Activation of p38 mitogen-activated protein kinase during normal mitosis in the developing retina

The p38 member of the mitogen-activated protein kinase superfamily is engaged by phosphorylation in response to environmental stress signals, and may have either permissive or inhibitor roles upon cell proliferation. The cell cycle in the proliferative zone of the retina is tightly controlled and proceeds in synchrony with interkinetic migration of the neuroblast nuclei. We examined the association of p38 kinase activity with the cell cycle in the normal, non-stressed retina of the developing rat, maintained either in vivo or in vitro. Using immunohistochemistry, we show that mitotic profiles in the developing retina are highly enriched for phosphorylated p38. Blockade of p38 activity with the chemical inhibitor SB203580 for 4 h transiently arrested cells at the metaphase-anaphase transition and induced cell death after 20 h. p38 inhibition induced an aberrant mitotic profile, with chromosomes arranged in one side of the cell. The data show that p38 is active during normal mitosis and we suggest that p38 is required for the proper cell cycle progression during metaphase-anaphase transition in retinal neuroblasts.

Differential effects of cyclin-dependent kinase blockers upon cell death in the developing retina

Pharmacological blockers of cyclin-dependent kinases (CDKs) can inhibit cell cycle progression. Deferoxamine (DFO) and mimosine (MIMO) arrest cells reversibly at the G1/S transition and olomoucine (OLO) inhibits the cell cycle at both G1/S and G2/M. We investigated the effect of these drugs upon cell death in histotypical explants taken from the retina of neonatal rats. Degeneration of retinal ganglions cells (RGC) induced by axotomy was inhibited by OLO (100 microM) but not by DFO (up to 2 mM) or MIMO (up to 1 mM). On the other hand, after 1 day in vitro, all cell cycle inhibitors induced cell death in the neuroblastic layer (NBL) of the explants. DFO and MIMO induced cell death only of proliferating cells, identified either by their incorporation of bromodeoxyuridine or by immunolabeling the proliferating cell nuclear antigen. In turn, OLO induced cell death of both proliferating and post-mitotic cells. However, the post-mitotic cells were unlabeled with markers of retinal differentiation. Our results indicate that cyclin-dependent kinases are involved in the control of sensitivity to cell death in the retina, and that retinal cells present differentiation-dependent responses to modulation of CDK activity.

Apoptosis underlies immunopathogenic mechanisms in acute silicosis

We investigated immunopathogenic roles for apoptosis in acute murine silicosis. Intratracheal silica instillation induced pulmonary inflammation and enlarged thoracic lymph nodes. Lymphocytes from silica-exposed lymph nodes showed reduced mitogenic responses to T cell receptor (TCR) stimulation, and markedly increased activation-induced cell death, compared with control lymphocytes from saline-exposed lymph nodes. CD4(+) T cell death was mediated by Fas ligand, because CD4(+) T cells from Fas ligand-deficient gld mice did not undergo activation-induced apoptosis. Silica deposition also resulted in increased apoptosis associated with inflammatory infiltrates in lung parenchyma. In vivo treatment with caspase inhibitors reduced neutrophil accumulation, and alleviated inflammation in the lungs of silica-treated mice. These results suggest that silica-induced apoptosis plays an inflammatory role in the lung parenchyma, and creates immunologic abnormalities in regional lymph nodes, with pathogenic implications for the host.

Stress-inducible protein 1 is a cell surface ligand for cellular prion that triggers neuroprotection

Prions are composed of an isoform of a normal sialoglycoprotein called PrP(c), whose physiological role has been under investigation, with focus on the screening for ligands. Our group described a membrane 66 kDa PrP(c)-binding protein with the aid of antibodies against a peptide deduced by complementary hydropathy. Using these antibodies in western blots from two-dimensional protein gels followed by sequencing the specific spot, we have now identified the molecule as stress-inducible protein 1 (STI1). We show that this protein is also found at the cell membrane besides the cytoplasm. Both proteins interact in a specific and high affinity manner with a K(d) of 10(-7) M. The interaction sites were mapped to amino acids 113-128 from PrP(c) and 230-245 from STI1. Cell surface binding and pull-down experiments showed that recombinant PrP(c) binds to cellular STI1, and co-immunoprecipitation assays strongly suggest that both proteins are associated in vivo. Moreover, PrP(c) interaction with either STI1 or with the peptide we found that represents the binding domain in STI1 induce neuroprotective signals that rescue cells from apoptosis.

