Multiple fluorescent ligands for dopamine receptors. II. Visualization in neural tissues

65 years of research on dopamine's role in classical fear conditioning and extinction: A systematic review

Dopamine, a catecholamine neurotransmitter, has historically been associated with the encoding of reward, whereas its role in aversion has received less attention. Here, we systematically gathered the vast evidence of the role of dopamine in the simplest forms of aversive learning: classical fear conditioning and extinction. In the past, crude methods were used to augment or inhibit dopamine to study its relationship with fear conditioning and extinction. More advanced techniques such as conditional genetic, chemogenic and optogenetic approaches now provide causal evidence for dopamine's role in these learning processes. Dopamine neurons encode conditioned stimuli during fear conditioning and extinction and convey the signal via activation of D1-4 receptor sites particularly in the amygdala, prefrontal cortex and striatum. The coordinated activation of dopamine receptors allows for the continuous formation, consolidation, retrieval and updating of fear and extinction memory in a dynamic and reciprocal manner. Based on the reviewed literature, we conclude that dopamine is crucial for the encoding of classical fear conditioning and extinction and contributes in a way that is comparable to its role in encoding reward.

Mechanisms of action of antipsychotic drugs of different classes, refractoriness to therapeutic effects of classical neuroleptics, and individual variation in sensitivity to their actions: Part I

Many issues remain unresolved about antipsychotic drugs. Their therapeutic potency scales with affinity for dopamine D2 receptors, but there are indications that they act indirectly, with dopamine D1 receptors (and others) as possible ultimate targets. Classical neuroleptic drugs disinhibit striatal cholinergic interneurones and increase acetyl choline release. Their effects may then depend on stimulation of muscarinic receptors on principle striatal neurones (M4 receptors, with reduction of cAMP formation, for therapeutic effects; M1 receptors for motor side effects). Many psychotic patients do not benefit from neuroleptic drugs, or develop resistance to them during prolonged treatment, but respond well to clozapine. For patients who do respond, there is a wide (>ten-fold) range in optimal doses. Refractoriness or low sensitivity to antipsychotic effects (and other pathologies) could then arise from low density of cholinergic interneurones. Clozapine probably owes its special actions to direct stimulation of M4 receptors, a mechanism available when indirect action is lost.

Inhibition of adult rat retinal ganglion cells by D1-type dopamine receptor activation

The spike output of neural pathways can be regulated by modulating output neuron excitability and/or their synaptic inputs. Dopaminergic interneurons synapse onto cells that route signals to mammalian retinal ganglion cells, but it is unknown whether dopamine can activate receptors in these ganglion cells and, if it does, how this affects their excitability. Here, we show D(1a) receptor-like immunoreactivity in ganglion cells identified in adult rats by retrogradely transported dextran, and that dopamine, D(1)-type receptor agonists, and cAMP analogs inhibit spiking in ganglion cells dissociated from adult rats. These ligands curtailed repetitive spiking during constant current injections and reduced the number and rate of rise of spikes elicited by fluctuating current injections without significantly altering the timing of the remaining spikes. Consistent with mediation by D(1)-type receptors, SCH-23390 [R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine] reversed the effects of dopamine on spikes. Contrary to a recent report, spike inhibition by dopamine was not precluded by blocking I(h). Consistent with the reduced rate of spike rise, dopamine reduced voltage-gated Na(+) current (I(Na)) amplitude, and tetrodotoxin, at doses that reduced I(Na) as moderately as dopamine, also inhibited spiking. These results provide the first direct evidence that D(1)-type dopamine receptor activation can alter mammalian retinal ganglion cell excitability and demonstrate that dopamine can modulate spikes in these cells by a mechanism different from the presynaptic and postsynaptic means proposed by previous studies. To our knowledge, our results also provide the first evidence that dopamine receptor activation can reduce excitability without altering the temporal precision of spike firing.

