Differential expression of D(1A) and D(1B) dopamine receptor mRNAs in the developing avian retina

In the chick retina, the D1 dopaminergic system differentiates very early, as shown by receptor-mediated increases in intracellular cyclic AMP concentration and the presence of [(3)H]SCH23390-specific binding sites. Here, we characterized, by RT-PCR, the expression of defined D1 receptor subtypes D(1A), D(1B), and D(1D) during the development of the chick retina. Total RNA was extracted from retinas of 6-day-old embryos (E6) to 1-day-old hatched chickens and reverse-transcribed. The resulting cDNA was amplified using D(1A)-, D(1B)-, or D(1D)-specific primers, and the PCR-amplified products were analyzed by electrophoresis. The fragment corresponding to D(1A) receptor was detected in developing retina as early as E7, whereas the fragment corresponding to D(1B) was observed starting around E10. No PCR product corresponding to D(1D) was observed in the retina, although it was detected in chick brain. As synaptogenesis in chick retina begins after E11 and [(3)H]SCH 23390 D1 binding sites increase after this stage, the present results show that expression of D(1B) receptor increases during synaptogenesis, whereas D(1A) is the receptor subtype associated with the D1-like actions of dopamine early in retina development.

Altered regulation of the D(1) dopamine receptor in mutant Chinese hamster ovary cells deficient in cyclic AMP-dependent protein kinase activity

To investigate the role of the cAMP-dependent protein kinase (PKA) in the desensitization and down-regulation of the D(1) dopamine receptor, we stably expressed the rat cDNA for this receptor in mutant Chinese hamster ovary (CHO) cell lines deficient in PKA activity. The 10260 mutant CHO cell line has been characterized as expressing less than 10% of type I and type II PKA activities relative to the parental 10001 CHO cell line. The 10248 mutant CHO line lacks type II PKA activity and expresses a defective type I PKA. The transfected parental and mutant cell lines were found to express approximately 1 pmol/mg D(1) receptor binding activity (B(max)) as determined using [(3)H]SCH-23390 binding assays. All three cell lines demonstrated similar levels of dopamine-stimulated adenylyl cyclase activity. Pretreatment of all three CHO cells with dopamine resulted in desensitization of the adenylyl cyclase response, although the maximum desensitization was attenuated by 20 and 40% in the 10260 and 10248 cell lines, respectively. Dopamine also promoted, in a time- and dose-dependent fashion, a >90% down-regulation of D(1) receptors in the parental cell line but only a 50 and 30% decrease in the 10260 and 10248 cells, respectively. Similarly, treatment of the cells with the membrane-permeable cAMP analog 8-(4-chlorophenylthio)-cAMP induced functional desensitization and down-regulation of the D(1) receptor, although it was not as great as that observed with agonist pretreatment. As with the agonist pretreatments, the 8-(4-chlorophenylthio)-induced responses were attenuated in the mutant cells with the 10248 line exhibiting the least desensitization/down-regulation. Our results suggest that PKA significantly contributes to the desensitization and down-regulation of D(1) receptors in CHO cells and that type II PKA may be the more relevant isoform with respect to regulating D(1) receptor function.

Veratridine- and glutamate-induced release of [3H]-GABA from cultured chick retina cells: possible involvement of a GAT-1-like subtype of GABA transporter

Four subtypes of GABA carriers (GAT1-GAT4) that transport GABA in a sodium-dependent manner were identified so far. In this report, the sodium-dependent release of GABA was investigated in cultured chick retinal cells. Opening of voltage-sensitive sodium channels by veratridine or activation of non-NMDA glutamate receptors induced the release of GABA from cultured cells. The release of GABA was calcium-independent, but could be completely prevented by the substitution of sodium chloride by lithium or choline chloride in the extracellular medium, suggesting that GABA release could be triggered by multiple mechanisms that led to the flux of sodium into these cells. Pharmacological experiments revealed that, while GABA uptake was almost completely inhibited by the GAT-1 blockers NNC-711 (50 microM) or nipecotic acid (1 mM), the release of this amino acid was inhibited by NNC-711, but not by nipecotic acid. The incubation with beta-alanine (10 mM), a GAT-2/GAT-3 inhibitor, blocked 50% of GABA uptake but had no effect on the release. Our data suggest that sodium-dependent GABA release from cultured chick retina cells is mediated by a GAT-1 like transporter that shows some, but not all, the pharmacological properties of the GAT-1 carrier.

