The effect of the dopamine neuronal reuptake inhibitor GBR 12909 on the dopaminergic neurotransmission parameters in the rat striatum in vivo

The aim of the present research was to investigate the effects of the selective dopamine re-uptake inhibitor GBR 12909 (10 mg/kg, i.p.) on dopaminergic neurotransmission in rat striatum. GBR 12909 increased DA levels in microdialysis samples from freely moving rats. In contrast, administration of GBR 12909 slightly decreased the NSD 1015 (50 mg/kg)--induced DOPA accumulation. This result may be explained as a decrease in DA biosynthesis in response to an increasing amount of DA in the synaptic cleft. Fast-scan cyclic voltammetric monitoring of electrically-evoked DA showed significant changes after the drug in DA re-uptake (Km), but not DA release. In conclusion, our results support the notion, that GBR 12909 is a specific DA uptake inhibitor without any transmitter releasing action.     

The influence of serotoninergic drugs on dopaminergic neurotransmission in rat substantia nigra,striatum and limbic forebrain in vivo

Summary The effects of serotoninergic drugs on dopaminergic neurotransmission in the substantia nigra, the striatum and the limbic forebrain of rat have been investigated. The accumulation of 3-methoxytyramine (3-MT) following inhibition of monoamine oxidase with pargyline was used as an indirect measure of dopamine (DA) activity in vivo. The effects of the following serotoninergic drugs were tested: the 5-HT_1A receptor agonist 8-OH-DPAT, the 5-HT_1B receptor agonist trifluoromethyl-phenylpiperazine (TFMPP), CGS 12066B and RU 24969, the 5-HT_1A/1B antagonist (±)pindolol, the 5-HT_2/1C receptor antagonist ritanserin, the 5-HT_2/1C receptor agonist DL-1-(2,5-dimethoxy-4-iodo-phenyl)-2-aminopropane (DOI), the 5-HT₃ receptor antagonist BRL 43 694, the unselective 5-HT₁ receptor antagonist methiothepin, and carbidopa+L-5-hydroxytryptophan (L5-HTP) to achieve a general, unselective stimulation of multiple 5-HT receptors. In the substantia nigra, carbidopa + 5-HTP treatment increased the 3-MT accumulation by 26% and decreased the DA concentration to 67% of controls, tentatively suggesting a 5-HTP-induced displacement of nigral DA. A minor, non dose-related reduction in nigral 3-MT was seen after the 5-HT_1A receptor agonist 8-OH-DPAT. None of the other serotonin receptor acting drugs induced any pronounced effect on the nigral 3-MT accumulation. Taken together, the findings provide little support for the idea that one single 5-HT₁ receptor subtype serves a modulatory function on DA activity in the substantia nigra. In the striatum and the limbic forebrain, trifluoromethylphenylpiperazine dose-dependently increased the 3-MT accumulation to maximally 200%–220% of controls. In the limbic forebrain also the highest dose of RU 24 969 (15 mg/kg) increased the 3-MT accumulation (78%), whereas in the striatum the lowest does of the drug (1.5 mg/kg) decreased it by 30%. The trifluoromethylphenylpiperazine-induced stimulation of 3-MT accumulation was not blocked by ritanserin. In the limbic forebrain, also DL-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane and carbidopa+Lr5-HTP treatment increased the 3-MT concentrations to 120% and 150% of controls, respectively. Paradoxically, methiothepin also induced an increase of the 3-MT accumulation in these brain regions, probably due to its DA receptor antagonism. None of the other serotoninergic drugs induced any pronounced effects on the 3-MT accumulation in these brain parts. The results may overall support the hypothesis that 5-HT₁ does modulate the DA activity in the striatum and limbic forebrain, tentatively via 5-HT_1B receptors in the striatum and 5-HT_1B and 5-HT₂ or 5-HT_1C receptors in the limbic forebrain. It may be speculated therefore, that clinical application of 5-HT₁ receptor-modulating drugs to influence central dopaminergic activity might be of therapeutical benefit, for example, in motor disorders like Parkinson's disease. Send offprint requests to H. Nissbrandt at the above address     

Effects of LSD and BOL on the catecholamine synthesis and turnover in various brain regions

