Timing and regulation of trkB and BDNF mRNA expression in placode-derived sensory neurons and their targets

BDNF Modulation by microRNAs: An Update on the Experimental Evidence

MicroRNAs can interfere with protein function by suppressing their messenger RNA translation or the synthesis of its related factors. The function of brain-derived neurotrophic factor (BDNF) is essential to the proper formation and function of the nervous system and is seen to be regulated by many microRNAs. However, understanding how microRNAs influence BDNF actions within cells requires a wider comprehension of their integrative regulatory mechanisms. Aim: In this literature review, we have synthesized the evidence of microRNA regulation on BDNF in cells and tissues, and provided an analytical discussion about direct and indirect mechanisms that appeared to be involved in BDNF regulation by microRNAs. Methods: Searches were conducted on PubMed.gov using the terms "BDNF" AND "MicroRNA" and "brain-derived neurotrophic factor" AND "MicroRNA", updated on 1 September 2023. Papers without open access were requested from the authors. One hundred and seventy-one papers were included for review and discussion. Results and Discussion: The local regulation of BDNF by microRNAs involves a complex interaction between a series of microRNAs with target proteins that can either inhibit or enhance BDNF expression, at the core of cell metabolism. Therefore, understanding this homeostatic balance provides resources for the future development of vector-delivery-based therapies for the neuroprotective effects of BDNF.

Tubular microdomains of Rab7-positive endosomes retrieve TrkA, a mechanism disrupted in Charcot-Marie-Tooth disease 2B

Axonal survival and growth requires signalling from tropomyosin receptor kinases (Trks). To transmit their signals, receptor-ligand complexes are endocytosed and undergo retrograde trafficking to the soma, where downstream signalling occurs. Vesicles transporting neurotrophic receptors to the soma are reported to be Rab7-positive late endosomes and/or multivesicular bodies (MVBs), where receptors localize within so-called intraluminal vesicles (herein Rab7 corresponds to Rab7A unless specified otherwise). Therefore, one challenging question is how downstream signalling is possible given the insulating properties of intraluminal vesicles. In this study, we report that Rab7-positive endosomes and MVBs retrieve TrkA (also known as NTRK1) through tubular microdomains. Interestingly, this phenotype is absent for the EGF receptor. Furthermore, we found that endophilinA1, endophilinA2 and endophilinA3, together with WASH1 (also known as WASHC1), are involved in the tubulation process. In Charcot-Marie-Tooth disease 2B (CMT2B), a neuropathy of the peripheral nervous system, this tubulating mechanism is disrupted. In addition, the ability to tubulate correlates with the phosphorylation levels of TrkA as well as with neurite length in neuronal cultures from dorsal root ganglia. In all, we report a new retrieval mechanism of late Rab7-positive endosomes, which enables TrkA signalling and sheds new light onto how neurotrophic signalling is disrupted in CMT2B. This article has an associated First Person interview with the first author of the paper.

BDNF and NGF signals originating from sensory ganglia promote cranial motor axon growth

After exiting the hindbrain, branchial motor axons reach their targets in association with sensory ganglia. The trigeminal ganglion has been shown to promote motor axon growth from rhombomeres 2/3 and 4/5, but it is unknown whether this effect is ganglion specific and through which signals it is mediated. Here, we addressed these questions by co-cultures of ventral rhombomere 8 explants with cranial and spinal sensory ganglia in a collagen gel matrix. Our results show that all cranial sensory ganglia and even a trunk dorsal root ganglion can promote motor axon growth and that ganglia isolated from older embryos had a stronger effect on the axonal growth than younger ones. We found that brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) are necessary and sufficient for this effect. Altogether, our results demonstrate that the promoting effect of sensory ganglia on cranial motor axon growth is stage dependent, but not ganglion specific and is mediated by BDNF and NGF signals.