Inhibition of carbachol-induced inositol phosphate accumulation in the embryonic retina promoted by kainate and veratridine

In the present study, we report that low concentrations of the glutamate ionotropic agonist kainate decreased the turnover of [3H]-phosphoinositides ([3H]-InsPs) induced by muscarinic receptors in the chick embryonic retina. When 100 microM carbachol was used, the estimated IC50 value for kainate was 0.2 microM and the maximal inhibition of approximately 50% was obtained with 1 microM or higher concentrations of the glutamatergic agonist. Our data also show that veratridine, a neurotoxin that increases the permeability of voltage-sensitive sodium channels, had no effect on [3H]-InsPs levels of the embryonic retina. However, 50 microM veratridine, but not 50 mM KCl, inhibited approximately 65% of the retinal response to carbachol. While carbachol increased [3H]-InsPs levels from 241.2 +/- 38.0 to 2044.5 +/- 299.9 cpm/mg protein, retinal response decreased to 861.6 +/- 113.9 cpm/mg protein when tissues were incubated with carbachol plus veratridine. These results suggest that the accumulation of phosphoinositides induced by activation of muscarinic receptors can be inhibited by the influx of Na+ ions triggered by activation of kainate receptors or opening of voltage-sensitive sodium channels in the chick embryonic retina.

Accumulation of 3H-phosphoinositides mediated by muscarinic receptors in the developing chick retina: inhibition of carbachol-induced response by glutamate receptors

In the present work we show the development of carbachol-induced accumulation of 3H-inositol phosphates (3H-InsPs) in the chick embryonic retina and its regulation by glutamate receptors. Although basal levels of 3H-InsPs increased during development, the retinal response to carbachol was high in the early developing stages and decreased after synaptogenesis in the retina. Eserine also stimulated the turnover of phosphoinositides in the embryonic but not in the mature retina. The effect of eserine could be blocked by atropine, suggesting that acetylcholine could be released from developing retina cells and further stimulate the turnover of InsPs in the embryonic tissue. Our data also show that muscarinic stimulation of turnover of 3H-InsPs could be blocked by stimulation of glutamatergic ionotropic receptors. Moreover, the effect of glutamate agonists did not seem to be mediated by the release of other neurotransmitters such as GABA, glycine, adenosine, or dopamine from the tissue because these neurotransmitters did not interfere with the retinal response to carbachol. These results suggest that muscarinic activation of phosphoinositide turnover occurs mainly in the embryonic retina and that activation of glutamate receptors can inhibit directly the muscarinic stimulation of hydrolysis of 3H-InsPs in this tissue.

Transient cyclic AMP accumulation mediated by dopamine D1 receptors in the chick embryo optic lobe

[3H]SCH 23390 bound with high affinity (Kd = 0.6 nM) and in a saturable manner (Bmax = 130 fmol/mg protein) to membrane preparations of the chick optic lobe. Pharmacological experiments, using several dopaminergic ligands, revealed that [3H]SCH 23390 bound stereospecifically to dopaminergic receptors of the D1 type in this tissue. Other experiments revealed that dopamine was able to induce cyclic AMP accumulation in the optic lobe (ED50 = 3 microM), an effect that was blocked by fluphenazine, a potent D1 antagonist (IC50 = 1.8 microM). The developmental profile of tissue dopamine-dependent cyclic AMP accumulation, however, was quite different from the differentiation pattern of [3H]SCH 23390 specific binding sites. While [3H]SCH 23390 binding sites increased 4-fold after the 12th embryonic day (E12), dopamine-dependent cyclic AMP accumulation was maximal in earlier stages, decreasing progressively after E10. In tissues from embryos at E16 or older, no difference was observed between basal and dopamine-stimulated levels of cyclic AMP. These data suggest that D1 receptors are coupled to adenylate cyclase in a limited period of the development of the optic lobe and that D1 receptors not coupled to the enzyme can be a common feature in the CNS.

