The oxidation of apomorphine and other catechol compounds by horseradish peroxidase: relevance to the measurement of dihydropteridine reductase activity

It has been reported by Shen et al. (Shen, R.-S., Smith, R.V., Davis, P.J. and Abell, C.W. (1984) J. Biol. Chem. 259, 8894-9000) that apomorphine and dopamine are potent, non-competitive inhibitors of quinonoid dihydropteridine reductase. In this paper we show that apomorphine, dopamine and other catechol-containing compounds are oxidized rapidly to quinones by the horseradish peroxidase-H2O2 system which is used to generate the quinonoid dihydropterin substrate. These quinones react non-enzymatically with reduced pyridine nucleotides, depleting the other substrate of dihydropteridine reductase. When true initial rates of dihydropteridine reductase-dependent reduction of quinonoid dihydropterins are measured, neither apomorphine nor any other catechol-containing compound that has been tested has been found to inhibit dihydropteridine reductase.

A mosaic-like distribution of dopamine receptors in rat neostriatum and its relationship to striosomes

The distribution of dopamine receptors in rat neostriatum was determined by means of light microscopic autoradiography of in vivo labeled binding sites for [3H]N-n-propylnorapomorphine and compared with the distribution of acetylcholinesterase histochemical staining. The distribution of dopamine receptors was non-homogeneous and patches of low receptor density were in register with acetylcholinesterase-poor striosomes. This suggests that the distribution pattern of dopamine receptors is specifically related to various input systems.

Modulation of dopamine receptors by thyrotropin-releasing hormone in the rat brain

Nanomolar concentration of thyrotropin-releasing hormone (TRH) in vitro caused a significant reduction of [3H]apomorphine binding sites (70% of the control) in the rat striatum and the limbic forebrain. [3H]Spiperone binding was not affected by TRH. On the other hand, dopamine and apomorphine displaced [3H]TRH binding partially, suggesting the presence of a TRH receptor subpopulation that has a high affinity for dopamine agonist. Most of the neuroleptics displaced [3H]TRH binding dose-dependently in the micromolar range. (-)-Sulpiride had no affinity to TRH receptors. These findings suggest that one of the important roles of TRH as a neuromodulator is to modulate receptors for classical neurotransmitters, and this receptor-receptor interaction may be of importance in explaining the well known stimulating effects of TRH on the dopaminergic system.

Dopamine binding to the alpha receptor in pregnant rabbit myometrium

This study evaluated in vitro binding of dopamine ligands to myometrial alpha adrenoceptors. With cell membranes from pregnant rabbits, receptor radioligand binding studies utilizing [3H] dihydroergocryptine +/- dopamine demonstrated receptor affinity (KD) = 0.75 +/- 0.10 nM (+/- SEM) and density (Bmax) = 533.2 +/- 45.2 fM/mg protein. Similar studies utilizing phentolamine or apomorphine gave essentially identical results. Competition binding studies demonstrated steriospecific butaclamol binding, along with significant binding of haloperidol, spiperone, apomorphine, and bromoergocryptine. These observations provide a mechanism for the observed uterotonic effects of dopamine.

In situ molecular size of agonist dopamine D-2 binding sites in rat striatum

Specific binding of [3H]N-propylnorapomorphine [( 3H]NPA) to 3,4-dihydroxyphenylethylamine (dopamine) D-2 receptors was investigated in rat striatum in vitro. For various dopamine receptor substances, the rank order of potency to inhibit [3H]NPA binding was spiroperidol greater than or equal to NPA greater than LY 171555 greater than SCH 23390 greater than SKF 38393. A single high-affinity binding site was found in membranes prepared in either Tris-citrate buffer or imidazole buffer; the affinity constants were 0.11 and 0.76 nM, respectively. The number of receptors (33 pmol/g wet weight) was independent of whether the membranes were prepared in Tris-citrate buffer or imidazole buffer and was similar to the number of receptors estimated by [3H]spiroperidol binding to dopamine receptors. Irradiation inactivation of frozen whole rat striata showed a monoexponential loss of [3H]NPA binding sites without a change in the binding affinity. The target size of the [3H]NPA binding site was 81,000 daltons, which shows that the functional molecular entity to bind the dopamine D-2 agonist was smaller than the molecular entity to bind the dopamine D-2 antagonist [3H]spiroperidol (target size, 137,000 daltons).

