Genetic tracing of the gustatory neural pathway originating from Pkd1l3-expressing type III taste cells in circumvallate and foliate papillae.

Polycystic kidney disease 1-like 3 (Pkd1l3) is expressed specifically in sour-sensing type III taste cells that have synaptic contacts with afferent nerve fibers in circumvallate (CvP) and foliate papillae (FoP) located in the posterior region of the tongue, although not in fungiform papillae (FuP) or the palate. To visualize the gustatory neural pathways that originate from type III taste cells in CvP and FoP, we established transgenic mouse lines that express the transneuronal tracer wheat germ agglutinin (WGA) under the control of the mouse Pkd1l3 gene promoter/enhancer. The WGA transgene was accurately expressed in Pkd1l3-expressing type III taste cells in CvP and FoP. Punctate WGA protein signals appeared to be detected specifically in type III taste cells but not in other types of taste cells. WGA protein was transferred primarily to a subset of neurons located in close proximity to the glossopharyngeal (GL) nerve bundles in the nodose/petrosal ganglion (NPG). WGA signals were also observed in a small population of neurons in the geniculate ganglion (GG). This result demonstrates the anatomical connection between taste receptor cells (TRCs) in the FoP and the chorda tympani (CT) nerves. WGA protein was further conveyed to neurons in a rostro-central subdivision of the nucleus of the solitary tract (NST). These findings demonstrate that the approximately 10 kb 5'-flanking region of the mouse Pkd1l3 gene functions as a type III taste cell-specific promoter/enhancer. In addition, experiments using the pkd1l3-WGA transgenic mice reveal a sour gustatory pathway that originates from TRCs in the posterior region of the tongue.

Mice lacking the p75 receptor fail to acquire a normal complement of taste buds and geniculate ganglion neurons by adulthood.

Brain-derived neurotrophic factor and neurotrophin-4 are required for normal taste bud development. Although these neurotrophins normally function via the tyrosine kinase receptor, trkB, they also bind to the pan-neurotrophin receptor, p75. The goal of the present study was to determine whether the p75 receptor is required for the development or maintenance of a full complement of adult taste buds. Mice with p75 null mutations lose 34% of their circumvallate taste buds, 36% of their fungiform papillae, and 26% of their fungiform taste buds by adulthood. The reduction of taste buds in the adult circumvallate papilla was similar to that observed previously at postnatal day 7 (Fan et al. Brain Res Dev Brain Res 2004;150:23-39). Taken together, these findings indicate that the p75 receptor is critical for the development of a full complement of taste buds, but is not required for maintenance of circumvallate taste buds in adulthood. Immunolabeling for p75 was not observed in taste buds, indicating that p75 signaling influences taste bud number indirectly. Geniculate ganglion neurons, which provides innervation to fungiform taste buds, express the p75 receptor. Mice with p75 null mutations also have fewer neurons in the geniculate ganglion. Together, these results suggest that the p75 receptor is important for the survival of geniculate neurons and geniculate neuron survival is required for the development of a full complement of taste buds by adulthood.

Retinal ganglion cell projections to the hamster suprachiasmatic nucleus, intergeniculate leaflet, and visual midbrain: bifurcation and melanopsin immunoreactivity.

The circadian clock in the suprachiasmatic nucleus (SCN) receives direct retinal input via the retinohypothalamic tract (RHT), and the retinal ganglion cells contributing to this projection may be specialized with respect to direct regulation of the circadian clock. However, some ganglion cells forming the RHT bifurcate, sending axon collaterals to the intergeniculate leaflet (IGL) through which light has secondary access to the circadian clock. The present studies provide a more extensive examination of ganglion cell bifurcation and evaluate whether ganglion cells projecting to several subcortical visual nuclei contain melanopsin, a putative ganglion cell photopigment. The results showed that retinal ganglion cells projecting to the SCN send collaterals to the IGL, olivary pretectal nucleus, and superior colliculus, among other places. Melanopsin-immunoreactive (IR) ganglion cells are present in the hamster retina, and some of these cells project to the SCN, IGL, olivary pretectal nucleus, or superior colliculus. Triple-label analysis showed that melanopsin-IR cells bifurcate and project bilaterally to each SCN, but not to the other visual nuclei evaluated. The melanopsin-IR cells have photoreceptive characteristics optimal for circadian rhythm regulation. However, the presence of moderately widespread bifurcation among ganglion cells projecting to the SCN, and projection by melanopsin-IR cells to locations distinct from the SCN and without known rhythm function, suggest that this ganglion cell type is generalized, rather than specialized, with respect to the conveyance of photic information to the brain.

Neurotrophin receptors in the geniculate ganglion.