In the rat, lysergic acid diethylamide (LSD) 0.5 mg/kg and 2-bromo lysergic acid diethylamide (BOL) 0.5 mg/kg increased the rate of the striatal in vivo tyrosine hydroxylation as measured by the DOPA accumulation after decarboxylase inhibition. Neither LSD nor BOL significantly changed the DOPA accumulation in the olfactory tubercle, a dopamine-rich part of the limbic system. LSD but not BOL increased the DOPA accumulation in the cerebral cortex and in the brain stem. LSD and BOL appeared not to alter the rate of -MT-induced disappearance of DA or of NA in the whole brain, nor did they change the rate of the -MT-induced disappearance of DA in the striatum. It is suggested that in the striatum LSD and BOL block autoreceptors (presynaptic receptors) regulating the tyrosine hydroxylation. These receptors may be DA receptors, but may also be 5-HT- or LSD-sensitive receptors.The regional differences observed between LSD and BOL suggest that LSD in the cerebral cortex and in the brain stem increases the DOPA accumulation by mechanism other than that functioning in the striatum. One possible explanation is that LSD and BOL may differ in their effects on 5-hydroxytryptaminergic systems in the cerebral cortex and in the brain stem.     

NSD 1034: An amino acid decarboxylase inhibitor with a stimulatory action on dopamine synthesis not mediated by classical dopamine receptors

Summary The accumulation rates of 3,4-dihydroxyphenyl-alanine (DOPA) and 5-hydroxytrypthophan (5-HTP) after inhibition of aromatic amino acid decarboxylase (AADC) by 3-hydroxybenzylhydrazine (NSD 1015) or 1-(dl-seryl)-2(2,3,4-trihydroxybenzyl)hydrazine (Ro 4-4602) have widely been used as measurements of the in vivo synthesis rates of monoamines. However, the values of dopamine (DA) turnover in rat striatum obtained using these drugs are much lower than values obtained by other methods. This discrepancy prompted us to further investigate the AADC inhibitor 1-(3-hydroxybenzyl)-1-methylhydrazine (NSD 1034) which earlier has been shown to give a DOPA accumulation rate in the striatum of the same magnitude as other measures of DA turnover. NSD 1034 was found to give a more than twofold higher DOPA accumulation rate than NSD 1015, NSD 1024, NSD 1039, NSD 1055 and Ro 4-4602 in the striatum. Also, in the limbic region and the hemispheres, but not in the substantia nigra, the DOPA accumulation was higher after NSD 1034 than after NSD 1015, but the difference was less pronounced. There was, however, no difference in 5-HTP accumulation between the drugs in any of the brain parts investigated. Although the DOPA accumulation rates are higher after NSD 1034 than after NSD 1015, the NSD 1015-induced DOPA accumulation seems to be more sensitive to changes in dopamine receptor occupancy. The different DOPA accumulation rates obtained with NSD 1015 and NSD 1034 are not due to differences in MAO inhibition, to interference with classical DA receptors, or to different degrees of AADC inhibition, but to an ability of NSD 1034 to stimulate DA synthesis. In addition, under certain conditions NSD 1034 also has a DA releasing action, like amphetamine. It is proposed that NSD 1034 and amphetamine stimulate DA synthesis and release by a common mechanism. The low value of DA synthesis rate, obtained when measured as DOPA accumulation after NSD 1015, is due to a substantial efflux of DOPA from the brain. The efflux of DOPA is equally large after NSD 1034 but the loss is compensated for by an increase in DOPA synthesis. Send offprint request to Hans Nissbrandt at the above address     

Haloperidol-induced changes in the regional cerebral pharmacokinetics of apomorphine

Haloperidol did not change significantly the rate of penetration of apomorphine to three investigated structures of the rat brain: striatum, limbic forebrain and cerebellum. It depressed, however, significantly the accumulation of apomorphine in the striatum and limbic forebrain; in the cerebellum, in which the accumulation of apomorphine was very low, haloperidol did not change it. In haloperidol-pretreated rats the pattern of elimination of apomorphine from the striatum changed, revealing the presence of two compartments; this was not observed in the limbic forebrain, from which the elimination was linear, both in the presence and absence of haloperidol.     

Effects of d-amphetamine on dopaminergic neurotransmission; a comparison between the substantia nigra and the striatum.

