Specificity of serotonin uptake by bovine retina: comparison with tryptamine

Synaptic circuitry of serotonin-synthesizing and serotonin-accumulating amacrine cells in the retina of the cane toad, Bufo marinus

The synaptic connections of amacrine cells synthesizing or accumulating serotonin in the retina of the cane toad, Bufo marinus, were studied by using preembedding double-labeling electron-microscopic immunocytochemistry. The binding sites of an anti-serotonin antibody were revealed by the diaminobenzidine reaction, whilst a colloidal gold-conjugated secondary antibody was used to detect an antibody to phenylalanine hydroxylase. Since the latter antibody recognizes tryptophan 5-hydroxylase, one of the synthesizing enzymes for serotonin, as well as tyrosine hydroxylase, the rate-limiting enzyme for catecholamine synthesis, the double labeling of the present study enabled us to identify three groups of labeled profiles at the ultrastructural level. The profiles of serotonin-synthesizing amacrine cells contained both diaminobenzidine reaction product and colloidal gold particles, whilst those of serotonin-accumulating and dopaminergic amacrine cells contained only diaminobenzidine reaction product or colloidal gold particles, respectively. The synapses of serotonin-synthesizing or serotonin-accumulating amacrine cells were distributed all through the inner plexiform layer of the retina. The profiles of serotonin-synthesizing amacrine cells predominantly received synapses from, and made synapses onto, unlabeled amacrine cell dendrites. They also received synapses from, and made synapses onto, bipolar cell terminals. They also made synapses onto presumed ganglion cell dendrites. However, the profiles of serotonin-accumulating cells made synapses only with unlabeled amacrine cell processes. There were close contacts between the profiles of serotonin-synthesizing and serotonin-accumulating amacrine cells. No synaptic relationships were observed between dopaminergic and serotonin-synthesizing or serotonin-accumulating amacrine cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Tryptamine: a metabolite of tryptophan implicated in various neuropsychiatric disorders

Although early interest in the biomedical relevance of tryptamine has waned in recent years, it is clear from the above discussion that the study of tryptamine is worthy of serious consideration as a factor in neuropsychiatric disorders. The study of [3H]-tryptamine binding sites indicates an adaptive responsiveness characteristic of functional receptors. The question raised by Jones (1982d) on whether tryptamine is acting centrally as a neurotransmitter or a neuromodulator still remains mostly unanswered, although the evidence cited within this review strongly suggests a modulatory role for this neuroactive amine (see also Juorio and Paterson, 1990). The synthesis and degradative pathways of tryptamine, as well as the intricate neurochemical and behavioral consequences of altering these pathways, are now more fully understood. It is not yet clear what the role of tryptamine is under normal physiological [homeostatic] conditions, however, its role during pathological conditions such as mental and physical stress, hepatic dysfunction and other disorders of metabolism (i.e. electrolyte imbalance, increased precursor availability, enzyme induction or alterations in enzyme co-factor availability) may be quite subtle, perhaps accounting for various sequelae hitherto considered idiopathic. The evidence for a primary role for tryptamine in the etiology of mental or neurological diseases is still relatively poor, although the observations that endogenous concentrations of tryptamine are particularly susceptible to pharmacological as well as physiological manipulations serve to reinforce the proposition that this indoleamine is not simply a metabolic accident but rather a neuroactive compound in its own right. Finally, one might wonder what proportion of the data attributed to modifications of 5-HT metabolism might, in fact, involve unrecognized changes in the concentrations of other neuroactive metabolites of tryptophan such as tryptamine.