Evidence that dopamine receptors identified by [3H]dopamine in the ventricles of guinea-pig heart are of DA2 type

Apomorphine and LY 171555, but not SKF 38393 displaced [3H]Dopamine binding to membranes of guinea-pig heart. Domperidone and 1-sulpiride but not SCH 23390 competitively antagonize the [3H]Dopamine binding performed with 100 microM cold dopamine as displacer. In conclusion the DA2 receptor agonists and antagonists used are able to interfere with the receptors labelled by [3H]Dopamine in cardiac muscle.

Evidence for dopaminergic binding sites in the human adrenal cortex

Dopamine may be a modulator of aldosterone secretion in man. Whether this effect is extraadrenal or is exerted directly at the adrenal gland via local dopaminergic receptors remains uncertain. This study examined the possibility that dopaminergic binding sites exist in the human adrenal cortex using [3H]spiperone, a butyrophenone with high affinity for dopaminergic receptors of the D2 subtype. [3H]Spiperone binding to membranes prepared from the outer adrenal cortex obtained from eight patients undergoing adrenalectomy was studied. Specific [3H]spiperone binding, defined as binding displacable by 250-fold excess of unlabeled spiperone reached equilibrium within 30 minutes at 4 degrees C and was readily reversible. Binding was consistent with both high affinity (Kd1 = 0.2 to 0.8 nmol/L) and low affinity (Kd2 = 20 to 127 nmol/L) binding states. Binding capacity was 27 to 276 fmol/mg for the high affinity and 63 to 597 fmol/mg for the low affinity binding state. The relative potency in inhibition of [3H]spiperone binding was as follows: antagonists, spiperone greater than domperidone greater than metoclopramide greater than ketanserin greater than (-) sulpiride greater than (+) sulpiride; agonists, dopamine, bromocriptine greater than NPA much much greater than epinephrine. Serotonin and norepinephrine did not affect [3H]spiperone binding. These data suggest the existence of dopaminergic binding sites possibly of the D2 subtype in the human adrenal cortex. The precise location of these sites remains to be determined.

Disposition of apomorphine in rat brain areas: relationship to stereotypy

Apomorphine-induced stereotyped behavior and apomorphine levels in plasma, striatum and nucleus accumbens were determined in rats at various intervals after a single i.p. injection of serial doses of apomorphine hydrochloride (0.625, 1.25, 2.5 and 5 mg/kg). Apomorphine disappeared from plasma in a mono-exponential mode with a half-life of about 10 min. In striatum and nucleus accumbens apomorphine concentrations peaked 10 min after administration, declining thereafter with a half-life comparable to that in plasma. Apomorphine was concentrated and distributed similarly in the two brain regions; the brain areas/plasma ratio was approximately seven for all doses tested. The rise of apomorphine levels in brain areas slightly preceded the behavioral response, whereas after the peak effect (20-30 min) the intensity of stereotypy declined almost parallel with the log drug concentrations. Plotting apomorphine levels in the tissues assayed against the drug response at the same interval for individual rats, regardless of dose, indicated a highly significant relation between the degree of behavioral effects and brain apomorphine levels. The threshold apomorphine concentrations for inducing stereotyped behaviour were 108 and 95 ng/g respectively in striatum and nucleus accumbens. These findings show that the time course and magnitude of the behavioral effects of apomorphine corresponded with its brain levels in 'dopaminergic' areas, suggesting that apomorphine-induced stereotyped behavior in rats can be described by a direct mechanism. Reserpine (5 mg/kg s.c.) enhanced the apomorphine stereotypy but did not affect apomorphine's disposition in brain and plasma.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of a long term dihydroergotoxine treatment on agonist and antagonist striatal dopamine binding sites are dose and age related

Chronic administration of dihydroergotoxine, at the two doses of 2.5 mg/kg and 5 mg/kg decreases the binding of dopamine 3H-agonists to striatal membranes. By contrast the binding of dopamine 3H-antagonists is decreased in the animals treated with the higher dose and increased in those treated with the lower one. In old rats, in which a partial loss of both 3H-antagonist and 3H-agonist binding sites is observed, the DHT treatment confirms to increase the binding of 3H-antagonists, without affecting that of 3H-agonists. Thus, aging and ergot alkaloids seem to discrimate between DA-agonist and DA-antagonist receptor sites suggesting that this receptors are separate entityes.

