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

Serotonin in retina

The expression of serotonin (5-HT) in the retina was first reported in the sixties. The detection of vesicular monoamine transporter and serotonin receptors in several retinal cells confirm that 5-HT is playing a neuromodulatory role in this structure. Whereas signaling pathways activated by 5-HT receptor binding has been poorly investigated so far, numerous data demonstrated that 5-HT is involved in retinal physiology, retinal physiopathology and photoreceptor survival.

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.

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.

5-Hydroxytryptamine1A agonists: potential use in glaucoma. Evidence from animal studies

Various classes of compounds exist to lower intraocular pressure (IOP) in the treatment of glaucoma. None of them is ideal since some patients respond better than others and the side effects vary between individuals. New classes of compounds need to be introduced to allow the clinician greater scope for effective treatment of all patients. It is now generally agreed that the cause of ganglion cell dysfunction in glaucoma is likely to be multifactorial and that concentrating solely on reducing IOP is inadequate. Irrespective of the reason for the dysfunction, the future goal must be to attenuate cell death. This may be achieved with drugs that interact with components of the retina, and is termed 'neuroprotection'. Thus, drugs that can both reduce IOP and act as neuroprotectants would be ideal for the treatment of glaucoma. In this article we summarise studies on animals which show serotonergic 5-HT1A agonists to both reduce IOP when topically applied to the rabbit eye and blunt the damaging effect to the rat retina and ganglion cells induced by glutamate toxicity or ischaemia. Reduction of IOP occurs via stimulation of 5-HT1A receptors associated with the ciliary processes. Neuroprotection of retinal neurones appears to involve the interaction of 5-HT1A agonists with membrane sodium channels and/or 5-HT1A or even possibly 5-HT7 receptors. Various 5-HT1A agonists are used in patients to treat depression, so classes of these drugs have a proven safety profile for use in patients. The animal studies summarised in this article suggest that 5-HT1A agonists need to be considered as a new class of drugs for the treatment of glaucoma.

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.

Serotonin elevates the c-wave of the electroretinogram of the rabbit eye by increasing the transepithelial potential

The influence of serotonin (5-hydroxytryptamine, 5-HT) and serotonin analogues on the direct current electroretinogram (d.c. ERG) and the standing potential of the albino rabbit eye (SP) was studied. After unilateral vitrectomy, corneal recordings were obtained during simultaneous intravitreal perfusion with a control solution alternating with 5-HT at concentrations of 25, 120 and 200 microM. The c-wave increased at 25 and 120 microM when changing from control solution to test solution (P < 0.05) but did not decrease significantly when changing back to control solution (P > 0.05). The c-wave was reversibly elevated at 200 microM (PHS-5-HT, P < 0.01; 5-HT-PHS, P < 0.05). To analyse further the influence on the c-wave, in vivo intraretinal microelectrode recordings were obtained during intravitreal perfusion with 5-HT. The transepithelial potential (TEP) increased (P < 0.01), while the slow PIII was not significantly affected (P > 0.05). The serotonin receptor agonists 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane, 5-methoxytryptamine, alpha-methyl-5-hydroxytryptamine and 2-methyl-5-hydroxytryptamine, caused a significant reversible elevation of the c-wave, whereas 5-carboxyamidotryptamine did not. Tropisetron did not block the serotonin effect and LY53857 had an effect of its own on the c-wave. The results seem to indicate that the influence of serotonin on the c-wave is mainly due to an effect on the retinal pigment epithelium (RPE) and that more than one type of serotonin receptor may be involved.

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.

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

The binding sites of 8-[3H]hydroxy-2-(di-n-propylamino)tetralin ([3H]DPAT) were characterized in the retina of goldfish in order to evaluate the selectivity of the ligand for serotonin1A (5HT1A) receptors. Specificity of the binding was performed in the presence of serotonergic and dopaminergic agonists and antagonists. Buspirone, spiroxatrine and 5-methoxy-N,N-dimethyltryptamine were potent inhibitors, followed by propranolol, citalopram, imipramine and desipramine. Serotonin was not a potent inhibitor, and its interaction with the binding sites of [3H]DPAT was complex. Nomifensine displayed an important inhibition, however, other dopamine uptake blockers, such as bupropion and GBR-12909, were less potent. Haloperidol was also a good inhibitor, but the D1 receptor agonist, SKF-38393, the D2 receptor antagonist, sulpiride, and dopamine did not inhibit the binding. GppNHp inhibited the binding in the micromolar range. The analysis of saturation experiments by isotopic dilution, using buspirone to determine nonspecific binding, revealed two sites. The number of binding sites defined by buspirone were higher than the ones defined by nomifensine. The specific binding, using buspirone for definition, was reduced by the intraocular injection of 6-hydroxydopamine. This investigation demonstrates that [3H]DPAT labels 5HT1A receptors in goldfish retina, but also interacts with a non-5HT receptor site. These receptors seem to be localized in dopaminergic neurons.