Serotonin binding proteins in bovine retina: binding of serotonin and catecholamines

Serotonin binding proteins (SBP) are present in the soluble fraction of bovine retina homogenates. These proteins can be precipitated with 30% ammonium sulphate and their binding and physicochemical characteristics are very similar to those of SBP in bovine and rat brain. Binding of [3H]serotonin to bovine retina SBP requires Fe2+ but not Fe3+. In the presence of an optimal concentration of Fe2+ (0.1 mM), these proteins behave as a single class of non-cooperative sites for [3H]serotonin (Bmax = 242 +/- 10 pmol/mg protein, KD = 0.22 +/- 0.44 microM). Competition binding studies reveal that serotonin analogs possessing an hydroxyl group on the indole ring and catecholamine analogs possessing an intact catechol moiety are potent competitors (K1 from 0.12 to 0.3 microM). In both cases, the affinity is strongly decreased if aromatic hydroxyl groups are methoxylated. Catecholamine SBP interactions can also be demonstrated directly by binding experiments with [3H]dopamine. Binding of this catecholamine is greatly enhanced by Fe2+, to a lesser extent by Cu2+ and Mn2+, but not by Fe3+. The Fe(2+)-dependent binding component is saturable (Bmax = 505 +/- 30 pmol/mg protein. KD = 0.34 +/- 0.04 microM). The SBP from bovine retina show the same physicochemical properties as SBP from bovine and rat brain: they elute immediately after the void volume on a Sephacryl S100 HR (1.6 x 140 cm) gel filtration column (reflecting aggregation) and they migrate with apparent molecular weights of respectively 43 kDa and 57 kDa on native polyacrylamide gel electrophoresis. The serotonin-storing role of SBP in serotonergic neurones has already been well documented.(ABSTRACT TRUNCATED AT 250 WORDS)

Putative serotonergic bipolar and amacrine cells in the chicken retina

Four populations of putative serotonergic cells could be detected in the chicken retina by histofluorescence and immunohistochemistry. Numerous (10,000/mm2) small (6 micron diameter) bipolar cells were located towards the middle of the inner nuclear layer, as were sparser (1000/mm2) larger (12 micron diameter) amacrine cells. Very sparse large (greater than 30 micron diameter) and more numerous small (12 micron diameter) ganglion cells were also detected. Prominent fibre plexuses were detected in the inner plexiform layer, close to the inner nuclear and ganglion cell layers, and appeared to be formed by the processes of the bipolar cells, amacrine cells and at least the large ganglion cells. Exogenous serotonin (5-HT) was detected in the chicken retina. From the effects of neurotoxins on 5-HT levels and 5-HT-like immunoreactivity (5-HTLI), most of this appeared to be associated with the amacrine cells. 5-HTLI bipolar cells were selectively destroyed by intravitreal injections of 5-10 nmol of kainic acid, while 5-HTLI amacrine cells were destroyed by N-methyl-D,L-aspartic acid and 5,7-dihydroxytryptamine. The sensitivity of the bipolar cells to kainic acid indicates that they are OFF-cells.

Synthesis and high affinity uptake of serotonin and dopamine by human Y79 retinoblastoma cells

Human Y79 retinoblastoma cells are capable of synthesizing the putative retinal neurotransmitters dopamine and serotonin. Separation of the catecholamines and indolamines by high performance liquid chromatography combined with electrochemical detection showed that the cells readily convert tyrosine to 3,4-dihydroxyphenylalanine (DOPA) and, to a lesser extent, dopamine. When DOPA was added, a large quantity of dopamine was produced, as well as norepinephrine, epinephrine, and 3,4-dihydroxyphenylacetic acid. Exogenous tryptophan added to the cells was partially converted to 5-hydroxytryptophan and serotonin. A larger quantity of serotonin was produced when 5-hydroxytryptophan was added. Y79 cells have a high- and low-affinity uptake system for dopamine and serotonin. The K'm and V'max for the high-affinity uptake of dopamine and serotonin are 2.34 +/- 0.64 and 3.63 +/- 1.15 microM and 4.77 +/- 1.12 and 3.20 +/- 1.20 pmol min-1 mg protein-1, respectively. These kinetic parameters are similar to those reported for other retinal preparations where dopamine and serotonin have been suggested to function as neurotransmitters. Tyrosine and tryptophan, the physiologic precursors of dopamine and serotonin, respectively, and phenylalanine are also taken up by high- and low-affinity transport systems. The kinetic parameters for their high-affinity uptake systems are all very similar, suggesting that they may be taken up by the same transporter. These studies show that a tumor cell line derived from the human retina synthesizes dopamine and serotonin and has high-affinity uptake systems for these compounds and their precursors.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on serotonin-accumulating cells in rabbit retinal cultures