Synthesis and dopaminergic binding of 2-aryldopamine analogues: phenethylamines, 3-benzazepines, and 9-(aminomethyl)fluorenes

A series of 2-aryldopamine analogues were synthesized and evaluated for their effects on D1 and D2 dopamine receptors. The 2-phenyldopamine and 6-phenylbenzazepine analogues exhibited weak binding to both D1 and D2 receptors. The 9-(aminomethyl)fluorenes also exhibited weak D2 binding; however, 2,5,6-trihydroxy-9H-fluorene-9-methanamine (4b) exhibited D1 binding comparable to apomorphine. The binding activity has been correlated with the calculated torsion angle of the biphenyl portion of these molecules. Good D1 dopamine binding occurs when the aromatic rings approach coplanarity; poor binding occurs when the aromatic rings are orthogonal.

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

The fact that SCH 23390, a selective dopamine (DA) D1 antagonist, blocks the effects of D2 agonists suggests a functional coupling of D1 and D2 receptors. Therefore, the binding of SCH 23390 to D2 receptors was investigated in vivo using 3H-N-n-propylnorapomorphine (NPA), a D2 agonist, and 3H-spiperone and 3H-raclopride, both D2 antagonists. SCH 23390 failed to inhibit 3H-spiperone or 3H-raclopride binding; however, SCH 23390 was relatively potent in inhibiting 3H-NPA binding. These results suggest that (some) antidopaminergic effects of SCH 23390 may result from antagonism of a D2 agonist conformation of the D2 receptor.

Dopamine antagonists reduce spontaneous electrical activity in cultured mammalian neurons from ventral mesencephalon

Mammalian neurons from ventral mesencephalon (VM) were grown in primary dissociated cell (PDC) culture. These neurons are predominantly non-dopaminergic. Many of these non-dopaminergic neurons have dopamine agonist and antagonist binding sites. Intracellular recordings were obtained from these neurons. When bathed in phosphate-buffered saline (PBS) solution they generated action potentials spontaneously. However, in the presence of haloperidol dissolved in PBS solution, the percentage of neurons which generated action potentials spontaneously was reduced in a dose-dependent manner (1-10 microM). This response was also obtained with (+) butaclamol (1 microM) but not with (-) butaclamol (1 microM). This neuroleptic inhibition of spontaneously generated action potentials was specific for neurons in PDC cultures of VM since neurons in PDC cultures of spinal cord did not demonstrate this phenomenon.

Dopaminergic 2-aminotetralins: affinities for dopamine D2-receptors, molecular structures, and conformational preferences

A combination of X-ray crystallography, NMR spectroscopy, and molecular mechanics (MMP2) calculations was used to determine the three-dimensional structures and conformational preferences of the enantiomers of 5-hydroxy-2-(di-n-propylamino)tetralin and their C1-methylated derivatives. In addition, the affinities of the compounds for striatal 3H-spiroperidol- and 3H-N-n-propylnorapomorphine-binding sites were determined. In the present series, the dopamine D2-receptor agonists have the S-configuration at the nitrogen-bearing carbon (C2), whereas the only established D2-receptor antagonist, 1S,2R-5-hydroxyl-1-methyl-2-(di-n-propylamino)tetralin (1S,2R-UH-242), has the opposite absolute configuration at C2. Two conformational parameters, the tetralin inversion angle (phi) and the dihedral angle tau(C1, C2, N, N-H or electron pair) (tau N), are shown to be critical for D2-receptor agonism; phi values around 0 degrees and tau N values around 60 degrees appear to be optimal. The low D2-affinity of 1S,2S-5-hydroxyl-1-methyl-2-(di-n-propylamino)tetralin seems to be related to its inability to assume a low-energy "D2-receptor agonistic conformation." It is noted that the common structural denominator between the D2-receptor antagonists 1S,2R-UH-242 and 6aS-apomorphine is their inability to assume "dopamine D2-receptor agonistic nitrogen electron pair orientations."

