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Serotonin stimulates inositol phosphate production and intracellular calcium mobilization in cultured rat retinal pigment epithelial (RPE) cells through interaction with 5-HT2A receptors, but decreases cAMP production in cultured human RPE cells via 5-HT1A receptors. Studies were therefore undertaken to investigate the effect of serotonin on the cAMP system in rat RPE cells. Exposure of cultured rat RPE cells to serotonin (100 microM) for 10 minutes had no effect on the basal levels of cAMP. However, a 5 minute preincubation with serotonin potentiated the production of cAMP induced by a 5 minute exposure to forskolin (5 microM), isoproterenol (1 microM) and 5'-[N-ethylcarboxamido]-adenosine (10 microM) by 133.0%, 296.8% and 651.9%, respectively. This effect of serotonin was dose-dependent on forskolin and 5'-[N-ethylcarboxamido]-adenosine with half-maximal effects close to those reported for its action on inositol phosphates production. The antagonists ketanserin, methysergide and spiperone attenuated the action of serotonin, while yohimbine and spiroxatrine were ineffectual, thus indicating that the potentiating effect was through the 5-HT1A receptor. Incubation of cultured rat RPE cells with bradykinin stimulates inositol phosphates production with half-maximal effect observed at 1 nM. Bradykinin also potentiates the action of forskolin, isoproterenol and 5'-[N-ethylcarboxamido]-adenosine on cAMP production in a dose-dependent manner with little effect on basal levels. RPE cells exposed to serotonin (500 microM) or phorbol 12-13 dibutyrate (1 microM) for 30 minutes showed translocation of protein kinase C to the membrane from the cytosol, with 53.3% and 29.4% increases in membrane activity, respectively. Forskolin- and 5'-[N-ethylcarboxamido]-adenosine-induced cAMP production was potentiated by phorbol 12-13 dibutyrate (1 microM) treatment. The effect of both serotonin and phorbol 12-13 dibutyrate on forskolin-induced cAMP production was attenuated by pretreatment of cell cultures with the protein kinase C antagonists staurosporin and calphostin C at 1 microM. Thus, the production of cAMP in cultured rat RPE cells is potentiated by 5-HT2A receptors through activation of protein kinase C. This effect is, however, not specific since bradykinin, which stimulates inositol phosphates turnover, also potentiates stimulated cAMP production.

Purpose: This study sought to identify, characterize, and localize subtypes of 5-hydroxytryptamine (HT) receptors in rabbit iris-ciliary body., Methods: Radioligand binding assays were performed with [3H]5-hydroxytryptamine on membranes prepared from rabbit iris-ciliary bodies and on tissue sections subsequently developed by autoradiography., Results: [3H]5-HT appeared to bind to a single population of receptors in membrane preparations of rabbit iris-ciliary body. The apparent affinity of the ligand (KD) was 2.19 nM, and the density of binding sites was 58.3 fmol/mg protein. Binding of [3H] 5-HT exhibited guanosine-5-triphosphate sensitivity. Competitive inhibition experiments were performed to differentiate between 5-HT receptor subtypes. A relative potency order of 5-CT > 5-HT = 8-OH-DPAT > ipsapirone > RU24969 > sumatriptan > ritanserin > ketanserin was demonstrated. The apparent inhibitory constants for the ligands tested fit with the profile expected of binding to 5-HT1A receptors. Inhibition studies with [3H] 5-HT plus 100 nM 8-OH-DPAT (which inhibits binding to 5-HT1A receptors only) representing total binding, indicated that no further displacement occurred when ligands preferentially selective for 5-HT1B, 5-HT1D alpha,1D beta, or 5-HT2C were tested. Total binding of [3H] 5-HT in tissue sections developed by autoradiography was displaced completely by 100 nM 8-OH-DPAT. Melatonin showed little affinity for the [3H] 5-HT binding sites., Conclusions: A population of 5-HT1A receptors is present in rabbit ciliary processes. There is no evidence to suggest the presence of 5-HT1D alpha, 5-HT1D beta, 5-HT2B, or 5-HT2C receptors in the iris-ciliary body.

