Brain-derived neurotrophic factor regulates expression of vasoactive intestinal polypeptide in retinal amacrine cells

Visual stimulation and brain-derived neurotrophic factor (BDNF) have protective effects in experimental autoimmune uveoretinitis

AIMS

To investigate the therapeutic potential of visual stimulation (VS) and BDNF in murine experimental autoimmune uveoretinitis (EAU).

MAIN METHODS

Mice were immunized by subcutaneous injection of interphotoreceptor retinoid-binding protein in Freund's complete adjuvant and intravenous injection of pertussis toxin, and were then exposed to high-contrast VS 12 h/day (days 1-14 post-immunization). EAU severity was assessed by examining clinical score, visual acuity, inflammatory markers, and immune cells in the retina. The transcriptome of activated retinal cells was determined by RNA-seq using RNA immunoprecipitated in complex with phosphorylated ribosomal protein S6. The retinal levels of protein products of relevant upregulated genes were quantified. The effect of BDNF on EAU was tested in unstimulated mice by its daily topical ocular administration (days 8-14 post-immunization).

KEY FINDINGS

VS attenuated EAU development and decreased the expression of pro-inflammatory cytokines/chemokines and numbers of immune cells in the retina (n = 10-20 eyes/group for each analysis). In activated retinal cells of control mice (n = 30 eyes/group), VS upregulated genes encoding immunomodulatory neuropeptides, of which BDNF and vasoactive intestinal peptide (VIP) also showed increased mRNA and protein levels in the retina of VS-treated EAU mice (n = 6-10 eyes/group for each analysis). In unstimulated EAU mice, BDNF treatment mimicked the protective effects of VS by modulating the inflammatory and stem cell properties of Müller cells (n = 5 eyes/group for each analysis).

SIGNIFICANCE

VS effectively suppresses EAU, at least through enhancing retinal levels of anti-inflammatory and neuroprotective factors, VIP and BDNF. Our findings also suggest BDNF as a promising therapeutic agent for uveitis treatment.

Relevance of Peptide Homeostasis in Metabolic Retinal Degenerative Disorders: Curative Potential in Genetically Modified Mice

The mammalian retina contains approximately 30 neuropeptides that are synthetized by different neuronal cell populations, glia, and the pigmented epithelium. The presence of these neuropeptides leaves a mark on normal retinal molecular processes and physiology, and they are also crucial in fighting various pathologies (e.g., diabetic retinopathy, ischemia, age-related pathologies, glaucoma) because of their protective abilities. Retinal pathologies of different origin (metabolic, genetic) are extensively investigated by genetically manipulated in vivo mouse models that help us gain a better understanding of the molecular background of these pathomechanisms. These models offer opportunities to manipulate gene expression in different cell types to help reveal their roles in the preservation of retinal health or identify malfunction during diseases. In order to assess the current status of transgenic technologies available, we have conducted a literature survey focused on retinal disorders of metabolic origin, zooming in on the role of retinal neuropeptides in diabetic retinopathy and ischemia. First, we identified those neuropeptides that are most relevant to retinal pathologies in humans and the two clinically most relevant models, mice and rats. Then we continued our analysis with metabolic disorders, examining neuropeptide-related pathways leading to systemic or cellular damage and rescue. Last but not least, we reviewed the available literature on genetically modified mouse strains to understand how the manipulation of a single element of any given pathway (e.g., signal molecules, receptors, intracellular signaling pathways) could lead either to the worsening of disease conditions or, more frequently, to substantial improvements in retinal health. Most attention was given to studies which reported successful intervention against specific disorders. For these experiments, a detailed evaluation will be given and the possible role of converging intracellular pathways will be discussed. Using these converging intracellular pathways, curative effects of peptides could potentially be utilized in fighting metabolic retinal disorders.