The involvement of dopamine in nociception: the role of D(1) and D(2) receptors in the dorsolateral striatum

Determination of the neuroanatomical and neurochemical factors that contribute to nociception is an essential element in the study and treatment of pain. Several lines of evidence have implicated nuclei and neurotransmitters within the basal ganglia in nociception. For example, previous studies have shown that dopamine receptors in the striatum are involved in acute nociception, however, it remains to be determined if dopamine receptors in the dorsolateral striatum are involved in persistent nociception. The purpose of the present study was therefore to determine whether activation or antagonism of dopamine receptors in the dorsolateral striatum influences the nociceptive responses of rats in the formalin test, a model of persistent pain. It was found that micro-injection of the non-selective dopamine antagonist haloperidol into the dorsolateral striatum increases formalin-induced nociception whereas injection of the non-selective dopamine agonist apomorphine reduces formalin-induced nociception. Injection of the D(1) antagonist SCH23390 or the D(1) agonist SKF38393 does not affect formalin-induced nociception. In contrast, injection of the D(2) antagonist eticlopride enhances formalin-induced nociception, whereas injection of the D(2) agonist quinpirole reduces formalin-induced nociception. These results provide additional evidence that dopamine receptors in the striatum are involved in nociception. Furthermore, this study strongly suggests that D(2), but not D(1), dopamine receptors in the dorsolateral striatum are involved in modulation of persistent nociception.

Dopamine promotes differentiation of olfactory neuron in vitro

In two previous in vitro experiments, we have shown that dopamine induced apoptosis or differentiation in an olfactory cell line while it reduced mitosis and triggered cell death in human olfactory biopsy cultures. The aims of the present study were to locate precisely D2 dopamine receptors within the olfactory epithelium and to monitor the effect of dopamine on olfactory neuronal differentiation in explant cultures. We show here that D2 dopamine receptors are expressed in supporting cells, neurons and basal cells in the olfactory epithelium. In vitro, dopamine was found to (1) trigger neuronal differentiation and maturation in a dose-dependent manner via D2 dopamine receptors, (2) be active only when not oxidised, (3) act directly on epithelial cells and not through other reactive cells in the underlying lamina propria. Altogether these data indicate that, in parallel to its action in odour processing, dopamine plays a growth factor-like role in the permanent neurogenesis observed in the olfactory epithelium.

Bromocriptine protects dopaminergic neurons from levodopa-induced toxicity by stimulating D(2)receptors

Neuroprotective properties of bromocriptine, a D(2) receptor agonist, were investigated using the in vitro neurotoxicity of levodopa for dopaminergic neurons from rat embryonic ventral mesencephalon. Levodopa, when added to the culture medium, showed toxicity which was specific for dopaminergic neurons. Bromocriptine was found to protect dopaminergic neurons from levodopa toxicity. Another D(2) agonist, 2-(N-phenethyl-N-propyl-amino-5-hydroxytetralin, showed similar protective effects. The neuroprotective effect of bromocriptine was inhibited by supplementation of the culture medium with sulpiride, a D(2) antagonist, or by D(2) receptor knockdown with an antisense oligonucleotide. Dopaminergic neurons treated with levodopa showed an increase in free radicals. These data suggest that neuroprotective properties of bromocriptine seen in this cellular model of neurotoxicity are dependent on dopamine D(2) autoreceptor binding and that levodopa toxicity may be related to increased free radical generation in dopaminergic neurons.

D1- and D2-like dopamine receptors are co-localized on the presynaptic varicosities of striatal and nucleus accumbens neurons in vitro

The neuromodulatory actions of dopamine in the striatum and nucleus accumbens are likely to depend on the distribution of dopamine receptors on individual postsynaptic cells. To address this, we have visualized D1- and D2-like receptors on living medium-spiny GABAergic neurons in cultures from the striatum and nucleus accumbens using receptor antagonist fluoroprobes. We labeled D1-like receptors with rhodamine-SCH23390, D2-like receptors with rhodamine-N-(p-aminophenethyl)spiperone and synaptic sites with K+-stimulated uptake of the activity-dependent endocytic tracer FM-143. The fluoroprobes were applied in sequence to assess co-localization. We found that D1- or D2-like receptors were present on about two-thirds of the cells, and co-localized on 22+/-3% (mean +/- S.E.M.) of striatal and 38+/-6% of nucleus accumbens cells. On either D1 or D2 labeled cells, postsynaptic labeling continuously outlined the cell body membrane and extended to proximal dendrites, but not axons. About two-thirds of synaptic varicosities showed D1 or D2 labeling. D1- and D2-like receptors were co-localized on 21+/-4% of striatal and 27+/-3% of nucleus accumbens varicosities. Presynaptic labeling was typically more intense than postsynaptic labeling. The distribution of presynaptic dopamine receptors contrasted with that of postsynaptic GABA(A) receptors, which were clustered in longer patches on neighboring postsynaptic membranes. The extensive presence of D1- and D2-like receptors on presynaptic varicosities of medium-spiny neurons suggests that the receptors are likely to play an important and interacting role in the presynaptic modulation of inhibitory synaptic transmission in the striatum and nucleus accumbens. The significant overlap in labeling suggests that D1-D2 interactions, which occur at the level of individual postsynaptic cells, the circuit level and the systems level, may also be mediated at the presynaptic level. Finally, the ability to visualize dopamine, as well as GABA(A), receptors on the individual synapses of living neurons now makes possible physiological studies of individual mesolimbic system synapses with known receptor expression.