Autoradiographic distribution of the D1 agonist [3H]SKF 38393, in the rat brain and spinal cord. Comparison with the distribution of D2 dopamine receptors

The regional distribution of the specific D1 agonist [3H]SKF 38393 (SKF 38393, 2,3,4,5-tetra-hydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine) has been studied autoradiographically in the rat CNS. The binding of [3H]SKF 38393 to striatal sections was saturable, stereospecific, reversible, of high affinity (Kd = 9.9 nM) and partly sodium sensitive; it occurred at a single population of sites and possessed the pharmacological characteristics of the dopamine D1 receptor. The highest levels of [3H]SKF 38393 binding sites were found in the caudate-putamen, nucleus accumbens, olfactory tubercle and substantia nigra. Moderately high concentrations of the [3H]ligand were observed in the amygdala, endopyriform nucleus, nucleus olfactorius anterior, lateral septum, primary olfactory cortex, cerebellum (molecular layer) and spinal cord. An intermediate labelling was found in the thalamus, habenula, subthalamic nucleus, hypothalamus, ventral tegmental area, superior colliculus, hippocampus and cerebral cortex. Moderate levels of [3H]SKF 38393 binding were observed in the globus pallidus and arcuate nucleus. The autoradiographic distribution of [3H]SKF 38393 overlapped with that of [3H]N,n-propylnorapomorphine, a radioligand which labels the D2 dopamine receptors, in a number of dopamine-rich brain areas but there were several areas which exhibited a high density of [3H]SKF 38393 binding sites but undetectable concentrations of [3H]N,n-propylnorapomorphine. Moreover, in the spinal cord, the subregional localization of these [3H]ligands clearly differed. Intrastriatal injection of ibotenic acid caused a large decrease in [3H]SKF 38393 and [3H]N,n-propylnorapomorphine binding in the striatum and provoked a reduction of [3H]SKF 38393 but not [3H]N,n-propylnorapomorphine binding in the substantia nigra confirming the view that nigral D1 but not D2 receptors are located on striatonigral fibres.

Tracer and maximal specific binding of tritiated spiperone or N-n-propylnorapomorphine to quantify dopamine receptors in rat brain regions in vivo

Specific tracer and maximal specific binding (Bmax) were determined in rat brain regions from radioactivity accumulation after intravenous administration of 3H N-n-propylnorapomorphine (NPA) or 3H spiperone at various specific activities. With NPA the highest Bmax-values (expressed in pmol.g-1 tissue) were found in the striatum (26 pmol.g-1) nucleus accumbens (about 27 pmol.g-1) and the olfactory tubercle (11 pmol.g-1). Saturable NPA binding was also found in the amygdaloid complex, medulla oblongata and inferior colliculi, but not in the frontal cortex. Bmax values for spiperone were high in the striatum (73 pmol.g-1), the nucleus accumbens (48 pmol.g-1), the olfactory tubercle (34 pmol.g-1) and the frontal cortex (18 pmol.g-1). A similar order was found for the tracer contents in these regions. There was no linear relationship between these contents and Bmax values. The possible implications of these findings and usefulness of NPA for brain imaging are discussed.

Evidence that the potential antipsychotic agent rimcazole (BW 234U) is a specific, competitive antagonist of sigma sites in brain

