Identification of D1-dopamine receptor in chicken embryo retina with [125I]SCH 23982

Pharmacology of signaling induced by dopamine D(1)-like receptor activation

Dopamine D(1)-like receptors consisting of D(1) and D(5) subtypes are intimately implicated in dopaminergic regulation of fundamental neurophysiologic processes such as mood, motivation, cognitive function, and motor activity. Upon stimulation, D(1)-like receptors initiate signal transduction cascades that are mediated through adenylyl cyclase or phosphoinositide metabolism, with subsequent enhancement of multiple downstream kinase cascades. The latter actions propagate and further amplify the receptor signals, thus predisposing D(1)-like receptors to multifaceted interactions with various other mediators and receptor systems. The adenylyl cyclase response to dopamine or selective D(1)-like receptor agonists is reliably associated with the D(1) subtype, while emerging evidence indicates that the phosphoinositide responses in native brain tissues may be preferentially mediated through stimulation of the D(5) receptor. Besides classic coupling of each receptor subtype to specific G proteins, additional biophysical models are advanced in attempts to account for differential subcellular distribution, heteromolecular oligomerization, and activity-dependent selectivity of the receptors. It is expected that significant advances in understanding of dopamine neurobiology will emerge from current and anticipated studies directed at uncovering the molecular mechanisms of D(5) coupling to phosphoinositide signaling, the structural features that might enhance pharmacological selectivity for D(5) versus D(1) subtypes, the mechanism by which dopamine may modulate phosphoinositide synthesis, the contributions of the various responsive signal mediators to D(1) or D(5) interactions with D(2)-like receptors, and the spectrum of dopaminergic functions that may be attributed to each receptor subtype and signaling pathway.

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.

Ontogeny of [3H]-SCH23390 and [3H]-YM09151-2 binding sites in human fetal forebrain

In order to determine the gestational age at which binding sites for the dopamine "D1-like" and "D2-like" receptor antagonists, [3H]-SCH23390 and [3H]-YM09151-2, respectively, can be reliably detected in the human and to identify any discrete anatomic distribution of these binding sites, fetal forebrain tissue sections from mid-first (n = 4) and mid-second (n = 4) trimester gestations were used for receptor autoradiography. Specific binding for both ligands was detectable at the earliest fetal age examined (gestational week 6). Age-related increases in maximum saturation binding were demonstrated for both ligands using tissue sections from basal forebrain. The Bmax for both [3H]-SCH23390 and [3H]-YM09151-2 binding increased ten-fold comparing gestational week 6 and gestational week 18 values. In the cortex at gestational day 120, [3H]-YM09151-2 specific binding could be seen at the gray-white matter boundary, which was more prominent by gestational day 140. In contrast, [3H]-SCH23390 specific binding to the cortex at gestational day 120 did not appear to differentiate specific areas and did not increase between gestational days 120 and 140. These preliminary observations in human fetal brain provide evidence that dopamine "D2-like" binding sites can be localized in a discrete cortical area in the course of normal human brain development. Characterizing these binding sites and the population of cells that demonstrates these binding sites may be relevant to neurodevelopmental hypotheses of psychiatric disorders.

Dopamine stimulates K+ efflux in the chick retina via D1 receptors independently of adenylyl cyclase activation

Dopamine (DA) stimulated K+ efflux (assessed as 86Rb+ efflux) in retinal suspensions of posthatched chicken. This effect was dose dependent (EC50 = 22 microM), was mimicked by the D1-selective agonist SKF-38393, and reversed by the D1-selective antagonist SCH-23390, indicating an involvement of D1 receptors. Analogues of cyclic AMP (cAMP) did not mimic the DA action. Moreover, DA failed to affect cAMP levels, suggesting that adenylyl cyclase (AC) was not involved. In contrast, forskolin (FSK) stimulated both K+ efflux and cAMP accumulation in the retina (EC50 of 10 microM for both effects). The FSK-elicited K+ efflux was not mimicked by 1,9-dideoxy-FSK (an analogue of FSK that does not activate AC), suggesting that FSK stimulated K+ efflux through the activation of AC. Both DA and FSK inhibited Na+,K(+)-ATPase activity in the retina. However, the DA-elicited K+ efflux was independent of this inhibition, whereas the FSK effect on K+ efflux was largely due to the inhibitory action of the diterpene of the ion pump. A possible role of protein kinase C (PKC) in the DA action was explored. The PKC activator 4 beta-phorbol 12-myristate 13-acetate (4 beta-PMA) potently (EC50 = 4 nM) stimulated K+ efflux. This action was not mimicked by the inactive isomer 4 alpha-PMA. When added together, DA and 4 beta-PMA behaved in an additive manner, suggesting separate mechanisms of action for these two drugs. Moreover, DA failed to stimulate retinal phosphoinositide hydrolysis, a well-known pathway leading to PKC activation.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Analysis of adenosine immunoreactivity, uptake, and release in purified cultures of developing chick embryo retinal neurons and photoreceptors