The purpose of this study was to investigate the effect of kainate on protein kinase C (PKC), gamma-aminobutyrate (GABA) and serotonin uptake in the rabbit retina. Kainate when injected into the vitreous humour produces a change in the GABA immunoreactivity within 6 hours. After 3 days, remnants of the normal GABA immunoreactivity still persist and additionally astrocyte and microglia-like elements "stain" positively for GABA. After 7 days exposure to kainate none of the normal GABA immunoreactivity is apparent, instead a number of round-shaped elements which may be reactive astrocytes and/or microglia stain positively for GABA. During these stages kainate does not affect the alpha PKC immunoreactivity associated with the on-bipolar cells. Six hours following kainate treatment the ability of certain GABA amacrine cells to take up exogenous serotonin is unaffected. After three days only a few of these cells can still take up exogenous serotonin and then not avidly. After seven days the GABA/serotonin amacrine cells cannot take up exogenous serotonin suggesting that all of these neurons are irreversibly damaged. One hour after treatment with kainate both calcium-dependent and -independent PKC species are translocated from the cytosolic to membrane compartments. After 5 hours and 7 days there was also evidence from the enzyme assay experiments that kainate caused the calcium-dependent and -independent PKC enzymes to be translocated but because the total enzyme activity was reduced due perhaps to down-regulation of the enzyme this was difficult to assess precisely.(ABSTRACT TRUNCATED AT 250 WORDS)

The aim of the described experiments was to use immunohistochemistry to visualize the release of GABA from specific retinal amacrine cells following ischaemia and to establish the involvement of defined glutamatergic receptors. In initial experiments, rabbit retinas were exposed in vitro to excitatory amino acid agonists alone or in combination with a putative antagonist, or in physiological solution lacking oxygen and glucose, or in solution containing potassium cyanide for 45 min at 37 degrees C. The nature of the GABA immunoreactivity was then examined by immunohistochemistry. In other in vitro experiments, retinas were first allowed to accumulate exogenous serotonin before exposing the tissues to the combinations as described. These tissues were then processed immunohistochemically for the localization of serotonin. In yet other experiments, the intraocular pressure of a rabbit's eye was raised to about 110 mmHg for 60 min and a reperfusion time of 45 min allowed before dissecting the retina and processing for the localization of GABA immunoreactivity. The other eye served as a control. Of the excitatory amino acid agonists tested, only N-methyl-D-aspartate, kainate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid caused a change in the GABA immunoreactivity. The N-methyl-D-aspartate effect was specifically antagonized by dizocilpine maleate, dextromethorphan and memantine, and was characterized by a reduction in the number of GABA-immunoreactive perikarya. The GABA "staining" in the inner plexiform layer also appeared as four clear bands. The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid- and kainate-induced effects were both antagonized by 6-cyano-2,3-dihydroxy-7-nitroquinoxaline-2,3-dione and partially by kynurenic acid at the concentrations used. Here, the amount of GABA-positive perikarya was greatly reduced and three immunoreactive bands appeared in the inner plexiform layer. However, for low concentrations of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid four GABA-immunoreactive bands could be identified in the inner plexiform layer. The normal GABA immunoreactivity of the inner plexiform layer also appeared to be in defined bands in retinas which received an ischaemic insult either by reducing the availability of glucose and oxygen, exposing the tissue to potassium cyanide or raising the intraocular pressure of an eye. In these cases the number of GABA-positive perikarya was also reduced. Only alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and kainate of the excitatory amino acid agonists tested caused a release of serotonin and this process was antagonized by 6-cyano-2,3-dihydroxy-7-nitroquinoxaline-2,3-dione and partially by kynurenic acid.(ABSTRACT TRUNCATED AT 400 WORDS)

From the evidence available it is clear that melatonin and serotonin receptors exist in the iris/ciliary processes of the rabbit. These receptors may be involved in maintaining the intraocular pressure (IOP). However, the published results on this point are often contradictory, perhaps because of the variation in the species of animals used and the methodology employed. It is also clear that the data obtained from studies on the rabbit cannot be directly applied to man. Nevertheless, present information points to the possibility that drugs influencing specific serotonin and/or melatonin receptors may be used to influence IOP in man and thus have a therapeutic effect.