Sensory Axon Growth Requires Spatiotemporal Integration of CaSR and TrkB Signaling

Neural circuit development involves the coordinated growth and guidance of axons. During this process, axons encounter many different cues, but how these cues are integrated and translated into growth is poorly understood. In this study, we report that receptor signaling does not follow a linear path but changes dependent on developmental stage and coreceptors involved. Using developing chicken embryos of both sexes, our data show that calcium-sensing receptor (CaSR), a G-protein-coupled receptor important for regulating calcium homeostasis, regulates neurite growth in two distinct ways. First, when signaling in isolation, CaSR promotes growth through the PI3-kinase-Akt pathway. At later developmental stages, CaSR enhances tropomyosin receptor kinase B (TrkB)/BDNF-mediated neurite growth. This enhancement is facilitated through a switch in the signaling cascade downstream of CaSR (i.e., from the PI3-kinase-Akt pathway to activation of GSK3α Tyr279). TrkB and CaSR colocalize within late endosomes, cotraffic and coactivate GSK3, which serves as a shared signaling node for both receptors. Our study provides evidence that two unrelated receptors can integrate their individual signaling cascades toward a nonadditive effect and thus control neurite growth during development.SIGNIFICANCE STATEMENT This work highlights the effect of receptor coactivation and signal integration in a developmental setting. During embryonic development, neurites grow toward their targets guided by cues in the extracellular environment. These cues are sensed by receptors at the surface that trigger intracellular signaling events modulating the cytoskeleton. Emerging evidence suggests that the effects of guidance cues are diversified, therefore expanding the number of responses. Here, we show that two unrelated receptors can change the downstream signaling cascade and regulate neuronal growth through a shared signaling node. In addition to unraveling a novel signaling pathway in neurite growth, this research stresses the importance of receptor coactivation and signal integration during development of the nervous system.

Neurotrophic factors and the regeneration of adult retinal ganglion cell axons

The adult central nervous system (CNS) has only a limited capacity to regenerate axons after injury. This is due to a number of factors including the presence of extrinsic inhibitory factors that limit plasticity, lack of effective trophic support, and intrinsic changes in neuronal responsiveness. In this review, we describe the expression and role of neurotrophins in retinal ganglion cells (RGCs) during development and adulthood, and the receptors and miscellaneous signaling systems that influence axonal regeneration after injury. The impact of exogenous neurotrophic factors on adult RGCs injured at different sites in the visual pathway is described for several modes of delivery, including recombinant factors, viral vectors, cell transplantation, as well as combinatorial treatments involving other pharmacotherapeutic agents. Indirect, off-target effects of neurotrophic factors on RGC axonal regeneration are also considered. There remain unresolved issues relating to optimal delivery of neurotrophic factors, and we emphasize the need to develop safe, reliable methods for the regulation of exogenous supply of these factors to the injured CNS.

Mice deficient in brain-derived neurotrophic factor have altered development of gastric vagal sensory innervation

Vagal sensory neurons are dependent on neurotrophins for survival during development. Here, the contribution of brain-derived neurotrophic factor (BDNF) to survival and other aspects of gastric vagal afferent development was investigated. Post-mortem anterograde tracing with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbo-cyanine perchlorate (DiI) was used to label selectively vagal projections to the stomach on postnatal days (P) 0, 3, 4, and 6 in wild types and heterozygous or homozygous BDNF mutants. Sampling sites distributed throughout the ventral stomach wall were scanned with a confocal microscope, and vagal axon bundles, single axons, putative mechanoreceptor precursors (intraganglionic laminar endings, IGLEs; intramuscular arrays, IMAs), and efferent terminals were quantified. Also, myenteric neurons, which are innervated by IGLEs, were stained with cuprolinic blue and counted. Quantitative comparisons across wild-type stomach compartments demonstrated that the adult distribution of IMAs was not present at P0 but began to form by P3-6. Among all the quantified elements, at P0, only IGLE density was significantly different in homozygous mutants compared with wild types, exhibiting a 50% reduction. Also, antrum innervation appeared disorganized, and some putative IMA precursors had truncated telodendria. At P3-6, the effect on IGLEs had recovered, the disorganization of antrum innervation had partially recovered, and some IMA telodendria were still truncated. The present results suggest that gastric IGLEs are among the vagal sensory neurons dependent on BDNF for survival or axon guidance. Alternatively, BDNF deficiency may delay gastric IGLE development. Also, BDNF may contribute to IMA differentiation and patterning of antral vagal innervation.

