D1 receptors mediate dopamine action in the fetal suprachiasmatic nuclei: studies of mice with targeted deletion of the D1 dopamine receptor gene

Studies in rodents suggest the presence of a dopaminergic system that influences the function of a biological clock in the hypothalamic suprachiasmatic nuclei (SCN). To provide insights into mechanisms of dopamine action in the SCN, we studied transgenic mice that had either one allele (+?-) or both alleles (-/-) of the D1 dopamine receptor gene deleted, along with normal (+/+) littermates. As expected, receptor labelling autoradiography studies using [125I]SCH 23982 showed a complete absence of D1 dopamine receptor binding sites in the SCN of -/- animals. When pregnant mice from +?- x +?- matings were injected with the D1 receptor agonist SKF 38393, or the dopamine reuptake blocker GBR 12909 at day 19 of gestation, c-fos mRNA expression was observed in the SCN of +/+ fetuses. In contrast, c-fos mRNA induction was not seen in -/- or +?- litter mates. Injection of cocaine into pregnant dams also resulted in robust SCN c-fos mRNA expression in +/+ mice. Increases in SCN c-fos mRNA expression were also seen in +?- and -/- mice suggesting that cocaine action in the SCN involves both D1 receptor-dependent and -independent mechanisms. Collectively, our studies of transgenic mice deficient in D1 receptors support the presence of a functional dopaminergic system in the fetal SCN. We also identify D1 receptors as the prominent transducer of dopamine action in the fetal SCN.     

Partial and full dopamine D1 agonists produce comparable increases in ventral pallidal neuronal activity: contribution of endogenous dopamine

Systemic administration of the partial DA D1 agonist SKF38393 often increases the firing rate of neurons in the VP of rats. This study extended this finding by comparing responses to (+/-)SKF38393 with those produced by two D1 agonists that have greater intrinsic efficacy, (+/-)SKF82958 and (+/-)DHX. The role of endogenous DA in D1 agonist-induced effects also was examined. Extracellular recordings of single VP neurons were obtained in chloral hydrate-anesthetized male rats, to which equimolar doses of SKF38393, SKF82958 or DHX were administered i.v. Each of the agonists increased firing rate in about 45% of the neurons tested. Moreover, each agonist produced the same maximal increase in activity (161% to 178% of spontaneous rate). Acute decreases in synaptic DA, produced by either GBL or combined treatment with reserpine and AMPT, potentiated the maximal increase in activity evoked by SKF38393 or SKF82958. These DA-depleting treatments did not alter the percentage of neurons that displayed this response to D1 agonist challenge. Low doses of the selective D1 antagonists SCH23390 or SCH39166 generally attenuated the agonist-induced changes in firing rate, supporting the conclusion that D1 receptors were activated by SKF38393, SKF82958 and DHX. Thus, these three D1 agonists, which produce different maximal increases in striatal adenylyl cyclase activity, had comparable efficacy to increase VP neuronal activity. A reduction in endogenous DA enhanced the D1 agonist-induced effects, possibly through a reduction in inhibitory influences on VP neurons that are mediated by other DA receptor subtypes.     

Constitutive activity of a chimeric D2/D1 dopamine receptor

Chimeric D1/D2 receptors were constructed to identify structural determinants of drug affinity and efficacy. We previously reported that chimeras that had D1 receptor transmembrane domain VII together with amino-terminal sequence from the D2 receptor were nonfunctional. D2/D1 chimeras were constructed that contained D2 receptor sequence at the amino- and carboxyl-terminal ends and D1 receptor sequence in the intervening region. Chimeric receptors with D2 sequence from transmembrane domain 7 to the carboxyl terminus together with D2 receptor sequence from the amino terminus through transmembrane helix 4 (D2[1-4,7]) and 5 (D2[1-5,7]) bound [3H]spiperone with high affinity, consistent with the hypothesis that D2 receptor transmembrane domain I or II is incompatible with D1 receptor transmembrane domain VII. D2[1-4,7] and D2[1-5,7] had affinities similar to D1 and D2 receptors for most nonselective dopamine antagonists and had affinities for most of the selective antagonists that were intermediate between those of the parent receptors. D2[1-4,7] and D2[1-5,7] mediated dopamine receptor agonist-induced stimulation and inhibition, respectively, of cAMP accumulation. The more efficient coupling of D2[1-5,7] to inhibition of cAMP accumulation, compared with the coupling of D2[5-7] and D2[3-7], supports the view that multiple D2 receptor cytoplasmic domains acting in concert are necessary for receptor activation of Gi. In contrast, D2[1-4,7], which contains only one cytoplasmic loop (the third) from the D1 receptor, is capable of activating Gs. D2[1-4,7] exhibited several characteristics of a constitutively active receptor, including enhanced basal (unliganded) stimulation of cAMP accumulation, high affinity for agonists even in the presence of GTP, and blunted agonist-stimulated cAMP accumulation. A number of dopamine receptor antagonists were inverse agonists at D2[1-4,7], inhibiting basal cAMP accumulation. Some of these drugs were also inverse agonists at the D1 receptor. Interestingly, several antagonists also potentiated forskolin-stimulated cAMP accumulation via D2[1-5,7] and via the D2 receptor, which could reflect inverse agonist inhibition of native constitutive activity of this receptor.     