Purpose: The effect of serotonin (5-hydroxytryptamine: 5-HT) and other agonists on rabbit intraocular pressure (IOP), pupil size, and the breakdown of blood-aqueous barrier were evaluated., Methods: Serotonin and various other agonists were applied topically to the rabbit eye, and intraocular pressure was followed over the next 3 hours using a Digilab 30D pneumatometer., Results: It was demonstrated immunohistochemically that topical 5-HT reached the anterior chamber within 1 hour. Serotonin raised the IOP in a dose-dependent manner over a period of up to 4 hours, with a maximum reached between 30 minutes and 1 hour. A similar effect was observed with the 5-HT1-agonist 5-carboxamidotryptamine (5-CT). Neither tryptamine, 5-hydroxytryptophan, melatonin, gepirone, nor 5,6/5,7-dihydroxytryptamine caused any changes in IOP. Serotonin did not cause a change in pupil size or a breakdown of the blood-aqueous barrier, nor did the aqueous cAMP change significantly after topical 5-HT administration., Conclusions: The data presented suggest a role for 5-HT in the control of IOP. Previously demonstrated receptors on the iris-ciliary body and the effect of the 5-HT1-agonist 5-CT suggest that the rise in IOP may be caused partly or entirely by an increase in aqueous secretion mediated by 5-HT1-like receptors. Whether or not 5-HT has a role in altering aqueous outflow resistance remains to be seen. An effect of serotonin on other aspects of aqueous dynamics or on the extraocular muscles to cause a change in IOP cannot be excluded.

The rabbit isolated iris sphincter muscle maintained in an isotonic state is unaffected by applied serotonin (5-hydroxytryptamine or 5-HT) whereas carbachol causes the muscle to contract. Serotonin does, however, produce a relaxation of the contracted muscle in a dose-dependent manner. This effect is also induced by the 5-HT receptor agonists 8-OH-DPAT (8-hydroxy-2-[di-n-propyl-amino] tetralin, RU 24969 (5-methoxy-3-[1,2,3,6, tetrahydro-4-pyridinyl]-1-indole) and ipsapirone, suggesting the involvement of 5-HT1A receptors. This view is supported by the finding that metergoline, methysergide and propranolol all counteracted the effect produced by serotonin. While 5-HT3 receptors are not involved in the described process, a minor involvement of 5-HT2 receptors cannot be excluded as methysergide partially counteracted the serotonin response. These data provide evidence that serotonin receptors, in particular the 5-HT1A subtype, may be associated with the iris sphincter muscle and suggest their involvement in the regulation of pupil size.

The accumulation, metabolism and potassium-induced release of 5-hydroxytryptamine (5-HT) in bovine retinae was studied. When isolated retina pieces were incubated at 37 degrees C in a medium containing [14C]5-HT, tissue:medium ratios of about 5 were obtained after only 10 min. Within this time the accumulated amine is hardly metabolized and even after a 60 min period, only approximately 12% of the amine is metabolized to form 5-hydroxyindoleacetic acid. The process responsible for this accumulation showed properties of an active transport system: it was temperature-sensitive, sodium-dependent and inhibited in particular by Lilly 110140, chlorimipramine and desimipramine. The uptake of [14C]5-HT was saturated with increasing 5-HT concentrations and could be accounted for by two saturable processes; a high-affinity system with a Km1 of 1.3 X 10(-7) M and Vmax1 of 0.65 pmol/mg/10 min, and a low affinity system with a Km2 of 1.2 X 10(-5) and Vmax2 of 29 pmol/mg/10 min. A rapid efflux of [14C]5-HT from tissue loaded with [14C]amine was observed when exposed to 70 mM KCl. This release was inhibited when the calcium content in the medium was replaced by sucrose. Cobalt ions added to the incubation medium also counteracted the effect of KCl, while chlorimipramine added to the medium enhanced the release of 5-HT caused by KCl. Bovine retinae also possess the enzymes necessary to produce 5-HT from tryptophan. These data generally support the idea that 5-HT is a transmitter in the retina, although as this study shows, the amine is present only in minute quantities.