The serotonin-accumulating neurones in rabbit retinal cultures were studied, using immunohistochemistry to localize serotonin. Double-labelling experiments showed that serotonin-accumulating cells in culture and intact retinas react positively to antiserum PGP 9.5, which is neurone-specific. The uptake process of serotonin is very specific; known serotonergic blockers, such as chlorimipramine, abolished transport, while benztropine, a dopamine uptake blocker, was ineffectual. Analogues of serotonin such as tryptamine, tryptophan and 5-hydroxytryptophan at concentrations 100-fold those of exogenous serotonin did not appear to compete with the transport of the amine. Newly dissociated retinal cells from 1-5-day postnatal rabbits which lack processes have the capacity to take up exogenous serotonin; these cells when kept in culture grew processes and appeared to reach maximum development after 6-15 days. Dissociated retinal cells subjected to density centrifugation resulted in the production of an enriched (4-fold) population of serotonin-accumulating cells. Since most of the endogenous serotonin was associated with this fraction, it is concluded that the serotonin-accumulating cells contain serotonin.

Indoleamine-sensitive adenylate cyclase in rabbit retina: characterization and distribution

Previous histological, electrophysiological, and biochemical reports have addressed the hypothesis that serotonin functions as a neurotransmitter in mammalian retinas. We have tested the effect on the levels of cyclic AMP of the application of exogenous serotonin, 5-methoxytryptamine, melatonin, and 5-methoxydimethyl-tryptamine to isolated, incubated rabbit retinas. All indoleamines tested significantly elevated intracellular levels of cyclic AMP in both light- and dark-adapted, incubated, intact retinas, provided a phosphodiesterase inhibitor was present. In homogenates of rabbit retina, all indoleamines tested also markedly increased adenylate cyclase activity over basal levels. Maximal activity was observed with 50 microM indoleamine; addition of GTP augmented this increase. The increase in enzyme activity persisted in the presence of known antagonists of dopamine and serotonin 5-HT2-receptors, but was blocked by the mixed 5-HT1, 5-HT2-antagonist lysergic acid diethylamide. The retinal locations of this response have also been identified using layer microdissection techniques on freeze-dried samples obtained from rabbit eyecups suprafused with indoleamine plus phosphodiesterase inhibitor. Cyclic AMP levels were measured in discrete retinal layers of both light- and dark-adapted suprafused eyecups, and increased levels were observed primarily in the inner and outer plexiform layers, which contain the synapses of the retinal neurons.

Postnatal development of serotonin-accumulating neurones in the rabbit retina and an immunohistochemical analysis of the uptake and release of serotonin

The serotonin-accumulating neurones in the rabbit and bovine retina were studied with the use of immunohistochemistry to localize serotonin. It was established that a subpopulation of amacrine cells in both tissues has the ability to take up and store serotonin. The uptake process is very specific; known serotonergic uptake blockers, viz. chlorimipramine and Lilly 110140, abolish transport, while benztropine, a dopamine-uptake blocker, is ineffectual. The serotonin accumulated by the serotonergic neurones can be released by potassium depolarization in a calcium-dependent manner. All these results form a strong case for serotonin being a likely transmitter in the mammalian retina. The subpopulation of serotonin-accumulating neurones in the rabbit retina appears to be determined prenatally, as they can be observed immediately after birth. On the basis of the serotonin content in retinas from animals of different ages, it is suggested that the serotonin-accumulating cells mature around the 24th postnatal day. The same maturation period has been proposed for the rabbit retinal dopamine cells (Lam et al., 1981).