Rimcazole (BW 234U) is a potential antipsychotic agent which in open-clinical trials appears to be effective in acute schizophrenic patients. In the present study, rimcazole was found to block the specific binding of [3H]-(+)-SKF 10,047 to sigma sites in rat and guinea pig brain (IC50 = 5.0 X 10(-7) M). The compound was 100 times weaker as a blocker of phencyclidine sites (IC50 = 4.3 X 10(-5) M). At 1 X 10(-5) M, rimcazole had only weak effects on mu, delta, kappa and epsilon opioid receptors. Scatchard analysis of the binding data from guinea pig brain revealed an apparent KD for [3H]-(+)-SKF 10,047 of 85 +/- 5 nM and a Bmax of 824 +/- 27 fmole/mg protein. In the presence of 5 X 10(-7) M BW 234U, the apparent KD was 165 +/- 35 nM, but the Bmax (892 +/- 146 fmoles/mg protein) was not affected. This suggests that rimcazole is a competitive inhibitor of sigma sites. The agent was also capable of blocking sigma sites in vivo (ID50 = 6 mg/kg i.p., mice) as judged by an in vivo sigma receptor binding assay. Thus, if the antipsychotic activity of rimcazole is confirmed in double-blind, placebo-controlled trials, it would be the first compound whose mechanism of antipsychotic activity may best be explained by a direct blockade of sigma sites and not by a direct blockade of dopamine (D2) receptors in brain.

Autoradiographic localization and quantification of dopamine D2 receptors in normal human brain with [3H]N-n-propylnorapomorphine

The precise distribution of dopamine receptors has been studied autoradiographically in the normal human brain using [3H]N-n-propylnorapomorphine ([3H]NPA) as a ligand. Preliminary experiments aimed at optimizing the binding assay conditions revealed that preincubation washing of caudate nucleus sections was a prerequisite to obtain a good ratio of specific to non-specific binding. The binding of [3H]NPA to caudate-putamen sections was saturable, stereospecific, reversible, of high affinity (Kd = 0.27-0.35 nM) and occurred at a single population of sites. Competition experiments with various drugs indicated that in the caudate-putamen the specific [3H]NPA binding sites possess the pharmacological features of the dopamine D2 receptor. The highest levels of [3H]NPA binding sites were found in the caudate nucleus, putamen, globus pallidus and nucleus accumbens. There were also intermediate to low concentrations of the 3H-ligand in the hippocampus, the insular and cingular cortices and in the occipito-temporal gyrus, while almost undetectable levels of binding were found in the anterior frontal cortex. Thorough examination of the subregional distribution of [3H]NPA binding sites in the caudate-putamen-pallidum complex revealed heterogeneous patterns of radioactivity. In these brain regions, the distribution of autoradiographic grains was punctate and islands of high and low densities were observed. Moreover, in the caudate nucleus, there was a subtle high lateral to low medial gradient in the topography of the [3H]NPA binding sites and a more pronounced gradient along the rostrocaudal axis; the highest levels of binding being located at the midbody of the nucleus. No gradients of [3H]NPA binding were observed in the putamen. The present data indicate that [3H]NPA is a suitable ligand for accurate autoradiographic labeling of dopamine D2 receptors in human postmortem brain tissue and that dopamine receptors are heterogeneously distributed and topographically organized in patches and gradients in the basal ganglia regions.

[3H]Fluphenazine binding to brain membranes: simultaneous measurement of D-1 and D-2 receptor sites

[3H]Fluphenazine was used to label both D-1 and D-2 dopamine receptors in mouse striatal membranes. The D-1 and D-2 specific binding of [3H]fluphenazine was discriminated by the dopamine antagonists SCH-23390 (D-1 selective) and spiperone (D-2 selective). Saturation analyses of these two sites yielded a D-1 receptor density in mouse striatum of 1,400 fmol/mg of protein and a D-2 receptor density of 700 fmol/mg of protein. The affinity of [3H]fluphenazine for the D-2 site was slightly greater than for the D-1 site; the equilibrium dissociation constant (KD) was 0.7 versus 3.2 nM, respectively. Assay conditions are described that reduce nonspecific binding of [3H]fluphenazine to acceptable levels (35% of total binding at 1 nM [3H]fluphenazine). By comparison of displacement curves from a series of dopaminergic and nondopaminergic ligands, the pharmacological specificity of [3H]fluphenazine binding in mouse striatum was demonstrated to be dopaminergic. Only small amounts of dopamine-specific (apomorphine-sensitive) [3H]fluphenazine binding were found in other brain regions. However, chlorpromazine displaced considerable [3H]fluphenazine from all brain regions, including cerebellum, suggesting the presence of a [3H]fluphenazine binding site with a phenothiazine specificity.