Retentive multipotency of adult dorsal root ganglia stem cells

Preservation of neural stem cells (NSCs) in the adult peripheral nervous system (PNS) has recently been confirmed. However, it is not clear whether peripheral NSCs possess predestined, bona fide phenotypes or a response to innate developmental cues. In this study, we first demonstrated the longevity, multipotency, and high fidelity of sensory features of postmigrating adult dorsal root ganglia (aDRG) stem cells. Derived from aDRG and after 4-5 years in culture without dissociating, the aDRG NSCs were found capable of proliferation, expressing neuroepithelial, neuronal, and glial markers. Remarkably, these aDRG NSCs expressed sensory neuronal markers vesicular glutamate transporter 2 (VGluT2--glutamate terminals), transient receptor potential vanilloid 1 (TrpV1--capsaicin sensitive), phosphorylated 200 kDa neurofilaments (pNF200--capsaicin insensitive, myelinated), and the serotonin transporter (5-HTT), which normally is transiently expressed in developing DRG. Furthermore, in response to neurotrophins, the aDRG NSCs enhanced TrpV1 expression upon exposure to nerve growth factor (NGF), but not to brain-derived neurotrophic factor (BDNF). On the contrary, BDNF increased the expression of NeuN. Third, the characterization of aDRG NSCs was demonstrated by transplantation of red fluorescent-expressing aDRG NSCs into injured spinal cord. These cells expressed nestin, Hu, and beta-III-tubulin (immature neuronal markers), GFAP (astrocyte marker) as well as sensory neural marker TrpV1 (capsaicin sensitive) and pNF200 (mature, capsaicin insensitive, myelinated). Our results demonstrated that the postmigrating neural crest adult DRG stem cells not only preserved their multipotency but also were retentive in sensory potency despite the age and long-term ex vivo status.

The relationship between neurotrophic factors and CaMKII in the death and survival of retinal ganglion cells

The scientific discourse relating to the causes and treatments for glaucoma are becoming reflective of the need to protect and preserve retinal neurons from degenerative changes, which result from the injurious environment associated with this disease. Knowledge, in particular, of the signal transduction pathways which affect death and survival of the retinal ganglion cells is critical to this discourse and to the development of a suitable neurotherapeutic strategy for this disease. The goal of this chapter is to review what is known of the chief suspects involved in initiating the cell death/survival pathways in these cells, and what still remains to be uncovered. The least controversial aspect of the subject relates to the potential role of neurotrophic factors in the protection of the retinal ganglion cells. On the other hand, the postulated triggers for signaling cell death in glaucoma remain controversial. Certainly, the restricted flow of neurotrophic factors has been cited as one possible trigger. However, the connections between glaucoma and other factors present in the retina, such as glutamate, long held to be a prospective culprit in retinal ganglion cell death are still being questioned. Whatever the outcome of this particular debate, it is clear that the downstream intersections between the cell death and survival pathways should provide important foci for future studies whose goal is to protect retinal neurons, situated as they are, in the stressful environment of a cell destroying disease. The evidence for CaMKII being one of these intersecting points is discussed.

Neurotrophin and Trk neurotrophin receptors in the inner ear of Salmo salar and Salmo trutta

Neurotrophins (NTs) and their signal transducing Trk receptors play a critical role in the development and maintenance of specific neuronal populations in the nervous system of higher vertebrates. They are responsible for the innervation of the inner ear cochlear and vestibular sensory epithelia. Neurotrophins and Trks are also present in teleosts but their distribution in the inner ear is unknown. Thus, in the present study, we used Western-blot analysis and immunohistochemistry to investigate the expression and cell localization of both NTs and Trk receptors in the inner ear of alevins of Salmo salar and Salmo trutta. Western-blot analysis revealed the occurrence of brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), but not nerve growth factor (NGF), as well as all three Trk receptors, i.e. TrkA, TrkB and TrkC, the estimated molecular weights of which were similar to those expected for mammals. Specific immunoreactivity for neurotrophins was detected mainly in the sensory epithelia. In particular, BDNF immunoreactivity was found in the maculae of the utricle and saccule, whereas NT-3 immunoreactivity was present in the sensory epithelium of the cristae ampullaris. As a rule the sensory epithelia of the inner ear lacked immunoreactivity for Trks, thus excluding possible mechanisms of autocrinia and/or paracrinia. By contrast, overlapping subpopulations of neurons in the statoacoustic ganglion expressed TrkA (about 15%), TrkB (about 65%) and TrkC (about 45%). The present results demonstrate that, as in mammals and birds, the inner ear of teleosts expresses the components of the neurotrophin-Trk system, but their roles remain to be elucidated.