Neurobiology of retinal dopamine in relation to degenerative states of the tissue

Neurobiology of retinal dopamine is reviewed and discussed in relation to degenerative states of the tissue. The Introduction deals with the basic physiological actions of dopamine on the different neurons in vertebrate retinae with an emphasis upon mammals. The intimate relationship between the dopamine and melatonin systems is also covered. Recent advances in the molecular biology of dopamine receptors is reviewed in some detail. As degenerative states of the retina, three examples are highlighted: Parkinson's disease; ageing; and retinal dystrophy (retinitis pigmentosa). As visual functions controlled, at least in part, by dopamine, absolute sensitivity, spatial contrast sensitivity, temporal (including flicker) sensitivity and colour vision are reviewed. Possible cellular and synaptic bases of the visual dysfunctions observed during retinal degenerations are discussed in relation to dopaminergic control. It is concluded that impairment of the dopamine system during retinal degenerations could give rise to many of the visual abnormalities observed. In particular, the involvement of dopamine in controlling the coupling of horizontal and amacrine cell lateral systems appears to be central to the visual defects seen.

Coexpression of striatal dopamine receptor subtypes and excitatory amino acid subunits

The striatal cellular coexpression patterns for the D(1A) and D2 dopamine (DA) receptor subtypes and the ionotropic excitatory amino acid (EAA) subunits of the N-methyl-D-aspartate (NMDA-R1) and the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) (GluR1 and GluR2/3) receptor subunits were examined morphologically. Their coincidence was assessed by visualization of mRNA transcripts, localization of encoded receptor proteins, and binding analysis using concurrently paired methods of fluorescence detection. The findings indicated that 1) mRNA transcripts for both receptor systems were detected in the medium-sized neuron population, and the distribution of receptor message closely reflected protein and binding patterns, with the exception of the GluR1 subunit; 2) both DA receptor mRNA transcripts were coexpressed with each ionotropic EAA receptor subunit examined and with each other, and NMDA and AMPA receptor subunits also showed coincident expression; 3) D(1A) DA receptor protein was detected in neurons which coexpressed EAA subunit proteins; and 4) GluR2/3 and NMDA-R1 subunit proteins were coexpressed in medium-sized neurons which also demonstrated D2 DA receptor binding sites. These findings suggest morphological receptor "promiscuity" since the coexpression patterns between DA and EAA receptors were found in all permutations. The results provide a spatial framework for physiological findings describing functional interactions between the two DA receptor types and between specific DA and EAA receptors in the striatum.

Antennal lobe neurons of the honey bee, Apis mellifera, express a D2-like dopamine receptor in vitro

We have used the D2-specific dopamine receptor ligand spiperone [N-(p-aminophenethyl) spiperone; NAPS] coupled to the fluorophore 7-nitrobenz-2-oxa-1,3-diazole-4-yl (NBD) to visualize dopamine receptors expressed in vitro by neurons of the primary antennosensory centers (antennal lobes) of the brain of the honey bee, Apis mellifera. Changes in the percentage of antennal lobe neurons exhibiting spiperone binding sites over time in culture and at different stages of metamorphic adult development have been investigated. Neurons obtained from animals at all stages of development exhibited spiperone binding sites, but only after 2 days or more in vitro. The percentage of antennal lobe neurons in vitro expressing spiperone binding sites increased significantly with the development of the antennal lobe neuropil. Fluorescently labelled spiperone (120 nM) could be displaced effectively by 1 mM dopamine but not by the same concentration of tyramine, octopamine, or serotonin. In addition, the D2 antagonist spiperone and the D2/D1 antagonist fluphenazine were more effective at displacing the fluorescent ligand than the D1-specific antagonist SCH23390. Our results indicate that Apis antennal lobe neurons in culture express a dopamine receptor and that this receptor is more likely to be D2-like than D1-like in nature. The receptor is expressed early in the metamorphic adult development of the antennal lobe neuropil of the brain.