A role for 5HT3 receptors in visual processing in the mammalian retina

We investigated the role of 5HT3 receptors in the mammalian retina using electrophysiological techniques to monitor ganglion cell activity. Activation of 5HT3 receptors with the selective agonist 1-phenylbiguanide (PBG) increased the ON responses of ON-center ganglion cells, while decreasing the OFF responses of OFF-center cells. The application of a selective 5HT3 antagonist had a reciprocal effect, namely it reduced the center response in ON-center cells and concomitantly increased the center responses in OFF-center cells. Since putative serotoninergic amacrine cells in the retina are connected specifically to the rod bipolar cell, these agents most likely affect the rod bipolar terminal. These data, together with previous studies, suggest that both 5HT2 and 5HT3 receptors mediate an excitatory influence which serves to facilitate the output from rod bipolar cells, the former via a phosphatidyl inositol second-messenger system, and the latter via a direct ion channel.

Type I calmodulin-sensitive adenylyl cyclase is neural specific

The distribution of type I calmodulin-sensitive adenylyl cyclase in bovine and rat tissues was examined by northern blot analysis and in situ hybridization. Northern blot analysis using poly(A)(+)-selected RNA from various bovine tissues indicated that mRNA for type I adenylyl cyclase was found only in brain, retina, and adrenal medulla, suggesting that this enzyme is neural specific. In situ hybridization studies using bovine, rabbit, and rat retina indicated that mRNA for type I adenylyl cyclase is found in all three nuclear layers of the neural retina and is particularly abundant in the inner segment of the photoreceptor cells. The neural-specific distribution of type I adenylyl cyclase mRNA and its restricted expression in areas of brain implicated in neuroplasticity are consistent with the proposal that this enzyme plays an important role in various neuronal functions including learning and memory.

[3H]serotonin binding sites in goldfish retinal membranes

Serotonin (5HT) binding sites were studied in goldfish retinal membranes by radioligand experiments. The binding site of [3H]5HT was sensitive to pre-treatment of the membranes at 40 degrees or 60 degrees C. 5HT and 5-methoxy-N,N-dimethyltryptamine were the best inhibitors of [3H]5HT binding to retinal membranes. The 5HT2 agonist, 1-(-naphthyl)piperazine, was also a potent inhibitor, however, (+)-1-2,5-dimethoxy-4-iodophenyl-2-aminopropane was less efficient. The catecholaminergic agents haloperidol and clonidine did not display an important inhibition. Propranolol, also reported as 5HT1B antagonist, was a relatively potent blocker. Monoamine uptake blockers did not show potent inhibition. The GTP analog, GppNHp, inhibited the binding. The iterative analysis of saturation curves revealed two classes of binding sites, a high affinity component (B(max) 2.45 pmol/mg of protein, kd 6.86 nM), and a low affinity component (B(max) 53.46 pmol/mg of protein, Kd 232.07 nM). Analysis of the association and dissociation kinetics suggested a binding site (Kd 2 nM). The semilogarithmic plot of the dissociation kinetics gave curves concave to the upper side. The selectivity of the binding and the inhibition by GppNHp suggest the existence of 5HT1 receptors in goldfish retina. The low affinity interaction probably represents the transporter of 5HT or a subtype of receptor expressed in glial cells.