Regulation of neuronal survival and death by extracellular signals during development

Cell death is a prominent feature of the developing vertebrate nervous system, affecting neurons, glial cells and their progenitors. The most extensively studied and best understood phase of cell death occurs in populations of neurons shortly after they begin establishing connections with other neurons and/or non-neural tissues. This phase of cell death makes appropriate adjustments to the relative sizes of interconnected groups of neurons and matches the size of neuronal populations that innervate non-neural tissues to the optimal requirements of these tissues. The fate of neurons during this period of development is regulated by a variety of secreted proteins that either promote survival or bring about cell death after binding to receptors expressed on the neurons. These proteins may be derived from the targets the neurons innervate, the afferents they receive or from associated glial cells, or they may be secreted by the neurons themselves. In this review, I will outline the established and emerging principles that modulate neuronal number in the developing nervous system.

Neuronal growth factors and development of respiratory control

Neurotrophic molecules, released by neurons and neural target tissues, play a pivotal role in regulating neuronal development and plasticity. This article reviews recent work demonstrating the pivotal role of two such molecules, brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF), in the growth and maturation of respiratory neurons and the expression of normal ventilatory behavior. Although BDNF and GDNF are structurally dissimilar and signal through wholly distinct receptors, they are both required for development of peripheral chemoafferent neurons that provide hypoxic drive to the brainstem respiratory network. Studies of genetically engineered mice carrying targeted deletions in the genes encoding BDNF and GDNF, as well as genetic linkage analysis in humans, indicate that these trophic molecules may be candidate genes for human developmental disorders of breathing.

Auditory system development: primary auditory neurons and their targets

The neurons of the cochlear ganglion transmit acoustic information between the inner ear and the brain. These placodally derived neurons must produce a topographically precise pattern of connections in both the inner ear and the brain. In this review, we consider the current state of knowledge concerning the development of these neurons, their peripheral and central connections, and their influences on peripheral and central target cells. Relatively little is known about the cellular and molecular regulation of migration or the establishment of precise topographic connection to the hair cells or cochlear nucleus (CN) neurons. Studies of mice with neurotrophin deletions are beginning to yield increasing understanding of variations in ganglion cell survival and resulting innervation patterns, however. Finally, existing evidence suggests that while ganglion cells have little influence on the differentiation of their hair cell targets, quite the opposite is true in the brain. Ganglion cell innervation and synaptic activity are essential for normal development of neurons in the cochlear nucleus.

Immunohistochemical localization of BDNF-, TrkB- and TrkA-like proteins in the teleost lateral line system

The lateral line system, formed of both superficial (pit organs) and canal neuromasts, is one of the major mechanosensory systems in fish. It has always been assumed that this system depends on neurotrophins and their cognate Trk receptors for development and maintenance, as has been shown in other mechanosensitive systems of vertebrates. However, until nowthis issue has not been specifically addressed. In this study we used immunohistochemistry to investigate the occurrence and localization both of neurotrophins (NGF-, BDNF- and NT-3-like) and of Trk-like proteins (TrkA-, TrkB-, TrkC-like) in alevins of Salmo salar and S. trutta. All cells in the pit organs of S. salar displayed strong immunoreactivity for TrkB-like and BDNF-like, whereas they were restricted to the hair cells in S. trutta. The hair, supporting and mantle cells of S. salar, and the mantle cells of S. trutta, also expressed TrkA-like immunoreactivity. In the canal neuromasts BDNF-, TrkA- and TrkB-like proteins were present in all cells, without differences between species. NGF-, NT-3- and TrkC-like immunoreactivity were never detected. The present results suggest that mechanoreceptive hair cells, as well as supporting cells, in the lateral line system are under the control of the BDNF-TrkB-like complex, and probably of ligands of TrkA-like receptors.

