Chlorimipramine,fenfluramine and quipazine decrease 5-hydroxytryptamine synthesis in discrete rat brain nuclei

A procedure for studying 5-hydroxytryptamine synthesis by determining the rate of accumulation of 5-hydroxytryptophan after administering m-hydroxybenzylhydrazine, an inhibitor of aromatic-l-amino acid decarboxylase, and large doses of l-tryptophan was characterized. The utility of this method as an index of 5-hydroxytryptamine neuronal activity was studied by determining the effects on 5-hydroxytryptophan accumulation of direct and indirect 5-hydroxytryptamine agonists; viz, chlorimipramine-a 5-hydroxytryptamine uptake inhibitor, fenfluramine-a 5-hydroxytryptamine releaser, and quipazine-a 5-hydroxytryptamine receptor agonist. In the absence of m-hydroxybenzylhydrazine pretreatment 5-hydroxytryptophan and the dopamine precursor 3,4-dihydroxyphenylalanine were not readily detectable in any brain region studied. They both accumulated after m-hydroxybenzylhydrazine treatment in a time-dependent manner with the 30 min time point being on the linear portion of the curve. Administration of l-tryptophan 60 min before sacrifice increased 5-hydroxytryptophan, but not 3,4-dihydroxyphenylalanine, in a dose-related manner with the peak effect occurring after 100–300 mg/kg. Chlorimipramine, fenfluramine and quipazine all decreased 5-hydroxytryptophan, but not 3,4-dihydroxyphenylalanine, in m-hydroxybenzylhydrazine and l-tryptophan-treated animals. Chlorimipramine produced these effects in a dose-related manner only after l-tryptophan loading and without affecting brain concentrations of l-tryptophan. These results suggest that the measurement of 5-hydroxytryptophan after l-tryptophan administration and aromatic-l-amino acid decarboxylase inhibition might serve as a useful index of 5-hydroxytryptamine synthesis.     

Simultaneous Determination of 5-Hydroxytryptophan and 3,4-Dihydroxyphenylalanine in Rat Brain by HPLC with Electrochemical Detection Following Electrical Stimulation of the Dorsal Raphe Nucleus

HPLC coupled with electrochemical detection was used to make concurrent measurements of the rate of accumulation of 5-hydroxytryptophan and 3,4-dihydroxyphenylalanine in selected brain regions (striatum, nucleus accumbens, septum, medial periventricular hypothalamus) and thoracic spinal cords of rats treated with NSD 1015, an inhibitor of aromatic-L-amino-acid decarboxylase. 5-Hydroxytryptophan and 3,4-dihydroxyphenylalanine accumulated in all brain regions 30 min after the intravenous infusion of various doses of NSD 1015; there were no significant differences in the responses to 12.5, 25, 50, and 100 mg/kg. After the intravenous administration of 25 mg/kg NSD 1015 the concentrations of 5-hydroxytryptophan and 3,4-dihydroxyphenylalanine increased linearly with time in all brain regions for at least 30 min. Electrical stimulation of 5-hydroxytryptamine neurons in the dorsal raphe nucleus for 30 min at 5 or 10 Hz increased 5-hydroxytryptophan accumulation in all brain regions but not in the spinal cord. Unexpectedly, this stimulation also increased the accumulation of 3,4-dihydroxyphenylalanine in the hypothalamus and spinal cord. These results suggest that 5-hydroxytryptophan accumulation following the administration of NSD 1015 is a valid index of 5-hydroxytryptamine neuronal activity in the brain.     

Effects of chronic antidepressant treatment on nigrostriatal and mesolimbic dopamine autoreceptors in the rat

Dopamine autoreceptors were studied by determining the effects of chronic antidepressant treatment on the ability of several doses of apomorphine to decrease 3,4-dihydroxyphenylalanine accumulation (an index of dopamine synthesis in vivo) after saline or γ-hydroxybutyric acid lactone (γ-butyrolactone). 3,4-Dihydroxyphenylalanine accumulation was measured in nigrostriatal [nucleus caudatus putamen] and mesolimbic [nucleus accumbens and tuberculum olfactorium] nerve terminals. Apomorphine decreased 3,4-dihydroxyphenylalanine accumulation in the nucleus caudatus putamen, tuberculum olfactorium and nucleus accumbens in a dose-related manner. Chronic imipramine (10 days) treatment attenuated the low and high dose apomorphine-induced decrease in 3,4-dihydroxyphenylalanine accumulation in the nucleus caudatus putamen to a greater extent than the tuberculum olfactorium or nucleus accumbens. In γ-butyrolactone-treated animals chronic treatment with imipramine, amitriptyline or bupropion (10 days) attenuated the low dose apomorphine effect in the nucleus caudatus putamen, but not the tuberculum olfactorium or nucleus accumbens. Only 2 days of imipramine treatment had no effect on the apomorphine-induced decrease in 3,4-dihydroxyphenylalanine accumulation in the nucleus caudatus putamen with or without γ-butyrolactone treatment. These data suggest that chronic treatment with three antidepressants produces dopamine autoreceptor subsensitivity in nigrostriatal neurons more than mesolimbic neurons and that this effect is not seen with short-term imipramine treatment.     

