8-[3H]hydroxy-2-(di-n-propylamino) tetralin binding sites in goldfish retina

Olfactory stimulation selectively modulates the OFF pathway in the retina of zebrafish

Cross-modal regulation of visual performance by olfactory stimuli begins in the retina, where dopaminergic interneurons receive projections from the olfactory bulb. However, we do not understand how olfactory stimuli alter the processing of visual signals within the retina. We investigated this question by in vivo imaging activity in transgenic zebrafish expressing SyGCaMP2 in bipolar cell terminals and GCaMP3.5 in ganglion cells. The food-related amino acid methionine reduced the gain and increased sensitivity of responses to luminance and contrast transmitted through OFF bipolar cells but not ON. The effects of olfactory stimulus were blocked by inhibiting dopamine uptake and release. Activation of dopamine receptors increased the gain of synaptic transmission in vivo and potentiated synaptic calcium currents in isolated bipolar cells. These results indicate that olfactory stimuli alter the sensitivity of the retina through the dopaminergic regulation of presynaptic calcium channels that control the gain of synaptic transmission through OFF bipolar cells.

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Olfactory stimuli regulate transmission of signals through retinal bipolar cells

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Modulation of synaptic gain and sensitivity occur in OFF bipolar cells but not ON

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An inhibitor of dopamine uptake blocks odor-induced changes in synaptic gain

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Dopamine potentiates presynaptic calcium channels in isolated bipolar cells

Serotonergic synapses modulate generation of spikes from retinal ganglion cells of teleosts

Serotonin [5-hydroxytryptamine (5-HT)] is a common neurotransmitter/neuromodulator found widely in the nervous system. Cellular morphology and retinal distribution of serotonergic amacrine cells in the channel catfish (Ictalurus punctatus) retina are identified using monoclonal anti-5HT antibody. These cells receive ribbon synapses from OFF-center (hyperpolarizing) bipolar cells as well as conventional synapses with other non-serotonergic amacrine cells. Output synapses from the serotonergic cells are mainly channel onto retinal ganglion cells. Output synapses from the serotonergic cells occur as "the branched synapses" onto the ganglion cell dendrites at the dyads of the ribbon synaptic sites, and are made onto the ganglion cells, apart from the ribbon synapses. Application of serotonin receptor agonist: 5HT(1A) serotonin receptor agonist, (+)-8-hydroxy-dipropylaminotetralin [8-OH-DPAT; 1-10 muM] is also known to activate 5HT(7) serotonin receptor, coupled with activation of adenylate cyclase, generated continuous repetitive spikes from large retinal ganglion cells of the adult goldfish (Carassius auratus) in flat-mounted preparations, using amphotericin-B-perforated patch-clamp. Under control conditions of bleached retina with continuous light illumination, goldfish large retinal ganglion cells had generated only few spikes. This is the first observation of positive neuromodulation promoting retinal ganglion cell excitation in the retina. The results confirm previous reports of a serotonergic system in the mammalian retina. These results support the presence of developed postsynaptic serotonin receptors in cyprinid fish retina together with other physiological and anatomical studies, and suggest that the action of serotonin in the retina may be more important than previously believed.

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.

Serotonin, serotonin 5-HT(1A) receptors and dopamine in blood peripheral lymphocytes of major depression patients

There are increasing evidences of cell markers present in the immune and the nervous systems. These include neurotransmitter receptors and transporters. Serotonin receptor subtypes are related to depression and also have been shown to be present in certain cells of the immune system. In the present report, we determined the presence of 5-HT(1A) receptors by the binding of the selective agonist 8-hydroxy-2-(di-n-propyl-amino)tetralin in lymphocytes of peripheral blood isolated by Ficoll/Hypaque gradients from controls and depressed patients. The capacity of these receptors was around 24 fmol/10(6) cells in both groups of subjects, without significant difference among them. The affinity was in the nM range and either differ between controls and patients. Serotonin, 5-hydroxyindoleacetic acid, dopamine and 3,4-dihydroxyphenylacetic acid were determined by HPLC with electrochemical detector. There were no significant differences between controls and major depression patients in the values obtained for rich and poor platelet plasma or in the isolated cells. However, there was a reduction in serotonin turnover rate indicated by an increase in the ratio serotonin/5-hydroxyindoleacetic acid, but not in that of dopamine, in lymphocytes of major depression patients. Thus, there is a serotonergic dysfunction in immune circulating cells of major depression patients, without changes in the number of 5-HT(1A) receptors, although the coupling of these receptors to transduction mechanisms could be affected and may be related to the alteration of 5-HT turnover rate.