Immunohistochemical localization of dopamine D1 receptors in rat retina

We have localized the dopamine D1 receptor in rat retina using a subtype-specific monoclonal antibody. Immunolabelling can be detected in the inner and outer plexiform layers and in a number of cells in the inner nuclear layer. In the inner plexiform layer, labelled processes form four distinct horizontal bands and a series of patches. In order further to characterize the labelling pattern of the D1 receptor antibody, double-labelling experiments were performed with antibodies against population-specific neuronal markers in the retina. Antibodies against tyrosine hydroxylase, choline acetyltransferase, calretinin, calbindin, the glutamate transporter GLT-1, protein kinase C, recoverin and parvalbumin were co-applied with the D1 receptor antibody. With these cell markers we demonstrate that horizontal cells, at least three types of cone bipolar cells and a small number of amacrine cells are immunolabelled for the D1 receptor. In the inner plexiform layer, processes labelled by the D1 receptor antibody are co-stratified with processes labelled by the GLT-1 antibody. D1 receptor-labelled processes are not co-localized with the processes of amacrine cells and ganglion cells labelled by antibodies against tyrosine hydroxylase, choline acetyltransferase or calretinin. Our results indicate that dopamine D1 receptors are localized predominantly to horizontal cells and cone bipolar cells. Furthermore, the spatial disparity between dopaminergic processes and the site of the majority of D1 receptors supports the idea that in the retina dopamine acts as a neuromodulator that diffuses through extracellular space. The localization of D1 receptors to a number of identified cell types enables future physiological work to be directed towards specific synaptic circuits within the retina.

Pharmacological characterization and autoradiographic localization of dopamine receptors in the rat adrenal medulla

The pharmacological profile and the anatomical localization of dopamine D1-like and D2-like receptors were studied in sections of rat adrenal medulla, with radioligand binding and autoradiographic techniques, respectively. [3H]([R]-(+)-chloro-2,3,4,5-tetrahydro-5-phenyl-1 H-3benzazepin-al hemimaleate) (SCH 23390) was used as a ligand for dopamine D1-like receptors and [3H]spiperone was used as a ligand for dopamine D2-like receptors. Radioligand binding and light microscope autoradiography did not show specific [3H]SCH 23390 binding in sections of rat adrenal medulla. This suggests that rat adrenal medulla does not express dopamine D1-like receptors. [3H]Spiperone was specifically bound to sections of rat adrenal medulla. The binding was time-, temperature- and concentration-dependent, with a dissociation constant (Kd) of 1.05 nM and a maximum density of binding sites (Bmax) of 100.2 +/- 3.8 fmol/mg tissue. The pharmacological profile of [3H]spiperone binding to rat adrenal medulla was similar to that displayed by neostriatum, which is known to express dopamine D2 receptors. Light microscope autoradiography showed the accumulation of specifically bound [3H]spiperone as silver grains within sections of adrenal medulla. Silver grains were found primarily over the cellular membrane of chromaffin cells. The above data indicate that chromaffin cells of the rat adrenal medulla express dopamine receptors belonging to the dopamine D2 receptor subtype. These receptors are probably involved in the modulation of catecholamine release from chromaffin cells, as documented by functional studies.

Localization of D1 dopamine receptors on live cultured striatal neurons by quantitative fluorescence microscopy

Single neurons in culture express a heterogeneity of neurotransmitter receptor subtypes. The study of the effects of neurotransmitters on neuronal function is complicated by this heterogeneity. It would therefore be useful to be able to identify live neurons that express the receptors of interest and then use these neurons for functional studies. We have used quantitative fluorescence microscopy to identify single live striatal neurons that express D1 dopamine receptors. The binding of the fluorescent D1 dopamine receptor antagonist bodipy-SCH 23390 was measured in 2-3-week-old primary striatal cultures derived from fetal rats (embryonic day 18). Binding of bodipy-SCH 23390 to live neurons was displaced by (+)-butaclamol, dopamine or SCH 23390, indicating that it specifically labelled D1 dopamine receptors. However, the fraction of bodipy-SCH 23390 binding that was specific varied substantially among individual neurons indicating heterogeneity of D1 dopamine receptor expression. Interestingly, bodipy-SCH 23390 also specifically labelled discrete spots of receptors on the neuronal processes. This technique should prove useful in the study of the effects of dopaminergic drugs on neuronal function in primary culture.