We examined the distribution of the high affinity neurotrophin receptors (trkA, trkB, and trkC) in the rat geniculate ganglion. Previous work had shown that during early (prenatal) development, trkB and its two ligands, BDNF and NT-4/5, were most important for survival of almost all neurons. Using nested polymerase chain reaction (PCR), we showed that trkA, trkB, and trkC transcripts were expressed, and the mRNAs for trkB and trkC were more abundant than that for trkA. We modified and improved the method for direct reverse transcription in situ PCR and localized trkB mRNA in approximately one third of the neurons in the ganglion. Immunohistochemical data confirmed that approximately the same fraction of neurons was immunoreactive with antibody vs. trkB, and an approximately equal fraction was immunoreactive with trkC antibody. These results are consistent with the notion that both BDNF/trkB and NT-3/trkC play important roles in maintenance of the geniculate ganglion neurons and possibly the peripheral taste system in the young postnatal rat.

[Morphologic characteristic of substance P immunoreactivity in facial nerve of normal cats].

This paper presents the morphologic charactistic of substance P immunoreactivity (SP-IR) in the facial nerve and geniculate ganglion of normal cats by using immunohistochemical technique and immunoelectronic microscopy. SP-IR positive cells are more in geniculate ganglion than the other parts of the facial nerve. These cells are black. The sizes of their somata are different, and the shapes are round, elliptic or irregular. The somata are connected with each other by their central and peripheral synapses in the shapes of spider's web or root of a tree. Among the cells the bipolar and pseudounipolar cells are also found. SP-IR positive fibers curve and some of them have obvious expansive bodies, which are beaded, different in diameter. Under electronic microscope SP-IR positive cells show thicker electronic density than the others, and SP-IR positive products with thick electronic density are found in the nuclei, nucleomembranes, outer membranes of mitochndria and plasma. The nerve fibers are myelinated and the myelin sheath is concentric, and in the axoplasms there are nervous microfilament and a bit of mitochndria.

Developmental changes in nerve growth factor (NGF) binding and NGF receptor proteins trkA and p75 in the facial nerve.

This study characterizes the temporal-spatial distribution of nerve growth factor (NGF) low (p75) and high-affinity (trkA) receptors in the facial nerve and geniculate ganglion (GG) of developing quail embryos (E-3 to E-14). We used 125I-labeled NGF (125I-NGF) to study binding dynamics in a temporal series of isolated primordia and an autoradiographic series of staged specimens to characterize the occurrence and distribution of NGF receptors in this cranial nerve and its ganglion. In addition, expression of trkA and p75 protein-like immunoreactivity in the facial nerve and GG was studied by Western blot, in order to distinguish between high- and low-affinity NGF receptors respectively. The quantitative study of binding show that isolated facial primordia ranging from E-3 to E-14 exhibit different levels of specific binding. High initial binding levels were observed on E-3 specimens, then an initial decrease on day 4 (E-4) followed by a steady increase from days E-4 to E-7. Maximum 125I-NGF binding was achieved on E-7, followed by a steady decline in binding on days 8 (E-8) and 9 (E-9), reaching near background levels on day 10 (E-10) of development and until the oldest stage assayed (E-14). Most of the cells bearing NGF receptors appeared to be non-neuronal crest-derived cells, but some placode-derived neurons and motor fibers of the VIIth cranial nerve transiently expressed the ability to bind 125I-NGF. The temporal pattern of p75 expression matches the pattern of quantitative binding of NGF, while the trkA expression is restricted to a few stages mainly E7 and E9, implying that most of the binding detected is via low-affinity receptors, except for a proportion of high-affinity receptors present at stages of maximum binding. This temporal pattern of NGF binding sites suggests that cells within the VIIth cranial nerve are responsive to and/or dependent upon NGF in vivo, so NGF may play a biological role during normal development of the facial nerve. In view of the developmental events that parallel the occurrence and type of NGF binding sites, we suggest that this role may be to modulate from earlier chemotaxis and cell proliferation to much later events, such as neuronal differentiation and neuron-glia interactions. The significance of these findings in regeneration during adult life remain to be investigated.

Detection of latent herpes simplex virus DNA and RNA in human geniculate ganglia by the polymerase chain reaction.

By using the polymerase chain reaction (PCR) we detected latent herpes simplex virus type 1 (HSV-1) in human geniculate and trigeminal ganglia obtained from autopsy cases. A pair of primers which were specific for a part of the HSV-1 thymidine kinase domain were used for detection of HSV DNA. We also examined the latency-associated transcript (LAT), known as latency-specific RNA, by means of reverse transcription-PCR with a pair of LAT-specific primers. HSV-1 DNA was detected in 16 of 17 (94%) trigeminal ganglia and in 15 to 17 (88%) geniculate ganglia of adults. We also demonstrated HSV-1 RNA derived from the LAT in both types of ganglia. These findings suggest that HSV-1 latently infects the majority of geniculate and trigeminal ganglia of adults, and that PCR and reverse transcription-PCR are useful tools for analysis of HSV latency.