8-[3H]-hydroxy-2-(di-n-propylamino)tetralin binding sites in blood lymphocytes of rats and the modulation by mitogens and immobilization

Serotonin 5-HT(1A) receptors were characterized in rat resting lymphocytes obtained by cardiac puncture with the use of the ligand [3H]8-hydroxy-2-(di-n-propylamino)tetralin. Selectivity of the specific binding was demonstrated by inhibition experiments with various serotonergic and nonserotonergic drugs. The rank order of potency for inhibition was WAY-100478>pindobind>NAN-190>buspirone>imipramine>serotonin. While pimozide, desipramine, nomifensine, haloperidol and sulpiride did not inhibit the binding. Kinetic parameters calculated from saturation experiments indicated one site of interaction, with an equilibrium dissociation constant of 2.50 nM and maximum binding capacity of 487.21 nmol/10(6) cells. Complete dissociation was obtained with serotonin as the displacement agent, and equilibrium dissociation constant calculated by association and dissociation experiments was 2.03 nM. Thus, serotonin 5-HT(1A) receptors are present in resting lymphocytes. The in vivo administration of the mitogens lipopolysacharide (0.1 mg/kg, 18 h) or concanavalin A (0.2 mg/kg, 18 h) increased the number of sites. The elevation produced by the latter was of higher magnitude than that of lipopolysacharide, and two sites of the binding were determined by isotopic dilution. Immobilization stress (1 h daily for 7 days) also resulted in a significant increase of binding capacity, but was smaller than that produced by the mitogens. The affinity of binding was not affect by the treatments. The results indicate that serotonin 5-HT(1A) receptors are modulated by unspecific and specific immune system activation, as well as by a potent stress condition, which might result in relevant functional modifications in the response of rat lymphocytes.

Serotonin transporter modulation in blood lymphocytes from patients with major depression

1. Serotonin is a neurotransmitter in the central nervous system which has been implicated in the aetiology and pathogenesis of affective disorders. The serononergic system also plays several roles in the immune system through the expression of a number of its receptor subtypes in the immune cells. 2. Following release serotonin is inactivated by reuptake into neurons and other cells by a specific serotonin sodium and chloride-dependent transporter molecule, whose structure has been elucidated. 3. Measurement [3H]paroxetine binding showed that human lymphocytes contain a high-affinity serotonin transporter. 4. To assess the serotonin function in major depression, we investigated serotonin transporter density in blood lymphocytes from patients with this disorder and selected according to the interview of the American Psychiatric Association. 5. Patients were divided into two groups and treated with two different antidepressant drugs, one group receiving fluoxetine, a selective serotonin reuptake inhibitor, and another mirtazapine, an antagonist of alpha2-adrenergic auto and heteroreceptors, for a period of 6 weeks. 6. Blood samples were obtained before and after the treatment, lymphocytes were isolated by Ficoll/Hypaque gradient, subjected to differential adhesion to plastic, and cell membranes were prepared for binding assay of [3H]paroxetine. 7. Lymphocytes serotonin transporter number was significantly reduced, while the affinity was unchanged, in patients with major depression disorder as compare to controls. 8. In addition, there was a partial recovery in lymphocytes serotonin (5HT) transporter number in the period posterior to the antidepressants administration, accompanied with clinical and depression rating scales improvement. Serotonin was determined in platelet-poor plasma and in lymphocytes before and after drugs administration, showing a significant decrease in the patients treated compared to untreated and controls. 9. These results are evidence of the potential interaction between the nervous and immune systems. The mechanisms underlying this interaction are under study, and might be related to modifications in the expression or function of the serotonin transporters in lymphocytes of depressed patients.

Further characterization of 5-HT1A receptors in the goldfish retina: role of cyclic AMP in the regulation of the in vitro outgrowth of retinal explants