Photoaffinity labeling of D1 and D5 dopamine receptors

Although human D1 and D5 dopamine receptors are encoded by distinct genes and share only 50% sequence homology at the amino acid level, their pharmacological properties are identical. Using a selective D1 receptor photoaffinity radioligand, (+/-)-7-[125I]iodo-8-hydroxy-3-methyl-1-(4-azidophenyl)-2,3,4,5-tetrahyd ro-1H-3-benzazepine ([125I]MAB), we have further probed the molecular properties of these receptors in transfected GH4C1 rat pituitary cells. Under reversible, non-covalent binding conditions, [125I]MAB bound to both the D1 and the D5 receptors with identical affinities, dopaminergic selectivity and stereospecificity. Upon photoactivation of the bound [125I]MAB, the label was incorporated into a approximately 64,000 mol. wt protein corresponding to the D1 dopamine receptor. However, there was no specific photoincorporation of the ligand observed in D5 receptors. The lack of [125I]MAB photolabeling of D5 receptors was independent of the cell line chosen, since similar results were obtained using other transfected cells. The data suggest that although both D1 and D5 receptors share structurally similar binding sites, the protein domains around the sites are different. Thus, although there are currently no specific compounds which bind preferentially to D1 or D5 receptors, these receptors can be distinguished from one another by the inability of [125I]MAB to photolabel D5, but not D1, receptors. Such selective targeting of a specific receptor may be useful in understanding the functional importance and/or interaction between closely related members of the same receptor family when co-expressed in the same cell.     

Visualization of D1 dopamine receptors on living nucleus accumbens neurons and their colocalization with D2 receptors

To examine the substrate for dopamine (DA) synaptic action in the nucleus accumbens (nAcc), we visualized the cellular and subcellular distribution of DA receptors on postnatal nAcc neurons in culture using fluoroprobe derivatives of DA receptor ligands. Previously, we have shown that rhodamine-N-(p-aminophenethyl)-spiperone (NAPS) (10 nM), a derivative of the D2 antagonist spiperone, labels D2-like receptors on living nAcc neurons. We now show that rhodamine-Sch-23390 (30 nM), a derivative of the D1 antagonist, labels D1-like receptors. Putative specific membrane labeling reached a plateau after about 20 min. Labeling was stereospecific, as it was unaffected by competition with (-)-butaclamol, but blocked with (+)-butaclamol. We found that 52 +/- 7% of nAcc medium-sized neurons showed D1 labeling, which extended onto the dendrites. Labeling was also seen on presynaptic terminals, often abutting D1-positive and D1-negative cell bodies, consistent with a presynaptic modulatory role for D1 receptors. Larger neurons, which may be GABAergic or cholinergic interneurons, were also labeled. By sequential labeling first with rhodamine-Sch-23390 and then rhodamine-NAPS, we found that 38 +/- 6% of medium-sized neurons express both D1- and D2-like receptors, indicating that D1-D2 interactions may occur at the level of single postsynaptic neurons.     