A retinal dark-light switch: a review of the evidence

We propose that there exists within the avian, and perhaps more generally in the vertebrate retina, a two-state nonadapting flip-flop circuit, based on reciprocal inhibitory interactions between the photoreceptors, releasing melatonin, the dopaminergic amacrine cells, and amacrine cells which contain enkephalin-, neurotensin-, and somatostatin-like immunoreactivity (the ENSLI amacrine cells). This circuit consists of two loops, one based on the photoreceptors and dopaminergic amacrine cells, and the other on the dopaminergic and ENSLI amacrine cells. In the dark, the photoreceptors and ENSLI amacrine cells are active, with the dopaminergic amacrine cells inactive. In the light, the dopaminergic amacrine cells are active, with the photoreceptors and ENSLI amacrine cells inactive. The transition from dark to light state occurs over a narrow (< 1 log unit) range of low light intensities, and we postulate that this transition is driven by a graded, adapting pathway from photoreceptors, releasing glutamate, to ON-bipolar cells to dopaminergic amacrine cells. The properties of this pathway suggest that, once released from the reciprocal inhibitory controls of the dark state, dopamine release will show graded, adapting characteristics. Thus, we postulate that retinal function will be divided into two phases: a dopamine-independent phase at low light intensities, and a dopamine-dependent phase at higher light intensities. Dopamine-dependent functions may show two-state properties, or two-state properties on which are superimposed graded, adapting characteristics. Functions dependent upon melatonin, the enkephalins, neurotensin, and somatostatin may tend to show simpler two-state properties. We propose that the dark-light switch may have a role in a range of light-adaptive phenomena, in signalling night-day transitions to the suprachiasmatic nucleus and the pineal, and in the control of eye growth during development.

Arylacetamide-derived fluorescent probes: synthesis, biological evaluation, and direct fluorescent labeling of kappa opioid receptors in mouse microglial cells

Fluorescein isothiocyanate isomer I (FITC-I) conjugates of 2-(3,4-dichlorophenyl)-N-methyl-N-[1-(3- or 4-aminophenyl)-2-(1-pyrrolidinyl)ethyl]acetamide (10 and 14) were prepared either without or with an intervening mono-, di-, or tetraglycyl linker. The 3-substituted fluorescent probes (2-5) were found to retain potent agonist activity in smooth muscle preparations as well as high kappa receptor affinity and selectivity in receptor binding assays. The 4-substituted series (6-9) were substantially less potent than the corresponding 3-substituted compounds. Flow cytometric analysis demonstrated high levels of direct kappa-specific staining of mouse microglial cells by the fluorescent probe 5 containing a tetraglycyl linker, as indicated by a 41% decrease in percent cells positively labeled and a 61% decrease in mean fluorescence intensity in the presence of the kappa-selective antagonist, norbinaltorphimine (norBNI). In similar studies, the probe 2 without a linker exhibited only nonspecific binding. This is the first report of direct, selective staining of kappa opioid receptors by a fluorescent nonpeptide opioid ligand. The results of the present study illustrate the importance of introducing hydrophilic linkers to reduce nonspecific binding of fluorescent probes for opioid receptors.

D3 and D2 dopamine receptors: visualization of cellular expression patterns in motor and limbic structures

The distribution of the D3 and D2 dopamine receptor subtypes in forebrain regions of the basal ganglia and mesocorticolimbic system was determined. This was assessed through combined fluorescent visualization of subtype selective anti-peptide antibodies for these cloned receptors and detection of their ligand recognition sites using the D2 subfamily antagonist,N-(p-aminophenethyl) spiperone (NAPS fluoroprobe). The double-labeling technique enabled direct comparison of the cloned receptor proteins and NAPS fluoroprobe binding in vitro. The application of these two methods together produced results comparable to single-labeling paradigms. Functional D3 receptors, defined as the coincident fluorescence of the D3 receptor antisera and fluoroprobe binding, were detected in the core region of the nucleus accumbens and exhibited a laminated expression pattern in the frontal cortex. D3 receptor protein was expressed robustly in neurons of the dorsolateral striatum, but showed an intense neuropil reaction in the globus pallidus. Functional D2 receptors, defined as the coincident fluorescence of the D2 receptor antisera and fluoroprobe binding, were detected in the frontal cortex and the medial shell of the nucleus accumbens. Thus, heterogeneities occurred in the cellular expression of functional D3 and D2 receptors in forebrain dopaminoceptive areas. D3 appears more related to basal ganglia and structures involved with motoric behavior, while D2 was associated with regions associated with cognitive/affective functions.