Cellular prion protein transduces neuroprotective signals

To test for a role for the cellular prion protein (PrP(c)) in cell death, we used a PrP(c)-binding peptide. Retinal explants from neonatal rats or mice were kept in vitro for 24 h, and anisomycin (ANI) was used to induce apoptosis. The peptide activated both cAMP/protein kinase A (PKA) and Erk pathways, and partially prevented cell death induced by ANI in explants from wild-type rodents, but not from PrP(c)-null mice. Neuroprotection was abolished by treatment with phosphatidylinositol-specific phospholipase C, with human peptide 106-126, with certain antibodies to PrP(c) or with a PKA inhibitor, but not with a MEK/Erk inhibitor. In contrast, antibodies to PrP(c) that increased cAMP also induced neuroprotection. Thus, engagement of PrP(c) transduces neuroprotective signals through a cAMP/PKA-dependent pathway. PrP(c) may function as a trophic receptor, the activation of which leads to a neuroprotective state.

Pituitary adenylyl cyclase-activating polypeptide prevents induced cell death in retinal tissue through activation of cyclic AMP-dependent protein kinase

Multiple neuroactive substances are secreted by neurons and/or glial cells and modulate the sensitivity to cell death. In the developing retina, it has been shown that increased intracellular levels of cAMP protect cells from degeneration. We tested the hypothesis that the neuroactive peptide pituitary adenylyl cyclase-activating polypeptide (PACAP) has neuroprotective effects upon the developing rat retina. PACAP38 prevented anisomycin-induced cell death in the neuroblastic layer (NBL) of retinal explants, and complete inhibition of induced cell death was obtained with 1 nm. A similar protective effect was observed with PACAP27 and with the specific PAC1 receptor agonist maxadilan but not with glucagon. Photoreceptor cell death induced by thapsigargin was also prevented by PACAP38. The neuroprotective effect of PACAP38 upon the NBL could be reverted by the competitive PACAP receptor antagonist PACAP6-38 and by the specific PAC1 receptor antagonist Maxd.4. Molecular and immunohistochemical analysis demonstrated PAC1 receptors, and treatment with PACAP38 induced phospho-cAMP-response element-binding protein immunoreactivity in the anisomycin-sensitive undifferentiated postmitotic cells within the NBL. PACAP38 produced an increase in cAMP but not inositol triphosphate, and treatment with the cAMP-dependent protein kinase inhibitor R(p)-cAMPS blocked the protective effect of PACAP38. The results indicate that activation of PAC1 receptors by PACAP38 modulates cell death in the developing retina through the intracellular cAMP/cAMP-dependent protein kinase pathway.

Cellular prion protein: on the road for functions

Cellular prion (PrPc) is a plasma membrane glycosyphosphatidylinositol-anchored protein present in neurons but also in other cell types. Protein conservation among species suggests that PrPc may have important physiological roles. Cellular and molecular approaches have established several novel features of the regulation of PrPc expression, cellular trafficking as well as its participation in copper uptake, protection against oxidative stress, cell adhesion, differentiation, signaling and cell survival. It is therefore likely that PrPc plays pleiotropic roles in neuronal and non-neuronal cells, and as such the loss of function of PrPc may be an important component of various diseases.

Sympathetic neuronal survival induced by retinal trophic factors

Neuronal survival in the vertebrate peripheral nervous system depends on neurotrophic factors available from target tissues. In an attempt to identify novel survival factors, we have studied the effect of secreted factors from retinal cells on the survival of chick sympathetic ganglion neurons. Embryonic day 10 sympathetic neurons undergo programmed cell death after 48 h without appropriate levels of nerve growth factor (NGF). Retina Conditioned Media (RCM) from explants of embryonic day 11 retinas maintained for 4 days in vitro supported 90% of E10 chick sympathetic neurons after 48 h. Conditioned medium from purified chick retinal Muller glial cells supported nearly 100% of E10 chick sympathetic neurons. Anti-NGF (1 microg/mL) blocked the survival effect of NGF, but did not block the trophic effect of RCM. Neither BDNF nor NT4 (0.1-50 ng/mL) supported E10 sympathetic neuron survival. Incubation of chimeric immunoglobulin-receptors TrkA, TrkB, or TrkC had no effect on RCM-induced sympathetic neuron survival. The survival effects were not blocked by anti-GDNF, anti-TGFbeta, and anti-CNTF and were not mimicked by FGFb (0.1-10 nM). LY294002 at 50 microM, but not PD098059 blocked sympathetic survival induced by RCM. Further, the combination of RCM and NGF did not result in an increase in neuronal survival compared with NGF alone (82% survival after 48 h). The secreted factor in RCM is retained in subfractions with a molecular weight above 100 kDa, binds to heparin, and is unaffected by dialysis, but is heat sensitive. Our results indicate the presence of a high-molecular weight retinal secreted factor that supports sympathetic neurons in culture.