All indoleamine-accumulating cells in the rabbit retina contain GABA

The indoleamine-accumulating amacrine cells of the rabbit retina are wide-field and numerous. They form a dense plexus in sublamina 5 of the inner plexiform layer where they make reciprocal synapses with rod bipolar cells. To provide a quantitative test for the colocalization of serotonin (5-HT) and gamma-aminobutyric acid (GABA) in the rabbit retina, we designed two parallel double-label experiments. In the first series, the indoleamine-accumulating cells were labeled with 5,7-dihydroxytryptamine (5,7-DHT), which was subsequently visualized by photooxidation in the presence of diaminobenzidine. This was combined with autoradiography for 3H-muscimol. In the second and complementary series, 3H-5-HT uptake was combined with postembedding GABA immunocytochemistry. These two experiments provided essentially identical results: over 98% of the indoleamine-accumulating amacrine cells were double-labeled. This means that, within the limit of experimental error, all the indoleamine-accumulating amacrine cells are GABAergic. The indoleamine-accumulating amacrine cells account for 15-20% of a large diverse group of GABA amacrine cells. In addition, the rare type 3 indoleamine-accumulating cells and fine processes running in the optic fiber layer were double-labeled. If there is insufficient 5-HT to support a transmitter role in the rabbit retina, our results suggest that the indoleamine-accumulating cells may use GABA as a neurotransmitter. Thus, rod bipolar cells, in common with other bipolar cell types, receive extensive negative feedback at GABA-mediated reciprocal synapses.

The effects of serotonin drugs on horizontal and ganglion cells in the rabbit retina

We have investigated the effects of a serotonin 5-HT2 antagonist and a 5-HT1A agonist on horizontal and ganglion cells in the rabbit retina. Simultaneous intracellular horizontal cell and extracellular ganglion cell recordings were obtained from a superfused in vitro rabbit eyecup preparation and the effects of bath applied drugs on these cells' light responses observed. Sinusoidally modulated current was also injected into horizontal cells while the extracellular spike activity of nearby, single-unit ganglion cells was monitored. Although the ON components of the light-evoked responses of ganglion cells were reduced by the 5-HT2 antagonist or the 5-HT1A agonist, the membrane potential and the light responses of horizontal cells and the b-wave of the ERG were simultaneously unaffected. However, the drugs blocked current-driven ganglion cell spike activity induced by current injections into nearby horizontal cells. These results are discussed with respect to the site of action of these serotonin drugs and with respect to the circuitry of serotonergic neurons.

Immunohistochemical localization of retinal serotonin cells in the lamprey (Lampetra fluviatilis)

Light-microscopic immunocytochemistry was used to study serotonin (5HT)-containing retinal cells in the lamprey (Lampetra fluviatilis). Observations of sections and flatmounted retinas enabled us to distinguish four principal types of 5HT-immunoreactive neurons, on the basis of the localization of their somata and the arrangement of their processes in the inner plexiform layer, (IPL). Type 1 cell bodies (9 micron mean diameter) were numerous and were found in the innermost row of the inner nuclear layer (INL). They sent their processes into a dense plexus in sublamina a of the IPL. Type 2 cell bodies (12 micron mean diameter) were observed near the inner limiting membrane, their processes forming a plexus in sublamina b of the IPL. Most of the type 3 cells were bistratified, their cell bodies (similar in dimension to type 1) were located in the INL and their dendrites projected to both plexuses. Type 4 cell bodies (15 micron mean diameter) were observed in the middle of the IPL and could be compared with the interstitial described elsewhere. Their processes probably ended in the 5HT plexus of sublamina b but because of their sinuous course in the IPL, we could not affirm this fact. Most of 5HT immunoreactive cells were thought to be amacrine cells, but the presence of some thin processes emerging either from the soma or the primary dendrite, principally in type 1 and 2 cells, raises the possibility that some ganglion cells could be 5HT immunoreactive. The organization of the 5HT processes into two plexuses located in sublaminae a and b of the IPL resemble the functional ON and OFF pathway seen in the other vertebrates.