Trachea enhances neurite regeneration from adult rat nodose ganglia in vitro

Trachea is intensely innervated with vagal afferent nerve fibers, and may play an important role in vagus nerve regeneration after axonal injury caused by trauma and surgical operation. We investigated the effects of tracheal tissue on neuronal cell survival and neurite regeneration in adult rat nodose ganglia (NG) in vitro. Co-culture with trachea significantly increased the average number of neurites regenerated from transected nerve terminals of NG explants, from 73.7 to 154.2 after 3 days, from 68 to 186.7 after 5 days, and from 31 to 101.5 after 7 days in culture. Dissociated NG neurons could continue to survive and extend neurites only in the co-existence with satellite cells in collagen gel. Co-cultured trachea improved the ratios of survival and neurite-bearing cells of NG neurons, from 56.7% and 11.1% to 72.3% and 37.6% after 4 days, and from 41.1% and 20.3% to 56.4% and 47.2% after 7 days in culture, respectively. These results imply that tracheal tissue secretes a factor, which could enhance neuronal cell survival and neurite regeneration in NG in the presence of satellite cells in vitro.

Dopaminergic retinal cell differentiation in culture: modulation by forskolin and dopamine

We examined the effects of dopamine and cAMP on the differentiation of dopaminergic retinal cells in the chick retina, using an in vitro system and tyrosine hydroxylase immunocytochemistry. Tyrosine hydroxylase-positive cells were detected in cultures prepared from embryonic day 10 retinas. These increased in number as a function of time in vitro and by treatment for 4 days with forskolin. Besides causing a 3.4-fold increase in the tyrosine hydroxylase-positive population, forskolin also caused these cells to developed morphogenetic features of more mature cells. As opposed to forskolin, cultures treated with dopamine exhibited a 55% reduction of the tyrosine hydroxylase-positive cell population, as compared to untreated cultures. Quinpirole was able to mimic the dopamine effect. This dopamine effect could only be blocked by clozapine, whereas raclopride and eticlopride were ineffective. Our results suggest the existence of a narrow window during development when undifferentiated dopaminergic cells are capable of being influenced by specific signals, possibly via cAMP production. The data also indicate that dopamine may act as a regulatory factor limiting the tyrosine hydroxylase-positive population in the retina.

Regulation of glial cell line-derived neurotrophic factor responsiveness in developing rat sympathetic neurons by retinoic acid and bone morphogenetic protein-2

There are several lines of evidence suggesting that, in addition to neurotrophins, member(s) of glial cell line-derived neurotrophic factor (GDNF) family play important roles in the development of sympathetic neurons. However, the mechanism regulating the responsiveness of the neurons to GDNF family members is not known. Previously, we reported on the cooperative roles of bone morphogenetic protein-2 (BMP2) and retinoic acid (RA) in the enhancement of neurotrophin-3 (NT3) responsiveness in cultured sympathetic neurons dissociated from perinatal rat superior cervical ganglia (SCG). In the present study, we further examined the effects of BMP2 and RA on the regulation of the responsiveness of SCG neurons to GDNF family members. Consequently, we found that RA alone induced the responsiveness of SCG neurons specifically to GDNF by upregulating the ligand-specifying receptor for GDNF (GFRalpha-1) at both the mRNA and protein levels. The expression levels of mRNAs for other ligand-specifying receptors for GDNF family (GFRalpha-2 and GFRalpha-3) were unaffected by RA. Although the upregulation of signal-transducing receptor Ret by the RA treatment was rather small, this treatment significantly increased the efficacy of tyrosine phosphorylation of Ret by GDNF. Experiments using synthetic retinoids suggested that RA acts through alpha-type of nuclear retinoic acid receptor to exert the induction of GDNF responsiveness. On the other hand, BMP2, which had no significant effect by itself on the GDNF responsiveness, promoted the action of RA to upregulate GFRalpha-1 and enhance the GDNF responsiveness. These results indicate that RA and BMP2 play important roles in the induction of GDNF responsiveness, as well as NT3 responsiveness, of developing SCG neurons.

