Daiichiro Nakahara

N-methylation of dopamine-derived 6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline, (R)-salsolinol, in rat brains : in vivo microdialysis study

Abstract: N‐Methylation of (R)‐1‐methyl‐6,7‐dihydroxy‐1,2,3,4‐tetrahydroisoquinoline [(R)‐salsolinol] derived from dopamine was proved by in vivo microdialysis study in the rat brain. The striatum was perfused with (R)‐salsolinol and N‐methylated compound was identified in the dialysate using HPLC and electrochemical detection with multichanneled electrodes. N‐Methylation of (R)‐salsolinol was confirmed in three other regions of the brain, the substantia nigra, hypothalamus, and hippocampus. In the substantia nigra, the amount of N‐methylated (R)‐salsolinol was significantly larger than in the other three regions. These results indicate that around dopaminergic neurons, particularly in the substantia nigra, (R)‐salsolinol was methylated into N‐methyl‐(R)‐salsolinol, which has a chemical structure similar to that of 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine, the selective dopaminergic neurotoxin. N‐Methylation of tetrahydroisoquinolines and β‐carbolines have already been proven to increase their toxicity to dopaminergic neurons and N‐methylation might be an essential step for these alkaloids to increase their toxicity. On the other hand, after perfusion of (R)‐salsolinol, release of dopamine and 5‐hydroxytryptamine was observed and inhibition of monoamine oxidase was indicated. (R)‐Salsolinol and its derivatives may be candidates for being dopaminergic neurotoxins.

increased dopamine and serotonin metabolism in rat nucleus accumbens produced by intracranial self-stimulation of medial forebrain bundle as measured by in vivo microdialysis

In the present study, we have used a newly developed microdialysis system to perfuse the nucleus accumbens (NAC) of conscious rats during spontaneous intracranial self-stimulation of the medial forebrain bundle (MFB). Chromatographic (HPLC-ECD) analysis of the perfusates showed that dopamine (DA) release increased, but with an unstable pattern during the actual period of self-stimulation. On the other hand, the main DA metabolites, 3,4-dihydroxyphenylacetic acid and homovanillic acid, and a serotonin metabolite 5-hydroxyindoleacetic acid, were all markedly enhanced by self-stimulation, but with different time courses. These findings indicate that self-stimulation of the MFB in rats induces increases in both DA and serotonin activities in the NAC. Such changes may be involved in mediating self-stimulation of the MFB.

Differential effect of self-stimulation on dopamine release and metabolism in the rat medial frontal cortex, nucleus accumbens and striatum studied by in vivo microdialysis

Changes in the extracellular levels of dopamine (DA) and its metabolites in the dopaminergic terminal regions, the medial frontal cortex (MFC), nucleus accumbens (NAC), and striatum (STR), were measured by microdialysis during self-stimulation of the medial forebrain bundle (MFB) in rats pretreated with the DA uptake inhibitor, nomifensine (1 mg/kg, i.p.). Self-stimulation of the MFB in nomifensine-pretreated rats caused an increase in the extracellular DA level in the MFC and NAC but not in the STR. Self-stimulation also increased the extracellular concentrations of the main DA metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) to a similar extent in the MFC and NAC and to a lesser extent in the STR. Thus, there was a regional difference in the neurochemical changes following self-stimulation with either the MFC or the NAC showing larger extracellular levels of DA, DOPAC, and HVA than the STR. Furthermore, these changes were observed on both hemispheres ipsilateral and contralateral to the stimulation. The results indicate that self-stimulation of the MFB preferentially activates the mesocorticolimbic DA systems, thereby bilateral increases in the release of DA and its metabolism being produced in their terminal regions, the MFC and NAC.

Acute effects of 1-methyl-4-phenylpyridinium ion (MPP+) on dopamine and serotonin metabolism in rat striatum as assayed in vivo by a micro-dialysis technique

The acute effect of 1-methyl-4-phenylpyridinium ion (MPP+), a neurotoxin derived from 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP), was examined by the in vivo micro-dialysis technique. A dialysis cannula was implanted into rat striatum, and the changes in the concentrations of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) in the perfusate every 20 min after administration of MPP+ were determined by high-performance liquid chromatography with electrochemical detection (HPLC-ED). After MPP+ administration the levels of DOPAC, HVA and 5-HIAA were markedly decreased. On the contrary the level of DA was markedly increased and reached a maximum 40 min after beginning of the MPP+ administration. By postmortem analysis of the striatal tissue MPP+ was proved to cause the inhibition of monoamine oxidase (MAO), especially MAO-B. These results suggest that the acute biochemical changes induced by MPP+ in vivo were MAO inhibition and release of DA.

The effect of uptake inhibition on dopamine release from the nucleus accumbens of rats during self- or forced stimulation of the medial forebrain bundle: a microdialysis study

Changes in dopamine (DA) release were measured in microdialysis samples taken from the nucleus accumbens (NAC) of rats pretreated with the DA uptake inhibitor, nomifensine (1 mg/kg, i.p.) during self- or forced stimulation of the medial forebrain bundle (MFB). Self-stimulation of the MFB in nomifensine-pretreated rats caused an increased release of DA in the NAC. In the same rats, similar increases in DA release were also found during forced stimulation, that is, during MFB stimulation in the absence of lever-pressing, but at current and rate parameters identical to those recorded in the previous self-stimulation session. The results indicate that self-stimulation of the MFB activates the mesolimbic DA system; similar neurochemical changes observed during self- and forced stimulation of the MFB suggest that the operant lever-pressing behavior itself did not influence DA release in the NAC.