Tryptamine transport in rat brain slices: a comparison with 5-hydroxytryptamine

The uptake of [14C]tryptamine (14C-T) and [3H]serotonin (3H-5HT) into slices of rat hypothalamus (HT), fronto-parietal cortex (CX), and caudate nucleus (Cau) has been investigated. In all three brain areas, the uptake of 3H-5HT at 37 degrees C was much greater than that in an ice-bath at 1.0-1.5 degrees C. In contrast, the uptake of 14C-T at 37 degrees C was not much greater than uptake at 1.0-1.5 degrees C. While markedly different amounts of 3H-5HT were accumulated by each of the brain areas studied, the regional uptake of 14C-T was quantitatively similar. In general the uptake of 14C-T was inhibited less than 3H-5HT by cocaine, DNP, ouabain, and decreased Na+ concentrations. Similarly, 14C-T was less susceptible to serotonin uptake inhibitors except in the caudate. It was concluded that though a common indoleamine uptake system accumulates both T and 5HT, a non-specific low affinity or diffusional process also transports both amines and is predominantly responsible for T, but not 5HT, uptake. The spontaneous release, or wash-out, of 14C-T from the caudate was much faster than that of 3H-5HT. In addition, while depolarizing stimuli caused little or no release of 14C-T, large releases of 3H-5HT were observed. T, therefore, does not behave like a conventional neurotransmitter.

Substance P in the bovine retina: localization, identification, release, uptake and receptor analysis

Substance P-like immunoreactivity is restricted to at least three different cell types in the bovine retina. They are amacrine cells, displaced amacrine cells and interplexiform neurones. High performance liquid chromatography and radioimmunoassay have shown that the bovine retina substance P-like immunoreactive material cannot be distinguished from authentic substance P. Isolated bovine retina does not take up exogenous [3H]substance P as revealed by autoradiography. By increasing the external K+ concentration it was shown that the release of endogenous substance P from the retina was stimulated. This release is Ca2+ dependent. The binding of [3H]substance P to crude membrane preparations of bovine retina was saturable and revealed a single population of binding sites with a KD value of 0.32 nM and a Bmax of 2.45 pmol/g wet wt. The characteristics of the binding site suggest the presence of substance P receptors. It is suggested that substance P is a likely neurotransmitter in the bovine retina.

Immunocytochemical demonstration of serotonergic neurons and processes in the retina and optic nerve of the stingray, Dasyatis sabina

Neurons and processes in the stingray retina can be stained using PAP immunohistochemistry and an antibody to serotonin, without pharmacological pretreatment. Most of the cell bodies are in the inner nuclear layer while the processes ramify in the inner plexiform layer suggesting the presence of a population of serotonin containing amacrine cells in this species. Scattered immunopositive axons were observed in the optic nerve from the optic chiasm to the optic nerve head.

The detection of specific [3H]imipramine binding in bovine retina

Using [3H]imipramine, specific imipramine binding was demonstrable in the crude membrane homogenate of bovine retina. Scatchard analysis of the saturation experiments revealed an apparent dissociation constant (Kd) of 6.4 nM and a maximal number of binding sites of 780 fmol/mg protein. These results and the substrate specificity show that the properties of imipramine binding to retinal membranes are essentially the same as described for the rat brain.

Serotonin-containing neurones in vertebrate retinas

It has been established by a combination of HPLC and electrochemical detection that frog, lizard, goldfish, rabbit, and bovine retinas contain both dopamine and serotonin. Immunohistological and immunoradiographical methods show that serotonin is localised in amacrine perikarya and processes situated in the inner plexiform layers of frog, lizard, and goldfish retinas. The amount of serotonin in the mammalian retina appears to be too low for detection in neurones. The serotonin in the bovine retina is located mainly in the inner nuclear and plexiform layers, suggesting that the amine is present in the same types of cells as found for frog, lizard, and goldfish retinas. Retinas incubated in [3H]serotonin showed that radioactivity is associated with processes in the inner plexiform layer and amacrine perikarya. These results suggest that the neuronal elements that contain endogenous serotonin also have the capacity to accumulate exogenous amine and are consistent with the opinion that serotonin has a neuronal function in retinas of a variety of vertebrates.

