The benzazepine, SCH 23390, inhibits 3H-NPA binding in mouse brain in vivo

Dopamine receptors participate in acquisition and consolidation of latent learning of spatial information in zebrafish (Danio rerio)

There is growing appreciation that various aspects of learning and memory are strongly influenced by dopamine neurotransmission, and that zebrafish hold particular promise in the study of neurotransmitter systems. In this study, we sought to investigate the effect of dopamine receptors on acquisition and consolidation of memory in zebrafish using a latent learning paradigm. To this end, fish were subjected to a 30 min training trial each day for 16 days during which fish were allowed to freely explore a complex maze with the left or right path blocked and without the presence of a reward. During 16 days fish were treated with dopaminergic agonists (apomorphine, SKF-38393, and quinpirole) and antagonists (SCH-23390 and eticlopride) before or after training trials. To assess cognitive performance of fish, a subsequent probe trial was performed on day 17 while all paths leading to a reward chamber were open and the maze now contained stimulus fish as a reward. Pre- and post-training exposure to apomorphine, SKF-38393, and quinpirole significantly impaired learning and memory in fish. In contrast, fish exposed to eticlopride before and after training exhibited improved performance in a latent learning task. Administration of SCH-23390 before training did not affect zebrafish learning ability, but produced significant memory enhancement when given after training trials. Taken together, these findings are the first indications that D1 and D2 receptors are critically involved in acquisition and consolidation of latent learning in zebrafish, with a more prominent role for D2 receptors. The current study opens the door to future studies to investigate the involvement of dopamine receptors in various aspects of cognitive processes.

In vivo vulnerability to competition by endogenous dopamine: Comparison of the D2 receptor agonist radiotracer (-)-N-[11C]propyl-norapomorphine ([11C]NPA) with the D2 receptor antagonist radiotracer [11C]-raclopride

(-)-N-Propyl-norapomorphine (NPA) is a full dopamine (DA) D2 receptor agonist and [11C]NPA is a suitable radiotracer to image D2 receptors configured in a state of high affinity for agonists with positron emission tomography (PET). In this study the vulnerability of the in vivo binding of [11C]NPA to acute fluctuation in synaptic DA was assessed with PET in baboons and compared to that of the reference D2 receptor antagonist radiotracer [11C]raclopride. Three male baboons were studied with [11C]raclopride and [11C]NPA under baseline conditions and following administration of the potent DA releaser amphetamine (0.3, 0.5, and 1.0 mg kg(-1) i.v.). Kinetic modeling with an arterial input function was used to derive the striatal specific-to-nonspecific equilibrium partition coefficient (V3"). [11C]Raclopride V3" was reduced by 24 +/- 10%, 32 +/- 6%, and 44 +/- 9% following amphetamine doses of 0.3, 0.5, and 1.0 mg kg(-1), respectively. [11C]NPA V3" was reduced by 32 +/- 2%, 45 +/- 3%, and 53 +/- 9% following amphetamine doses of 0.3, 0.5, and 1.0 mg kg(-1), respectively. Thus, endogenous DA was more effective at competing with [11C]NPA binding compared to [11C]raclopride binding, a finding consistent with the pharmacology of these tracers (agonist vs. antagonist). These results also suggest that 71% of D2 receptors are configured in a state of high affinity for agonists in vivo. In conclusion, [11C]NPA might provide a superior radiotracer to probe presynaptic DA function with PET in health and disease.

Specific binding of [(11)C]raclopride and N-[(3)H]propyl-norapomorphine to dopamine receptors in living mouse striatum: occupancy by endogenous dopamine and guanosine triphosphate-free G protein