We have investigated the presence of endogenous adenosine and of mechanisms for adenosine uptake and release in chick embryo retinal neurons and photoreceptors grown in purified cultures in the absence of glial cells. Simultaneous autoradiographic and immunocytochemical analysis showed that endogenous adenosine and the uptake mechanism for this nucleoside colocalize in practically all the photoreceptors, but only in approximately 20% of the neurons. Approximately 25% of the neurons showed either immunocytochemical labeling or autoradiographic labeling, while greater than 50% of the neurons were unlabeled with both techniques. [3H]Adenosine uptake was saturable and could be inhibited by nitrobenzylthioinosine and dipyridamole and by pretreatment of the [3H]adenosine with adenosine deaminase. Although these observations indicate that the uptake is specific for adenosine, only 35% of accumulated radioactivity was associated with adenosine, with the remaining 65% representing inosine, hypoxanthine, and nucleotides plus uric acid. Adenosine as well as several of its metabolites were released by the cells under basal as well as K(+)-stimulated conditions. Potassium-enhanced release was blocked by 10 mM CoCl2 or in Ca2(+)-free, Mg2(+)-rich solutions. The results indicate that retinal cells that synthesize, store, and release adenosine differentiate early during embryogenesis and are therefore consistent with a hypothetical role for adenosine in retinal development.

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.

Retinal dopamine D1 and D2 receptors: characterization by binding or pharmacological studies and physiological functions

1. In the retinal inner nuclear layer of the majority of species, a dopaminergic neuronal network has been visualized in either amacrine cells or the so-called interplexiform cells. 2. Binding studies of retinal dopamine receptors have revealed the existence of both D1- as well D2-subtypes. The D1-subtype was characterized by labeled SCH 23390 (Kd ranging from 0.175 to 1.6 nM and Bmax from 16 to 482 fmol/mg protein) and the D2-subtype by labelled spiroperidol (Kd ranging from 0.087 to 1.35 nM and Bmax from 12 to 1500 fmol/mg protein) and more selectively by iodosulpiride (Kd 0.6 nM and Bmax 82 fmol/mg protein) or methylspiperone (Kd 0.14 nM and Bmax 223 fmol/mg protein). 3. Retinal dopamine receptors have been also shown to be positively coupled with adenylate cyclase activity in most species, arguing for the existence of D1-subtype, whereas in some others (lower vertebrates and rats), a negative coupling (D2-subtype) has been also detected in peculiar pharmacological conditions implying various combinations of dopamine or a D2-agonist with a D1-antagonist or a D2-antagonist in the absence or presence of forskolin. 4. A subpopulation of autoreceptors of D2-subtype (probably not coupled to adenylate cyclase) also seems to be involved in the modulation of retinal dopamine synthesis and/or release. 5. Light/darkness conditions can affect the sensitivity of retinal dopamine D1 and/or D2-receptors, as studied in binding or pharmacological experiments (cAMP levels, dopamine synthesis, metabolism and release). 6. Visual function(s) of retinal dopamine receptors were connected with the regulation of electrical activity and communication (through gap junctions) between horizontal cells mediated by D1 and D2 receptor stimulation. Movements of photoreceptor cells and migration of melanin granules in retinal pigment epithelial cells as well as synthesis of melatonin in photoreceptors were on the other hand mediated by the stimulation of D2-receptors. 7. Other physiological functions of dopamine D1-receptors respectively in rabbit and in embryonic avian retina would imply the modulation of acetylcholine release and the inhibition of neuronal growth cones.