TrkB expression and early sensory neuron survival are independent of endogenous BDNF

Sensory neurons initially survive independently of neurotrophins in culture during the stage of development when their axons are growing to their targets. Because mRNAs encoding brain-derived neurotrophic factor (BDNF) and its receptor tyrosine kinase TrkB are detectable in subsets of sensory neurons from the earliest stages of their development, we investigated whether a BDNF autocrine loop is responsible for sustaining the survival of these neurons during this early stage in their development. Low-density dissociated cultures of nodose and dorsal root ganglion neurons were established from wild type and BDNF(-/-) mouse embryos at this stage and were grown in defined medium without added neurotrophins. Wild type and BDNF-deficient neurons survived equally well under these conditions, indicating that a BDNF autocrine loop does not play a role in sustaining the survival of sensory neurons during the earliest stages of their development. As sensory axons approach their targets, TrkB expression increases in a subset of neurons that becomes dependent on BDNF produced by other cells. Because numerous studies have shown that neurotrophins, including BDNF, increase expression of their receptors, we investigated whether endogenous BDNF is required for the increase in TrkB expression observed during stage of development. Quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) showed that the developmental increase in TrkB mRNA expression occurred normally in the sensory ganglia of BDNF(-/-) embryos. Taken together, our studies of sensory neuron development in BDNF-deficient embryos have demonstrated that endogenous BDNF is neither required for the early survival of these neurons nor for the induction of TrkB expression.

Abundant production of brain-derived neurotrophic factor by adult visceral epithelia. Implications for paracrine and target-derived Neurotrophic functions

Brain-derived neurotrophic factor (BDNF) plays a crucial role for the survival of visceral sensory neurons during development. However, the physiological sources and the function of BDNF in the adult viscera are poorly described. We have investigated the cellular sources and the potential role of BDNF in adult murine viscera. We found markedly different amounts of BDNF protein in different organs. Surprisingly, BDNF levels in the urinary bladder, lung, and colon were higher than those found in the brain or skin. In situ hybridization experiments revealed that BDNF mRNA was made by visceral epithelial cells, several types of smooth muscle, and neurons of the myenteric plexus. Epithelia that expressed BDNF lacked both the high- and low-affinity receptors for BDNF, trkB and p75(NTR). In contrast, both receptors were present on neurons of the peripheral nervous system. Studies with BDNF-/-mice demonstrated that epithelial and smooth muscle cells developed normally in the absence of BDNF. These data provide evidence that visceral epithelia are a major source, but not a target, of BDNF in the adult viscera. The abundance of BDNF protein in certain internal organs suggests that this neurotrophin may regulate the function of adult visceral sensory and motor neurons.

Selective regulation of trkC expression by NT3 in the developing peripheral nervous system

We have studied the influence of neurotrophin-3 (NT3) on the expression of its receptor tyrosine kinase, trkC, in embryonic mice. The expression of trkC transcripts encoding full-length and kinase-deficient receptors was almost entirely restricted to neurons in the trigeminal ganglion and increased markedly throughout development. In NT3(+/-) embryos, the level of trkC mRNA in the trigeminal ganglion was much lower than that in wild-type embryos, although there was no significant reduction in the total number of neurons in the ganglion. This demonstrates that endogenous NT3 regulates trkC expression in trigeminal neurons independently of changes in population size. In NT3(-/-) embryos, the number of neurons in the trigeminal ganglion was much lower than in wild-type embryos, and there was a further reduction in the mean neuronal level of trkC mRNA. Direct regulation of trkC mRNA expression in cultured trigeminal neurons was also observed, although the finding that trkC mRNA levels were sustained better in explant cultures than in dissociated cultures irrespective of the presence of NT3 suggests that trkC mRNA expression is regulated by additional factors within the ganglion. In contrast to trigeminal neurons, the level of trkC mRNA was sustained at normal levels in neurons of the sympathetic chain of NT3(-/-) embryos and was not increased by NT3 in sympathetic neuron cultures. TrkC mRNA expression in developing cutaneous tissues was also unaffected by the NT3 null mutation. In summary, our findings provide the first clear evidence that the expression of a trk receptor, tyrosine kinase, is regulated by physiological levels of its ligand in vivo and show that regulation by NT3 is cell type-specific.