Behavioral effects of selective and nonselective dopamine agonists on young rats after irreversible antagonism of D1 and/or D2 receptors

In general, preweanling and adult rats respond similarly when challenged with competitive dopamine (DA) agonists or antagonists. In contrast, results using a noncompetitive antagonist suggest that the D1 and D2 receptor systems of preweanling and adult rats differ in some critical way. To further assess this phenomenon, the behavioral effects of irreversible receptor blockade were assessed across 8 days in NPA (a nonselective DA agonist), quinpirole (a D2 agonist), or SKF 38393 (a D1 agonist) treated 17-day-old rat pups. The irreversible antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) did not block the locomotor activity and rearing of NPA- or quinpirole-treated rat pups, nor did EEDQ reduce SKF 38393-induced grooming. Moreover, pretreatment with EEDQ appeared to potentiate the normal increases in locomotor activity and rearing produced by NPA, but only when D2 receptors were not protected by a previous injection of sulpiride (a D2 antagonist). Taken together, these results are consistent with the presence of large reserves of D1 and D2 receptors in the preweanling rat pup.     

D1 dopamine receptor binding and mRNA levels are not altered after neonatal 6-hydroxydopamine treatment: evidence against dopamine-mediated induction of D1 dopamine receptors during postnatal development

The role of dopaminergic innervation on the postnatal developmental expression of D1 dopamine receptors was investigated. Bilateral destruction of dopamine-containing neurons was achieved by treating rats intracisternally with 6-hydroxydopamine (6-OHDA) on postnatal day 3, and rats were killed on day 21. To ensure effective reduction of D1 receptor activation by residual dopamine, a group of 6-OHDA-lesioned rats was given twice daily injections of the D1 receptor antagonist SCH-23390, from day 4 to 20. D1 dopamine receptor binding was assessed in the caudate-putamen, nucleus accumbens, and olfactory tubercle by quantitative autoradiographic analysis of [3H]SCH-23390 binding. In addition, the relative amount of D1A receptor mRNA was assessed by in situ hybridization of a 35S-labeled riboprobe. In the developing rats, neither the amount of [3H]SCH-23390 binding nor the amount of D1A receptor mRNA was altered by 6-OHDA lesioning followed by chronic treatment with SCH-23390. Thus, bilateral destruction of dopamine-containing neurons and treatment with SCH-23390 in neonatal rats did not interfere with the developmental expression of D1 receptors or alter the levels of mRNA that code for this receptor protein. Treatment of intact rats with SCH-23390 from postnatal day 4 to 20 also did not alter [3H]SCH-23390 binding or levels of D1 receptor mRNA. However, adult rats treated chronically with SCH-23390 exhibited increased [3H]SCH-23390 binding but did not show a significant change in D1 receptor mRNA levels.     

N-methyl-D-aspartate receptor blockade attenuates D1 dopamine receptor modulation of neuronal activity in rat substantia nigra

Recent technical advances in CT have renewed interest in the development of CT angiography (CTA). CT angiography is a minimally invasive method of visualising the vascular system and is becoming an alternative to conventional arteriography in some situations. Spiral technology allows a volume of data to be obtained on a single breath-hold with no respiratory misregistration. Fast machines with second or subsecond acquisition times mean the images are obtained while there are high circulating levels of contrast medium giving peak vascular opacification from a peripheral intravenous injection. Accurate timing will ensure either the arterial or venous phase is imaged. Multiple overlapping axial images can be obtained from the data set with no increase in radiation dose to the patient and from these scans computer generated multiplanar and 3D images are obtained which can be viewed from numerous angles. CT angiography can be performed more quickly, less invasively and at reduced cost compared to conventional angiography.     

D2 dopamine receptor antisense oligodeoxynucleotide inhibits the synthesis of a functional pool of D2 dopamine receptors

In vivo administration of an antisense oligonucleotide targeted toward the D2 dopamine (DA) receptor mRNA (D2 AS) markedly inhibited D2 receptor-mediated behaviors but produced only a relatively small reduction in the levels of D2 DA receptors in mouse striatum. This apparent dissociation between DA receptor-mediated behaviors and the levels of D2 DA receptors was addressed by inhibiting the total number of D2 DA receptors by intraperitoneal administration of the selective, irreversibly acting D2 DA receptor antagonist fluphenazine-N-mustard (FNM) and then determining the effects of D2 AS, administered intracerebroventricularly, on the rate of synthesis of D2 DA receptors and on the recovery of D2 receptor-mediated behaviors. FNM inactivated approximately 90% of D2 DA receptors within 4 hr of treatment, after which the receptors returned to normal levels by approximately 8 days. D2 AS treatment significantly inhibited the rate of recovery of D2 DA receptors in striatum of FNM-treated mice. FNM treatment also produced a number of behavioral alterations, including catalepsy, and the inhibition of stereotypic behavior induced by the D2/D3 DA receptor agonist quinpirole. Both of these behaviors returned to normal within 8 days after FNM treatment. D2 AS treatment delayed the restoration of these FNM-induced behaviors. Thus, it reduced the rate of disappearance of the cataleptic behavior induced by FNM and significantly delayed the restoration of the stereotypic behavior induced by quinpirole. The changes induced by D2 AS on D2 receptor-mediated behaviors were reversed on cessation of D2 AS treatment. A random oligomer given in the same amount and for the same length of time as that of the D2 AS had no significant effects on either D2 DA receptor synthesis or DA receptor-mediated behaviors. These studies demonstrate that in vivo administration of D2 AS decreased the rate of recovery of D2 DA receptors and inhibited the recovery of D2 DA receptor-mediated behaviors after irreversible receptor inactivation and suggest that D2 AS treatment inhibits the synthesis of a functional pool of D2 DA receptors.     