Localisation analysis of nerves in the mouse pancreas reveals the sites of highest nerve density and nociceptive innervation

BACKGROUND

Neuropathy and neuro-inflammation drive the severe pain and disease progression in human chronic pancreatitis and pancreatic cancer. Mice, especially genetically induced-mouse models, have been increasingly utilized in mechanistic research on pancreatic neuropathy, but the normal "peripheral neurobiology" of the mouse pancreas has not yet been critically compared to human pancreas.

METHODS

We introduced a standardized tissue-harvesting technique that preserves the anatomic orientation of the mouse pancreas and allows complete sectioning in an anterior to posterior fashion. We applied immunohistochemistry and quantitative colorimetry of all nerves from the whole organ for studying pancreatic neuro-anatomy.

KEY RESULTS

Nerves in the mouse pancreas appeared as "clusters" of nerve trunks in contrast to singly distributed nerve trunks in the human pancreas. Nerve trunks in the mouse pancreas were exclusively found around intrapancreatic blood vessels, and around lymphoid structures. The majority of nerve trunks were located in the pancreatic head (0.15 ± 0.08% of tissue area) and the anterior/front surface of the corpus/body (0.17 ± 0.27%), thus significantly more than in the tail (0.02 ± 0.02%, P = .006). Nerves in the tail included a higher proportion of nociceptive fibers, but the absolute majority, ie, ca. 70%, of all nociceptive fibers, were localized in the head. Mice heterozygous for Bdnf knockout allele (Bdnf^+/- ) exhibited enrichment of nitrergic nerve fibers specifically in the head and corpus.

CONCLUSIONS & INFERENCES

Neuro-anatomy of the "mesenteric type" mouse pancreas is highly different from the "compact" human pancreas. Studies that aim at reproducing human pancreatic neuro-phenomena in mouse models should pay diligent attention to these anatomic differences.

Multiple cell types form the VIP amacrine cell population

Amacrine cells are a heterogeneous group of interneurons that form microcircuits with bipolar, amacrine and ganglion cells to process visual information in the inner retina. This study has characterized the morphology, neurochemistry and major cell types of a VIP-ires-Cre amacrine cell population. VIP-tdTomato and -Confetti (Brainbow2.1) mouse lines were generated by crossing a VIP-ires-Cre line with either a Cre-dependent tdTomato or Brainbow2.1 reporter line. Retinal sections and whole-mounts were evaluated by quantitative, immunohistochemical, and intracellular labeling approaches. The majority of tdTomato and Confetti fluorescent cell bodies were in the inner nuclear layer (INL) and a few cell bodies were in the ganglion cell layer (GCL). Fluorescent processes ramified in strata 1, 3, 4, and 5 of the inner plexiform layer (IPL). All tdTomato fluorescent cells expressed syntaxin 1A and GABA-immunoreactivity indicating they were amacrine cells. The average VIP-tdTomato fluorescent cell density in the INL and GCL was 535 and 24 cells/mm² , respectively. TdTomato fluorescent cells in the INL and GCL contained VIP-immunoreactivity. The VIP-ires-Cre amacrine cell types were identified in VIP-Brainbow2.1 retinas or by intracellular labeling in VIP-tdTomato retinas. VIP-1 amacrine cells are bistratified, wide-field cells that ramify in strata 1, 4, and 5, VIP-2A and 2B amacrine cells are medium-field cells that mainly ramify in strata 3 and 4, and VIP-3 displaced amacrine cells are medium-field cells that ramify in strata 4 and 5 of the IPL. VIP-ires-Cre amacrine cells form a neuropeptide-expressing cell population with multiple cell types, which are likely to have distinct roles in visual processing.

Neurotrophins in the Brain: Interaction With Alcohol Exposure During Development

Fetal alcohol spectrum disorders (FASDs) are a result of the teratogenic effects of alcohol on the developing fetus. Decades of research examining both individuals with FASDs and animal models of developmental alcohol exposure have revealed the devastating effects of alcohol on brain structure, function, behavior, and cognition. Neurotrophic factors have an important role in guiding normal brain development and cellular plasticity in the adult brain. This chapter reviews the current literature showing that alcohol exposure during the developmental period impacts neurotrophin production and proposes avenues through which alcohol exposure and neurotrophin action might interact. These areas of overlap include formation of long-term potentiation, oxidative stress processes, neuroinflammation, apoptosis and cell loss, hippocampal adult neurogenesis, dendritic morphology and spine density, vasculogenesis and angiogenesis, and behaviors related to spatial memory, anxiety, and depression. Finally, we discuss how neurotrophins have the potential to act in a compensatory manner as neuroprotective molecules that can combat the deleterious effects of in utero alcohol exposure.

