Olfactory sensory axon growth and branching is influenced by sonic hedgehog.

Olfactory sensory neuron (OSN) axons extend from the olfactory epithelium to the olfactory bulb without branching until they reach their target region, the glomerulus. In this report, we present evidence to support the involvement of sonic hedgehog in promoting rat olfactory sensory axons to branch and to enter into the glomerulus. Sonic hedgehog (Shh) protein is detected in the glomeruli of the olfactory bulb, whereas its transcript is expressed in the mitral and tufted cells, suggesting that Shh in the glomeruli is produced by mitral and tufted cells. In primary OSN cultures, Shh-N peptide promotes olfactory axon branching. When Shh function is neutralized in vivo by its antibody, growth of newly generated OSN axons into the glomeruli is vastly reduced.

Each sensory nerve arising from the geniculate ganglion expresses a unique fingerprint of neurotrophin and neurotrophin receptor genes.

Neurons in the geniculate ganglion, like those in other sensory ganglia, are dependent on neurotrophins for survival. Most geniculate ganglion neurons innervate taste buds in two regions of the tongue and two regions of the palate; the rest are cutaneous nerves to the skin of the ear. We investigated the expression of four neurotrophins, nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3), and NT-4, and five neurotrophin receptors, trkA, trkB, trkC, p75, and truncated trkB (Trn-B) in single sensory neurons of the adult rat geniculate ganglion associated with the five innervation fields. For fungiform papillae, a glass pipette containing biotinylated dextran was placed over the target papilla and the tracer was iontophoresed into the target papilla. For the other target fields, Fluoro-Gold was microinjected. After 3 days, geniculate ganglia were harvested, sectioned, and treated histochemically (for biotinylated dextran) or immunohistochemically (for Fluoro-Gold) to reveal the neurons containing the tracer. Single labeled neurons were harvested from the slides and subjected to RNA amplification and RT-PCR to reveal the neurotrophin or neurotrophin receptor genes that were expressed. Neurons projecting from the geniculate ganglion to each of the five target fields had a unique expression profile of neurotrophin and neurotrophic receptor genes. Several individual neurons expressed more than one neurotrophin receptor or more than one neurotrophin gene. Although BDNF is significantly expressed in taste buds, its primary high affinity receptor, trkB, was not prominently expressed in the neurons. The results are consistent with the interpretation that at least some, perhaps most, of the trophic influence on the sensory neurons is derived from the neuronal somata, and the trophic effect is paracrine or autocrine, rather than target derived. The BDNF in the taste bud may also act in a paracrine or autocrine manner on the trkB expressed in taste buds, as shown by others.

Developmental expression of neurotrophin receptor genes in rat geniculate ganglion neurons.

Individual neurons dissected from immunohistochemically stained paraffin sections of the developing rat geniculate (VIIth cranial) ganglion were assayed for their content of mRNA of the neurotrophin receptor genes, p75 , trkA , trkB and trkC. Fetal and postnatal rats, from the 13th embryonic day (E13) until the 20th postnatal day (P20), were used. Single cells were subjected to RNA amplification, followed by treatment with reverse transcriptase and DNA amplification by the polymerase chain reaction (PCR). The identity of the PCR products was verified by subcloning and sequencing. A total of 227 neurons were examined, of which 212 (93%) gave a PCR signal for at least one neurotrophin receptor. We found: (1) Approximately half of the neurons expressed more than one receptor. (2) A truncated version of trkB , possessing the ligand-binding region but lacking the tyrosine kinase domain, occurred quite frequently, often in combination with the full-length trkB, with trkA or both. (3) The pattern of staining for trkB-like immunoreactivity was usually predictive that either its full length or truncated mRNA would be present. This was not the case for trkC-like immunoreactivity. Western blots on E15 brain tissue showed no band for full-length trkC ( approximately 150 kDa), suggesting the antibody may have been immunoreactive with a truncated ( approximately 120 kDa) but not a full-length version of the trkC receptor. (4) The pattern of neurotrophin receptor gene expression changed during development. (5) p75 expression occurred infrequently--in only 7 of the 212 neurons that gave a signal for any receptor.

Altered epithelial density and expansion of bulbar projections of a discrete HSP70 immunoreactive subpopulation of rat olfactory receptor neurons in reconstituting olfactory epithelium following exposure to methyl bromide.

A previously described subpopulation of rat olfactory receptor neurons, the 2A4(+)ORNs, is 1) distinguished by intense constitutive cytoplasmic immunoreactivity to antibodies to the 70-kD heat shock protein (HSP70); 2) occurs sparsely but consistently through ventral and lateral olfactory epithelium (OE); and 3) projects to just two to three consistently located glomeruli in each olfactory bulb (OB) (Carr et al. [1994] J Comp Neurol 348:150-160). Immunoreactivity appears not to be stress-related. To examine the persistence of these features following destruction and reconstitution of the OE, rats were subjected to methyl bromide-induced OE lesion (Schwob et al. [1995] J Comp Neurol 59:15-37; Schwob et al. [1999] J Comp Neurol 412:439-457] and their OE and OBs examined with antibodies to HSP70 6-10.5 weeks postlesion. Lesioned OE showed significantly increased 2A4(+)ORN densities but no alteration of 2A4(+)ORN zonal distribution. The OBs of lesioned animals showed marked expansions of 2A4(+)ORN bulbar projections, with 2-15-fold increases in numbers of glomeruli showing 2A4(+)axons, and projection expansions were greater in animals maintained on chronic food restriction prior to lesioning. Examination of archival 5-month post-MeBr lesion material indicates that altered projection patterns are maintained.

The septal organ of the rat during postnatal development.

The septal organ of Masera (SO) is a small, isolated patch of olfactory epithelium, located in the ventral part of the nasal septum. We investigated in this systematic study the postnatal development of the SO in histological sections of rats at various ages from the day of birth (P1) to P666. The SO-area increases to a maximum at P66-P105, just as the animals reach sexual maturity, and decreases thereafter, significantly however only in males, indicating a limited neurogenetic capacity for regeneration. In contrast, the main olfactory epithelium area continues to expand beyond P300. The modified respiratory epithelium ('zwischen epithelium') separating the SO and the main olfactory epithelium contains a few olfactory neurons up to age P66. Its length increases postnatally so that the SO becomes more ventral to the OE. Although the position of the SO relative to other anatomical landmarks changes with development it is consistently located just posterior to the opening of the nasopalatine duct (NPAL). Thus, a possible function of the SO is in sensing chemicals in fluids entering the mouth by licking and then delivered to the nasal cavity via the NPAL; therefore the SO may be involved in social/sexual behavior as is the vomeronasal organ (VNO). We suggest that the SO is a separate accessory olfactory organ with properties somewhat different from both OE and VNO and may exist only in species where the NPAL does not open into the VNO.