Functional implications of multiple dopamine receptor subtypes: the D1/D3 receptor coexistence

The D3 dopamine receptor, a D2-like receptor, is selectively expressed in the ventral striatum, particularly in the shell of nucleus accumbens and islands of Calleja, where it is found in medium sized substance P neurons. The latter co-express the D1 receptor whose interaction with the D3 receptor was studied by treating rats with selective agonists and antagonists. In agreement with the opposite cAMP response, they mediate in cultured neuroblastoma cells, the D1 and D3 receptors exerted opposite influences on c-fos expression in islands of Calleja. However, in agreement with the synergistic influence of cAMP on D3 receptor-mediated mitogenesis on the same cultured cells, D1 and D3 receptor stimulation in vivo synergistically enhanced preprotachykinin mRNA in the shell of accumbens. This indicates that the two receptor subtypes may affect neurons in either synergy or opposition according to the cell or signal generated. Levodopa-induced behavioral sensitization in hemiparkinsonian rats is another example of D1/D3 receptor interaction. Hence repeated levodopa administration induces the ectopic appearance of the D3 receptor in substance P/dynorphin, striatonigral neurons of the dorsal striatum. This induction is secondary to D1 receptor stimulation in neurons of the denervated side and fully accounts for the sensitization, i.e. the increased behavioral responsiveness to levodopa. During brain development, a similar process could operate to control the late appearance of the D3 receptor in D1-receptor bearing neurons of the ventral striatum at a time at which they start to be innervated by dopamine neurons. Finally, taking into account a variety of genetic, developmental, neuroimaging and pharmacological data, we postulate that imbalances between the levels of D1 and D3 receptors in the same neurons could be responsible for schizophrenic disorders.     

Opposing roles for dopamine D1 and D2 receptors in the regulation of hypothalamic tuberoinfundibular dopamine neurons

The purpose of the present study was to characterize pharmacologically dopamine D1 receptor-mediated inhibition of tuberoinfundibular dopamine neurons in males rats, and to determine if inhibitory dopamine D1 receptors oppose stimulatory dopamine D2 receptors and account for the inability of mixed dopamine receptor agonists to alter the activity of these neurons. Tuberoinfundibular dopamine neuronal activity was estimated by measuring the concentrations of the dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence, the region of the hypothalamus containing terminals of these neurons. Administration of the dopamine D1 receptor agonist (+/-)-1 phenyl-2,3,4,5-tetrahydro-(1 H)-3-benzazepine-7,8-diol (SKF38393) decreased median eminence DOPAC and increased plasma prolactin concentrations, whereas administration of the dopamine D1 receptor antagonist ((-)-trans,6,7,7a,8,9,13b-hexahydro-3-chloro-2-hydroxy-N-methyl-5H -benzo[d]naphtho-[2,1 b]azepine (SCH39166) increased median eminence DOPAC concentrations but had not effect on plasma prolactin. The inhibitory effect of SKF38393 on median eminence DOPAC concentrations was blocked by SCH39166. These results demonstrate that acute activation of dopamine D1 receptors inhibits the activity of tuberoinfundibular dopamine neurons and thereby increases prolactin secretion, and that under basal conditions dopamine D1 receptor-mediated inhibition of tuberoinfundibular dopamine neurons is tonically active. Administration of the dopamine D2 receptor agonist (5aR-trans)-5,5a,6,7,8,9,9a,10-octahydro-6-propyl-pyridol[2, 3-g]quinazolin-2-amine (quinelorane) increased median eminence DOPAC concentrations, and SKF38393 caused a dose-dependent reversal of this effect. Administration of the mixed dopamine D1/D2 receptor agonist R(-)-10,11-dihydroxy-apomorphine (apomorphine) had no effect per se, but blocked quinelorane-induced increases in DOPAC concentrations in the median eminence. These results reveal that concurrent activation of dopamine D1 and D2 receptors nullifies the actions of each of these receptors on tuberoinfundibular dopamine neurons, which likely accounts for the lack of an acute effect of mixed dopamine D1/D2 receptor agonists on these hypothalamic dopamine neurons.     