Effects of GTP, forskolin, sodium fluoride, serotonin, dopamine, and carbachol on adenylate cyclase in Teleost retina

Adenylate cyclase activity can be stimulated in goldfish retina by forskolin, GTP, NaF, dopamine and serotonin. Pharmacological characterisation of the dopamine and serotonin responses shows them to be mediated through specific receptors. A synergistic increase in the level of C-AMP is observed following application of forskolin together with NaF, GTP, dopamine, or serotonin. Dopamine and serotonin with or without GTP produce an additive response. When NaF and GTP are both together their combined effect in elevating C-AMP levels in the presence or absence of forskolin is less than additive. These results suggest that forskolin may be interacting with a Gs protein as well as directly stimulating adenylate cyclase. Increases in the level of C-AMP observed following application of forskolin or dopamine are decreased by carbachol in a dose-dependent manner. The carbachol response is blocked by pertussis toxin and is insensitive to the phosphodiesterase inhibitor, IBMX, suggesting an involvement of a Gi protein. Carbachol attenuation of elevated C-AMP levels is inhibited by atropine while pirenzapine has little effect suggesting the presence of a M2-type receptor.

Serotonin levels in the rabbit retina are elevated following intraocular injection of forskolin

Analysis of the mammalian retina for serotonin immunoreactivity suggests an absence of the amine. However, following an intraocular injection of forskolin (1 microM) into a rabbit eye 1 h before analysis of the retina, serotonin immunoreactivity is associated with a subpopulation of amacrine cells. These cells correspond in size and position to the "indoleamine-accumulating cells" of the retina. Biochemical experiments show that forskolin treatment produces an increase in levels of endogenous serotonin and 5-hydroxytryptophan but has no effect on the uptake of serotonin or tryptophan or the metabolism of 5-hydroxytryptophan. These results suggest that the "indoleamine-accumulating cells" in the retina are "serotonergic cells" and that the level of amine is elevated sufficiently for localisation following forskolin treatment. It would appear that forskolin either directly or indirectly activates tryptophan hydroxylase.

HIOMT-like immunoreactivity in the vertebrate retina: a species comparison

Localization of the melatonin-synthesizing enzyme, hydroxyindole-O-methyltransferase (HIOMT)-like immunoreactivity was examined by light and electron microscopic immunocytochemistry, in the retinas of several species that have been used as animal models to study the retinal melatonin system. HIOMT-like immunoreactivity was observed in the retinal photoreceptors of rabbit, pigmented rat, guinea-pig, chick, goldfish. African clawed toad, and leopard frog. Additionally, most species displayed HIOMT immunoreactivity in a population of bipolar cells in the inner nuclear layer. At the ultrastructural level, HIOMT-like immunoreactivity was localized to the cytoplasm of rod and cone photoreceptors, and a population of cone bipolar cells. These observations are identical to earlier observations in the human retina. The similar pattern of HIOMT-like immunoreactivity among species suggests a phylogenetic conservation of the melatonin-synthesizing capability of retinal photoreceptors and some bipolar cells.

Neuropharmacological analysis of the role of indoleamine-accumulating amacrine cells in the rabbit retina

In order to elucidate the role of putative indoleaminergic amacrine cells in visual processing, we have employed pharmacological agents specific for the two classes of serotonin receptor, 5-HT2 and 5-HT1, which have been identified in both the retina and brain. Perfusion of the rabbit retina with 5-HT2 selective antagonists decreases the ON-excitation of all classes of ganglion cell as well as the spontaneous activity of these cells. The effect on OFF-responses depends on the cell type: OFF-excitation of center-surround brisk and sluggish cells is increased or not affected by these drugs, but OFF excitation of ON/OFF direction selective cells is reduced. No disruption of the trigger features of direction selective or orientation selective cells was discovered, suggesting that indoleaminergic amacrine cells do not play a role in the generation of the complex properties of these cells. 5-HT1 receptors are heterogeneous and classified as a, b, or c subtypes. Since no selective antagonists are available for these sites, we have employed specific agonists. The most specific of these are for the 5-HT1A receptor. Perfusion with these agonists had physiological effects similar to those with perfusion of 5-HT2 antagonists. Thus, we have suggested that these two classes of serotonin receptors mediate opponent processes in the neural pathway. Since indoleaminergic cells make reciprocal synaptic connections with rod bipolar cell terminals, which are depolarizing in the rabbit retina, we hypothesize that 5-HT2 receptors facilitate the synaptic transmission from the depolarizing rod bipolar cell thus facilitating ON-excitation in the retinal network while 5-HT1A receptors mediate an inhibitory process. Similar self-opponent processing appears to take place in the hypothalamic and hippocampal serotonergic systems as well as in the dopaminergic retinal system. Thus, it is likely that this organization is a general feature of monoamine signal transmission in the central nervous system.