The effects of d-amphetamine (d-AMP) on dopaminergic neurotransmission in the cell body/dendritic region of the nigrostriatal pathway, the substantia nigra, have been investigated and compared to the effects obtained in the terminal region of the pathway, the striatum. The rate of synthesis of dopamine (DA) was quantified as accumulation of 3,4-dihydroxyphenylalanine (DOPA), after inhibition of aromatic L-amino acid decarboxylase with 3-hydroxybenzyl-hydrazine (NSD 1015). As measures of the metabolism of DA the concentrations of the metabolites of DA, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 3-methoxytyramine (3-MT) were determined. As indices of release of DA, the accumulation of 3-methoxytyramine (3-MT), after inhibition of monoamine oxidase with pargyline and the disappearance of DA, after inhibition of its synthesis with alpha-methyl-p-tyrosine were assessed. d-Amphetamine insignificantly increased the concentration of DA in the striatum but profoundly decreased it in the substantia nigra (to 60% of controls). Both in the striatum and in the substantia nigra treatment with d-AMP induced clearcut decreases of the concentrations of DOPAC. Also the concentration of HVA was profoundly decreased in the striatum but only marginal effects on HVA were observed in the substantia nigra. In both structures of the brain, d-AMP increased the concentration of 3-MT. Depending on the dose, d-AMP increased or had no effect on the accumulation of DOPA in the striatum but consistently decreased it in the substantia nigra. In the striatum, d-AMP increased the pargyline-induced accumulation of 3-MT, without affecting the concentration of DA. However, in the substantia nigra the concentration of DA was profoundly decreased (to 50% of controls) in combination with unaltered accumulation of 3-MT, unless the rats were pretreated with haloperidol. If so, the effects of d-AMP on the concentration of DA and accumulation of 3-MT were the same in the substantia nigra and in the striatum. The results indicate that d-AMP depleted stores of DA in the substantia nigra, due to its releasing action, in combination with its decreasing effect on the rate of synthesis of DA. The decrease in rate of synthesis of DA is suggested to be due to the d-AMP-induced decrease in the firing rate of dopaminergic neurones.     

Regional effects of amphetamine, cocaine, nomifensine and GBR 12909 on the dynamics of dopamine release and metabolism in the rat brain.

1. The effects of single-dose regimens of amphetamine, cocaine, nomifensine and GBR 12909 on the dynamics of dopamine (DA) release and metabolism were evaluated in the frontal cortex, hypothalamus, nucleus accumbens and striatum. The regimens selected are known to produce substantial behavioural effects. 2. 3-Methoxytyramine (3MT) and 3,4-dihydroxyphenylacetic acid (DOPAC) rates of formation were used to assess DA metabolism by catechol-O-methyltransferase and monoamine oxidase respectively. The rate of formation of 3MT was used as an index of synaptic DA. The ratio and sum, respectively, of 3MT and DOPAC rates of formation were used to assess DA reuptake inhibition and turnover. 3. The effects of amphetamine on 3MT production and DOPAC steady-state levels were similar in all regions, suggesting similar pharmacodynamic actions. Amphetamine increased 3MT formation and steady-state levels, and reduced DOPAC steady-state levels. DOPAC formation was significantly reduced only in the nucleus accumbens and striatum. Total DA turnover remained unchanged except in the nucleus accumbens. Apparently, the amphetamine-induced increase in DA release occurred at the expense of intraneuronal DA metabolism and did not require stimulation of synthesis. 4. Nomifensine elevated 3MT formation in all regions. A similar effect was produced by cocaine except in the nucleus accumbens. GBR 12909 elevated 3MT production only in the hypothalamus, the striatum and the nucleus accumbens. 5. Cocaine selectively reduced DOPAC formation in the frontal cortex. Nomifensine increased and reduced, respectively, DOPAC formation in striatum and hypothalamus. GBR 12909 elevated DOPAC formation in all regions except the cortex, where pargyline did not reduce DOPAC levels in GBR 12909-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects ofd-amphetamine and methylphenidate on hyperactivity produced by neonatal 6-hydroxydopamine treatment

Neonatal intracisternal administration of 6-hydroxydopamine (6-OHDA, 50 g on day 1 after birth) caused a marked hyperactivity when the rats were tested as adults. These rats also showed severe DA depletions in striatum and nucleus accumbens. Pretreatment with the noradrenaline (NA) uptake inhibitor desipramine provided protection against NA depletion in frontal cortex and nucleus accumbens. Pretreatment with DNA uptake inhibitors, amfolenic acid or GBR 12909, before 6-OHDA, provided full protection against DA depletion but produced marked NA depletion in frontal cortex. These rats did not demonstrate any degree of hyperactivity. Low doses ofd-amphetamine (0.25 mg/kg SC) or methylphenidate (1 mg/kg SC) reversed the hyperactivity in DA-depleted rats but increased motor activity in vehicle-treated and NA-depleted rats. Higher doses ofd-amphetamine (1 mg/kg) or methylphenidate (4 mg/kg) produced potentiated levels of locomotion but attenuated levels of rearing in DA-depleted animals. The results further suggest the utility of the neonatal DA lesion in rats as a potential animal model for derivation of therapeutic agents that may be efficacious in the treatment of the hyperkinetic syndrome.     

