(Figure 3D).  In comparison to trkB cRNA hybridization, trkC labeling was more uniformly distributed in the cortex and olfactory bulbs; it did not appear laminated.  Tyrosine kinase C labeling was high in the mesoderm, whereas trkB labeling was not detected here.  At later ages, expression of trkC mRNA in the mesoderm declined, while labeling distinctly increased in the olfactory bulbs. Tyrosine kinase C mRNA was not detected in the olfactory epithelium at any of the observed ages.

Neurotrophin-3 immunoreactivity:
Neurotrophin-3 protein expression was not seen in the olfactory bulbs of rats at postnatal day 4.   However, immunoreactivity was detected in olfactory axons distributed between the olfactory epithelium and the olfactory bulb (Figure 4A).  No immunoreactivity was detected in the sensory neuron cell bodies in the olfactory epithelium.

Tyrosine receptor kinase B immunoreactivity:
At postnatal day 4, trkB antibody stained the mitral cells of the olfactory bulbs.   The cell bodies were densely stained; lower levels of staining were seen in their dendrites within the external plexiform layer and the glomeruli (Figure 4B). Developing granule cells and their dendrites were faintly labeled at this age. Tyrosine kinase B immunoreactivity was also seen in a few cells in the glomerular layer of the olfactory bulb.

Discussion

studies have provided evidence that neurotrophins can be anterogradely transported in the central nervous system (Figure 1).^3,4,7,6  Studies of the developing chick visual system have shown that radiolabeled NT3 and BDNF are anterogradely transported from the retina to the tectum when injected into the eye.⁶   In the retinotectal pathway, neurotrophin mRNA is expressed in the retina while trk mRNA is expressed in the postsynaptic target, the tectum. Such patterns of distribution suggest anterograde transport of endogenous neurotrophins in this pathway. Additional evidence for anterograde transport of trophic factors has come from studies of the motor-cortical region of the zebra finch brain.³  If presynaptic input from the lateral magnocellular nucleus of the anterior neostriatum is removed, there is massive neuronal death in the target, the robust nucleus of the archistriatum (RA).   However, if lesions are followed by infusions of neurotrophins into the RA, cell death in the RA is suppressed.³  Further evidence of anterograde transport in the brain has also been obtained in the rat. Brain-derived neurotrophic factor protein is present in processes in the striatum, not in cell bodies, but BDNF mRNA is not expressed here.  Inhibition of axonal transport decreases BDNF immunoreactivity in the striatum and increases it in regions that project to the striatum.⁴   Taken together, these observations suggest that anterograde transport of neurotrophins may be a mechanism common to many brain systems.

In the developing rat olfactory system, patterns of neurotrophin mRNA expression are correlated with olfactory nerve innervation of the developing forebrain.  At embryonic day 12 to 13, olfactory axons first contact the telencephalon; by embryonic day 14, the olfactory bulb primordium is morphologically distinguishable from the adjacent neocortex.^8,9  The coincident arrival of olfactory nerve afferents in the telencephalon with bulb formation suggests that the olfactory nerve contributes to olfactory bulb development.  This hypothesis is supported by studies of arhinencephalic mutant mouse embryos.  In these mice, the olfactory nerve axons fail to make contact with the developing forebrain, resulting in apparent death of developing mitral cells and the complete absence of olfactory bulbs.¹¹  Furthermore, surgical removal of the olfactory placode in amphibian species results in a shrunken telencephalon and/or the absence of olfactory bulbs.⁸  Data suggested that olfactory nerve fibers may contribute to bulb formation by altering cell cycle kinetics within the contacted olfactory bulb primordium, causing more cells in this area to exit the cell cycle and begin differentiation as compared to nearby neocortex.⁹   Neurotrophins or other substances provided by the olfactory nerve may be causing changes in cell proliferation patterns in the olfactory bulb primordium.