Changes in monoamine levels in mouse brain elicited by forced-swimming stress, and the protective effect of a new monoamine oxidase inhibitor, RS-8359

As a stress model, a forced swimming test was applied to mice; and a typical behavioral change, an immobile posture, was recognized. This affected the brain monoamine levels significantly. The norepinephrine concentration was reduced, while that of its product was increased; and in the case of dopamine, both the amount of the amine and its product were increased. Stress increased the levels of serotonin and its product in the brain. The effects of RS-8359, (±)-4-(4-cyanophenyl)amino-6,7-dihydro-7-hydroxy-5H-cyclopenta[d]-pyrimidine, a new inhibitor of type A monoamine oxidase, on the behavioral and biochemical changes caused by forced swimming were also investigated. RS-8359 significantly improved the immobile posture elicited by the forced swimming test. It reduced the increased turnover of norepinephrine and serotonin systems caused by swimming. These results suggest that the effect of RS-8359 on behavioral and biochemical changes by stress may be mainly due to its effects on norepinephrine and serotonin systems, presumably by the inhibition of type A monoamine oxidase.

Endogenous synthesis of N-methylsalsolinol, an analogue of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, in rat brain during in vivo microdialysis with salsolinol, as demonstrated by gas chromatography—mass spectrometry

N-Methylsalsolinol, an analogue of 1,2,3,6-tetrahydropyridine, is present in the brains of patients with Parkinsons disease. To determine the metabolic pathway for the synthesis of N-Methylsalsolinol in the brain, salsolinol was perfused through the striatum or the substantia nigra of the rat brain by in vivo microdialysis. N-Methylsalsolinol was detected in the brain dialysate samples during microdialysis with salsolinol using gas chromatography-mass spectrometry with selected-ion monitoring. These results demonstrate that endogenous N-methylation of salsolinol into N-methylsalsolinol occurs in the brain in vivo.

The mechanism of perturbation in monoamine metabolism by L-DOPA therapy: in vivo and in vitro studies

In the cerebrospinal fluid of the patients with Parkinsons disease treated with L-DOPA, L-3-O-methyldopa was the major metabolite of administered L-DOPA. Using a dopaminergic cell model, clonal rat phenochromocytoma PC 12h cells, and by microdialysis of the rat striatum it was proved that L-3-O-methyldopa was taken up into monoamine neurons by transport system specific for aromatic L-amino acids and inhibited transport of L-DOPA and other amino acids competitively. L-3-O-Methyldopa depleted allosteric regulation of the biopterin cofactor on activity of tyrosine hydroxylase, the rate-limiting enzyme of catecholamine synthesis. Depletion of the allostery may perturb the buffer action of endogenous L-DOPA synthesis that stabilizes dopamine level in the brain. By these mechanisms L-3-O-methyldopa may reduce clinical effectiveness of administrated L-DOPA and be involved in wearing-off phenomenon. L-DOPA inhibited the activity of tryptophan hydroxylase and thus serotonin synthesis, which may be related to psychiatric side-effects in the patients under L-DOPA therapy.

Naturally-occurring isoquinolines perturb monoamine metabolism in the brain: studied by in vivo microdialysis

Naturally occurring isoquinolines affected the monoamine metabolism in the rat striatum, as proved by in vivo microdialysis technique. By analysis of monoamines and their metabolites in the dialysate, dopamine-derived 6,7-dihydroxy-1,2,3,4-tetrahydroisoquinolines were found to inhibit monoamine oxidase and catechol-O-methyltransferase activity. 1-Methyl- and 2-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline were found to inhibit activity of type A monoamine oxidase most markedly. To compare the structure-activity relationship, corresponding isoquinolines without a catechol structure were also examined. The inhibition by catechol isoquinolines was more manifest than those without a catechol structure. Among latter isoquinolines, N-methyl-isoquinolinium ion was the most potent inhibitor of monoamine oxidase. In addition, catechol isoquinolines increased monoamine levels in the brain. The number and the site of the methyl group are essentially required for the inhibition of monoamine oxidase and a catechol structure for that of catechol-O-methyl-transferase. These results are discussed in relation to possible involvement of these isoquinolines to the clinical features of some neuro-psychiatric diseases, such as alcoholism or in L-DOPA therapy.

Selective release of serotonin by endogenous alkaloids, 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinolines, (R)- and (S)salsolinol, in the rat striatum; in vivo microdialysis study

Dopamine-derived 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinolines, (R)- and (S)salsolinol, released an enormous amount of serotonin in the rat striatum; the concentration of serotonin increased from undetectable level to 2.53 +/- 0.12 and 3.69 +/- 0.01 microM after perfusion of (R)- and (S)salsolinol, respectively. Salsolinols increased extracellular dopamine level, but to a much lesser degree than serotonin. Other naturally occurring isoquinolines with catechol structure released serotonin and dopamine, but salsolinols were the most potent and selective releaser of serotonin. Serotonin release by salsolinols may be involved in some psychiatric symptoms in L-DOPA therapy or alcoholism.