Serotonin was biochemically shown to be present in the iris-ciliary body of the frog, pigeon, goldfish and guinea-pig eyes at a concentration of between 55 and 95 ng per g. The aqueous fluid, in contrast, had no measurable serotonin, though a small amount of dopamine was present. Immunohistochemistry of the iris-ciliary body and cornea of all species failed to demonstrate the localization of serotonin. However, when tissues were first incubated with exogenous serotonin and then processed immunohistochemically for the localization of serotonin, positive staining was observed. Serotonin-accumulating fibres were present in the corneal stroma of the frog, pigeon and guinea-pig but not in the goldfish. In this species only a few unidentified non-neural cells in the corneal epithelium took up exogenous serotonin. The evidence of serotonin-accumulating fibres in the frog, rat or goldfish iris-ciliary body complex was not conclusive. This contrasted with the pigeon iris-ciliary body where there were some delicate fibres in the epithelium layers which take up exogenous serotonin. It is however, in the guinea-pig iris-ciliary body that the serotonin-accumulating cells are most numerous with fibres being situated in the muscular and epithelial areas. The distribution of tyrosine-hydroxylase immunoreactivity and serotonin-accumulating fibres in the guinea-pig iris-ciliary body was similar. The rat tissue also demonstrated a perfuse distribution of tyrosine-hydroxylase-positive fibres but lacked clear serotonin-accumulating fibres. The serotonin-accumulating fibres, therefore, do not give an indication of catecholaminergic fibres in all species.

Using high performance liquid chromatography (HPLC) and electrochemical detection, serotonin and its precursor 5-hydroxytryptophan (5-HTP) were identified in crude perchloric acid extracts from bovine cornea. In addition, dopamine, noradrenaline and adrenaline were identified. Through the use of immunohistochemical procedures it was established that the serotonin is localized in nerve fibers in the cornea and they were always found subepithelially or in the stroma. The occurrence of serotonergic fibres in the cornea suggests that the amine has a specific functional role in this tissue.

The central ganglia of the leech, Hirudo medicinalis, were processed for the immunohistochemical localisation of bombesin-, substance P-, cholecystokinin-, vasoactive intestinal polypeptide-, enkephalin-, serotonin- and dopamine-beta-hydroxylase-related substances. To varying extents all of the substances were localised in neuropile processes, and all, with the exception of substance P, were associated with specific perikarya. The most prominent neuropeptides, in terms of the number of immunoreactive neurones, were cholecystokinin and vasoactive intestinal peptide. The dopamine-beta-hydroxylase positive neurones are thought to be octopaminergic, and the serotonin monoclonal antibody revealed positive staining in the Retzius cells. We were unable to demonstrate the coexistence of pairs of substances in any neurones in the leech ganglia.

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.

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.

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).

Isolated ganglia possess the ability to concentrate tryptamine from an external medium by a process which is temperature sensitive and independent of sodium and other cations. Kinetic analysis of the accumulation process showed the influx of tryptamine to be a single mechanism with Km and Vmax values of 1.4 X 10(-4)M and 5 X 10(-8) mole/g/min. The influx of tryptamine is an unspecific process and is insensitive to a number of metabolic inhibitors and various analogues. The process of tryptamine influx is thus similar in principle to the low affinity uptake mechanism for 5-HT (see Osborne et al., 1975). The present data, which include some experiments on the release of 5-HT and tryptamine, are discussed from the point of view of a functional role for 5-HT and tryptamine in the snail CNS.

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.

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.