D2 dopamine receptor protein location: Golgi impregnation-gold toned and ultrastructural analysis of the rat neostriatum

The neostriatal distribution of D2 dopamine receptor protein has been assessed using subtype-selective polyclonal antibodies generated against three unique polypeptide sequences of the receptor. The experimental tissues were processed by peroxidase based immunohistochemical procedures for routine light microscopy, Golgi impregnation-gold toned morphological characterization, and correlative light/electron microscopy. The results demonstrated a regional gradient of D2-like dopamine receptor expression in the neostriatum, where lateral portions in the nucleus exhibited more reactive cell bodies than medial portions. D2-like expression was detected in the three populations of neostriatal neurons, i.e., the medium-sized spiny projection neurons, and the medium- and large-sized aspiny interneuron types. Morphometric measurements of labeled neurons verified that medium and large diameter neurons expressed the D2-like receptor subtype. D2-like immunoreactivity was distributed throughout the cytoplasm in dendritic processes, and in presynaptic terminal boutons. Immunoreactivity for the receptor protein was also detected in small, thinly myelinated axons, suggesting the possibilities of anterograde transport of the receptor from cell bodies in the substantia nigra to their neostriatal terminal fields, as well as from local axon collaterals of neostriatal projections neurons. These findings provide evidence of widespread distribution of the D2-like receptor protein in neostriatal neurons, and showed that the presynaptic D2 receptors contain analogous epitopes to the postsynaptic receptor subtype.

Dopamine receptor distribution in the rat CNS: elucidation using anti-peptide antisera directed against D1A and D3 subtypes

Anti-peptide antibodies were generated against amino acid sequences of intracellular and extracellular portions of the native proteins for the cloned rat D1A and D3 dopamine receptor subtypes in order to determine the cellular distribution of these specific forms in the brain. These polyclonal antisera exhibited high specific titers, assessed by ELISA and immunofluorescent detection of functional recombinant receptor proteins expressed in stably transfected Chinese hamster ovary (CHO) cells. Central nervous system (CNS) areas of the male rat were examined using standard immunofluorescent methods in fresh frozen tissues. This paradigm detected D1A-like and D3-like dopamine receptor staining primarily in larger-sized neurons throughout layers 3 and 5 of the cortex, in medium-diameter somata of the striatum, and in the densely packed cells of the olfactory tubercle and hippocampal formation. More attenuated immunoreactivity for both dopamine receptor subtypes was noted in the substantia nigra, not associated with perikarya. Differences in cellular staining patterns and intensity were evident between the D1A-like and D3-like dopamine receptor subtypes. Equivalent morphological elements exhibited dopamine receptor expression following incubation using antisera generated against either extracellular or intracellular epitopes of either the D1A or D3 native proteins. Dopamine receptor immunoreactivity could not be detected in the cerebellum at equivalent antisera dilutions used to discriminate cellular staining patterns within the forebrain. Fluorescent-labeled latex microspheres were infused into the substantia nigra terminal fields to retrogradely identify the cell bodies of the striatonigral projection system. This paradigm showed that 80% of striatonigral neurons expressed D1A-like receptors, while 65% demonstrated D3-like dopamine receptor staining. This distribution for the D1A-like and D3-like receptor subtypes suggests that overlap may occur in the expression of the receptors in the striatonigral neuron population. Our previous results localizing cellular D2-like receptor expression patterns in this projection system of the rat neostriatum implies that all three of these dopamine receptor subtypes may be co-expressed in this efferent system.

Dopamine receptor binding on identified striatonigral neurons

Dopamine receptors have been divided into two families, known as D1 and D2, based on their ability to bind distinct ligands, and their use of separate post-synaptic transduction systems. Determining the specific cellular location for these dopamine receptors in the striatum is important to the design of drug treatments for disorders with suspected dopaminergic involvement such as Parkinson's disease. This study examined the binding of D1 and D2 antagonist ligands on identified striatonigral neurons using in vitro fluorescent techniques. The results indicate that striatonigral neurons express both pharmacological subfamilies of dopamine receptor binding sites.