Aporphines 65: chemical, microbial synthesis and characterization by gas chromatography/mass spectrometry of (R)-(-)-10-hydroxy 11-methoxy-N-n-propylnoraporphine, a potential metabolite of N-n-propylnorapomorphine

The title compound has been synthesized by a multistep sequence from (R)(-)-morphine, and compared with the product obtained by microbial O-demethylation of (R)- and (S)-10,11-dimethoxy-N-n-propylnoraporphine (N-n-propylnorapomorphine dimethyl ether). The comparison was based on an analysis of the trifluoroacetyl derivatives of the microbial products and the synthesized compounds using gas chromatography/mass spectrometry in the electron impact mode. Examination by gas chromatography/electron impact/mass spectrometry of the trifluoroacetyl derivatives of 11-hydroxy-10-methoxyaporphine (apocodeine, 3) and 10-methoxy-11-hydroxyaporphine (isoapocodeine, 5) has revealed their excellent chromatographic resolution, and the preferential loss of the methyl group as in 3 [M-15]+ or the trifluoroacetyl group from the 10-position of the aporphine ring as in 5 [M-97]+. Characteristic fragmentations of these isomeric aporphines were used to confirm their structures and were compared with the spectra of authentic synthetic samples.

Reduced apomorphine sensitivity of dopamine metabolism in rat striatum after repeated administration of methamphetamine

Rats received daily injections of saline or methamphetamine (MAP; 4 mg/kg/day) for 14 days. Seven days after the completion of this regime, dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) levels were measured in the striatum following intraperitoneal injections of either saline plus gamma-butyrolactone (GBL; 750 mg/kg) or apomorphine (2 mg/kg) plus GBL. The saline plus GBL challenges produced no difference in the DA or DOPAC levels between saline- and MAP-treated rats. By contrast, the apomorphine plus GBL challenges produced higher DOPAC levels in MAP-treated rats than saline-treated rats, although they produced no difference in the DA levels between the two groups. These results indicate that apomorphine depresses the striatal DA metabolism less in MAP-treated rats than in saline-treated control rats. Repeated MAP administration might produce this effect through apomorphine subsensitivity of presynaptic DA autoreceptors.

Identification of dopamine "D3" and "D4" binding sites, labelled with [3H]2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene, as high agonist affinity states of the D1 and D2 dopamine receptors, respectively

On the basis of affinity differences for spiperone, two binding sites for [3H](+/-)-2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene ([3H]ADTN) in the rat brain could be distinguished: "D3" with a low and "D4" with a high affinity for spiperone. Evidence is provided that D3 and D4 sites are related to high agonist affinity states of the D1 and D2 dopamine receptors, respectively. Various well-known selective D1 and D2 agonists and antagonists showed potencies at these sites in agreement with this hypothesis. A comparison of the Bmax values for [3H]ADTN binding to D3 and D4 sites with the numbers of D1 receptors (labelled by [3H]SCH 23390) and of D2 receptors (labelled by [3H]spiperone), both in the striatum and in the mesolimbic system, indicated that under the conditions used for 3H-agonist binding experiments, both populations of D1 and D2 receptors were converted to their high agonist affinity states to a considerable, although different extent. In fact, when competition experiments with [3H]spiperone were performed under the conditions otherwise used for [3H]ADTN binding experiments (instead of the conditions usually used for antagonist binding), substantial shifts of the displacement curves of 3,4-dihydroxyphenylethylamine (dopamine) and ADTN toward higher affinities were observed. A comparison of the effects of various agonists and antagonists in the [3H]ADTN binding experiments and in functional tests revealed a significant correlation between their potencies at D4 binding sites and at D2 receptors modulating the release of [3H]acetylcholine from striatal slices. However, in the situation of the D1/D3 pair, when the measurement of adenylate cyclase activity was taken as a functional test for D1 receptors, agonists were more active in the binding than in the functional test, whereas for many antagonists the opposite was found. The results are discussed with regard to the classification and functional aspects of brain dopamine receptors.