Long-term effects of electroconvulsive shock therapy on synthesis turnover and uptake of brain monoamines

The effects of electroconvulsive shocks administered once daily for 7 days (ECS x VII) on the synthesis, turnover, and uptake of dopamine (DA), noradrenalin (NA), and 5-hydroxytryptamine (5-HT) in rat brains were studied in vivo 1–7 days after the last ECS.The syntheses of catecholamines (CA) and 5-HT were estimated by measuring the accumulation of dihydroxyphenylalanine (Dopa) and 5-hydroxytryptophan (5-HTP), respectively, 30 min after administration of the inhibitor of L-amino acid decarboxylase NSD 1015 (3-hydroxybenzyl hydrazine HCl), in four brain regions; the DA-rich striatum, limbic parts, the NA-rich hemispheres, and the rest of the brain (rest). The turnover of CA and 5-HT were determined in whole brains on day 3 by their depletion 2.5 h after administration of the synthesis inhibitors H 44/68 (-methyl-p-tyrosine methylester HCl) and H 22/54 (-propyldopacetamide), respectively. The activities of the uptake mechanisms for CA and 5-HT at the level of the nerve cell membranes were estimated in whole brains on day 3 by the H 77/77 (4,-dimethyl-metatyrosine)-induced depletion of CA and the H 75/12-induced depletion of 5-HT, respectively. H 77/77 and H 75/12 were administered 4 and 2 h before sacrifice.The Dopa accumulation was increased in hemispheres and rest and unchanged in striatum and the limbic part by ECS x VII on days 1–7. The 5-HTP accumulation was unchanged by ECS x VII at all time intervals. The turnover of NA appeared slightly increased whereas the turnover of DA and 5-HT was unchanged by ECS x VII. The H 77/77-induced depletion of NA was partially antagonized by ECS x VII, suggestive of a slightly reduced uptake of NA. The H 77/77-induced depletion of DA and the H 75/12-induced depletion of 5-HT were unaffected by ECS x VII.The results indicate that ECS x VII induces a sustained increased activity in NA neurons but not in DA or 5-HT neurons.     

Discontinuation of chronic clonidine treatment: Evidence for facilitated brain noradrenergic neurotransmission

Summary Clonidine, 0.1 mg/kg i.p., or saline was administered to mice twice daily for 12 days. After withdrawal of the drug (14–18.5 h after the last injection), the accumulation of Dopa during 30 min after inhibition of central aromatic amino acid decarboxylase by NSD 1015 (3-hydroxy-benzyl hydrazine, 150 mg/kg i.p.) was significantly increased in the noradrenaline (NA)-rich, but dopamine (DA)-poor, brain stem but not in the DA-rich, but NA-poor corpus striatum. An increased Dopa accumulation was also found in the limbic system and, probably, in the hemispheres. The central accumulation of 5-hydroxytryptophan (5-HTP) was significantly increased in the limbic system. Clonidine, 0.1 mg/kg i.p., administered to mice during the withdrawal phase caused reduction of the accumulation of Dopa and 5-HTP (during 30 min after NSD 1015, 150 mg/kg i.p.) in all brain regions studied to approximately the same levels, as when given to saline-pretreated controls. The disappearance rates of brain NA and DA after synthesis inhibition (-methyl-p-tyrosine methylester HCl 250 mg/kg i.p., 4 h) were probably not altered by clonidine withdrawal.The locomotor stimulation by the central catecholamine receptor agonists apomorphine plus clonidine (1.5 mg/kg i.p. both) after reserpine (10 mg/kg, 4 h) pretreatment was significantly enhanced in clonidine withdrawal mice. However, the motor stimulation by apomorphine (1.5 mg/kg i.p.) alone after reserpine pretreatment was not affected. The spontaneous motor activity was not significantly altered by the withdrawal of clonidine.The data show that brain NA and 5-HT systems are affected by clonidine withdrawal in the opposite direction to that seen after acute administration of low doses of the drug, when they are inhibited. Since postsynaptic central -adrenoceptors, which are stimulated by high but not by low doses of clonidine, showed an enhanced response to high doses of clonidine following discontinuation of a chronic low dose regimen, supersensitivity due to functional noradrenergic denervation might be implicated.     