Microanatomy of the dopaminergic system in the rainbow trout retina

We have investigated the morphology of dopaminergic interplexiform cells as well as the distribution of two classes of dopamine receptors in the retina of the rainbow trout. Interplexiform cells were visualized using an antiserum against tyrosine hydroxylase and PAP immunocytochemistry. In whole amounts, these cells have a density of between 91 and 182 cells per mm2 with highest values in the lower temporal quadrant. Their cell bodies lie at the inner margin of the inner nuclear layer with only 12-17 cells per retina displaced to the ganglion cell layer. There are three levels of stratification in the inner plexiform layer, one at the distal and proximal borders respectively, and one in the middle. They arise mostly from a radially oriented, stout primary dendrite. Tangential processes are about 1 micron in diameter and show a number of varicosities. The density of processes is greatest in sublayer 5, but no major difference in the general organization is apparent between the three sublayers. In the outer retina, there are two levels of dense ramification confined to the layer of horizontal cells. Light and electron microscopic analysis shows synaptic input to horizontal cells, but not to photoreceptors. The distribution of D1 receptors was assessed by studying the binding pattern of a specific, fluorescent-labelled antagonist, SCH 23390, in unfixed frozen sections. We found displaceable binding in the inner and outer plexiform layers and in the region of horizontal cell perikarya. We used an anti-peptide antibody directed to an extracellular domain of the rat D2 receptor and a fluorescent secondary antiserum to study the localization of D2 receptors. In addition to marked label in both plexiform layers, the outer, and especially the inner segments of rods and cones show specific immunoreactivity. In addition, there is distinct label at the level of the horizontal cell bodies; in the inner retina, specific fluorescence is found in somata of some amacrine cells. The significance of the connectivity pattern and the distribution of the two receptor types is discussed with respect to the role of dopamine in controlling adaptational processes in the outer retina, such as retinomotor movements and changes in horizontal cell morphology and physiology.

Localization of D2 dopamine receptors in vertebrate retinae with anti-peptide antibodies

Dopamine plays an important role in modulating various aspects of retinal signal processing. The morphology of dopaminergic neurons and its physiological effects are well characterized. Two classes of receptor molecules (D1 and D2) were shown pharmacologically to mediate specific actions, with differences between individual groups of vertebrates. In an attempt to better understand dopaminergic mechanisms at the cellular level, we used antisera against D2 receptors and investigated the localization of the dopamine D2 receptor in the retinae of rat, rabbit, cow, chick, turtle, frog, and two fish species with immunofluorescence techniques. Antisera were raised in rabbits to two oligopeptides predicted from rat D2 receptor cDNA; one specific for the splice-variant insertion in the third cytoplasmic loop and the other directed towards the extracellular amino terminal region shared by both short and long isoforms. Preadsorption with the synthetic peptide resulted in a significant reduction of label, indicating the presence of specific binding in all species except turtle and goldfish. The pattern of labelling produced by the two antisera was essentially identical; however, the staining obtained with antiserum to the extracellular motif was always more intense. Specific staining was present in photoreceptor inner and outer segments, and in the outer and inner plexiform layers of all species. In mammals and chick, strongly fluorescent perikarya were observed in the ganglion cell layer and at the proximal margin of the inner nuclear layer. Label may be present in the pigment epithelium but could not be established beyond doubt. This pattern of labelling is in accordance with previous observations on D2 receptor localization by means of radioactive ligand binding and in situ hybridization techniques. It suggests that retinal dopamine acts as a neuromodulator as well as a transmitter. In the distal retina, it may reach its targets via diffusion over considerable distances, even crossing the outer limiting membrane; in the inner and outer plexiform layers, conventional synaptic transmission seems to coexist with paracrine addressing of more distant targets, and D2 receptors are expressed by both amacrine and ganglion cells.

D2-like dopamine receptors in amphibian retina: localization with fluorescent ligands

Dopamine induces several light adaptive changes in amphibian retina via receptors with D2-like pharmacology, but the identity of the primary target cells has not been determined. Using a fluorescent probe consisting of a selective D2 antagonist, N-(p-aminophenethyl)-spiperone (NAPS), derivatized with the fluorophore Bodipy (NAPS-Bodipy), we identified the distribution of dopamine binding sites in the retina of two amphibians, post-metamorphic Xenopus laevis and larval Ambystoma tigrinum. Specific labeling was defined as staining that was displaced by D2 selective ligands (eticlopride or sulpiride), but insensitive to D1 selective drugs (SCH 23390), adrenergic catecholamines (epinephrine or norepinephrine), or serotoninergic analogues (ketanserin). Both rod and cone cells showed specific dopamine D2-like binding sites arranged in clustered arrays on discrete membrane domains of the inner segment. Labeling of photoreceptor outer segments was continuous and was not displaced by competition with D2 selective ligands; this labeling was considered nonspecific. In addition, in both species, clustered binding of the D2-probe was found on Müller cells and on a subset of inner retinal cells with the morphology of amacrine/interplexiform cells. Our data provide direct evidence for D2 receptors on both rods and cones, and suggest that the receptors may be clustered into patches within a discrete cellular domain, the inner segment.