Serotonin Agonists Reduce Dopamine Synthesis in the Striatum Only when the Impulse Flow of Nigro-Striatal Neurons Is Intact

The effects of 5-methoxy-N, N-dimethyltryptamine (5-MeO-DMT) and m-chlorophenylpiperazine (CPP), two 5-hydroxytryptamine (5-HT, serotonin) agonists, on the accumulation of 3,4-dihydroxyphenylalanine (DOPA] were studied in the striatum of rats treated with gamma-butyrolactone (GBL). Unlike 2 mg/kg i.p. apomorphine, neither 5 mg/kg i.p. 5-MeO-DMT nor 2.5 mg/kg i.p. CPP significantly reduced the GBL-induced increase in DOPA accumulation in the striatum. 5-MeO-DMT and CPP significantly reduced DOPA accumulation in animals that had received the aromatic amino acid decarboxylase inhibitor Ro 4-4602 but not GBL. 5-HT (10 micrograms in 0.5 microliter) injected in the substantia nigra, pars compacta, like GBL, significantly increased Ro 4-4602-induced accumulation of DOPA in the striatum. The data indicate that 5-HT agonists can reduce 3,4-dihydroxyphenylethylamine (DA, dopamine) synthesis in the striatum of rats only when the impulse flow of DA neurons is intact. An indirect effect through mechanisms controlling DA synthesis in the striatum, for instance cholinergic and GABA-ergic neurons, is suggested.     

Evidence that histamine-stimulated prolactin secretion is not mediated by an inhibition of tuberoinfundibular dopaminergic neurons.

The effects of histamine on prolactin secretion and the activity of tuberoinfundibular dopaminergic (DA) neurons were examined in male rats. Tuberoinfundibular DA neuronal activity was estimated in situ by measuring the metabolism [concentration of 3,4-dihydroxyphenylacetic acid (DOPAC)] and synthesis [accumulation of 3,4-dihydroxyphenylalanine (DOPA) after administration of a decarboxylase inhibitor] of dopamine in the median eminence. Intracerebroventricular (icv) injection of histamine produced a dose- and time-dependent increase in plasma prolactin levels but had no effect on DOPA accumulation or DOPAC concentrations in the median eminence. These results indicate that the stimulation of prolactin secretion following icv histamine is not mediated by an inhibition of tuberoinfundibular DA neurons.     

Effect of 3,4-Methylenedioxymethamphetamine on 3,4-Dihydroxyphenylalanine Accumulation in the Striatum and Nucleus Accumbens

The effect of the racemic mixture of 3,4-methylenedioxymethamphetamine (MDMA) on the synthesis of dopamine in the terminals of nigrostriatal and mesolimbic neurons was estimated by measuring the accumulation of 3,4-dihydroxyphenylalanine (DOPA) in the striatum and nucleus accumbens 30 min following the administration of the L-aromatic amino acid decarboxylase inhibitor, 3-hydroxybenzylhydrazine. MDMA produced an increase in DOPA accumulation in the striatum which was greater in magnitude and longer in duration than that in the nucleus accumbens. Although the concentrations of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in both the striatum and nucleus accumbens were reduced 3 h following an injection of MDMA (20 mg/kg), 5-HT and 5-HIAA concentrations were significantly reduced only in the striatum 7 days after the administration of MDMA. Pretreatment with a 5-HT2 antagonist, ketanserin, significantly attenuated the reduction in 5-HT concentration in the striatum 3 h following MDMA administration and completely blocked 5-HT depletion at 7 days post administration. Moreover, ketanserin completely blocked MDMA-induced DOPA accumulation in the striatum. The results obtained in these studies suggest that MDMA activates nigrostriatal dopaminergic pathways via 5-HT2 receptors. In addition, these data are supportive of the hypothesis that dopamine plays a role in MDMA-induced 5-HT depletion.     

Microspectrofluorimetric analysis of the formaldehyde-induced fluorophores of 5-hydroxytryptamine and dopamine in intrapulmonary neuroepithelial bodies after administration ofl-hydroxytryptophan andl-DOPA

Summary To demonstrate the intracellular store of 5-hydroxytryptamine and dopamine in pulmonary neuroepithelial bodies of the neonatal rabbit after treatment with the corresponding amino-acid precursorsl-5-hydroxytryptophan orl-3,4-dihydroxyphenylalanine, formaldehyde-induced flourescence in combination with microspectrofluorimetric analysis has been used. Emission spectra and excitation spectra in an extended wavelength range from 240 to 460 nm, the displacement of excitation peaks after exposure to hydrochloric acid vapour, and calculation of peak ratio values 410/260, 380/320, 320/260 for phenylethylamine fluorophores and 385/315 for indolylethylamine fluorphores were performed. Thus, the presence of 5-hydroxytryptamine without occurrence of 5-hydroxytryptophan was demonstrated in pulmonary neuroepithelial bodies after administration of the corresponding biological precursor, while dopamine combined with 5-hydroxytryptamine were clearly revealed after administration ofl-3,4-dihydroxyphenylalanine. The rate of photodecomposition always corroborated these findings.Dedicated to Prefessor Dr. T.H. Schiebler on the occasion of his 65th birthdaySupported by grant nr. 3.0059.81 (to D.W.S.) from the Fund for Medical Scientific Research (Belgium)     