According to the ternary complex model of G-protein linkage to receptors, agonists increase the affinity of the receptors for the G protein. The model predicts that an endogenous agonist's constant of inhibition toward an agonist radioligand is lower than that toward an antagonistic radioligand. The authors hypothesized that competition from endogenous dopamine in striatum of living mice should have a greater effect on the binding of the D2,3 partial agonist N-[3H]propylnorapomorphine than on the binding of the D2,3 antagonist [(11)C]raclopride. The baseline binding potential (pB(0)), defined as the ratio of bound-to-unbound ligand in the absence of competition from endogenous dopamine, was simultaneously measured in mouse striatum for [(11)C]raclopride (pB(0) = 8.5) and N-[(3)H]propylnorapomorphine (p'B(0) = 5.3). The baseline was established by treatment with alpha-methyl-p-tyrosine and reserpine. Relative to these baseline values in saline-treated mice, the pB of N-[(3)H]propylnorapomorphine decreased 52% whereas the pB of [(11)C]raclopride decreased only 30%, indicating greater sensitivity of the former compound to inhibition by synaptic dopamine. Furthermore, amphetamine decreased the pB of N-[(3)H]propylnorapomorphine to a greater extent (73%) than that of [(11)C]raclopride (43%) relative to the reserpine condition. For both radioligands, the occupancy of the dopamine receptors by endogenous agonist obeyed Michaelis-Menten kinetics over a wide range of agonist concentrations established by the pharmacologic treatments. The apparent inhibition constant of endogenous dopamine depended on the dopamine occupancy and decreased to a value 1.66 times greater for N-[(3)H]propylnorapomorphine than for [(11)C]raclopride at its highest occupancies. The results are consistent with the hypothesis that agonist binding is more sensitive than antagonist binding to competition from endogenous dopamine. Therefore, dopamine agonist ligands may be superior to benzamide antagonist ligands for the estimation of dopamine receptor occupancy by endogenous synaptic dopamine. The analysis of the effect of dopamine occupancy on the inhibition of N-[(3)H]propylnorapomorphine binding indicated a limited supply of G protein with a maximum ternary complex fraction of 40% of maximum agonist binding capacity.

The dopamine inhibitor GBR 12909: selectivity and molecular mechanism of action

The neurochemical profile of GBR 12909 (1-(2-bis(4-fluorphenyl)-methoxy)-ethyl)-4-(3-phenyl-propyl)pipera zine) was investigated. GBR 12909 was a potent and selective inhibitor of synaptosomal dopamine uptake (KI = 1 nM), with a 20-fold lower affinity for the histamine H1-receptor and a more than 100-fold affinity for the noradrenaline and 5-HT uptake carriers, the dopamine D-1, D-2, 5-HT2, 5-HT1A and alpha 1-receptors and voltage-dependent sodium channels. GBR 12909 (3 microM) was without effect on muscarinic, alpha 2, beta 1 + 2, gamma-aminobutyric acid (GABA) and benzodiazepine receptors, and on choline and GABA uptake carriers. The selective dopamine uptake inhibitory profile of GBR 12909 was confirmed by ex vivo uptake experiments. GBR 12909 inhibited uptake in vitro in a competitive manner as did cocaine and methylphenidate. [3H]GBR 12935 binding was competitively inhibited by GBR 12909 as well as by dopamine, cocaine and methylphenidate. Off-rate analysis of the [3H]GBR 12935 binding excluded the presence of allosteric binding sites on the dopamine carrier complex. Instead, the data favored the notion that GBR 12909 inhibits dopamine uptake by binding to the dopamine binding site on the carrier protein itself, thereby blocking the carrier process. In conclusion, GBR 12909 is a highly selective inhibitor of dopamine uptake, both in vivo and in vitro. At the moment GBR 12909 is the only compound with this neurochemical profile. The selective effect of GBR 12909 on this neuronal system makes it an interesting experimental tool and a potential antidepressant agent.

Kinetic properties of the in vivo accumulation of 3H-(-)-N-n-propylnorapomorphine in mouse brain

(1) The influence of various dopamine (DA) receptor agonists and antagonists on the kinetic properties of the specific binding of 3H(-)-N-n-propylnorapomorphine (NPA) in the mouse striatum in vivo was studied. The specific binding of 3H-NPA, defined as the difference between the radioactivity in the striatum and cerebellum, was completely antagonized by the selective D-2 receptor antagonist raclopride but not by the selective D-1 antagonist SCH 23390, showing that the binding occurs exclusively to the D-2 receptors. (2) The selective D-2 receptor agonists pergolide and quinpirole inhibited the 3H-NPA binding biphasically at low doses, indicating that these DA receptor agonists have high affinities for a subfraction (10 to 30%) of the NPA binding sites. (3) Increasing the synaptic DA concentration by DA release [(+)-amphetamine] or uptake blockade (amfonelic acid and methylphenidate) inhibited the 3H-NPA binding in a competitive manner (unchanged Bmax, increased KD). Depletion of the DA in the synapses by gamma-butyrolactone or reserpine decreased the apparent KD value. (4) The possibility of estimating changes in the synaptic DA concentration from changes in the apparent KD is discussed. According to the results obtained, the normal concentration of DA in the synaptic cleft in mouse striatum in vivo is about 40 nmol/l and this concentration is increased 2 to 3 times by (+)-amphetamine and amfonelic acid in doses which evoke hyperactivity and stereotypic behaviour.

