No abstract

The neurotransmitter serotonin is synthesized in the retina by one type of amacrine neuron but accumulates in bipolar neurons in many vertebrates. The mechanisms, functions and purpose underlying serotonin accumulation in bipolar cells remain unknown. Here, we demonstrate that exogenous serotonin transiently accumulates in a distinct type of bipolar neuron. KCl-mediated depolarization causes the depletion of serotonin from amacrine neurons and, subsequently, serotonin is taken-up by bipolar neurons. The accumulation of endogenous and exogenous serotonin by bipolar neurons is blocked by selective reuptake inhibitors. Exogenous serotonin is specifically taken-up by bipolar neurons even when serotonin-synthesizing amacrine neurons are destroyed; excluding the possibility that serotonin diffuses through gap junctions from amacrine into bipolar neurons. Further, inhibition of monoamine oxidase A prevents the degradation of serotonin in bipolar neurons, suggesting that monoamine oxidase A is present in these neurons. However, the vesicular monoamine transporter 2 is present only in amacrine cells suggesting that serotonin is not transported into synaptic vesicles and reused as a transmitter in the bipolar neurons. We conclude that the serotonin-accumulating bipolar neurons perform glial functions in the retina by actively transporting and degrading serotonin that is synthesized in neighboring amacrine cells.

N-acetylserotonin (NAS) is synthesized from serotonin by arylalkylamine N-acetyltransferase (AANAT), which is predominantly expressed in the pineal gland and retina. NAS activates TrkB in a circadian manner and exhibits antidepressant effects in a TrkB-dependent manner. It also enhances neurogenesis in hippocampus in sleep-deprived mice. Here we report the identification of NAS derivatives that possess much more robust neurotrophic effects with improved pharmacokinetic profiles. The compound N-[2-(5-hydroxy-1H-indol-3-yl)ethyl]-2-oxopiperidine-3-carboxamide (HIOC) selectively activates TrkB receptor with greater potency than NAS. It potently protects retinas from light-induced retinal degeneration (LIRD), which is tightly coupled with pronounced TrkB activation in retinas. Pharmacokinetic studies demonstrate that this compound is stable in serum and liver microsomes. It can pass the blood-brain barrier and bloodretinal barrier. Hence, HIOC is a good lead compound for further drug development for treating retinal degenerative diseases.circadian rhythm | melatonin | small molecule | neurotrophins A rylalkylamine N-acetyltransferase (AANAT) acetylates Nacetylserotonin (NAS) from the neurotransmitter serotonin in the pineal gland and retina. NAS is subsequently methylated and converted into melatonin by hydroxyindole-O-methyltransferase (HIOMT). The three components of the melatonin synthetic pathway, namely 5-HT, NAS, and melatonin, all display dramatic circadian rhythms (1). It has long been thought that NAS only acts as a precursor of melatonin in the process of melatonin biosynthesis. Recently, we demonstrated that NAS activates the TrkB receptor and exerts antidepressant effects in a TrkB-dependent manner (2). Moreover, we reported that NAS but not melatonin stimulates neurogenesis in the hippocampus of sleep-deprived mice via activating TrkB receptor in dentate gyrus (3). In addition to these findings, NAS has been shown to facilitate memory (4), regulate hypothermic body temperature (5), induce analgesia in central nervous system (6), and exert antioxidative actions (7).Brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family that also includes nerve growth factor (NGF), Neurotrophin-3 (NT-3), and NT-4/5, exerts its biological functions through two transmembrane receptors: the p75 neurotrophin receptor (p75NTR) and TrkB receptor tyrosine kinase. BDNF binding to TrkB triggers its dimerization and autophosphorylation of tyrosine residues in its intracellular domain, leading to activation of the three major downstream signaling cascades, including mitogen-activated protein kinase (MAPK), phosphatidylinositol 3-kinase (PI3K), and phospholipase C-γ1 (PLC-γ1) (8, 9). Through these pathways, BDNF mediates a variety of neuronal activities involved in neuronal survival, neurogenesis, and synaptic plasticity. Accumulating evidence demonstrates that neurotrophins play critical roles in the development of the retina and visual system (10-12). Limiting amounts of neurotrophins in the target area of th...

A circumferential marginal zone (CMZ) of retinal progenitors has been identified in most vertebrate classes, with the exception of mammals. Little is known about the formation of the CMZ during late stages of embryonic retinal histogenesis. Thus, the purpose of this study was to characterize the formation and patterning of the CMZ in the embryonic chicken retina. We identified progenitors by assaying for the expression of proliferating cell nuclear antigen (PCNA), N-cadherin and the nestinrelated filament transitin, and newly generated cells by using BrdU-birthdating. We found that there is a gradual spatial restriction of progenitors into a discreet CMZ during late stages of embryonic development between E16 and hatching, at about E21. In addition, we found that retinal neurons remain immature for prolonged periods of time in far peripheral regions of the retina. Early markers of neuronal differentiation (such as HuD, calretinin and visinin) are expressed by neurons that are found directly adjacent to the CMZ. By contrast, genes that are expressed with a delay (7-10 days; PKC, calbindin, red/green opsin) after terminal mitosis in the central retina are not expressed until as much as 30 days after terminal mitosis in the far peripheral retina. We conclude that the CMZ is gradually formed during late stages of embryonic development and that the neurons that are generated by late-stage CMZ progenitors differentiate much more slowly than neurons generated during early stages of retinal development. We propose that the microenvironment within the far peripheral retina at late stages of development permits the maintenance of a zone of progenitors and slows the differentiation of neurons.