Serotonergic and serotonin-synthesizing cells of the Xenopus retina

Neuroanatomical Distribution of the Serotonergic System in the Brain and Retina of Holostean Fishes, The Sister Group to Teleosts

Among actinopterygian fishes, holosteans are the phylogenetically closest group to teleosts but they have been much less studied, particularly regarding the neurochemical features of their central nervous system. The serotonergic system is one of the most important and conserved systems of neurotransmission in all vertebrates. By means of immunohistochemistry against serotonin (5-hydroxytryptamine), we have conducted a comprehensive and complete description of this system in the brain and retina of representative species of the 3 genera of holostean fishes, belonging to the only 2 extant orders, Amiiformes and Lepisosteiformes. Serotonin-immunoreactive cell groups were detected in the preoptic area, the hypothalamic paraventricular organ, the epiphysis, the pretectal region, the long and continuous column of the raphe, the spinal cord, and the inner nuclear layer of the retina. Specifically, the serotonergic cell groups in the preoptic area, the epiphysis, the pretectum, and the retina had never been identified in previous studies in this group of fishes. Widespread serotonergic innervation was observed in all main brain regions, but more abundantly in the subpallium, the hypothalamus, the habenula, the optic tectum, the so-called cerebellar nucleus, and the area postrema. The comparative analysis of these results with those in other groups of vertebrates reveals some extremely conserved features, such as the presence of serotonergic cells in the retina, the pineal organ, and the raphe column, while other characteristics, like the serotonergic populations in the preoptic area, the paraventricular organ, the pretectum, and the spinal cord are generally present in all fish groups, but have been lost in most amniotes.

The serotonergic system in fish

Neurons using serotonin (5-HT) as neurotransmitter and/or modulator have been identified in the central nervous system in representatives from all vertebrate clades, including jawless, cartilaginous and ray-finned fishes. The aim of this review is to summarize our current knowledge about the anatomical organization of the central serotonergic system in fishes. Furthermore, selected key functions of 5-HT will be described. The main focus will be the adult brain of teleosts, in particular zebrafish, which is increasingly used as a model organism. It is used to answer not only genetic and developmental biology questions, but also issues concerning physiology, behavior and the underlying neuronal networks. The many evolutionary conserved features of zebrafish combined with the ever increasing number of genetic tools and its practical advantages promise great possibilities to increase our understanding of the serotonergic system. Further, comparative studies including several vertebrate species will provide us with interesting insights into the evolution of this important neurotransmitter system.

Serotonin released from amacrine neurons is scavenged and degraded in bipolar neurons in the retina

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.

Day/night variation of tryptophan hydroxylase and serotonin N-acetyltransferase mRNA levels in the ovine pineal gland and retina

In mammals, the photoperiodic information, received by the retina, is transmitted to the pineal gland. In both organs, melatonin is produced and functions as a neurohormone giving temporal information to the organism. A four-step enzymatic pathway, involving in particular the tryptophan hydroxylase (TPOH), the rate-limiting enzyme in serotonin synthesis, and the serotonin N-acetyltransferase (NAT) that converts serotonin to N-acetylserotonin, allows the synthesis of melatonin. Many studies on melatonin synthesis modulation have focused on the enzyme NAT, but the regulation of TPOH is less well understood. We report here a quantitative study, using a reverse transcription polymerase chain reaction (RT-PCR) analysis, of the nycthemeral expression of TPOH and NAT mRNAs in the ovine retina and pineal gland. In both organs, we show a nocturnal increase in mRNA levels of the two enzymes. suggesting a role of transcriptional mechanisms in the regulation of melatonin synthesis. However, the amplitude of the observed increase in TPOH and NAT mRNAs expression can not entirely explain the 7-fold nocturnal increase in the plasma melatonin level. Our results suggest that, in the sheep, post-transcriptional mechanisms might also be involved in the day/night modulation of melatonin production.

Excitatory amino acids and serotonin uptake blockers reveal two physiologically distinct serotonin systems in the retina of the skate, Raja erinacea

The retina of the skate (Raja erinacea) contains at least 2 types of cell (amacrines and bipolars) that can be visualized with an antiserum against serotonin. We have employed serotonin immunocytochemistry in combination with pharmacological manipulation of retinal tissue to analyze physiological properties of serotonergic amacrine cells and serotonin-accumulating bipolar cells. Excitatory amino acids (NMDA, aspartate) had no detectable effects on serotonin-immunoreactivity in bipolar cells but decreased staining in amacrine cells. High K+ Ringer increased staining in bipolar cell somata, however, it depleted the inner plexiform layer of the retina of serotonin. Zimelidine, a serotonin uptake inhibitor, completely blocked serotonin accumulation by bipolar cells but had no effect on amacrine cells, whereas incubation of the retinas in fluoxetine (Prozac), a different inhibitor of serotonin uptake, did not block serotonin accumulation into bipolar cells which was actually enhanced in some cases. We conclude that amacrine and bipolar cells of the skate retina employ two different serotonin uptake carrier systems, thus generating two distinct pharmacological components that are capable of interacting with each other as they compete for extracellular serotonin. Similar mechanisms may exist in the vertebrate CNS and further examination of the interaction of these systems could provide important insights into the action and possible side effects of serotonin-related drugs.