Comparison of the pharmacological characteristics of [3H]raclopride and [3H]SCH 23390 binding to dopamine receptors in vivo in mouse brain

In vivo binding of the benzamide derivative [3H]raclopride was studied in mouse brain. The binding was saturable, reversible and stereospecific. Non-specific binding was 5-15% of the total binding. Pharmacological characterization of the binding indicated labelling of dopamine D2 receptors since the binding was potently inhibited by compounds with high affinity for this receptor in vitro. On the other hand, compounds with low affinity in vitro i.e., dopamine D1-selective compounds were weak or inactive as inhibitors of [3H]raclopride binding. A comparison of the pharmacological characteristics of [3H]raclopride and [3H]SCH 23390 binding in vivo indicated that compounds with selectivity in vitro retained this selectivity in vivo. Thus, spiroperidol, haloperidol, 1-sulpiride, clebopride, LY 171555 and (-)-NPA ((-)-N-propyl-norapomorphine) were D2 selective while SCH 23390, SKF 38393 and SKF 75670 were D1 selective. Clozapine, tilozepine, cis-flupentixol, chlorpromazine and butaclamol were non-selective both in vitro and in vivo. However, a few compounds changed profile in vivo compared to in vitro. Thus, fluperlapine and fluphenazine had a dual D1-D2 receptor profile in vitro but were D1- or D2-selective in vivo, respectively. Pergolide and molindone which were D2-selective in vitro both had a dual D1-D2 receptor profile in vivo. In conclusion, [3H]raclopride, in vivo, selectively labels the dopamine D2 receptor. Comparison of the pharmacological characteristics of [3H]raclopride and [3H]SCH 23390 binding in vivo supported the that the dopamine D1 receptor is an important target for a variety of neuroleptics, especially of the clozapine type. This may indicate that blockade of the dopamine D1 receptor conveys antipsychotic action.

Interaction of the D1 receptor antagonist SCH 23390 with the central 5-HT system: radioligand binding studies, measurements of biochemical parameters and effects on L-5-HTP syndrome

The interaction of SCH 23390 with dopamine (DA) and serotonin (5-HT) systems has been examined in vivo and in vitro. Like selective 5-HT2 blockers, SCH 23390 inhibited in vivo [3H]spiperone binding in the rat frontal cortex (ID50: 1.5 mg/kg) without interacting at D2 sites. SCH 23390 was equipotent to cinanserin and methysergide. In vitro, SCH 23390 inhibited [3H]ketanserin binding to 5-HT2 sites (IC50 = 30 nM). Biochemical parameters linked to DA and 5-HT were not changed excepted in striatum where SCH 23390 increased HVA and DOPAC. In the L-5-HTP syndrome model, SCH 23390 clearly showed antagonism of 5-HT2 receptors. SCH 23390 had weak affinity for 5-HT1B (IC50 = 0.5 microM), 5-HT1A (IC50 = 2.6 microM) and alpha 1-adrenergic receptors (IC50 = 4.4 microM).

Target size of dopamine D-1 receptors in rat corpus striatum estimated by binding of the benzazepine 3H-SCH 23390

The binding characteristics of 3H-SCH 23390 in rat striatum prepared in potassium phosphate buffer are described. The amount of binding sites labeled by 3H-SCH 23390 decreased monoexponentially when exposed to increasing radiation dose. The molecular weight was determined to 78,000 daltons similar to the molecular mass of 3H-piflutixol binding to D-1 receptor previously reported, however lower than the molecular mass of dopamine D-1 agonist binding sites (Gredal & Nielsen 1987).

Effect of prolonged treatment with neuroleptics on dopamine D-1 and D-2 receptor density in corpus striatum of mice

The effects of long-term treatment with neuroleptics on D-1 and D-2 receptor parameters in corpus striatum were studied in mice. Mice were treated daily for 12 days with the selective D-1 antagonist, the benzazepine SCH 23390, the mixed D-1/D-2 antagonist zuclopenthixol or the selective D-2 antagonist haloperidol. Three days after withdrawal Bmax and KD for 3H-SCH 23390 binding to D-1 receptors and 3H-spiperone binding to D-2 receptors were determined. The study showed that the density of D-2 receptors increased after haloperidol whereas no changes were seen after SCH 23390 or zuclopenthixol. No changes in D-1 receptor density were seen after either treatment. The results are in full agreement with published behavioural results where haloperidol induces tolerance whereas SCH 23390 or zuclopenthixol do not. Some changes in KD values were seen. The cause and meaning of these changes await further investigation.