1. The effect of 5,7-dihydroxytryptamine (5,7-DHT) on an identified 5-hydroxytryptamine (5-HT)-containing neurone in the CNS of the snail was studied by histochemical, biochemical and electrophysiological methods. 2. Low concentrations of 5,7-DHT decreased the endogenous 5-HT content of the neurone without affecting the amino acids, while relatively large amounts of the drug proportionately lowered 5-HT and in addition slightly decreased the tryptophan and methionine content of the cell. 3. 5,7-DHT blocked the uptake of [3H]-5-HT into the neurone; the close analogue 5,6-DHT was more potent. 4. As well as slightly influencing the accumulation of [3H]-tryptophan by the neurone 5,7-DHT inhibited the metabolism of this amino acid to form 5-HT, probably by affecting the enzyme tryptophan-hydroxylase. 5. 5,7-DHT produced a postsynaptic blockade of transmission from the neurone by blocking the 5-HT receptors of the follower neurones. This effect appeared to be specific for 5-HT receptors. 6. The data support the idea that 5,7-DHT is neurotoxic for indoleamine-containing neurones.

The rabbit iris-ciliary body contains 78 +/- 6 ng/gm serotonin (5-HT) while the amine content in the aqueous humor is less than 0.01 ng/100 microliter. The low levels of endogenous 5-HT in the iris-ciliary body could not be directly detected by immunocytochemistry. However, pretreatment in vivo and in vitro with L-tryptophan and pargyline resulted in the localization of a sparse population of 5-HT fibers. These fibers could not be studied by exposing the tissue to exogenous 5-HT since the amine was taken up by noradrenergic fibers as well. This was confirmed in studies involving superior cervical ganglionectomy. It is concluded that 5-HT is taken up by both serotonergic and adrenergic fibers of the iris-ciliary body. In the presence of lithium, 5-HT stimulated an increase in the 3H-inositol phosphate accumulation in a dose-dependent manner in tissue where the phosphoinositide pool was labeled with 3H-inositol. A variety of agonists and antagonists were used to show that the 5-HT response is mediated, at least partly, by 5-HT2 receptors. The 5-HT-mediated effect on inositol phosphates is unaffected by superior cervical ganglionectomy. The effect of noradrenaline, which also stimulates inositol phosphate accumulation (but via alpha 1-adrenergic receptors in the iris-ciliary body), was elevated following superior cervical ganglionectomy.

Immunohistochemical procedures revealed that the serotonin-containing cell situated in each metacerebral ganglion of the snail Helix also contains a cholecystokinin-like peptide, but is devoid of substance P. In radioimmunoassays the cholecystokinin-like material showed similarities to the carboxy terminal regions of mammalian cholecystokinin and gastrin. Since much is known about the morphology and synaptic connections of this serotonin neuron, the role of the cholecystokinin-like peptides can now be investigated by neurophysiological methods, thus opening the way to discovering whether a neuron can use more than one transmitter.

The same cells in both the intact retina and retinal cultures of the rabbit retina take up exogenous serotonin or 5,7-dihydroxytryptamine. Both substances can be localised by immunohistochemistry using a monoclonal antibody directed against serotonin. The 5,7-dihydroxytryptamine can also be revealed directly in both fixed and living tissues using appropriate U.V. light. Some of the cells in the intact retina and retinal cultures which take up 5,7-dihydroxytryptamine also stain positively for the immunohistochemical localisation of gamma-aminobutyric acid (GABA). Cell counts in the cultures show that 80% of all 5,7-dihydroxytryptamine-positive cells stain positively for GABA, while 20% of the GABA-immunoreactive cells also accumulate 5,7-dihydroxytryptamine. These results demonstrate unambiguously that a subpopulation of GABA cells has the capacity to take up exogenous 5,7-dihydroxytryptamine and therefore utilise GABA and, probably, serotonin. Findings in favour of the 'co-occurrence' of GABA and serotonin come from studies where tissues were exposed to radioactive GABA and unlabelled serotonin so that the dual localisation of these substances by autoradiography and immunohistochemistry could be followed. It could be shown both in cultures and in intact retinal pieces that certain cells take up GABA and serotonin while other cells take up either GABA or serotonin.

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.