Evidence for two independent mechanisms of GABA release induced by veratridine and glutamate in monolayer cultures of chick embryo retinal cells

GABA is a major inhibitory neurotransmitter in the central nervous system, including the retina. In the present paper we present evidence for the existence of two independent mechanisms for GABA release in cultured retina cells. Eight-day-old chick embryo retinas were dissociated and plated in 35-mm plastic dishes and cultured for 3 or 7 days at 37 degrees C. An increase of 3 to 5-fold in GABA release was observed in cultures of 3 or 7 days in vitro preloaded with 0.5 microCi [3H]GABA and stimulated with glutamate (100 microM) or veratridine (100 microM). Tetrodotoxin (1 microM) blocked the release induced by veratridine but not by glutamate. In contrast, the non-N-methyl-D-aspartate (NMDA) glutamate antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 100 microM) was able to inhibit GABA release promoted by glutamate but not by veratridine. These results indicate that depolarization of retinal cells by opening of voltage-dependent sodium channels or activation of non-NMDA glutamate receptors can trigger intracellular events that lead to calcium-independent GABA release.

Glutamic acid decarboxylase of embryonic avian retina cells in culture: regulation by gamma-aminobutyric acid (GABA)

1. Retina-cell aggregate cultures expressed glutamate decarboxylase activity (L-glutamate 1-carboxylase; EC 4.1.1.15) as a function of culture differentiation. 2. Glutamic acid decarboxylase (GAD) activity was low in the initial phases of culture and increased eight-fold until culture day 7, remaining high up to day 13 (last stage studied). 3. The addition of GABA to the culture medium 24 h after cell seeding almost totally prevented the expression of GAD activity. 4. In association with decreased enzyme activity, aggregates exposed to GABA did not display immunoreactivity for GAD, suggesting that GAD molecules were either lost from GABAergic neurons or significantly altered with GABA treatment. 5. Control, untreated aggregates showed intense GAD immunoreactivity in neurons. Positive cell bodies were characterized by a thin rim of labeled cytoplasm with thickest labeling at the emergence of the main neurite. 6. Heavily labeled patches were also observed throughout the aggregates, possibly reflecting regions enriched in neurites. 7. The GABA-mediated reduction of GAD immunoreactivity was a reversible phenomenon and could be prevented by picrotoxin.

Developmental immunoreactivity for GABA and GAD in the avian retina: possible alternative pathway for GABA synthesis

Although the distribution of GABAergic neurons in chick retina has been previously described by several investigators, the early appearance of these neurons has not been reported. In the present study immunohistochemical methods were used to localize GABAergic neurons with antisera to both GABA and its synthesizing enzyme, glutamate decarboxylase (GAD), in embryonic chick retina at several stages of development and beyond hatching. GABA-positive neuroblast-like cells were clearly detected in retinas as early as embryonic day 6. In contrast, GAD-containing cells were not observed in retinas until embryonic day 10. These findings indicated that immunocytochemically detectable amounts of GAD were not present in young GABAergic cells. Our data on the developmental appearance of GABA and GAD immunoreactivities are consistent with previous biochemical data for the development of GABA concentration and GAD activity in the chick retina. Together, these data suggest that retina cells from the early stages of development may synthesize GABA from an alternative pathway in which the most likely precursor is putrescine.

D1 dopamine receptors in neurite regions of embryonic and differentiated retina are highly coupled to adenylate cyclase in the embryonic but not in the mature tissue

[3H]SCH 23390 binds stereospecifically and with high affinity to D1 dopaminergic receptors in the developing chick retina. Autoradiographic experiments revealed that in retinas from 3-day-old chicken and embryos with 12, 14 and 16 days of development, specific labeling of [3H]SCH 23390 was mainly observed over the plexiform layers of the tissue, showing that dopaminergic D1 receptors are localized in retina cell neurites since the initial stages of neurite formation. The total number of [3H]SCH 23390 binding sites increased 5-fold during the differentiation of the retina, while the dopamine-dependent cyclic adenosine monophosphate (AMP) accumulation was significantly decreased. Consequently, the ratio between dopamine-dependent cyclic AMP accumulation and [3H]SCH 23390 binding sites decreased 10-fold as retina differentiated, indicating that a significant portion of D1 receptors in retinas from adult chicken are not effectively coupled to adenylate cyclase molecules.