The presence of serotonin 5-HT1A receptors and their physiological role were further characterized in the goldfish retina. The effects of the 5-HT6/7 receptor antagonists pimozide, fluphenazine and amoxapine, the 5-HT1A receptor antagonist WAY-100,135, and the alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline, on the 5-HT1A receptor agonist [3H]8-hydroxy-2-(di-n-propylamino)tetralin binding to retinal membranes, were evaluated. In addition, the effects of serotonin, 8-hydroxy-2-(di-n-propylamino)tetralin, WAY-100,135, the adenylate cyclase inhibitors SQ22536 and MDL12330A, and the cyclic AMP analog 8-bromoadenosine-3':5' cyclic monophosphate were also studied on neuritic outgrowth from retinal explants. WAY-100,135 but not 5-HT6/7 receptor antagonists inhibited [3H]8-hydroxy-2-(di-n-propylamino)tetralin binding to retinal membranes N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline decreased [3H]8-hydroxy-2-(di-n-propylamino)tetralin binding sites up to 70%, while receptor turnover was similar to that reported in other tissues. Serotonin and 8-hydroxy-2-(di-n-propylamino)tetralin stimulated cyclic AMP production, both ex vivo and in vitro, and these increases were related to inhibition of neuritic outgrowth. The inhibitory effect was reduced by SQ22536 and by WAY-100,135, and was mimicked by 8-bromoadenosine-3':5'cyclic monophosphate. This study supports previous findings about the role of serotonin as a regulator of axonal outgrowth during in vitro regeneration of the goldfish retina and demonstrates that this effect is mediated, at least in part, by 5-HT1A receptors through a mechanism which involves an increase of cyclic AMP levels.

Modulation of outgrowth from goldfish retinal explants by a 5-HT2 receptor agonist and [3H]ketanserin binding sites in goldfish and rabbit retina

The binding of [3H]ketanserin to goldfish and rabbit retinal membrane preparations and the possible role of 5-HT2A receptors in the in vitro outgrowth from goldfish retina were evaluated. Saturation experiments indicated a high-affinity binding site, and positive cooperativity for both tissues. The 5-HT2A/2C agonist (+/-)-2,5-dimetoxy-4-iodoamphetamine and serotonin inhibited outgrowth from goldfish retinal explants. These effects were blocked by the 5-HT2 antagonists ketanserin and 1-(1-naphthyl)piperazine and by the 5-HT2C antagonist mesulergine, respectively. Results make to suggest that [3H]ketanserin binds to 5-HT2A receptors in the rabbit and goldfish retina, but also to a monoamine transporter in the latter tissue. Subtypes of 5-HT2 receptors might mediate the 5-HT modulatory role on in vitro outgrowth of the goldfish retina.

Differential effect of taurine and serotonin on the outgrowth from explants or isolated cells of the retina

The sulfur amino acid taurine and the indoleamine serotonin increases and decreases, respectively, the outgrowth from goldfish retinal explants. Taurine seems to be acting, at least partially, through an increase in calcium fluxes, and the serotonin-inhibiting effect appears to be mediated by serotonin1A receptors and cAMP. Isolated cells of postcrush goldfish retina and of retina from 5-day-old rats were cultured in the presence of taurine or serotonin. In the goldfish, the classical morphology of postcrush ganglion cells was observed. An antibody against the glycoprotein Thy-1 labelled three types of cells in the cultures of goldfish retina. The number of cells outgrowing and the length of the main neurite was measured at 5 days in culture in both species. The number of cells presenting neurites was increased in the goldfish retina by the addition of taurine, and decreased by serotonin. However, the length of the neurites was unaffected by the addition of the modulators. In the rat, only a slight decrease in the number of cells outgrowing was observed in the presence of serotonin. The incorporation of [3H]thymidine was not modified after 5 days in culture in the presence of taurine or serotonin, either in the goldfish or in the rat retina. The antibody Thy 1.1 can label retinal cells of the goldfish in vitro, one of them being ganglion cells. The trophic effect exerted by taurine in the postcrush goldfish retina needs the integrity of the tissue favoring the interaction of cells and factors, because outgrowth increases in retinal explants, but not in isolated cells.

5HT1A receptor agonist differentially increases cyclic AMP concentration in intact and lesioned goldfish retina. In vitro inhibition of outgrowth by forskolin