Postmortem instability of dopamine and its metabolites in rat striatum and limbic forebrain.

The level of dopamine (DA) and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 3-methoxytyramine (3-MT) were determined in the brains of rats kept 24 h after death at two different temperatures, 4 degrees C and 22 degrees C. The estimations were carried out in the striatum and limbic forebrain containing: nucleus accumbens, septum, limbic cortex, amygdala, tuberculum olfactorium. Brain tissue of control rats was dissected immediately after decapitation, frozen over solid CO2 and stored at -70 degrees C until assayed. DA and its metabolites were measured, using high-performance liquid chromatography (HPLC) with electrochemical detection. The levels of DA, DOPAC and HVA in the striatum were significantly decreased (from 50% to 80%) when rats were kept 24 h after death. The changes were more pronounced at 22 degrees C than at 4 degrees C. As the decrease in DA concentration was stronger than that of its final metabolite HVA, the ratio of HVA/DA concentration measured as an index of the rate of DA metabolism was even increased (from 8 to 11). Different changes occurred in the limbic region, where the levels of DA and HVA did not change neither at 4 degrees C nor 22 degrees C. The level of intraneuronally formed DA metabolite-DOPAC was elevated (by about 60%). The level of 3-MT, extraneuronally formed DA metabolite, was significantly increased both in the striatum (200%) and limbic DA structures (500%). These data demonstrate regional postmortal differences in stability of DA and its metabolite levels, which are in the striatum temperature-, time-, and storage-dependent. That implicates a careful assessment of postmortem studies when measuring the neurotransmitter dynamics in human necropsy material.     

Effects of amphetamine isomers, methylphenidate and atomoxetine on synaptosomal and synaptic vesicle accumulation and release of dopamine and noradrenaline in vitro in the rat brain

D- and L-amphetamine sulphate isomers, methylphenidate and atomoxetine, are effective treatments for attention-deficit hyperactivity disorder (ADHD). This study provides a detailed comparison of their effects on the synaptosomal and vesicular accumulation of dopamine (DA) and noradrenaline (NA) and release in vitro in rat prefrontal cortex and striatum. D-amphetamine was more potent than L-amphetamine at inhibiting accumulation of DA or NA in synaptosomes and vesicles. All drugs were weaker at inhibiting the accumulation of vesicular DA and NA compared to synaptosomal accumulation and more potently inhibited NA accumulation than DA. Methylphenidate was weak at inhibiting vesicular accumulation of DA and NA compared to its potent synaptosomal effects. The D-isomer had greater potency than the L-isomer on basal and electrically stimulated striatal DA release; however the L-isomer was 2-fold more potent than the D-isomer on basal fronto-cortical NA release. The selective DA reuptake inhibitor, GBR-12909 and NA reuptake inhibitors, maprotiline and atomoxetine, had different release profiles both on the potency and magnitude of basal and stimulated DA and NA release compared to the amphetamine isomers. These results identify distinct pharmacological action by the amphetamine isomers on dopaminergic and noradrenergic neurotransmission, which may impact on their therapeutic effects in the treatment of ADHD.     

A study of the neuronal and non-neuronal stores of dopamine in rat and rabbit kidney.