Effects of prenatal ionizing irradiation on the development of the ganglion cell layer of the mouse retina

Prenatal exposure to ionizing irradiation has been shown to be an effective method to eliminate selectively certain neuronal population. This investigation studied the effects on the ganglion cell layer of the retinae of adult mice exposed to a gamma source (total dose=3 Gy) at 16 days gestation. There was a significant reduction in the total number of neurons (displaced amacrine+ganglion cells) in the ganglion cell layer (33%) that was mainly caused by a pronounced loss (59%) of displaced amacrine cells. The diameters of the surviving retinal ganglion cells were consistently larger than those of the controls. Prenatal irradiation is the first experimental approach that partially eliminates displaced amacrine cells. It is suggested that the morphogenesis of retinal ganglion cells may be affected by displaced amacrine cells.

Fas ligand triggers pulmonary silicosis

We investigated the role of Fas ligand in murine silicosis. Wild-type mice instilled with silica developed severe pulmonary inflammation, with local production of tumor necrosis factor (TNF)-alpha, and interstitial neutrophil and macrophage infiltration in the lungs. Strikingly, Fas ligand-deficient generalized lymphoproliferative disease mutant (gld) mice did not develop silicosis. The gld mice had markedly reduced neutrophil extravasation into bronchoalveolar space, and did not show increased TNF-alpha production, nor pulmonary inflammation. Bone marrow chimeras and local adoptive transfer demonstrated that wild-type, but not Fas ligand-deficient lung macrophages recruit neutrophils and initiate silicosis. Silica induced Fas ligand expression in lung macrophages in vitro and in vivo, and promoted Fas ligand-dependent macrophage apoptosis. Administration of neutralizing anti-Fas ligand antibody in vivo blocked induction of silicosis. Thus, Fas ligand plays a central role in induction of pulmonary silicosis.

Depletion of cortical target induced by prenatal ionizing irradiation: effects on the lateral geniculate nucleus and on the retinofugal pathways

Studies using neonatal surgical lesions to reduce the target area of the retina have supported the idea that developing axons show only a limited specificity in their targeting. This investigation tested whether retinogeniculate axons adjust for partial target depletion by repositioning of axons. We used adult Swiss mice exposed to gamma rays at the time when layer IV cells are generated in the ventricular zone (16 days of gestation). Nissl-stained brain sections were used for histological analyses in thalamus and cortex. Retinal ganglion cells were backfilled from the optic tract with horseradish peroxidase. Intraocular injections of horseradish peroxidase were used to study the retinal projections. In the posterior cortex there was a nearly complete absence of layer IV. The irradiated animals showed a 75% reduction of the dorsal lateral geniculate nucleus. The ventral division, superior colliculus, and other visually related nuclei were not affected. The loss in the ganglion cells (15.7%) was significant but clearly smaller than that observed in the dorsal lateral geniculate nucleus (75%). Therefore, the shrinkage of the dorsal lateral geniculate nucleus led to a reduction in the area available for retinal projections. Despite partial target loss, pattern of retinal projections did not differ from that of the controls. The effect on the dorsal lateral geniculate nucleus is discussed in the light of differences between prenatal and neonatal damage of the presumptive visual cortex. The absence of aberrant retinal projections suggests that repositioning of axons is not the first mechanism employed by retinal axons to match connections in numerically disparate populations.

Differentiation-dependent sensitivity to cell death induced in the developing retina by inhibitors of the ubiquitin-proteasome proteolytic pathway

The effects of inhibitors of proteasome function were studied in the retina of developing rats. Explants from the retina of neonatal rats at postnatal day (P) 3 or P6 were incubated with various combinations of the proteasome inhibitor carbobenzoxyl-leucinyl-leucinyl-leucinal (MG132), the protein synthesis inhibitor anisomycin, or the adenylyl cyclase activator forskolin. MG132 induced cell death in a subset of cells within the neuroblastic (proliferative) layer of the retinal tissue. The cells sensitive to degeneration induced by either MG132 or anisomycin, were birthdated by bromodeoxyuridine injections. This showed that the MG132-sensitive population includes both proliferating cells most likely in their last round of cell division, and postmitotic undifferentiated cells, at a slightly earlier stage than the population, sensitive to anisomycin-induced cell death. The results show that sensitivity to cell death induced by proteasome inhibitors defines a window of development in the transition from the cell cycle to the differentiated state in retinal cells.