BDNF is a target-derived survival factor for arterial baroreceptor and chemoafferent primary sensory neurons

Brain-derived neurotrophic factor (BDNF) supports survival of 50% of visceral afferent neurons in the nodose/petrosal sensory ganglion complex (NPG; Ernfors et al., 1994a; Jones et al., 1994; Conover et al., 1995; Liu et al., 1995; Erickson et al., 1996), including arterial chemoafferents that innervate the carotid body and are required for development of normal breathing (Erickson et al., 1996). However, the relationship between BDNF dependence of visceral afferents and the location and timing of BDNF expression in visceral tissues is unknown. The present study demonstrates that BDNF mRNA and protein are transiently expressed in NPG targets in the fetal cardiac outflow tract, including baroreceptor regions in the aortic arch, carotid sinus, and right subclavian artery, as well as in the carotid body. The period of BDNF expression corresponds to the onset of sensory innervation and to the time at which fetal NPG neurons are BDNF-dependent in vitro. Moreover, baroreceptor innervation is absent in newborn mice lacking BDNF. In addition to vascular targets, vascular afferents themselves express high levels of BDNF, both during and after the time they are BDNF-dependent. However, endogenous BDNF supports survival of fetal NPG neurons in vitro only under depolarizing conditions. Together, these data indicate two roles for BDNF during vascular afferent pathway development; initially, as a target-derived survival factor, and subsequently, as a signaling molecule produced by the afferents themselves. Furthermore, the fact that BDNF is required for survival of functionally distinct populations of vascular afferents demonstrates that trophic requirements of NPG neurons are not modality-specific but may instead be associated with innervation of particular organ systems.

Regulation of neurotrophin receptor expression by retinoic acid in mouse sympathetic neuroblasts

We have studied the effect of retinoic acid on the expression of the neurotrophin receptors trkA, trkC, and p75 by neuroblasts and neurons at different axial levels along the embryonic mouse paravertebral sympathetic chain. In dissociated cultures of sympathetic neuroblasts, retinoic acid inhibited the developmental increase in trkA mRNA expression and the developmental decrease in trkC mRNA expression that normally occurs in these cells but did not affect p75 mRNA expression. At higher concentrations, retinoic acid also increased the proliferation of sympathetic neuroblasts. After sympathetic neuroblasts became postmitotic, retinoic acid no longer affected receptor expression. Studies with retinoic acid receptor agonists and antagonists indicated that the effects of retinoic acid on neurotrophin receptor expression were mediated mainly by alpha retinoic acid receptors, not beta or gamma receptors. The observation that alpha-antagonists increased trkA mRNA expression in intact sympathetic ganglion explants suggests that endogenous retinoic acid is a physiological regulator of trkA receptor expression.

BDNF and trkB mRNAs oscillate in rat brain during the light-dark cycle

In this study, we investigated whether in basal conditions the different functional states occurring during a 24-h cycle are reflected by the expression of brain-derived neurotrophic factor (BDNF) and its receptor, trkB, in rat cerebral cortex and hippocampus. Using semiquantitative RT-PCR assay, the levels of both BDNF and trkB mRNAs were found to undergo significant variation in a 24-h period. The strongest variation was detected in the hippocampus, where the ratio between maximum and minimum levels was about 3.5 and 17.5 for BDNF and trkB, respectively. These findings provide the first evidence that, in the absence of any experimental manipulation, the expression of a neurotrophin and its receptor undergoes diurnal oscillation, possibly related to the physiological variations of the activity level.