Development of serotoninergic chick retinal neurons

Numerous neurotransmitters have been studied in detail in the developing retina. Almost all known neurotransmitters and neuromodulators were demonstrated in vertebrate retinas using formaldehyde-induced fluorescence, uptake autoradiography or immunohistochemistry procedures. Serotoninergic (5HT) amacrine neurons were described in the inner nuclear layer (INL) of the retina with their dendrites spreading within the inner plexiform layer (IPL). The present work describes the morphological pattern of development of serotoninergic amacrine neurons with a stratified dendritic branching pattern in the chick retina from embryonic day 12 to postnatal day 7. Serotoninergic-bipolar neurons are also described. SHT-amacrine neurons have round or pear-shaped somata and primary dendritic trees oriented toward the IPL that runs through the INL, showing several varicosities. Secondary dendrites then go through the INL, without any collateral branch. At the outer and inner margin of the IPL the primary and secondary dendrites originate an outer and an inner serotoninergic network, respectively. When the primary dendritic tree reaches the IPL it deflects laterally in sublayer 1-the outer serotoninergic network. Tertiary branches then arise from the secondary dendrite and deflect in the innermost sublayer of the IPL-the inner serotoninergic network. The final pattern of branching of 5HT amacrine cells was present at embryonic day 14 and was completely developed at hatching. Serotoninergic (5HT) bipolar neurons were also present in the INL at hatching. They are weakly immunoreactive and are probably a subset of bipolar cells that accumulate serotonin from the intersynaptic cleft and are not "true" 5HT neurons.

Two classes of bipolar cell in the retina of the skate Raja erinacea

We have used immunoreactions against serotonin and protein kinase C to visualize two distinct classes of bipolar cell in the all-rod retina of the skate, Raja erinacea. To enhance the immunoreaction in serotonin-accumulating bipolar cells, prior to fixation, some retinas were incubated in Ringer's solution containing serotonin and pargyline. We found the somata of serotonin-accumulating bipolar cells to be located slightly distal to the midline of the inner nuclear layer. With increasing eccentricity from the visual streak, the size of the perikarya increases, concomitant with a decline in density of their distribution. Dendrites emanate from stout primary stalks and branch out before reaching the outer plexiform layer. Axons are bistratified within the inner plexiform layer with ramifications at the border of strata 1 and 2 and in stratum 4. The overall morphology of serotonin-accumulating bipolar cells is similar to that of serotonin-accumulating OFF bipolar cells of other non-mammalian vertebrates. Protein kinase C immunoreactive cells display the typical appearance of rod bipolar cells. Somata of protein kinase C immunoreactive bipolar cells are spindle-shaped and located distal to the serotonin-accumulating bipolar cells. Dendrites of these bipolars do not ramify before reaching the outer plexiform layer. Thin axons of protein kinase C immunoreactive bipolar cells end in large, club-shaped terminals in stratum 5 of the inner plexiform layer, bearing a striking similarity to axon terminals of mammalian ON rod bipolar cells. Our findings suggest that the all-rod retina of the skate contains at least two distinct vertical pathways including an OFF bipolar cell pathway in addition to a classical rod ON bipolar pathway.

Centrifugal innervation of the rat retina

Centrifugal fibers innervating the retina have been shown in all classes of vertebrate, except for mammals where conventional tract-tracing methods have not been able to unmistakably demonstrate their existence. In a previous study, a unilateral, intravitreal injection of 5,7-dihydroxytryptamine was used to reveal indoleamine-accumulating centrifugal fibers which were visualized by an immunoreaction against serotonin. In the present study, I employed a modification of this method to stain retinopetal neurons in the rat. Terminals were located preferentially in the outer retina; labeled fibers could be traced back along an ipsilateral pathway to somata in the dorso-caudal portions of the chiasm or the medio-lateral preoptic area, and thence towards the suprachiasmatic nuclei. The unique beaded appearance of the fibers distinguishes them from retinal ganglion cell axons. The labeling of central cell bodies strongly suggests that they possess terminals in the retina. Thus, at least some mammalian retinas receive centrifugal innervation. This indoleamine-accumulating retinopetal pathway may be involved in retinal melatonin synthesis, coordination of circadian rhythms, and interocular phenomena.