The role of D1 and D2 receptors in the heightened locomotion induced by direct and indirect dopamine agonists in rats with hippocampal damage: an animal analogue of schizophrenia

Rats with limbic system damage display increases in responsivity to sensory stimulation and changes in the sensitivity to amphetamine, suggesting that their condition may parallel that of human schizophrenia. This experiment examined locomotion and stereotyped behavior in mature, male rats that had received aspirative lesions of the hippocampus, control lesions of the overlying parietal cortex, or were unoperated controls. Locomotion, measured as photocell beam breaks, was recorded during 2- or 3-h test sessions. Behavioral stereotypy was simultaneously rated. Hippocampal lesioned rats exhibited a selective enhancement in locomotion following D-amphetamine (0.0-5.6 mg/kg) when compared to animals in the control groups. Similar results were observed following injections of apomorphine (0.0-0.25 mg/kg), a mixed D1 and D2 agonist. In order to determine if D1 or D2 receptors were involved in this increased locomotion, the D1 agonist SKF 38393 (0.0-15 mg/kg) and the D2 agonist quinpirole (0.0-0.5 mg/kg) were tested alone and in combination. Hippocampal-ablated rats showed significantly increased locomotion only in response to quinpirole, suggesting that these lesion-induced increases were largely mediated by D2 receptors. When both drugs were administered together, SKF 38393 further enhanced the locomotor stimulating effects of quinpirole in hippocampal lesioned rats, indicating a synergistic interaction between D1 and D2 receptors in the modulation of locomotion. These findings provide further evidence of hippocampal modulation of locomotion and suggest that dopaminergic mechanisms in the nucleus accumbens, probably involving changes in receptor sensitivity, are involved. The results are discussed in relation to the functional roles of the nucleus accumbens and in terms of their implications for mental diseases including schizophrenia.     

Interaction of 5-HT1B/D ligands with recombinant h 5-HT1A receptors: intrinsic activity and modulation by G-protein activation state

Many 5-HT1B/D receptor ligands have affinity for 5-HT1A receptors. In the present study, the intrinsic activity of a series of 5-HT1B/D ligands was investigated at human 5-HT1A (h 5-HT1A) receptors by measuring G-protein activation in recombinant C6-glial and HeLa membranes, using agonist-stimulated [35S]GTPgammaS binding. In these two membrane preparations, the density of h 5-HT1A receptors (i.e., 246 to 320 fmol mg(-1) protein) and of their G-proteins, and the receptor: G-protein density ratio (0.08 to 0.18) appeared to be similar. It was found that: (i) the maximal [35S]GTPgammaS binding responses induced by the 5-HT1B/D receptor ligands in the HeLa preparation at 30 microM GDP were comparable to that of the native agonist 5-HT; (ii) as compared to 5-HT (1.00), similar potencies but lower maximal responses were observed in the C6-glial preparation at 0.3 microM GDP for zolmitriptan (0.89), dihydroergotamine (0.81), rizatriptan (0.71), CP122638 (0.69), naratriptan (0.60) and sumatriptan (0.53); and that (iii) maximal [35S]GTPgammaS binding responses induced by 5-HT1B/D ligands in the C6-glial preparation were either unaffected or significantly enhanced by increasing the GDP concentration from 0.3 to 30 microM and higher concentrations. These features differ from those observed with 5-HT1A receptor agonists; the latter display the same rank order of potency and efficacy in both membrane preparations, and increasing the amount of GDP with C6-glial membranes results in an attenuation of both the agonist's maximal effect and the apparent potency of partial agonists. The differential regulation of 5-HT1A and 5-HT1B/D agonist responses by GDP suggests that different G-protein subtypes are involved upon 5-HT1A receptor activation by 5-HT1A and 5-HT1B/D agonists.     