Expression of functional D2 dopamine receptors following differentiation of Y-79 human retinoblastoma cells

Y-79 human retinoblastoma cells grown in serum-free medium in monolayer culture have previously been shown to undergo differentiation in response to dibutyryl cyclic AMP (Bt2cAMP). We report here that Y-79 cells treated in this manner also express very high levels of functional D2 dopamine receptors. In control Y-79 cells, cultured in suspension, D2 dopamine receptors, quantified via saturation analysis with the D2 antagonist [3H]methylspiperone, are expressed at a level of approximately 3 fmol/10(6) cells (approximately 1,800 receptor sites/cell). Differentiation is initiated by attachment of the cells to the culture dish with poly-D-lysine and fibronectin and continued culture in serum-free medium. After 8 days in serum-free culture, differentiation is further induced with continuous Bt2cAMP treatment. Using this differentiation protocol, D2 receptor levels increase up to a maximum of 30 fmol/10(6) cells (18,000 receptors/cell) on day 20, the limit of culture viability. Cultures of 15-17 days (7-9 days of Bt2cAMP treatment) expressing receptor levels of 15-20 fmol/10(6) cells are used for pharmacological and functional characterization of D2 dopamine receptors. The pharmacology of competition for [3H]methylspiperone binding to differentiated Y-79 (dY-79) cell membranes by a series of dopaminergic antagonists verifies the D2 receptor nature of this site, exhibiting appropriate affinities and the following rank order of potency: YM-09151-2 approximately spiperone greater than domperidone approximately (+)-butaclamol approximately fluphenazine greater than chlorpromazine greater than (-)-sulpiride greater than (+)-sulpiride greater than promethazine greater than (+)-SCH 23390 much greater than (-)-butaclamol.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of A1 adenosine receptors in the chick embryo retina

Adenosine inhibits cyclic AMP synthesis induced by dopamine in embryonic but not in post-hatched chick retinas. N6-Cyclohexyladenosine (CHA), which preferentially activates A1 receptors as well as 2-chloroadenosine, inhibits cyclic AMP accumulation induced by dopamine in retinas from 10-day-old embryos (E10) with IC50's of 0.1 and 0.5 microM, respectively, but this effect is not detectable after hatching. In order to verify if this developmental change reflects variations in the number or affinity of A1 adenosine receptors, their development during chick retina ontogeny was studied. Binding studies using 3(H)CHA revealed the presence of A1 receptors at all stages of development examined, including the post-hatched retina. The number of binding sites increased between E10 and E17, and then decreased in post-hatched animals. In the latter, 3(H)CHA binding was to a single site with a Bmax of 128.6 +/- 13.4 fmol/mg protein and a Kd of 2.1 + 0.2 nM. Various ligands showed similar hierarchies of affinity for the A1 receptor in embryonic and post-hatched retinas, namely, CHA greater than R-N6-phenylisopropyladenosine (1-PIA) greater than 5'-N-ethylcarboxamideadenosine (NECA) greater than isobuthylemethyl-xanthine (IBMX). Given that CHA inhibited forskolin-induced cyclic AMP production and Gpp(NH)p inhibited 3(H)CHA binding in both embryonic and post-hatched retinas, it appears that receptor coupling to adenylate cyclase is present since early embryonic stages. The results suggest that the A1 receptors may have different functions in the embryonic as compared to the mature chick retina.

Brain receptor autoradiography with [3H]-YM 09151-2: a ligand for labeling dopamine D-2 receptors

Using the technique of in vitro receptor autoradiography to slide-mounted tissue sections, we studied the suitability of [3H]-YM-09151-2 as a ligand for labeling D-2 receptors in adult F344 rat brains. Specific [3H]-YM-09151-2 binding accounted for 70-80% of the total bound ligand and reached equilibrium after a 60-90 minute incubation. Scatchard analysis revealed a Kd of 626 pM. The apparent Bmax was 23.2 fmol/tissue section. Autoradiographs demonstrated high grain densities in the striatum and olfactory tubercle. Diffuse specific binding was also observed in the cortex.