Tissue biology of apoptosis. Ref-1 and cell differentiation in the developing retina

Programmed cell death by apoptosis plays a major role in neurogenesis. The sensitivity to apoptosis in developing nervous tissue is strongly dependent on cell interactions taking place within a highly structured environment, composed of various cell types at distinct stages of differentiation. In this article, we review evidence gathered both in vivo and in a histotypical retinal explant preparation in vitro that the bifunctional AP endonuclease/redox factor Ref-1 (HAP1, APE, APEX) may be an anti-apoptotic protein associated with cell differentiation in the developing retina.

Paracrine neuroprotective effect of nitric oxide in the developing retina

The retina of newborn rats consists of the ganglion cell layer (GCL), the inner plexiform layer (IPL), the inner nuclear layer (INL) containing amacrine cells and the neuroblastic layer (NBL). In retinal explants, the GCL enters cell death after sectioning of the optic nerve, whereas there is almost no cell death in the NBL. When protein synthesis is inhibited with anisomycin, cell death is blocked in the GCL and induced in the NBL. We tested the roles of nitric oxide (NO) on cell death in the retina in vitro. Either L-arginine, the substrate for NO synthase or the NO donor S:-nitroso-acetylpenicillamine (SNAP) blocked cell death induced by anisomycin in the NBL, but had no effect in the GCL. Sepiapterin, a precursor of the nitric oxide synthase (NOS)-cofactor tetrahydrobiopterin also had a protective effect against anisomycin. The use of 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, an inhibitor of soluble form of guanylyl cyclase, showed that anti-apoptotic effect of SNAP is partially mediated by cGMP generated by activation of guanylyl cyclase. NADPH-diaphorase histochemistry stained cells only in the GCL and INL. Thus, the degenerative effect of anisomycin is observed within the NBL, whereas the localization of NOS is restricted to the GCL and INL. The protective effect of both the NO substrate and cofactor upon cell death induced by anisomycin in the NBL, indicates that NO produced by amacrine and ganglion cells is a paracrine modulator of cell death within the retinal tissue.

Chloramphenicol induces apoptosis in the developing brain

Programmed cell death was studied in the superior colliculus of the developing rat brain following injections of chloramphenicol. Neonatal rats were either subject to unilateral eye removal or left untouched. Following a 3-h post-operative survival, the animals were perfused with fixatives and frozen sections of their brains were examined for apoptosis after either neutral-red staining, in situ nick-end labeling of fragmented DNA, or immunocytochemistry to activated caspase-3. Chloramphenicol induced apoptosis in control brains and potentiated cell death in deafferented superior colliculi. The results show that CMP has a general pro-apoptotic effect in the developing brain.

Laminin modulates neuritogenesis of developing rat retinal ganglion cells through a protein kinase C-dependent pathway

Dissociated cells from rat retinae (P2-P21) were cultured to investigate interactions between brain-derived neurotrophic factor (BDNF), various substrates (poly-L-lysine, collagen, and laminin), and protein kinases upon the neuritogenesis of retinal ganglion cells (RGCs). We found that BDNF-promoted neuritogenesis was enhanced by forskolin in RGCs from rats at P2-P21 plated on either poly-L-lysine or collagen. In contrast, in cultures with a laminin substrate, the enhancer effect of forskolin was observed only in RGCs taken from the retina of rats at P2-P6. Laminin blocked the enhancement of BDNF-induced RGCs neuritogenesis by forskolin, in RGCs from either P14 or P21, and induced a tenfold increase of protein kinase C (PKC) activity compared to poly-L-lysine. This blockade was reverted with a selective PKC inhibitor and was reproduced in poly-L-lysine cultures of P14-P21 RGCs with a PKC activator. Because axotomized RGCs need both BDNF and forskolin to regenerate, we suggest that laminin can hinder this effect by simultaneous PKC activation according to a developmentally regulated pattern. We further propose a model of interaction in the optic pathways triggered by BDNF, forskolin, and laminin that may be useful in elucidating some of the biological effects seen with regenerating axons.