Injury-associated PACAP expression in rat sensory and motor neurons is induced by endogenous BDNF

Peripheral nerve injury results in dramatic upregulation in pituitary adenylate cyclase activating polypeptide (PACAP) expression in adult rat dorsal root ganglia and spinal motor neurons mirroring that described for the neurotrophin brain derived neurotrophic factor (BDNF). Thus, we posited that injury-associated alterations in BDNF expression regulate the changes in PACAP expression observed in the injured neurons. The role of endogenous BDNF in induction and/or maintenance of PACAP mRNA expression in injured adult rat motor and sensory neurons was examined by intrathecally infusing or intraperitoneally injecting BDNF-specific antibodies or control IgGs immediately at the time of L4-L6 spinal nerve injury, or in a delayed fashion one week later for 3 days followed by analysis of impact on PACAP expression. PACAP mRNA in injured lumbar sensory and motor neurons was detected using in situ hybridization, allowing quantification of relative changes between experimental groups, with ATF-3 immunofluorescence serving to identify the injured subpopulation of motor neurons. Both the incidence and level of PACAP mRNA expression were dramatically reduced in injured sensory and motor neurons in response to immediate intrathecal anti-BDNF treatment. In contrast, neither intraperitoneal injections nor delayed intrathecal infusions of anti-BDNF had any discernible impact on PACAP expression. This impact on PACAP expression in response to BDNF immunoneutralization in DRG was confirmed using qRT-PCR or by using BDNF selective siRNAs to reduce neuronal BDNF expression. Collectively, our findings support that endogenous injury-associated BDNF expression is critically involved in induction, but not maintenance, of injury-associated PACAP expression in sensory and motor neurons.

Neuropeptides and diabetic retinopathy

Diabetic retinopathy, a common complication of diabetes, develops in 75% of patients with type 1 and 50% of patients with type 2 diabetes, progressing to legal blindness in about 5%. In the recent years, considerable efforts have been put into finding treatments for this condition. It has been discovered that peptidergic mechanisms (neuropeptides and their analogues, activating a diverse array of signal transduction pathways through their multiple receptors) are potentially important for consideration in drug development strategies. A considerable amount of knowledge has been accumulated over the last three decades on human retinal neuropeptides and those elements in the pathomechanisms of diabetic retinopathy which might be related to peptidergic signal transduction. Here, human retinal neuropeptides and their receptors are reviewed, along with the theories relevant to the pathogenesis of diabetic retinopathy both in humans and in experimental models. By collating this information, the curative potential of certain neupeptides and their analogues/antagonists can also be discussed, along with the existing clinical treatments of diabetic retinopathy. The most promising peptidergic pathways for which treatment strategies may be developed at present are stimulation of the somatostatin-related pathway and the pituitary adenylyl cyclase-activating polypeptide-related pathway or inhibition of angiotensinergic mechanisms. These approaches may result in the inhibition of vascular endothelial growth factor production and neuronal apoptosis; therefore, both the optical quality of the image and the processing capability of the neural circuit in the retina may be saved.

VIP, PACAP-38, BDNF and ADNP in NMDA-induced excitotoxicity in the rat retina

PURPOSE

To evaluate the effect of intravitreal injection of N-methyl-D-aspartate (NMDA) on brain-derived neurotrophic factor (BDNF), pituitary adenylate cyclase-activating peptide-38 (PACAP-38), vasoactive intestinal peptide (VIP) and the VIP-associated glial protein activity-dependent neuroprotective protein (ADNP) in the rat retina. These elements have well-documented neuroprotective properties and may thus be integrated in endogenous neuroprotective mechanisms in the retina which break down in NMDA excitotoxicity.

