Chapter 4 The properties of the serotoninergic system in the retina

Fluoxetine degrades luminance perceptual thresholds while enhancing motivation and reward sensitivity

Selective serotonin reuptake inhibitors (SSRIs) increase serotonin activity in the brain. While they are mostly known for their antidepressant properties, they have been shown to improve visual functions in amblyopia and impact cognitive functions ranging from attention to motivation and sensitivity to reward. Yet, a clear understanding of the specific action of serotonin to each of bottom-up sensory and top-down cognitive control components and their interaction is still missing. To address this question, we characterize, in two adult male macaques, the behavioral effects of fluoxetine, a specific SSRI, on visual perception under varying bottom-up (luminosity, distractors) and top-down (uncertainty, reward biases) constraints while they are performing three different visual tasks. We first manipulate target luminosity in a visual detection task, and we show that fluoxetine degrades luminance perceptual thresholds. We then use a target detection task in the presence of spatial distractors, and we show that under fluoxetine, monkeys display both more liberal responses as well as a degraded perceptual spatial resolution. In a last target selection task, involving free choice in the presence of reward biases, we show that monkeys display an increased sensitivity to reward outcome under fluoxetine. In addition, we report that monkeys produce, under fluoxetine, more trials and less aborts, increased pupil size, shorter blink durations, as well as task-dependent changes in reaction times. Overall, while low level vision appears to be degraded by fluoxetine, performances in the visual tasks are maintained under fluoxetine due to enhanced top-down control based on task outcome and reward maximization.

Three weeks of SSRI administration enhances the visual perceptual threshold - a randomized placebo-controlled study

RATIONALE

The serotonergic system has been repeatedly linked to visual attention in general, but the effects of selective serotonin reuptake inhibitor (SSRI) on specific components of visual attention remain unknown. Changes in distinct perceptual and cognitive processes are not readily evident in most attention paradigms.

OBJECTIVE

In this study, we isolate basic components of visual attention to investigate potential effects of longer-term SSRI administration on non-emotional aspects of visual attention in healthy males.

METHODS

In a randomized double-blind placebo-controlled design, 32 young healthy males were tested on multiple attentional parameters, before and after a 3-week SSRI intervention with fluoxetine (40 mg daily) or placebo. Data were modeled with a computational theory of visual attention to derive independent estimates of five distinct components of visual attention.

RESULTS

The SSRI intervention selectively and significantly lowered the threshold for conscious visual perception. Specifically, we demonstrate that this improvement does not stem from a general increase in the speed of visual processing, as previously suggested, but specifically from a change in the perceptual threshold.

CONCLUSIONS

The study provides a novel description of the attentional dynamics affected by SSRI, while supporting previous findings on attentional effects of SSRI. Furthermore, it accentuates the utility of employing accuracy-based measures of attentional performance when conducting psychopharmacological research.

Melatonin modulates M4-type ganglion-cell photoreceptors

In the retina, melatonin is secreted at night by rod/cone photoreceptors and serves as a dark-adaptive signal. Melatonin receptors have been found in many retinal neurons including melanopsin-containing intrinsically photosensitive retinal ganglion cells (ipRGCs), suggesting it could modulate the physiology of these inner retinal photoreceptors. Here, we investigated whether melatonin modulates the alpha-like M4-type ipRGCs, which are believed to mediate image-forming vision as well as non-image-forming photoresponses. Applying melatonin during daytime (when endogenous melatonin secretion is low) caused whole-cell-recorded M4 cells' rod/cone-driven depolarizing photoresponses to become broader and larger, whereas the associated elevation in spike rate was reduced. Melanopsin-based light responses were not affected significantly. Nighttime application of the melatonin receptor antagonist luzindole also altered M4 cells' rod/cone-driven light responses but in the opposite ways: the duration and amplitude of the graded depolarization were reduced, whereas the accompanying spiking increase was enhanced. These luzindole-induced changes confirmed that M4 cells are modulated by endogenous melatonin. Melatonin could induce the above effects by acting directly on M4 cells because immunohistochemistry detected MT1 receptors in these cells, although it could also act presynaptically. Interestingly, the daytime and nighttime recordings showed significant differences in resting membrane potential, spontaneous spike rate and rod/cone-driven light responses, suggesting that M4 cells are under circadian control. This is the first report of a circadian variation in ipRGCs' resting properties and synaptic input, and of melatoninergic modulation of ipRGCs.