The present study has examined the effects of 6-hydroxydopamine (6-OHDA) alone and in combination with pargyline, desipramine and GBR 12909 and denervation as induced by occlusion of the renal artery (RAO) on the endogenous dopamine (DA) and noradrenaline (NA) contents in rat and rabbit renal tissues; the effects of chemical denervation on catecholamine levels in the left ventricle were also studied. In rat and rabbit renal medulla and rat renal cortex, 6-OHDA and pargyline plus 6-OHDA selectively reduced NA (85-92% reduction) without a parallel decrease in DA tissue content (19-27% reduction). This 6-OHDA- and pargyline plus 6-OHDA-insensitive DA pool was found to be resistant to denervation as induced by RAO. The NA-depleting effect of 6-OHDA in these renal areas was found to be prevented by the previous administration of desipramine, but not with that of GBR 12909. In the rabbit renal cortex, 6-OHDA selectively reduced NA (90% reduction) without a parallel depletion of DA (20% reduction); previous treatment with pargyline abolished this selectivity. Again, only desipramine, but not GBR 12909, was found to prevent the NA and DA depleting effect of 6-OHDA in the rabbit renal cortex. Denervation induced by RAO was also found to produce a parallel depletion of DA and NA tissue levels in this renal area. In the left ventricle, 6-OHDA alone or in combination with pargyline produced a parallel depletion of DA and NA tissue levels (79-88% reduction) in both species. These results provide evidence against the presence of independent dopaminergic neurones in rat and rabbit kidney and suggest that in rat and rabbit renal medulla and rat renal cortex most of DA is stored in a non-neuronal compartment; in rabbit renal cortex some of the DA appears to be located in noradrenergic neurones, in a store different from that which contains NA.     

Effects of taurine,homotaurine and GABA on hypothalamic and striatal dopamine metabolism

Summary To elucidate the effects of taurine on hypothalamic and striatal dopaminergic neurotransmission we compared its effects to those of -aminobutyric acid (GABA) and homotaurine (a GABA_A-receptor agonist) on hypothalamic and striatal concentrations of dopamine (DA) and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and, in the case of striatum, 3-methoxytyramine (3-MT) in rats. In addition, hypothalamic and striatal 5-hydroxytryptamine (5-HT) und 5-hydroxyindoleacetic acid, hypothalamic noradrenaline (NA) and 3-methoxy-4-hydroxyphenylglycol sulfate, and pituitary DA concentrations were also measured. The amino acids were injected into the lateral brain ventricles of conscious male rats in doses of 10 and 36 mol/rat, and rat were sacrificed 15 and 60 min later, respectively.Homotaurine (by 11010) but not the other two amino acids elevated striatal DA, whereas hypothalamic DA was increased by both taurine (36%) and homotaurine (31%). All three amino acids at 36 mol elevated striatal DOPAC, homotaurine (51%) more than taurine (31%) or GABA (30%), and hypothalamic DOPAC, both taurine (102%) and homotaurine (82%) clearly more than GABA (34%). Neither striatal nor hypothalamic HVA was altered by any of the amino acids. At 10 mol the amino acids decreased striatal 3-MT by about 40%. At 36 mol taurine and homotaurine reduced 3-MT by about 70%, whereas increasing the dose of GABA did not further reduce 3-MT. Both taurine and homotaurine at 36 mol decreased hypothalamic NA content. Neither hypothalamic nor striatal 5-HT metabolism was altered. In the neurointermediate lobe of the pituitary gland taurine at 10 mol but not at 36 mol slightly (20%) increased DA.These results show that taurine and homotaurine clearly alter the hypothalamic DA metabolism, and give further support to the suggestion that taurine and homotaurine similarly to GABA reduce the release of DA in the striatum. Homotaurine seems to alter the striatal DA metabolism more effectively than taurine or GABA. On the other hand, hypothalamic DA seems to be equally sensitive to taurine and homotaurine, and is clearly less sensitive to GABA. The pronounced elevation of hypothalamic DA and DOPAC after taurine and homotaurine could be related to changes in hypothalamic NA. Send offprint requests to L. Ahtee at the above address     

The effects of cinepazide on the content, biosynthesis and turnover of noradrenaline, dopamine and 5-hydroxytryptamine in the rat brain under room air and hypoxia

The effects of cinepazide, a vasodilator, on the content, biosynthesis and turnover of noradrenaline (NA), dopamine (DA) and 5-hydroxytryptamine (5-HT) in the rat brain were examined under room air and hypoxia (10% O2, 90% N2). Under room air, cinepazide had no significant effects on the content of NA, DA, 5-HT and 5-hydroxyindoleacetic acid (5-HIAA), the accumulation of 3,4-dihydroxyphenylalanine (DOPA) and 5-hydroxytryptophan (5-HTP) after central decarboxylase inhibition, and the depletion of NA, DA and 5-HT after synthesis inhibition. After 2 hr-exposure to hypoxia, the content of NA, 5-HT and 5-HIAA was decreased, whereas the content of DA was unchanged. The accumulation of DOPA and 5-HTP was decreased. The depletion of DA and 5-HT was inhibited by hypoxia, whereas the depletion of NA was unaffected. Under hypoxic conditions, cinepazide had no effects on the content of NA, DA and 5-HT, the accumulation of DOPA and 5-HTP, and the depletion of NA and DA, whereas cinepazide increased both the rate of 5-HT depletion and the content of 5-HIAA. The present data suggest that cinepazide selectively stimulates the functional activities of 5-HT neurons in the brain, which are depressed by hypoxia.     