METHODS

A volume of 2 μl of 100 nmol NMDA was intravitreally injected into one eye of rats, the untreated eye served as a control. Time-dependent effects of NMDA on VIP, PACAP-38 and BDNF were detected by radioimmunoassay and ELISA, and the effect on the expression of VIP, PACAP-38 and ADNP was evaluated by quantitative RT-PCR 20 days after NMDA injection. Topical flunarizine served to find out whether the effect of NMDA is counteracted.

RESULTS

Compared to PACAP-38, VIP levels significantly decreased on days 1, 7, 14, 28 and 56 after NMDA injection indicating that VIPergic cells are more vulnerable than PACAP-38-expressing cells. The expression of VIP and ADNP but not of PACAP-38 was found to be reduced, and application of topical flunarizine counteracted the decrease of VIP. BDNF levels significantly increased after days 1 and 3.

CONCLUSION

The early upregulation of BDNF seems to act neuroprotectively and leads to a delay of ganglion cell loss. Although there is no direct evidence, the decrease of VIP and ADNP - the consequence of the presence of NMDA receptors on these peptide-expressing cells - might contribute to the breakdown of endogenous neuroprotective mechanisms given that the decrease of the VIP-related ADNP runs in parallel with the decrease of VIP. Activating and maintaining these mechanisms must be the primary aim in the therapy of diseases with retinal neuronal degeneration.

Setting the pace for retinal development: environmental enrichment acts through insulin-like growth factor 1 and brain-derived neurotrophic factor

Environmental enrichment strongly affects visual system maturation both at retinal and cortical levels. Which molecular pathways are activated by an enriched environment (EE) to regulate visual system development has not been clarified. Here, we show that early [postnatal day 1 (P1) to P7] insulin-like growth factor 1 (IGF-1) injections in the eyes of non-EE rat pups mimic EE effects both in increasing BDNF levels in the retinal ganglion cell layer at P10 and in determining a more adult-like retinal acuity, assessed with pattern electroretinogram at P25. Blocking IGF-1 action in EE animals during the same early postnatal time window by injecting the IGF-1 receptor antagonist JB1 prevents EE effects both on BDNF expression and on retinal acuity maturation. Reducing BDNF expression in the retina of IGF-1-treated rats prevents IGF-1 effects on retinal acuity development. Finally, we show that tyrosine hydroxylase (TH) expression is increased in the retina of P10 EE and IGF-1-treated rats and that blocking TH expression in EE animals prevents EE from affecting retinal acuity development. Thus, early levels of IGF-1 play a key role in mediating EE effects on retinal development through an action that requires BDNF and involves dopaminergic amacrine cell network.

The neuropeptide NAP provides neuroprotection against retinal ganglion cell damage after retinal ischemia and optic nerve crush

BACKGROUND

NAP, an 8-amino acid peptide (NAPVSIPQ=Asn-Ala-Pro-Val-Ser-Ile-Pro-Gln) derived from activity-dependent neuroprotective protein (ADNP), plays an important role in neuronal differentiation and the survival of neurons in different pathological situations. We already discovered that NAP increases the survival of retinal ganglion cells (RGC) in vitro, and supports neurite outgrowth in retinal explants at femtomolar concentrations. The aim of this study was to investigate the effects of NAP on RGC survival after transient retinal ischemia and optic nerve crush.

METHODS

RGC of male Wistar rats were labelled retrogradely with 6 l FluoroGold injected stereotactically into both superior colliculi. Seven days later, retinal ischemia was induced by elevating the intraocular pressure to 120 mm Hg for 60 minutes or by crushing one optic nerve for 10 s after a partial orbitotomy. NAP was either injected intraperitoneally in the concentration of 100 microg/kg [corrected] 1 day before, directly after, and on the first and the second days after damage, or intravitreally (0.05 or 0.5 microg/eye) [corrected] directly after the optic nerve crush. Controls received the same concentrations of a control peptide. Densities of surviving RGC and activated microglial cells (AMC) were quantified in a masked fashion 10 days after damage by counting FluoroGold-labelled cells.