Retinopetal axons in mammals: emphasis on histamine and serotonin

Since 1892, anatomical studies have demonstrated that the retinas of mammals, including humans, receive input from the brain via axons emerging from the optic nerve. There are only a small number of these retinopetal axons, but their branches in the inner retina are very extensive. More recently, the neurons in the brain stem that give rise to these axons have been localized, and their neurotransmitters have been identified. One set of retinopetal axons arises from perikarya in the posterior hypothalamus and uses histamine, and the other arises from perikarya in the dorsal raphe and uses serotonin. These serotonergic and histaminergic neurons are not specialized to supply the retina; rather, they are a subset of the neurons that project via collaterals to many other targets in the central nervous system, as well. They are components of the ascending arousal system, firing most rapidly when the animal is awake and active. The contributions of these retinopetal axons to vision may be predicted from the known effects of serotonin and histamine on retinal neurons. There is also evidence suggesting that retinopetal axons play a role in the etiology of retinal diseases.

Darkness induced neuroplastic changes in the serotoninergic system of the chick retina

Sensory experience is critical for the formation of neuronal circuits and it is well known that neuronal activity plays a crucial role in the formation and maintenance of synapses. In the vertebrate retina, exposure to different environmental conditions results in structural, physiological, neurochemical and pharmacological changes. Serotoninergic (5HT) amacrine cells of the chicken retina are bistratified interneurons whose primary dendrites descend through the inner nuclear layer (INL) to branch in the inner plexiform layer (IPL) forming two plexi, an outer network, localized to sublamina 1, and an inner network, localized to sublamina 4 and 5 of the IPL. Their development is temporally correlated with the establishment of synapses in the retina and with the emergence of the typical adult electroretinogram. It is unknown, however, which role these cells play in processing visual information and whether visual deprivation modifies their phenotype. Here, we show that, in the chicken, red-light rearing from hatching to postnatal day 12 significantly alters the stratification pattern of 5HT amacrine cells, inhibiting their age-dependent pruning measured with morphometric and densitometric procedures; as well as increasing serotonin immunoreactivity measured as relative optical density. This change in dendritic arborization, accompanied by an increase in serotonin concentration in dark adapted conditions, may decrease visual threshold, thus increasing visual sensitivity.

Control of programmed cell death by neurotransmitters and neuropeptides in the developing mammalian retina

It has long been known that a barrage of signals from neighboring and connecting cells, as well as components of the extracellular matrix, control cell survival. Given the extensive repertoire of retinal neurotransmitters, neuromodulators and neurotrophic factors, and the exhuberant interconnectivity of retinal interneurons, it is likely that various classes of released neuroactive substances may be involved in the control of sensitivity to retinal cell death. The aim of this article is to review evidence that neurotransmitters and neuropeptides control the sensitivity to programmed cell death in the developing retina. Whereas the best understood mechanism of execution of cell death is that of caspase-mediated apoptosis, current evidence shows that not only there are many parallel pathways to apoptotic cell death, but non-apoptotic programs of execution of cell death are also available, and may be triggered either in isolation or combined with apoptosis. The experimental data show that many upstream signaling pathways can modulate cell death, including those dependent on the second messengers cAMP-PKA, calcium and nitric oxide. Evidence for anterograde neurotrophic control is provided by a variety of models of the central nervous system, and the data reviewed here indicate that an early function of certain neurotransmitters, such as glutamate and dopamine, as well as neuropeptides such as pituitary adenylyl cyclase-activating polypeptide and vasoactive intestinal peptide is the trophic support of cell populations in the developing retina. This may have implications both regarding the mechanisms of retinal organogenesis, as well as pathological conditions leading to retinal dystrophies and to dysfunctional cellular behavior.

