (3) a small group of cells located laterally in an area corresponding to SII. This change in the regional distribution of cortical neurons projecting to the spinal cord is interpreted as an economically adaptive way for the cortex to interconnect highly organized sensory and motor systems.

Transganglionic transport of HRP from the circumvallate papilla of the rat

To learn whether horseradish peroxidase (HRP) injections in gustatory papillae on the tongue can be used to study central topographical projections of taste buds and papillae, injections were made into the circumvallate papilla in rats. Labeled central projections after papilla injections were compared to projections after applying HRP to the cut glossopharyngeal nerve. Papilla injections result in HRP transport by afferent and efferent fibers of the glossopharyngeal nerve, and the pattern of central projections is similar to that after labeling the cut nerve. Projections include a separation in the brainstem of afferent, dorsally located fibers and efferent, ventrally located fibers. Afferent fibers project to the solitary nucleus and the trigeminal system. Efferent projections label muscle motorneurons in the nucleus ambiguus and the cells of origin of parasympathetic preganglionic fibers, which from the inferior salivatory nucleus. The parasympathetic neurons labeled after papilla projections are preganglionic fibers to Remaks ganglia in the tongue; post-ganglionic fibers of these ganglia are the secretomotor supply to the von Ebners glands. In summary, injections of HRP into gustatory papillae reliably label central projections of the papilla and can be used for studies to discern topography in central projections of the taste system. Injections into the circumvallate papilla also have demonstrated that the parasympathetic neurons innervating von Ebners glands are located in the inferior salivatory nucleus.

Central correlates of peripheral pattern alterations in the trigeminal system of the rat. III. Neurons of the principal sensory nucleus

We provide evidence that the discrete clustered distribution of the trigeminothalamic projection neurons in the principal sensory nucleus of the trigeminal is dependent on the integrity of the trigeminal nerve during development. Nerve section at birth abolishes all evidence of cellular clustering in the principal sensory nucleus, while more discrete damage to specific rows of follicles results in changes in the cellular clustering pattern only in the portion of the nucleus related to the afflicted rows of vibrissae.

The heavy neurofilament protein is expressed in regenerating adult but not embryonic mammalian optic fibers in vitro

Axonal neurofilaments are composed of light (NF-L), medium (NF-M), and heavy (NF-H) subunits which are sequentially expressed during axonal development. In retina, NF-L and NF-M appear prenatally, followed postnatally by NF-H which occurs at about the time axons are reaching their target tissue. Phosphorylation of the NF-H protein occurs after it has first appeared in the axon. Phosphorylated NF-H is also downregulated in regenerating peripheral nerves. These observations lead to the hypothesis that NF-H stabilizes axons, thereby inhibiting their ability to grow. We have previously shown that adult mouse retinal ganglion cells (RGCs) can extend neurites in vitro and that these neurites reexpress the developmental protein GAP-43. Here we ask whether the expression of neurofilament proteins in the growing adult RGC axons involves the recapitulation of development. Adult mice which had a priming lesion of the optic nerve and Embryonic Day 15 mouse retinas were explanted onto laminin substrates and grown in culture. After 2-4 days the growing neurites were stained with a battery of monoclonal antibodies against differentially phosphorylated versions of the neurofilament subunits. Both adult and embryonic neurites were highly immunoreactive for NF-L and NF-M. Only the adult neurites stained with antibodies against phosphorylated NF-H. There was no immunoreactivity in the embryonic explants. This indicates that regrowing adult RGC axons maintain their adult cytoskeletal properties and can nevertheless regenerate.

Expression of Polysialylated NCAM but Not L1 orN-Cadherin by Regenerating Adult Mouse Optic Fibersin Vitro

This study asks if there might be irreversible maturational changes in adult neurons that limit their capacity to regenerate. Retina from adult and embryonic mouse were placed in culture on laminin substrates so that regenerating adult optic fibers could be compared to growing embryonic fibers. Several cell adhesion molecules (CAMs) known to mediate the growth of embryonic neurites on astrocytes were assayed by immunocytochemistry: L1, N-cadherin, and NCAM. Thy 1.2, a potential CAM with inhibitory activity, was also examined. As in vivo, embryonic fibers were found to express both L1 and N-cadherin. In contrast, regenerating adult fibers had no detectable amounts of either of these CAMs. N-Cadherin is normally down regulated during development so its absence in adult fibers suggests it can not be reexpressed during regeneration. L1 is normally found in the proximal regions of adult optic fibers so its absence indicates it is not expressed or transported in regenerating fibers. Adult regenerating fibers expressed high levels of Thy 1.2, which was undetectable in embryonic optic fibers. Thy 1.2 is normally found in mature fibers, indicating this phenotypic feature is preserved during regeneration. Both adult and embryonic fibers showed strong reactivity for NCAM, which in vivo is normally found in embryonic and at lower levels in adult fibers. Surprisingly, both embryonic and regenerating adult fibers expressed high levels of polysialic acid, which is normally absent in adult fibers. NCAM may be one of few CAMs available to adult optic fibers for regeneration on astrocytes.

Differential Effect of Serum on Laminin-Dependent Outgrowth of Embryonic and Adult Mouse Optic Axons in Vitro

Laminin is an extracellular matrix molecule which promotes neurite outgrowth from a variety of neurons in culture. However, the reported effectiveness of laminin on neurite outgrowth from retinal ganglion cells in rat and that from chick have been contradictory. In chick, embryonic retinal ganglion cells show strong laminin-dependent neurite outgrowth which is lost during development. In contrast, in rat, laminin promotes neurite outgrowth from adult, but not embryonic retinal ganglion cells. We have reexamined the response of adult and embryonic mouse retinal ganglion cells to laminin in culture. We found, first, that both adult and embryonic retinal neurites respond to laminin with neurite outgrowth in serum-free medium. Second, we found that the addition of serum to the medium greatly inhibits the outgrowth of neurites from embryonic, but not adult, retinal explants. This inhibition appears specific for neurite outgrowth involving laminin receptors, since embryonic explants showed extensive neurite outgrowth in the presence of serum when astrocytes were used as a substrate. These developmental differences in the effect of serum on laminin-dependent neurite outgrowth may indicate maturational changes in surface molecules on optic fibers related to regenerative failure in the adult CNS.