RESULTS

After retinal ischemia, intraperitoneal injections of NAP increased the number of surviving RGC by 40% (p < 0.005) compared to the control group. After optic nerve crush, NAP raised the number of surviving RGC by 31% (p = 0.07) when injected intraperitoneally and by 54% (p < 0.05) when administered intravitreally.

CONCLUSIONS

NAP acts neuroprotectively in vivo after retinal ischemia and optic nerve crush, and may have potential in treating optic nerve diseases.

Brain-derived neurotrophic factor (BDNF)-like immunoreactivity localization in the retina and brain of Cichlasoma dimerus (Teleostei, Perciformes)

Brain-derived neurotrophic factor (BDNF) is a neurotrophin involved in the development and maintenance of vertebrate nervous systems. Although there were several studies in classical animal models, scarce information for fish was available. The main purpose of this study was to analyze the distribution of BDNF in the brain and retina of the cichlid fish Cichlasoma dimerus. By immunohistochemistry we detected BDNF-like immunoreactive cells in the cytoplasm and the nuclei of the ganglion cell layer and the inner nuclear layer of the retina. In the optic tectum, BDNF-like immunoreactivity was detected in the nucleus of neurons of the stratum periventriculare and the stratum marginale and in neurons of the intermediate layers. In the hypothalamus we found BDNF-like immunoreactivity mainly in the cytoplasm of the nucleus lateralis tuberis and the nucleus of the lateral recess. To confirm the nuclear and cytoplasm localization of BDNF we performed subcellular fractionation, followed by Western blot, detecting a 39 kDa immunoreactive-band corresponding to a possible precursor form of BDNF in both fractions. BDNF-like immunoreactivity was distributed in areas related with photoreception (retina), the integration center of retinal projections (optic tectum) and the control center of background and stress adaptation (hypothalamus). These results provide baseline anatomical information for future research about the role of neurotrophins in the adult fish central nervous system.

Glucagon-related peptides in the mouse retina and the effects of deprivation of form vision

BACKGROUND

In chickens, retinal glucagon amacrine cells play an important role in emmetropization, since they express the transcription factor ZENK (also known as NGFI-A, zif268, tis8, cef5, Krox24) in correlation with the sign of imposed image defocus. Pharmacological studies have shown that glucagon can act as a stop signal for axial eye growth, making it a promising target for pharmacological intervention of myopia. Unfortunately, in mammalian retina, glucagon itself has not yet been detected by immunohistochemical staining. To learn more about its possible role in emmetropization in mammals, we studied the expression of different members of the glucagon hormone family in mouse retina, and whether their abundance is regulated by visual experience.

METHODS

Black wildtype C57BL/6 mice, raised under a 12/12 h light/dark cycle, were studied at postnatal ages between P29 and P40. Frosted hemispherical thin plastic shells (diffusers) were placed in front of the right eyes to impose visual conditions that are known to induce myopia. The left eyes remained uncovered and served as controls. Transversal retinal cryostat sections were single- or double-labeled by indirect immunofluorescence for early growth response protein 1 (Egr-1, the mammalian ortholog of ZENK), glucagon, glucagon-like peptide-2 (GLP-2), glucose-dependent insulinotropic polypeptide (GIP), peptide histidine isoleucine (PHI), growth hormone-releasing hormone (GHRH), pituitary adenylate cyclase-activating polypeptide (PACAP), secretin, and vasoactive intestinal polypeptide (VIP). In total, retinas of 45 mice were studied, 28 treated with diffusers, and 17 serving as controls.