Distribution of 5-hydroxytriptamine2C receptor mRNA in rat retina

There are several factors that suggest serotonin [5-hydroxytryptamine (5-HT)] plays a role as a neurotransmitter/neuromodulator within the retina. The presence of mRNAs encoding 5-HT receptors (5-HTR) of the types 5-HT2CR and 5-HT5AR within the rat retina was investigated using in situ hybridization of digoxigenin-labeled probes. The 5HT5AR probe produced no labeling, whereas the 5HT2CR probe hybridized in cells scattered in the inner nuclear and ganglion cell layers. Thus, the 5HT2CR gene is expressed by retinal neurons, some of which represent third-order neurons, either amacrine or ganglion cells. This suggests that 5-HT may modulate the outgoing signal from the retina.

Identification of 5-HT(3A) and 5-HT(3B) receptor subunits in mammalian retinae: potential pre-synaptic modulators of photoreceptors

Although serotonin (5-HT) is found in the mammalian retina only at low levels, considerable evidence suggests that it plays a role in visual processing. Pharmacological experiments indicate that numerous receptors for 5-HT are present in the mammalian retina. One of these is the ionotropic 5-HT(3) receptor. So far, two subunits for this receptor have been identified in the nervous system, 5-HT(3A) and 5-HT(3B). Co-expression of these subunits in Xenopus oocytes is sufficient to reconstitute native 5-HT(3) receptor properties. Thus, it is believed that a native neuronal 5-HT(3) receptor is multimeric similar to the related acetylcholine receptor family. To determine whether this receptor is expressed in the mammalian retina, we first performed reverse transcription polymerase chain reaction and first demonstrated the presence of transcripts for both the 5-HT(3A) and 5-HT(3B) receptor subunits. Then using a well-characterized polyclonal antiserum against the 5-HT(3A) receptor subunit, we demonstrated 5-HT(3A) receptor immunoreactivity (IR) in the rabbit, rat, and human retina. This IR was localized specifically to the rod photoreceptor terminals in all three species, suggesting that this receptor may modulate the rod signaling pathway by controlling the output at the rod terminals.

5-HT(1A) and 5-HT(7) receptor expression in the mammalian retina

Serotonin or 5-hydroxytryptamine (5-HT) is a common neurotransmitter found widely in the nervous system. Here, using RT-PCR, we have identified both the 5-HT(1A) and 5-HT(7) receptor transcripts in the rabbit retina. Furthermore, we found the same two receptors in the rat retina which was previously believed not to have a serotoninergic system. These results confirm previous reports of 5-HT(7) gene expression in retina and together with other biochemical, physiological and anatomical studies, they support the presence of multiple 5-HT receptors in the mammalian retina and suggest that the action of serotonin in the retina may be more complicated than previously believed.

Development of nitric oxide neurons in the chick embryo retina

Nitric oxide (NO) is a gas involved in neurotransmission in the central nervous system (CNS) and in vertebrate retinas. This paper describes five types of nitrergic neurons in developing and adult chick retina using the nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) reaction. Three of them, nitrergic types 1, 2 and 3, were observed in the inner nuclear layer, while nitrergic type 4 was observed in the ganglion cell layer; nitrergic type 5 were the retinal photoreceptors. Cell processes formed four nitrergic networks, which could be observed in the inner plexiform layer (IPL), at sublayers 1, 3a, 3b and 4. Another nitrergic network was observed in the outer plexiform layer (OPL). From hatching, the dendritic branches were completely developed in the IPL and in the OPL, forming the mentioned networks. Current evidence suggests that NO is coexpressed with other neurotransmitters in neurons of the CNS. Double-staining procedures, using NADPHd and 5HT immunohistochemistry in chicken retina, in a sequential or in an alternative manner, did not reveal the coexistence of these two neurotransmitters in the same neurons, but their networks matched in sublayers 1 and 4 of the IPL.