Development of adenosine-dependent cyclic AMP accumulation in the avian optic tectum

The present work shows the existence of adenosine-dependent cyclic adenosine monophosphate (AMP) accumulation in the chick optic tectum. When tecta from 18-day-old embryos were incubated with the phosphodiesterase inhibitor IBMX and RO 20-1724, the cyclic AMP level increased from 39.2 to 73.3 and 285.5 pmol/mg protein, respectively. The high level obtained with RO 20-1724 could be inhibited by increasing concentrations of IBMX or by adenosine deaminase, but not by dipyridamole. 2-Chloroadenosine promoted a dose-dependent cyclic AMP accumulation in tecta incubated with RO 20-1724 and adenosine deaminase. This effect was blocked by IBMX and varied substantially during the development of the tissue. The degree of stimulation increased after day 11 of incubation, attaining maximal levels on day 14. The effect of 2-chloroadenosine remained constant until day 18, a period when both the protein content and the basal cyclic AMP levels are increasing in the developing tectum. The cyclic AMP increase elicited by 2-chloroadenosine was greatly reduced in tecta from 20-day-old embryos and 2-day-old chicks. The putative transmitters glutamate and glycine and the neurotransmitter analogs isoproterenol and carbachol had no stimulatory effect on the cyclic AMP accumulation of tecta from 10- and 17-day-old embryos.

Differential ontogenesis of D1 and D2 dopaminergic receptors in the chick embryo retina

The differentiation of D1 and D2 dopamine receptors was investigated during the ontogenesis of the chick embryo retina. Our results reveal an interesting complexity in dopaminergic differentiation, with one major receptor system developing before synapses and another one developing after. The dopamine-dependent increase of chick retina cAMP level differentiates early during retina ontogeny. By the embryonic day 10-11 10(-4) M dopamine and ADTN elicit a 13-fold increase in cAMP content of the retina. However, [3H]spiperone (D2 ligand) binds very little to crude membrane preparation of retinas from embryos in the same developmental stage (12-13 fmol/mg protein). High specific binding of [3H]spiperone is only detected after the embryonic day 17-18, attaining 80 to 100 fmol of specific spiperone binding sites in the retinas from post-hatched animals. Apomorphine also promotes the accumulation of cAMP of retinas from early embryonic stages. However, it is only 20-30% as effective as ADTN or dopamine. In addition, while the dopamine responsiveness of the tissue decreases sharply during its ontogeny, the apomorphine effect remains practically constant throughout this period. Both dopamine and apomorphine are equally effective in eliciting cAMP accumulation of retinas from post-hatched animals. Moreover, apomorphine is a potent inhibitor of dopamine-induced cAMP level of the embryonic tissue. The results presented here indicate that D1 and D2 receptors differentiate independently from each other, and that apomorphine elevates retina cAMP levels via a subclass of D1 receptors that does not desensitize significantly during retina development.

Regulation of dopamine- and adenosine-dependent adenylate cyclase systems of chicken embryo retina cells in culture

We have obtained evidence that receptor-stimulated adenylate cyclase activity [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] is regulated physiologically in both embryonic and mature neurons. In a series of experiments using cultured retina cells from chicken embryos, we found that dopamine-sensitive adenylate cyclase activity spontaneously desensitized as cultures differentiated. The cellular response to dopamine reached a maximum after 5 days in culture and then decreased to 40% during the next 5 days. This spontaneous desensitization appeared to be caused by functional dopaminergic transmission because it could be blocked by the dopamine antagonist haloperidol. The ability of added dopamine at 100 microM to cause near-complete desensitization is consistent with this conclusion. Pharmacologically induced desensitization required 31 hr for maximal effect and was half-maximal at 1-10 microM dopamine. Analogous desensitization of the adenosine-dependent adenylate cyclase system also was noted. When dopamine was removed from the medium of chronically treated cultures, cells resensitized to subsequent stimulation at a very slow rate. Resensitization likely depended on replacement of dopamine receptors because chronic dopamine treatment caused the disappearance of binding sites for the ligand [3H]spiroperidol. In a second series of experiments, using hatched animals, we found that similar regulation of dopamine receptor binding sites and activity could be elicited by manipulation of environmental light, a treatment thought to influence dopaminergic transmission. Retinas from animals in constant light had less specific [3H]spiroperidol binding (35 fmol/mg of protein) than did retinas from animals in constant darkness (66 fmol/mg of protein) and made less cAMP in response to added dopamine. Our results indicate that regulation of the dopamine receptor system begins early in development and continues to function in mature synapses.