Response surface analysis applied to the preparation of tablets containing a high concentration of vegetable spray-dried extract

This work relates to the formulation of tablets containing a high proportion of spray-dried extracts (SDEs) from Passiflora edulis leaves. The tablets were prepared by direct compression. Colloidal silicon dioxide was selected as a glidant and moisture adsorbent, cross-linked carboxymethycellulose was used as the disintegrant, microcrystalline cellulose was the filler/binder, and tricalcium phosphate as a spray-drying adjuvant. The colloidal silicon dioxide and cross-linked carboxymethycellulose quantities and their influences on the tablet hardness and disintegration time were studied by a central composite design. The model equations were fitted to the experimental data and then validated. It could be concluded that the colloidal silicon dioxide proportion increased the hardness, and the cross-linked carboxymethycellulose proportion determined a linear decrease of the disintegration time. The optimal values chosen were 2.0% Aerosil 200 and 2.5% Ac-Di-Sol. The tablets showed a hardness of 85.02 N and a disintegration time of 7.35 min.

The anti-death league: associative control of apoptosis in developing retinal tissue

Apoptosis, the major form of programmed cell death (PCD), is executed through a proteolytic cascade that can be differentially engaged by various extracellular signals. Modulation of both the sensitivity to PCD and of the actual sequence of apoptotic events is, therefore, strongly dependent on cell interactions. This paper reviews the use of a retinal explant preparation as a model of the organized nervous tissue, to study the effects of neural messengers in the control of sensitivity to apoptosis. Studies of retinal explants showed that dopamine, glutamate and nitric oxide may have anti-apoptotic effects upon developing retinal cells. At least the effects of nitric oxide are clearly paracrine. In addition, preliminary evidence has been gathered of a role for gap junctional communication in the control of sensitivity of retinal cells to the induction of apoptosis. These findings underscore the importance of selective cell interactions in the control of PCD in the developing nervous system.

Evidence that the bifunctional redox factor / AP endonuclease Ref-1 is an anti-apoptotic protein associated with differentiation in the developing retina

Retinal cell differentiation leads to resistance to apoptosis induced by inhibition of protein synthesis, suggesting the accumulation of anti-apoptotic proteins. The redox factor/AP endonuclease Ref-1 (APE, APEX, HAP1) affects both DNA repair and the activity of various transcription factors, and controls sensitivity to genotoxic insults. We studied the expression of Ref-1 in the retina and brain of developing rats. Ref-1 immunoreactivity increased progressively within the nucleus of differentiating retinal cells, whereas it decreased in the developing hippocampal formation. During both natural and experimentally-induced cell death, Ref-1 disappeared from the nucleus of apoptotic cells. Degradation of Ref-1 in axotomized ganglion cells preceded the morphological characteristics of apoptosis. The sensitivity to apoptosis triggered by either thapsigargin or okadaic acid was the highest in photoreceptors, that contain the least Ref-1 among differentiated retinal cells. In both these differentiated cell types, inhibition of protein synthesis prevented the loss of Ref-1 and rescued the neurons. The data suggest that Ref-1 is an anti-apoptotic protein associated with cell differentiation in the retina.

Selective sensitivity of early postmitotic retinal cells to apoptosis induced by inhibition of protein synthesis

In previous work we showed that apoptosis in retinal tissue from developing rats can be induced by inhibition of protein synthesis (Rehen et al. 1996, Development, 122, 1439-1448). Here we show that recent postmitotic cells are the cells sensitive to apoptosis triggered by blockade of protein synthesis. To label all proliferating cells in the retina, a series of injections of the nucleotide analogue, bromo-deoxy-uridine (BrdU, 60 mg/kg b.w.), was given in rat pups. Then, explants of the retina were incubated in vitro with the inhibitor of protein synthesis anisomycin (1.0-3.2 microg/mL) for 1 day to induce apoptosis. Detection of apoptotic bodies under differential interference contrast microscopy was combined with immunocytochemistry for BrdU, proliferating cell nuclear antigen (PCNA) or for various markers of retinal cell differentiation. Despite the large number of BrdU- and PCNA-labelled cells in the tissue, the vast majority of the cells that underwent apoptosis were postmitotic cells which have left the mitotic cycle 3-4 days before. However, these cells were not labelled with antibodies to calretinin, calbindin, rhodopsin or to a Muller glial cell marker, suggesting that these are early postmitotic neurons. We suggest that during migration and initial differentiation, the apoptotic machinery is blocked by suppressor proteins, thus allowing recent postmitotic cells to find their final positions and differentiate while protected from apoptosis.