D2 dopamine receptor distribution in the rodent CNS using anti-peptide antisera

D2 dopamine receptors were identified immunohistochemically in rodent tissues using anti-peptide antisera to distinguish regional and cellular staining patterns. These subtype selective polyclonal antibodies were directed against both extracellular and intracellular regions of the native protein and showed that the D2 dopamine receptors are widely distributed within the nervous system. The highest expression of D2-like dopamine receptor immunoreactivity was visualized in the forebrain and components of the basal ganglia, supportive of previous investigations of the D2 dopamine receptor distribution using in vitro autoradiographic ligand binding or in situ hybridization for its messenger RNA. The anti-peptide antisera could detect the dopamine receptor in both perfusion-fixed and fresh-frozen tissue preparations. The reactive cells and their processes could be distinguished using experimental incubations from 1:8,000 (in immunofluorescence processing) to 1:80,000 (in immunoperoxidase processing) in the most reactive nervous system region, the neostriatum. The antisera are selectively directed against extracellular or intracellular epitopes in both the long and short isoforms of the D2 dopamine receptor, and should prove useful in subsequent studies of the subcellular distribution of this receptor in particular, and the dopamine system in general.

2-[2-[4-[2-[2-[ 1,3-Dihydro- 1,1-bis (4-hydroxyphenyl)-3-oxo-5-isobenzofuranthioureidyl]ethylaminocarbonyl]ethyl]phenyl] ethylamino]-5'-N-ethylcarboxamidoadenosine (FITC-APEC): A Fluorescent Ligand For A2a-Adenosine Receptors

The fluorescein conjugate, FITC-APEC (2-[2-[4-[2-[2-[1,3-dihydro-l,l-bis(4-hydroxyphenyl)-3-oxo-5-isobenzofuranthioureidyl]ethylaminocarbonyl]ethyl]phenyl]ethylamino]-5'-N-ethylcarboxamidoadenosine), is a novel ligand derived from a series of functionalized congeners that act as selective A2a-adenosine receptor agonists. The binding of FITC-APEC to bovine striatal A2a,-adenosine receptors measured by fluorescence techniques was saturable and of a high affinity, with a Bmax, of 2.3 ± 0.3 pmol/mg protein and KD of 57 ± 2 nM. The KD value estimated by fluorescence was consistent with the Ki (11 ± 0.3 nM) obtained by competition studies with [3H]CGS 21680. Additionally, the Bmax, value found by FITC-APEC measurement was in agreement with Bmax, values obtained using radioligand binding. FITC-APEC exhibited rapid and reversible binding to bovine striatum. The potencies of chemically diverse A2a-adenosine receptor ligands estimated by inhibition of FITC-APEC binding were in good agreement with their potencies determined using radioligand binding techniques (r = 0.97, P = 0.0003). FITC-APEC binding was not altered by purine derivatives that do not recognize A2a-adenosine receptors. These findings demonstrate that the novel fluorescent ligand FITC-APEC can be used in the quantitative characterization of ligand binding to A2a-adenosine receptors.

Multiple fluorescent ligands for dopamine receptors. I. Pharmacological characterization and receptor selectivity

We report the synthesis and pharmacological characterization of novel fluorescently labeled ligands with high affinity and specificity for D1 and D2 dopamine receptors. D1-selective antagonist probes have been synthesized using (R,S)-5-(4'-aminophenyl)-8-chloro-2,3,4,5-tetrahydro-3-methyl-[1H]-3- benzazepin-7-ol, the 4'-amino derivative of the high affinity D1-selective antagonist, SCH-23390, while D2-selective antagonist probes were synthesized using the high affinity, D2-selective agonist, N-(p-aminophenethyl)spiperone (NAPS). In addition, we have synthesized fluorescent probes using an amino-derivative of the high affinity, D2-selective agonist, 2-(N-phenethyl-N-propyl)amino-5-hydroxytetralin (PPHT or N-0434). These ligands were coupled to the fluorescent moieties, fluorescein, rhodamine, coumarin, Texas red, Cascade blue, or Bodipy. This resulted in a wide variety of dopaminergic ligands which fluoresce at different wavelengths: Cascade blue and coumarin are blue fluorophores, fluorescein and Bodipy, are yellow-green, and Texas red and rhodamine are red. The interaction of these fluorescent ligands with dopamine and serotonin receptors was evaluated by examining their ability to compete for radioligand binding to D1 and D2 dopamine receptors and 5-HT1A, 5-HT1C and 5-HT2 serotonin receptors. We report here that these novel fluorescent ligands exhibit high affinity and, in general, selectivity for either D1 or D2 dopamine receptors. In addition, we demonstrate that the fluorescent derivatives of PPHT retain the full agonist efficacy exhibited by the parent compound.(ABSTRACT TRUNCATED AT 250 WORDS)