5HT1A receptors occur in the retina of various species and the administration of 5HT1A agonists results in the inhibition of outgrowth from postcrush goldfish retinal explants. The levels of cyclic AMP (cAMP) play a role in the modulation of the outgrowth of the nevous system. Moreover, the stimulation of central 5HT1A receptors with the agonist 8-hydroxy-2-(di-n-propylamino)tetralin has been reported to produce an increase or decrease in the activity of adenylate cyclase. In the present investigation we studied the effect of adenylate cyclase stimulation by forskolin, as well as the modulatory effects of 5HT1A receptor agonists and antagonists on the production of cAMP in the goldfish retina, and on the outgrowth of this tissue in vitro. 8-Hydroxy-2-(di-n-propylamino)tetralin produced a significant and dose-dependent increase in cAMP concentration. This effect was not additive to the stimulation produced by forskolin. By contrast, as previously described, the 5HT1A agonist decreased cAMP concentration in the hippocampus of the rat. Both effects were significantly impaired by the 5HT1A antagonist WAY-100,135. A significant effect of the antagonist alone was observed only in the goldfish retina. The increase in cAMP levels was greater in the intact than in the postcrush retina. In addition, forskolin decreased the outgrowth of postcrush retinal explants in a dose-dependent manner, suggesting the importance of critical levels of cAMP in this process. Taken together, 5HT1A receptors seem to be positively coupled to adenylate cyclase in the goldfish retina, where cAMP plays a role as a modulator of outgrowth and regeneration. The inhibitory effect of 5HT1A receptor agonists on retinal outgrowth might be mediated through the production of cAMP. The activation of other subtypes of 5HT receptors positively coupled to adenylate cyclase by the 5HT1A agonist, such as 5HT7, cannot be discarded.

Synthesis of serotonin from 5-hydroxytryptophan in the post-crush retina: inhibition of in vitro outgrowth by the intraocular administration of the precursor

Serotonin is present in the retina of many species, in which plays roles as a neurotransmitter, as a modulator of regeneration, and as the precursor of melatonin. The turnover of serotonin in the goldfish retina is modified by the lesion of the optic nerve and, in postcrush goldfish retinal explants, serotonin inhibits the outgrowth. In the present study, the modification of the serotonergic system of the retina induced by the process of regeneration was explored. The addition of the precursor of serotonin, 5-hydroxytryptophan, to retinal explants, increased the levels of serotonin in a concentration-dependent manner. The concentration of serotonin differentially increased in control and postcrush explants cultured in the presence of 5-hydroxytryptophan for various periods of time, indicating a greater accumulation of the indoleamine at early periods of time in the control than in the postcrush tissue culture. This observation, together with the fact that serotonin concentration in postcrush retina cultured in the absence of 5-hydroxytryptophan and exposed to the precursor for 60 min increased less than in the control, indicates a saturation of the serotonergic system produced by the lesion. The addition of imipramine or citalopram, serotonin uptake blockers, did not significantly change the concentration of serotonin in the cultures, thus, the elevation of serotonin accumulation, especially in the post-crush tissue, might not be due to the transport from the medium. The intraocular injection of 5-hydroxytryptophan after the crush of the optic nerve resulted in a decrease in the outgrowth of retinal explants, supporting the in vivo role of serotonin during the regenerating process in situ. The lesion of the optic nerve did not affect the specific cells, since the number of serotonin-immunoreactive neurons in the retina were not modified by the crush. Taken together, retinal serotonin system is regulated after producing a lesion of the optic nerve, a modulation which has been demonstrated in vivo and in vitro. Thus, there is a reciprocal interaction, since serotonin influences outgrowth in the postcrush retina and the serotonergic system is modulated by the crush, indicating a mechanism of feed-back regulation.

Serotonin inhibits outgrowth of goldfish retina and impairs the trophic effect of taurine

The regeneration of explants prepared from goldfish retinas with a prior crush of the optic nerve is stimulated by the sulphur amino acid, taurine. Serotonin has been reported to modify survival, proliferation, and outgrowth of nervous tissue. In the present work we evaluated the effect of serotonin and some serotonergic agonists on the neuritic outgrowth from goldfish retinal explants. Serotonin, its precursor, 5-hydroxytryptophan, and the 5HT1A receptor agonists, 8-hydroxy-2-(di-n-propylamino)tetralin and buspirone, inhibited the outgrowth. The blockers of serotonin uptake, imipramine and citalopram, were also inhibitors of neurite sprouting. Imipramine favoured the inhibitory effect of serotonin at 10 days in culture. The concentration of serotonin and its metabolite, 5-hydroxyindoleacetic acid, decreased in the retina at 3 and 5 days after the crush of the optic nerve. Serotonin levels started to recover after 5 days post-lesion, and the metabolite also increased. This indicates that the lesion increases the turnover rate of serotonin and this may be related to its role in regeneration. Serotonin concentration was elevated by the intraocular administration of its precursor, 5-hydroxytryptophan, indicating that the capacity for synthesis was preserved after the crush, but that it was smaller in the post-lesioned retinas. The trophic effect of taurine was impaired by a low concentration of serotonin, probably by opposing the final effect on growth via different targets. These results support a role of serotonin in the regeneration of goldfish retina probably through 5HT1A receptors.