RESULTS

Glucagon itself was not detected in mouse retina. VIP, PHI, PACAP and GIP were localized. VIP was co-localized with PHI and Egr-1, which itself was strongly regulated by retinal illumination. Diffusers, applied for various durations (1, 2, 6, and 24 h) had no effect on the expression of VIP, PHI, PACAP, and GIP, at least at the protein level. Similarly, even if the analysis was confined to cells that also expressed Egr-1, no difference was found between VIP expression in eyes with diffusers and in eyes with normal vision.

CONCLUSIONS

Several members of the glucagon super family are expressed in mouse retina (although not glucagon itself), but their expression pattern does not seem to be regulated by visual experience.

Expression of brain-derived neurotrophic factor in cholinergic and dopaminergic amacrine cells in the rat retina and the effects of constant light rearing

Brain-derived neurotrophic factor (BDNF) regulates many aspects of neuronal development, including survival, axonal and dendritic growth and synapse formation. Despite recent advances in our understanding of the functional significance of BDNF in retinal development, the retinal cell types expressing BDNF remains poorly defined. The goal of the present study was to determine the localization of BDNF in the mammalian retina, with special focus on the subtypes of amacrine cells, and to characterize, at the cellular level, the effects of constant light exposure during early postnatal period on retinal expression of BDNF. Retinas from 3-week-old rats reared in a normal light cycle or constant light were subjected to double immunofluorescence staining using antibodies to BDNF and retinal cell markers. BDNF immunoreactivity was localized to ganglion cells, cholinergic amacrine cells and dopaminergic amacrine cells, but not to AII amacrine cells regardless of rearing conditions. Approximately 75% of BDNF-positive cells in the inner nuclear layer were cholinergic amacrine cells in animals reared in a normal lighting condition. While BDNF immunoreactivity in ganglion cells and cholinergic amacrine cells was significantly increased by constant light rearing, which in dopaminergic amacrine cells was apparently unaltered. The overall structure of the retina and the density of ganglion cells, cholinergic amacrine cells and AII amacrine cells were unaffected by rearing conditions, whereas the density of dopaminergic amacrine cells was significantly increased by constant light rearing. The present results indicate that cholinergic amacrine cells are the primary source of BDNF in the inner nuclear layer of the rat retina and provide the first evidence that cholinergic amacrine cells may be involved in the visual activity-dependent regulation of retinal development through the production of BDNF. The present data also suggest that the production or survival of dopaminergic amacrine cells is regulated by early visual experience.

Specific alterations of tyrosine hydroxylase immunopositive cells in the retina of NT-4 knock out mice

To assess the effect of NT-4 deprivation on maturation of retinal circuitry, we investigated a mouse with targeted deletion of the gene encoding nt-4 (nt-4(-/-)). In particular, we studied neurons immunostained by an antibody recognizing tyrosine hydroxylase (TH), the rate limiting enzyme for dopamine (DA) synthesis. We found that TH immunopositive processes were altered in the retina of nt-4(-/-). Alteration of TH immunopositive processes in nt-4(-/-) mice resulted in changes of DA turnover, as assessed by high-pressure liquid chromatography measurements. These findings suggest that retinal NT-4 plays a role in the morphological maturation of dopaminergic retinal cells.

Retinal functional development is sensitive to environmental enrichment: a role for BDNF

Retina has long been considered less plastic than cortex or hippocampus, the very sites of experience-dependent plasticity. Now, we show that retinal development is responsive to the experience provided by an enriched environment (EE): the maturation of retinal acuity, which is a sensitive index of retinal circuitry development, is strongly accelerated in EE rats. This effect is present also in rats exposed to EE up to P10, that is before eye opening, suggesting that factors sufficient to trigger retinal acuity development are affected by EE during the first days of life. Brain derived neurotrophic factor (BDNF) is precociously expressed in the ganglion cell layer of EE with respect to non-EE rats and reduction of BDNF expression in EE animals counteracts EE effects on retinal acuity. Thus, EE controls the development of retinal circuitry, and this action depends on retinal BDNF expression.