Elevation of acetylcholine levels in striatum of rat brain by LY163502, trans-(-)-5,5a,6,7,8,9a,10-octahydro-6-propylpyrimido less than 4,5-g greater than quinolin-2-amine dihydrochloride, a potent and stereospecific dopamine (D2) agonist

LY163502, a partial ergoline and a trans-levorotatory enantiomer, does not stimulate adenylate cyclase in striatal membranes but inhibits 50% binding of 3H-apomorphine, 3H-pergolide and 3H-spiperone at 10, 13 and 151 nM (IC50), respectively. The racemic mixture (LY137157) is less effective, with 3, 2.7 and 1.4 times higher IC50 values, respectively, whereas the dextrorotatory isomer (LY175877) is inactive. LY163502 inhibits binding of 3H-clonidine with an IC50 value of 2600 nM, but not the binding of 3H-WB4101, 3H-dihydroalprenolol, 3H-serotonin, 3H-quinuclidinyl benzilate and 3H-pyramilamine or the uptake of serotonin, norepinephrine or dopamine, suggesting selective affinity toward dopamine receptors in vitro. Both LY163502 and LY137157 elevate striatal acetylcholine (Ach) levels. The elevation of Ach levels by LY163502 is reversed by dopamine antagonists haloperidol, cis-flupenthixol and metoclopramide. Therefore, the levorotatory enantiomer exhibits pharmacology of a D2 type of dopamine agonist.

Concentration-response relations for apomorphine effects on heart rate in conscious rats

The pharmacokinetics of apomorphine in plasma and brain tissue have been studied in relation to the time courses of effects on heart rate in conscious rats. The kinetic behaviour was investigated after 2 mg kg-1 i.v. and 5 mg kg-1 s.c., respectively. Apomorphine showed a high total plasma clearance (165-207 ml min-1 kg-1) and, despite a relatively large volume of distribution (3.4-4.1 litre kg-1), a biological half-life of about 14 min was obtained irrespective of route of administration. The kinetics in whole brain were identical with those in plasma. Apomorphine produced biphasic effects on the heart rate during the time courses of subcutaneous single doses: a low dose (50 micrograms kg-1) induced pure bradycardia while the doses of 100 micrograms kg-1 and 5 mg kg-1 produced responses oscillating between bradycardia and tachycardia. When we evaluated the relation between apomorphine concentrations and effects on the heart frequency with a composed sigmoid Emax model, apomorphine exhibited a U-shaped steady-state plasma concentration-response curve. Bradycardia appeared after low concentrations, reached a maximum and then decreased with increasing concentrations. A further augmentation of apomorphine concentration resulted in the opposite effect, i.e. tachycardia. Separate concentration-response curves for bradycardia and tachycardia were calculated. The changes in biophase concentration that occur during the absorption and disposition may thus cause the fluctuations between contrasting effects seen during the time course of a single dose.

Long-term consequence of early iron-deficiency on dopaminergic neurotransmission in rats

Nutritional iron-deficiency (ID) induced in rats caused a reduction in peripheral as well as central iron metabolism. This effect was markedly greater in the liver than the brain. Although the decrease in the rate of brain non-haem iron was slower than that of serum and liver, significant diminutions of behavioral response to apomorphine (2 mg/kg) and maximum [3H]spiperone binding (Bmax) in caudate nucleus were noted in these animals. These effects of ID can be reserved by iron supplementation in young (21-day-old) and adult (48-day-old) rats. In contrast, if ID is induced in new born (10-day-old) animals, the diminished brain non-haem iron, behavioral response to apomorphine and [3H]-spiperone binding in caudate nucleus will not recover even after 6 weeks of iron supplementation. However, these animals have normal serum iron, haemoglobin and liver iron. These data point to the profound effect early ID can have on the development of dopaminergic neurotransmission, since brain iron concentration increases its maximum in the 4-5 weeks after birth. The implications of the present finding is that the prevalence of ID in children occurs in the first decade of life, when brain iron accumulation reaches values observed in adults. The profound cognitive changes associated with ID in children is thought to be dopamine-dependent and is not always reversible with iron therapy.