3-Methoxytyramine accumulation: Effects of typical neuroleptics and various atypical compounds

Summary The accumulation of 3-methoxytyramine (3-MT), the O-methylated metabolite of dopamine (DA), in rat striatum was used to assess the effects of drugs on dopaminergic activity. This was accomplished by pretreating rats with pargyline to completely inhibit 3-MT catabolism. Under the conditions used, 3-MT accumulation was linear over time for at least 90 minutes. Apomorphine and -butyrolactone, drugs which depress the activity of DA-containing neurons, decreased striatal 3-MT accumulation; whereas typical neuroleptics (haloperidol, fluphenazine, chlorpromazine), which increase the activity of DA-containing neurons, increased striatal 3-MT accumulation. In addition, a number of other drugs which block DA receptors and exert various atypical actions on dopaminergic functioning were examined. These atypical compounds (clozapine, buspirone, molindone) also increased striatal 3-MT accumulation, but were generally less potent than the typical neuroleptics examined. Moreover, the potencies of the typical neuroleptics and atypical compounds that were tested appear to be somewhat related to their affinities for D-2 DA receptors, as measured by their abilities to displace ³H-spiperone from rat striatal membrane preparations. Interestingly, this relationship was less evident when NaCl was omitted from the ³H-spiperone binding assay buffer. The potential antipsychotic drugs, BW 234U and SCH 23390, were also investigated for their effects on 3-MT accumulation and ³H-spiperone binding, and they were relatively inactive in both of these measures of dopaminergic activity. The observation that SCH 23390, a D-1 DA receptor antagonist, had negligible effects on 3-MT accumulation suggests that D-1 receptor blockade, unlike D-2 blockade, may have minimal effects on DA release. The relevance of the above findings to understanding the effects of drugs on dopaminergic transmission and the advantages of measuring 3-MT accumulation instead of basal 3-MT concentrations are discussed.     

Formation of deaminated metabolites of dopamine in noradrenaline neurons

Summary The deaminated monoamine metabolites 3-methoxy-4-hydroxyphenylethyleneglycol (MOPEG), 3,4-di-hydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were determined electrochemically following organic solvent extraction and reverse-phase, high performance, liquid chromatography in four regions of the mouse brain. In the noradrenaline (NA)-predominant regions (hemispheres, brain stem), the ratio of the concentrations of DOPAC plus HVA to NA plus dopamine (DA) was approximately the same as in the DA-predominant regions (corpus striatum, limbic system). Yohimbine and reserpine elevated the concentrations of DOPAC and HVA both in the NA-and the DA-predominant regions. The effect of yohimbine was somewhat enhanced by the ₁-receptor blocking agent prazosin in the NA-predominant regions. The concentration of MOPEG was increased by yohimbine and decreased by reserpine.The concentrations of DOPAC and HVA were lowered by clonidine, but not by apomorphine in the NA-predominant regions of reserpine-treated mice. In, the DA-predominant regions, apomorphine, but not clonidine, reduced the concentrations of DOPAC and HVA. The effects of clonidine and apomorphine were reversed by yohimbine and haloperidol, respectively.The results indicate that the concentrations of the acid DA metabolites DOPAC and HVA in the NA-predominant regions reflect the rate of synthesis of DA in the NA neurons.     

Further in vitro and in vivo studies with the putative presynaptic dopamine agonist N,N-dipropyl-7-hydroxy-2-aminotetralin