Uptake, localization and release of serotonin in the chick retina

1. High pressure liquid chromatography and electrochemical detection have shown that serotonin exists in the chick retina (127 ng/g wet weight). No other indoleamine was identified. 2. Immunofluorescent histological studies showed that the endogenous serotonin was localized apparently in those cell bodies and processes which took up exogenous [3H]serotonin as revealed by autoradiography. These serotonergic neurones can be destroyed by injecting kainic acid into the eye. 3. Isolated chick retina accumulated exogenous [3H]serotonin. Kinetic analysis revealed the presence of two saturable uptake systems: a 'high affinity' mechanism with an apparent Km of 5.9 X 10(-8) M and a Vmax of 0.143 X 10(-13) mol/mg wet weight . min and a low affinity mechanism with an apparent Km2 of 1.8 X 10(-3) M and Vmax of 0.12 X 10(-9) mol/mg wet weight . min. 4. The uptake of serotonin was temperature-sensitive and sodium-dependent and Lilly 110140 and chlorimipramine were potent inhibitors of the amine uptake. 5. Autoradiographic studies indicated that neuronal processes associated with the innermost and outermost areas of the inner plexiform layer and perikarya situated in the inner nuclear layer are the sites which accumulated exogenous [3H]serotonin. 6. [3H]Serotonin accumulated in the retina was released by increasing the external K+ concentration. This release was Ca2+-dependent. Additionally, autoradiographic studies show that [3H]serotonin taken up by the serotonin neurones was also released by Ca2+-dependent K+ depolarization of the retina.

Autoradiography of (3H)-5-hydroxytryptamine uptake in the retina of some mammals

The uptake of indoleamines into the retina of rats, rabbits, cows, pigs, baboons. Cynomolgus monkeys, and man was studied by fluorescence microscopy and autoradiography. Indoleamines were either injected intravitreally or the retinas were incubated with them. Fluorescence microscopy failed to show any indoleamine accumulating neurons in all species investigated except rabbit, confirming previous observations. However, autoradiography showed uptake in a distinct class of neurons in cows and pigs. These neurons had their cell bodies among the amacrine cells and most of their processes branched in the middle of the inner plexiform layer. This is in contradistinction to the dopaminergic neurons, which in cows and pigs have all their processes in the outermost sublamina of the inner plexiform layer. The fluorescence microscopy is quite sensitive to small variations in the indoleamine molecule. The discrepancy between the results with fluorescence microscopy and autoradiography therefore suggest that there is an active uptake mechanism for indoleamines in cows and pigs but that the substances are rapidly transformed to compounds not possible to detect in the fluorescence microscope. No specific indoleamine accumulating mechanism was detected in the retina of rats, baboons, cynomolgus monkeys, or man.

Binding of 3H-ADTN, a dopamine agonist, to membranes of the bovine retina

The binding of 3H-ADTN, a potent dopamine receptor agonist, to crude membrane preparations of bovine retina was studied, using a filtration method to isolate membrane-bound ligand. Specific binding was found to be saturable and occurred at a single binding site with an affinity constant of 7.3 nM. Binding was sodium-independent, slightly enhanced by Triton X-100 treatment, but drastically reduced by both trypsin and sodium laurylsulphate. The binding sites demonstrated a high degree of pharmacological specificity, with dopamine, apomorphine, and epinine being potent displacers of 3H-ADTN. A higher degree of 3H-ADTN binding was associated with subcellular fractions enriched with conventional synaptosomes rather than fractions enriched with photoreceptor synaptosomes.