Muscarinic agonists with antipsychotic-like activity: structure-activity relationships of 1,2,5-thiadiazole analogues with functional dopamine antagonist activity

Muscarinic agonists were tested in two models indicative of clinical antipsychotic activity: conditioned avoidance responding (CAR) in rats and inhibition of apomorphine-induced climbing in mice. The standard muscarinic agonists oxotremorine and pilocarpine were both active in these tests but showed little separation between efficacy and cholinergic side effects. Structure-activity relationships of the alkylthio-1,2,5-thiadiazole azacyclic type muscarinic partial agonists are shown, revealing the exo-6-(3-propyl/butylthio-1,2, 5-thiadiazol-4-yl)-1-azabicyclo[3.2.1]octane analogues (4a,b and 9a, b) to be the most potent antipsychotic agents with large separation between efficacy and cholinergic side effects. The lack of enantiomeric selectivity suggests the pharmacophoric elements are in the mirror plane of the compounds. A model explaining the potency differences of closely related compounds is offered. The data suggest that muscarinic agonists act as functional dopamine antagonists and that they could become a novel treatment of psychotic patients.     

Colocalization of dopamine D1 and D2 receptor mRNAs in rat placenta

Dopamine is present in the human placenta. The major function of dopamine is the inhibition of human placental lactogen (hPL) release from human trophoblastic cells. This effect is mediated by cAMP through dopamine D2 receptors. However, studies on the effects of cAMP in the control of hPL release have yielded conflicting results. The purpose of this study is to explore the distribution of dopamine receptors in the rat placenta. Dopamine D1 and D2 receptor mRNAs were colocalized in the rat placenta by in situ hybridization histochemistry using radiolabeled cRNA probes. Dopamine D1 and D2 receptor mRNAs were detected in large cells of the endometrium of the uterus on day 10 of gestation. On days 12-16 of gestation, hybridization signals were localized mainly in the spongiotrophoblast and giant cells of the junctional zone of the placenta. With the development of the placenta, signals were moving from the junctional zone to the labyrinth zone. Pit-1 mRNA was detected in the placental lactotrophs and was also colocalized in neighboring placental sections. Our results clearly showed that dopamine D1 and D2 receptor mRNAs were coexpressed in the placental lactotrophs that express Pit-1 mRNA.     

Agonist-specific coupling of a cloned Drosophila melanogaster D1-like dopamine receptor to multiple second messenger pathways by synthetic agonists

The mechanism of coupling of a cloned Drosophila D1-like dopamine receptor, DopR99B, to multiple second messenger systems when expressed in Xenopus oocytes is described. The receptor is coupled directly to the generation of a rapid, transient intracellular Ca2+ signal, monitored as changes in inward current mediated by the oocyte endogenous Ca2+-activated chloride channel, by a pertussis toxin-insensitive G-protein-coupled pathway. The more prolonged receptor-mediated changes in adenylyl cyclase activity are generated by an independent G-protein-coupled pathway that is pertussis toxin-sensitive but calcium-independent, and Gbetagamma-subunits appear to be involved in the transduction of this response. This is the first evidence for the direct coupling of a cloned D1-like dopamine receptor both to the activation of adenylyl cyclase and to the initiation of an intracellular Ca2+ signal. The pharmacological profile of both second messenger effects is identical for a range of naturally occurring catecholamine ligands (dopamine > norepinephrine > epinephrine) and for the blockade of dopamine responses by a range of synthetic antagonists. However, the pharmacological profiles of the two second messenger responses differ for a range of synthetic agonists. Thus, the receptor exhibits agonist-specific coupling to second messenger systems for synthetic agonists. This feature could provide a useful tool in the genetic analysis of the roles of the multiple second messenger pathways activated by this receptor, given the likely involvement of dopamine in the processes of learning and memory in the insect nervous system.     

Dopamine-induced recruitment of dopamine D1 receptors to the plasma membrane

The recruitment of G protein-coupled receptors from the cytoplasm to the plasma membrane generally is believed to be a constitutive process. We show here by the use of both confocal microscopy and subcellular fractionation that, for at least one such receptor, this recruitment is regulated and not constitutive. Cells from a proximal tubular-like cell line, LLCPK1 cells, were incubated with either a D1 agonist, a dopamine precursor, or an inhibitor of dopamine metabolism to increase dopamine availability in the cell. Each of the three procedures led to a rapid translocation of dopamine D1 receptors from the cytosol to the plasma membrane.