Localization of 5-hydroxytryptamine1A and 5-hydroxytryptamine7 receptors in rabbit ocular and brain tissues

Serotonin is thought to play a physiological role in various tissues of the rabbit eye, yet little is known about the relative distribution of the different serotonin receptors. Demonstration of the receptor subtypes present in the various ocular tissues is essential in order to understand the function of serotonin in the eye. Using a combination of in situ hybridization histochemistry, in vitro receptor autoradiography and polymerase chain reaction studies, we have explored the distribution of the 5-hydroxytryptamine1A and 5-hydroxytryptamine7 receptors in the rabbit eye. As these receptors have not been sequenced in the rabbit, we initially established the suitability of the oligonucleotide probes by analysis of brain tissue. The distributions of 5-hydroxytryptamine1A and 5-hydroxytryptamine7 receptor messenger RNAs in rabbit brain correlated well with those in other species, confirming the specificity of the probes for detection of the messenger RNAs in rabbit tissues. In the eye, the expression of 5-hydroxytryptamine1A receptors appears to be restricted to the epithelial cell layer of the ciliary processes, although very low levels may appear in the retina. In contrast, the expression of 5-hydroxytryptamine7 receptor messenger RNA is more widespread with positive signals evident in the ciliary processes, retina and iris. The results confirm the existence of 5-hydroxytryptamine1A receptors in the ciliary body and their localization in the ciliary epithelium supports the hypothesis that they are involved in the secretion of aqueous humour. Unexpectedly, there was little evidence to support the idea that 5-hydroxytryptamine1A receptors are present in the retina and iris sphincter. However, the subsequent finding of 5-hydroxytryptamine7 receptor messenger RNA in the retina and iris may explain the apparent absence of 5-hydroxytryptamine1A receptors in these tissues. The presence of both 5-hydroxytryptamine1A and 5-hydroxytryptamine7 receptors in the ciliary processes may account for the complex intraocular pressure response of the rabbit to serotonin.

Day and night dysfunction in intraretinal melatonin and related indoleamines metabolism, correlated with the development of glaucoma-like disorder in an avian model

As previous studies have suggested that melatonin and serotonin may be involved in the regulation of intraocular pressure, retinal concentrations of melatonin, 5-HT, and related indoleamines measured at day and at night were studied during the development of a glaucoma-like disorder with increased intraocular pressure in the al mutant quail. Indoleamine levels were determined by HPLC with electrochemical detection in 1-month-, 3-month-, and 7-month-old al mutant and control quails. Morphology and numbers of melatonin-synthesizing and 5-HT-containing cells, labelled immunohistochemically with an anti-hydroxyindol-0-methyltransferase (HIOMT) antibody and an anti-5-HT antibody, respectively, were studied. Major findings were that: (1) no significant changes in morphology of melatonin-synthesizing cells or in the morphology and density of 5-HT-containing amacrine cells were observed during the development of glaucoma: (2) 5-HT metabolism was modified during the night at 1 month of age and during the day after 3 months; and (3) melatonin metabolism was modified during the night at 7 months and during the day after 3 months. These results demonstrate a relationship between the temporal evolution of this avian glaucoma and a dysfunction in indoleamine retinal metabolism.

Evidence for an endogenous clock in the retina of rainbow trout: II. Circadian rhythmicity of serotonin metabolism

The purpose of the present study was to investigate patterns of circadian rhythmicity in the retina of a salmonid fish, the rainbow trout (Oncorhynchus mykiss). Our data present the first demonstration of intraretinal variations of serotonergic substances under light/dark-conditions (LD) and during continuous darkness (DD). All substances examined (serotonin, N-acetyl serotonin, 5-hydroxytryptophol, 5-hydroxyindole acetic acid) were rhythmic in LD. Serotonin, N-acetyl-serotonin, and 5-hydroxyindole acetic acid showed a preservation of specific features of rhythmicity in DD indicating the involvement of an endogenous pacemaker in the regulation of serotonin metabolism in the rainbow trout eye.

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.