Gamma irradiation leads to two waves of apoptosis in distinct cell populations of the retina of newborn rats

Gamma radiation induces apoptosis in the proliferative zone (neuroblastic layer) of the developing rat retina. We asked whether sensitivity to apoptosis might be related to distinct phases of the cell cycle. Explants of newborn rat retina or newborn pups were gamma-irradiated and apoptosis was detected by chromatin condensation, DNA fragmentation in situ and DNA electrophoresis. After 6 hours, early appearing apoptotic bodies were located mainly towards the outer tier of the neuroblastic layer. In contrast, after 24 hours, late-appearing apoptotic cells were located towards the inner margin of the neuroblastic layer, a region associated with the S phase of the cell cycle. Labeling of a cohort of cells with the nucleotide analog bromo-deoxyuridine (BrdU) at the time of irradiation, showed that these cells die in the late wave of apoptosis. BrdU given 3 hours before fixation labeled a large number of late apoptotic cells, but no early apoptotic cells. After labeling of all cycling cells with BrdU, 40% of the early apoptotic profiles were unlabeled, and thus post-mitotic. The same schedules of cell death were identified after gamma irradiation in vivo. The results show that irradiation leads to two waves of apoptosis in distinct cell populations. An early wave comprises both post-mitotic cells and proliferating cells out of the S phase. The late wave comprises cells in S phase, which pass through this phase again to die. The antioxidant pyrrolidinedithiocarbamate prevented the early but not the late wave of apoptosis following irradiation, and blocked lipid peroxidation at 6 hours after the insult, suggesting that the two waves of apoptosis are indeed mediated by distinct mechanisms.

BDNF and NT-4 differentially modulate neurite outgrowth in developing retinal ganglion cells

We show here that neurite outgrowth of ganglion cells (RGCs) was selectively enhanced following treatment with BDNF or NT-4 in short-term cultures of dissociated cells derived from the neuroretina of postnatal rats. NT-4 was more effective than BDNF. The effect of NT-3 was variable, whereas NGF and CNTF had no effects upon neurite elongation. The neuritogenic responses of RGCs to both BDNF and NT-4 were prevented by competition with soluble TrkB receptor, and abolished by K252a, a selective inhibitor of the tyrosine kinase activity of Trks. These results indicate that the differentiating effects of BDNF and NT-4 are mediated by TrkB receptors, naturally expressed by RGCs. Developing RGCs treated with these TrkB ligands displayed distinct, albeit partially overlapping, patterns of neurite morphology. BDNF supported predominantly polarized outgrowth, whereas NT-4 induced the appearance of intensely branched symmetrical arbors. The lack of RGCs showing combined morphologies (e.g., highly arborized unipolar cells) suggests distinct mechanisms underlying either elongation or branching, and implicates distinct responses of RGC subsets. We conclude that neurite growth in vitro is extensively promoted by neurotrophins in developing RGCs. Moreover, highly homologous neurotrophins such as BDNF and NT-4, presumably activating via TrkB receptors, selectively control the differentiation of distinct ganglion cell neuritic morphologies.

Evidence for an antiapoptotic role of dopamine in developing retinal tissue

Inhibition of protein synthesis leads to apoptosis in the undifferentiated neuroblastic layer of the retina of newborn rats. We have shown previously that an increase in the intracellular concentration of cyclic AMP prevented apoptosis induced in the retinal neuroblastic layer by inhibition of protein synthesis. In this study, we tested the effects of dopamine on retinal apoptosis and related these effects to the intracellular concentration of cyclic AMP. Both dopamine (100 microM) and the D1-like agonists SKF-38393, 6-chloro-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (6-Cl-PB), and (+/-)-2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene (100 microM) blocked apoptosis induced in the neuroblastic layer by the protein synthesis inhibitor anisomycin. The antiapoptotic effects of the D1-like agonists were not reversed by the D1-like antagonist SCH-23390 (5-100 microM). Both dopamine and D1-like agonists induced a five- to sevenfold increase in the intracellular concentration of cyclic AMP in the retina of newborn rats. The concentration of cyclic AMP induced by the D1-like agonists in the presence of 100 microM SCH-23390 was still at least two- to threefold as high as control values, showing that the activation of adenylyl cyclase by D1-like agonists was reversed only partially by the specific antagonist. The isoquinolinesulfonamide H-89 (20 microM), an inhibitor of cyclic AMP-dependent protein kinase, partially prevented the antiapoptotic effect of 6-Cl-PB. The data show that an early effect of dopamine in the developing retina is the control of programmed cell death. The antiapoptotic effect of dopamine is mediated, at least in part, through an atypical D1-like receptor coupled to stimulation of adenylyl cyclase, followed by activation of cyclic AMP-dependent protein kinase.