The effects of serotonin agonists and antagonists on the response properties of complex ganglion cells in the rabbit's retina

Selective agonists and antagonists were employed to determine the role of indoleaminergic amacrine cells in the generation of the light-evoked responses and spontaneous activity of direction and orientation selective cells. Perfusion with 5-HT2 antagonists reduced the spontaneous activity and both the leading and trailing edge responses of ON/OFF direction selective cells. 5-HT1a agonists had a similar effect on this class of cell, namely, a reduction of light-evoked and spontaneous activity. Results from ON-center and OFF-center orientation selective cells were consistent with those obtained from direction selective cells in that no disruption of direction or orientation selectivity was observed during perfusion of these drugs. These data suggest that the indoleaminergic cells are not directly involved in the generation of the trigger features of complex ganglion cells, but may be facilitating synaptic transmission in the inner retina. This function is discussed relative to the connectivity of the rod bipolar cells and the putative indoleaminergic amacrine cells. The similarity of the effects of 5-HT1a agonists and 5-HT2 antagonists supports the hypothesis, developed during our prior studies of brisk ganglion cells, that these two receptor classes mediate antagonistic processes in the target neurons.

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.

Serotonergic and cholinergic stimulation of inositol phosphate formation in the rabbit retina. Evidence for the presence of serotonin and muscarinic receptors

A direct assay which involves prelabelling with [3H]inositol has been performed to characterize receptor-mediated breakdown of inositol phospholipids in the rabbit retina. In the presence of 10 mM lithium, the receptor agonists, carbachol and serotonin (5-HT), evoked an increase in the accumulation of tritiated inositol phosphates in a dose-dependent manner. A variety of 5-HT and other antagonists were used to show that at least part of the 5-HT-induced response is mediated by 5-HT2 receptor sites, adding weight to the theory that 5-HT plays a neurotransmitter role in the mammalian retina. The very high rate of carbachol-stimulated inositol phosphate accumulation observed in retinas from young animals (10 days) decreases significantly during development of the rabbit to the adult stage. In contrast 5-HT only induces significant stimulation of inositol phosphate accumulation in retinas of animals matured to at least 17 days. Separation of total accumulated inositol phosphates (after stimulation with both carbachol and 5-HT) showed that the vast majority of tritium label was associated with the monophosphate fraction.

5-HT2 antagonists reduce ON responses in the rabbit retina

We have investigated the effects of serotonin (5-HT2) antagonists in the rabbit retina. These antagonists reduce the ON responses of ON-center cells as well as the surround (ON) responses of OFF-center cells, and enhance the center (OFF) responses of the latter cells. The result is consistent with the anatomy of the indoleamine-accumulating cells in the rabbit retina, which ramify in sublamina b (ON) of the inner plexiform layer and contact primarily bipolar cells that are depolarizing in the rabbit. This suggests that at least part of the surround (ON) responses to OFF-center cells is generated in the inner plexiform layer.

Melatonin: parallels in pineal gland and retina

It is apparent that several relationships exist between the pineal gland and retina. The similarities in development and morphology have been obvious for many years. A recent resurgence of interest in this field has led to a further understanding of many functional similarities between these two organs. A notable feature of the pineal gland and retina is their common ability to synthesize the indolamine hormone, melatonin. Many investigators suspect that the cyclic rhythm of retinal melatonin synthesis may be related to other cyclic events which normally occur in the retina.

Adenosine-mediated regulation of cyclic AMP levels in isolated incubated retinas

The effect of adenosine on the modulation of retinal cAMP levels was assessed in several mammalian species including mouse, rat, guinea pig and rabbit. Adenosine had no effect when added to incubated rat, mouse and guinea pig retinas. However, levels of cAMP were elevated in dose-dependent manner by adenosine in both light- and dark-adapted incubated rabbit retinas. Isobutylmethylxanthine (IBMX) blocked the elevation elicited by adenosine. Norepinephrine and dopamine also elevated cAMP in incubated rabbit retinas and these effects were not blocked by IBMX. The elevations of cyclic AMP levels produced by adenosine were additive with the effects of dopamine or norepinephrine. These results indicate that an adenosine-sensitive cAMP system exists in rabbit retina, and it is probably distinct from the dopamine and norepinephrine regulated cyclic AMP systems.