Summary The in vitro binding of the putative dopamine autoreceptor agonist [³H]DP-7-ATN to rat striatal membrane homogenates was investigated. The maximum number of binding sites B _max was 497.5 ± 50.2 fmol/mg protein and the affinity constant K _D was 8.3 ± 1.5 nM using 10 M (+) butaclamol to define non-specific binding. Lesion of the left medium forebrain bundle by 6-hydroxydopamine resulted in an almost complete loss of dopamine in the striatum but did not affect the binding of [³H]DP-7-ATN. The binding of [³H]DP-7-ATN to the homogenates of the dopaminergic cell bodies in the substantia nigra revealed a B _max of 542.4 ± 40.1 fmol/mg protein and a K _D of 11.1 ± 1.3 nM. The pharmacological profile of the binding was characterized as being to D-2 receptors. No direct in vitro evidence could be found for a selective binding to DA autoreceptors. The dopamine uptake inhibitor GBR 12909 interacted in a noncompetitive manner with the in vitro binding of [³H]DP-7-ATN and the latter compounds uptake into isolated synaptosomes was not through the specific dopamine uptake system but rather through diffusion. GBR 12909 failed to reveal any agonistic or antagonistic activity in the GBL model but was able to antagonize the hypomotility in rats induced by 0.25 mg/kg DP-7-ATN. The inhibitory effect of DP-7-ATN on DA release was also demonstrated using in vivo brain dialysis in conscious rats. Based on the above results, the possibility is discussed that the release regulating DA autoreceptors, which might be coupled to the reuptake complex, and the DA biosynthesis regulating autoreceptors, are different entities. Send offprint requests to A. S. Horn     

The relative role of dopamine and norepinephrine receptor blockade in the action of antipsychotic drugs: Metoclopramide,thiethylperazine, and molindone as pharmacological tools

The aim of the present studies was to further delineate the role of striatal and limbic dopaminergic versus limbic noradrenergic blockade in the pharmacologic and perhaps therapeutic action of antipsychotic drugs. Metoclopramide shares many properties of antipsychotic neuroleptic drugs, including the production of extrapyramidal side effects, but is not an efficacious antipsychotic agent. This drug was a potent blocker of dopamine (DA) receptors in both striatum and limbic forebrain (comparable in potency to that of chlorpromazine) as evidenced from the increase in homovanillic acid in both brain structures. In contrast, metoclopramide was a weak inhibitor of the norepinephrine (NE) receptor-coupled adenylate cyclase system in the limbic forebrain. Molindone, which is reported not to block DA-sensitive adenylate cyclase, was a potent in vivo blocker of DA receptors in both striatum and limbic forebrain and also inhibited markedly the cyclic AMP response to NE in the limbic forebrain. The phenothiazine derivative, thiethylperazine, was also a potent blocker of DA receptors in both brain areas and displayed an IC₅₀ for NE blockade in the limbic forebrain comparable to that of chlorpromazine. The present results support the view that the ability and potency of drugs to block DA receptors parallels their ability and potency to cause extrapyramidal symptoms in man. Moreover, these results suggest that the blockade of NE receptor-coupled adenylate cyclase systems in brain may be relevant to both the pharmacologic and therapeutic activity of antipsychotic drugs.     

Increased dopamine turnover in the ventral striatum by 8-OH-DPAT administration in the rat

The administration of the 5-HT1A agonist 8-OH-DPAT (0.8 mumols kg-1 s.c.-40 min) produced an increase in dopamine (DA) turnover, estimated by the quotient (DOPAC + HVA) DA-1, in the ventral striatum of the rat. No statistically significant effects were obtained in the dorsal striatum. The accumulation of 3-MT in pargyline-treated animals (375 mumols kg-1 s.c.-60 min) was not affected by 8-OH-DPAT treatment (0.15-2.4 mumols kg-1 s.c.-30 min). These findings indicate that 8-OH-DPAT has weak antagonist properties at striatal DA receptors in normal rats. Both the 5-HT1A agonist flesinoxan (0.06-17.8 mumos kg-1 s.c. -50 min) and the mixed 5-HT1 and 5-HT2 agonist 5-MeODMT (1.6-26.0 mumols kg-1 s.c.-50 min) produced a decrease in forebrain 5-HTP accumulation (striatum and neocortex), following decarboxylase inhibition by means of NSD-1015 in reserpine treated rats, indicating stimulation of central 5-HT receptors by these two compounds. At the same time, the DOPA accumulation by the ventral striatum was decreased by flesinoxan and increased by 5-MeODMT treatment. These observations show that, under these conditions, the decrease in DA synthesis is not directly coupled to the decreased 5-HT synthesis produced by flesinoxan, as previously demonstrated for 8-OH-DPAT. Taken together with previous observations, the present results suggest that 8-OH-DPAT, depending on the experimental conditions, is an agonist or antagonist at striatal DA receptors, in all probability due to partial DA receptor agonist properties of the compound.