Dark-rearing-induced reduction of GABA and GAD and prevention of the effect by BDNF in the mouse retina

Gamma-aminobutyric acid (GABA) is an important retinal neurotransmitter. We studied the expression of GABA, glutamate decarboxylase 65 (GAD65) and GAD67 by immunocytochemistry and Western blot, in the retinas of control and dark-reared C57BL/6J black mice. This study asked three questions. First, is visual input necessary for the normal expression of GABA, GAD65 and GAD67? Second, can the retina recover from the effects of dark-rearing if returned to a normal light-dark cycle? Third, does BDNF prevent the influence of dark-rearing on the expression of GABA and GAD? At postnatal day 10 (P10), before eye opening, GABA immunoreactivity was present in the ganglion cell layer (GCL), in the innermost rows of the inner nuclear layer (INL) and throughout the inner plexiform layer (IPL) of control and dark-reared retinas. In P30 control retinas, GABA immunoreactivity showed similar patterns to those at P10. However, in P30 dark-reared retinas, the density of GABA-immunoreactive cells was lower in both the INL and GCL than in control retinas. In addition, visual deprivation retarded GABA immunoreactivity in the IPL. Western blot analysis showed corresponding differences in the levels of GAD65 but not of GAD67 expression between control and dark-rearing conditions. In our study, dark-rearing effects were reversed when the mice were put in normal cyclic light-dark conditions for 2 weeks. Moreover, dark-reared retinas treated with BDNF showed normal expression of both GABA and GAD65. Our data indicate that normal expression of GABA and GAD65 is dependent on visual input. Furthermore, the data suggest that BDNF controls this dependence.

TrkB/BDNF signaling regulates photoreceptor progenitor cell fate decisions

Neurotrophins, via activation of Trk receptor tyrosine kinases, serve as mitogens, survival factors and regulators of arborization during retinal development. Brain-derived neurotrophic factor (BDNF) and TrkB regulate neuronal arborization and survival in late retinal development. However, TrkB is expressed during early retinal development where its functions are unclear. To assess TrkB/BDNF actions in the early chick retina, replication-incompetent retroviruses were utilized to over-express a dominant negative truncated form of TrkB (trunc TrkB), or BDNF and effects were assessed at E15. Clones expressing trunc TrkB were smaller than controls, and proliferation and apoptosis assays suggest that decreased clone size correlated with increased cell death when BDNF/TrkB signaling was impaired. Analysis of clonal composition revealed that trunc TrkB over-expression decreased photoreceptor numbers (41%) and increased cell numbers in the middle third of the inner nuclear layer (INL) (23%). Conversely, BDNF over-expression increased photoreceptor numbers (25%) and decreased INL numbers (17%). Photoreceptors over-expressing trunc TrkB demonstrated no increase in apoptosis nor abnormalities in lamination suggesting that TrkB activation is not required for photoreceptor cell survival or migration. These studies suggest that TrkB signaling regulates commitment to and/or differentiation of photoreceptor cells from retinal progenitor cells, identifying a novel role for TrkB/BDNF in regulating cell fate decisions.

Airway-related vagal preganglionic neurons express brain-derived neurotrophic factor and TrkB receptors: implications for neuronal plasticity

Recent evidence indicates that brain-derived neurotrophic factor (BDNF) is present in neurons and may affect neurotransmitter release, cell excitability, and synaptic plasticity via activation of tyrosine kinase B (TrkB) receptors. However, whether airway-related vagal preganglionic neurons (AVPNs) produce BDNF and contain TrkB receptors is not known. Hence, in ferrets, we examined BDNF and TrkB receptor expression in identified AVPNs using in situ hybridization and immunohistochemistry. BDNF protein levels were measured within the rostral nucleus ambiguus (rNA) region by ELISA. We observed that the subpopulation of AVPNs, identified by neuroanatomical tract tracing, within the rNA region express BDNF mRNA, BDNF protein, as well as TrkB receptor. In addition, brain tissue from the rNA region contained measurable amounts of BDNF that were comparable to the hippocampal region of the brain. These data indicate, for the first time, that the BDNF-TrkB system is expressed by AVPNs and may play a significant role in regulating cholinergic outflow to the airways.