Effects of serotonergic agonists and antagonists on ganglion cells in the goldfish retina

Serotonin-induced brain glycogenolysis in rainbow trout (Oncorhynchus mykiss)

In this study, we evaluated the serotonin-mediated control of cerebral glycogen levels in the rainbow trout, Oncorhynchus mykiss. Intracerebroventricular (i.c.v.) administration of serotonin (5-HT) to normoglycemic trout (time and dose response) decreased glycogen levels in the brain and increased brain glycogen phosphorylase activity (time response). In hypoglycemic fish (that had been fasted for 5 and 10 days), there was a time-dependent decrease in brain glycogen levels; under these conditions, i.c.v. administration of 5-HT also reduced the brain glycogen content in fish that had been fasted for 5 days. In fish with local cerebral hypoglycemia (induced by 2-DG administration), the glycogen levels decreased and, as above, i.c.v. administration of 5-HT also lowered the glycogen content. In hyperglycemic fish, 5-HT did not affect glycogen levels. Administration of receptor agonists 5-HT1A (8-OH-DPAT), 5-HT1B (anpirtoline and CP93129) or 5-HT2 (α-m-5-HT) decreased the brain glycogen levels. This effect was antagonized by the administration of receptor antagonists 5-HT1A (WAY100135 and NAN190), 5-HT1B (NAS181) and 5-HT2B/C (SB206553). Administration of the receptor agonists (±)-DOI (5-HT2A/2C), m-CPP (5-HT2B/2C), BW723C86 (5-HT2B) and WAY 161503 (5-HT2C) led to decreases in the levels of brain glycogen. We found that 5-HT is involved in the modulation of brain glycogen homeostasis in the rainbow trout, causing a glycogenolytic effect when fish are in a normoglycemic or hypoglycemic state, but not when they are in a hyperglycemic state. 5-HT1A, 5-HT1B, 5HT2B and 5-HT2C-like receptors appeared to be involved in the glycogenolytic action of 5-HT, although the effect mediated by 5-HT1A or 5-HT1B was apparently stronger.

TRPV1 function in mouse colon sensory neurons is enhanced by metabotropic 5-hydroxytryptamine receptor activation

Using whole-cell patch-clamp methods, we examined the hypothesis that serotonin [5-hydroxytryptamine (5-HT)] receptor activation enhances TRPV1 function in mouse colon sensory neurons in lumbosacral dorsal root ganglia, which were identified by retrograde labeling with DiI (1,1'-dioctadecyl-3,3,3',3-tetramethlindocarbocyanine methanesulfonate) injected into multiple sites in the wall of the descending colon. 5-HT increased membrane excitability at a temperature below body temperature in response to thermal ramp stimuli in colon sensory neurons from wild-type mice, but not from TRPV1 knock-out mice. 5-HT significantly enhanced capsaicin-, heat-, and proton-evoked currents with an EC50 value of 2.2 microm. 5-HT (1 microm) significantly increased capsaicin-evoked (100 nm) and proton-evoked (pH 5.5) currents 1.6- and 4.7-fold, respectively, and significantly decreased the threshold temperature for heat current activation from 42 to 38 degrees C. The enhancement of TRPV1 by 5-HT was significantly attenuated by selective 5-HT2 and 5-HT4 receptor antagonists, but not by a 5-HT3 receptor antagonist. In support, 5-HT2 and 5-HT4 receptor agonists mimicked the facilitating effects of 5-HT on TRPV1 function. Downstream signaling required G-protein activation and phosphorylation as intracellularly administered GDP-beta-S [guanosine 5'-O-(2-thiodiphosphate], protein kinase A inhibitors, and an A-kinase anchoring protein inhibitor significantly blocked serotonergic facilitation of TRPV1 function; 5-HT2 receptor-mediated facilitation was also inhibited by a PKC inhibitor. We conclude that the facilitation of TRPV1 by metabotropic 5-HT receptor activation may contribute to hypersensitivity of primary afferent neurons in irritable bowel syndrome patients.

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.

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.

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.