High-performance liquid chromatographic analysis of monoamines and some of theirγ-glutamyl conjugates produced by the brain and other tissues ofHelix aspersa (gastropoda)

1. Earlier reports from this and other laboratories have indicated that wide variations exist in estimates of the concentrations of norepinephrine in the brain and heart of the snail Helix aspersa. This is a report on investigations of norepinephrine concentrations in Helix aspersa tissues using high-performance liquid chromatography with electrochemical detection. In addition, the effects of treatment with some amino acid precursors or enzyme inhibitors on the concentrations of norepinephrine, dopamine, 5-hydroxytryptamine, and some of their metabolites were investigated. 2. The levels of norepinephrine in the brain were low (46 ng/g) in comparison to dopamine (2.1) micrograms/g) and 5-hydroxytryptamine (2.6 micrograms/g). Epinephrine was not observed in either snail heart of snail nervous tissue. 3. Administration of L-3,4-dihydroxyphenylalanine resulted in elevated snail brain dopamine, while 3,4-dihydroxyphenylserine treatment increased norepinephrine. Treatment with blockers of tyrosine hydroxylase and aromatic-L-amino acid decarboxylase reduced dopamine concentrations without affecting 5-hydroxytryptamine. 4. The dopamine metabolite 3,4-dihydroxyphenylacetic acid was observed only after administration of L-3,4-dihydroxyphenylalanine or dopamine and then only in very small amounts. At no time was the dopamine metabolite homovanillic acid or the 5-hydroxytryptamine metabolite 5-hydroxyindoleacetic acid observed in brain, heart, or whole-body extracts of the snail. 5. Incubation of nervous tissue with either dopamine or 5-hydroxytryptamine resulted in the production of electrochemically active metabolites which were identified by oxidation characteristics and cochromatography with synthesized standards as the gamma-glutamyl conjugates of the amines. Treatment of snails with 5-hydroxytryptamine or dopamine also resulted in the production of gamma-glutamyl conjugates. 6. The present experiments show that great care must be exercised when measuring monoamines and their metabolites in gastropod tissues by high-performance liquid chromatography with electrochemical detection.     

Effect of Precursor Loading on the Synthesis Rate and Release of Dopamine and Serotonin in the Striatum: A Microdialysis Study in Conscious Rats

The effects of systemic administration of tyrosine and phenylalanine on the extracellular levels of tyrosine and dopamine were determined by microdialysis in the striatum of awake rats. In addition, the effects of these precursors on in vivo 3,4-dihydroxyphenylalanine (DOPA) formation were determined during continuous infusion of a decarboxylase inhibitor. Both precursors increased the dialysate levels of tyrosine sixfold, but only phenylalanine administration stimulated DOPA formation. However, neither precursor affected the release of dopamine. When the precursor administration was repeated in rats in which the release of dopamine was stimulated by haloperidol pretreatment, again no effect was seen on the release of dopamine. Systemic administration of tryptophan (100 mg/kg, i.p.) during continuous infusion of a decarboxylase inhibitor induced a threefold increase in the formation of 5-hydroxytryptophan and caused an increase in the release of serotonin during infusion of an uptake inhibitor to about 150% of controls. Finally, we investigated whether dietary precursors were able to influence neurotransmitter formation and release. Rats trained to consume their daily food in a period of 2 h were implanted with microdialysis probes. Scheduled eating induced a small increase in the extracellular levels of tyrosine (135% of controls), but the release of dopamine and the formation of 5-hydroxytryptophan during continuous infusion of a decarboxylase inhibitor were not affected.     

Activation of tuberohypophysial dopamine neurons following intracerebroventricular administration of the selective kappa opioid receptor antagonist NOR-binaltorphimine.

The effect of the kappa opioid receptor antagonist nor-binaltorphimine (NOR-BNI) was examined on the activity of dopamine (DA) neurons comprising the nigrostriatal, mesolimbic, and tuberohypophysial systems in the male rat. DA neuronal activity was estimated by measuring: (1) the concentration of the DA metabolite 3,4-dihydroxyphenylacetic acid and, (2) the accumulation of 3,4-dihydroxyphenylalanine after administration of a decarboxylase inhibitor in brain (striatum, nucleus accumbens) and pituitary regions (intermediate lobe, neural lobe) containing terminals of these neurons. The intracerebroventricular administration of NOR-BNI produced a dose- and time-related increase in the activity of tuberohypophysial DA neurons, but failed to alter the activity of nigrostriatal or mesolimbic DA neurons. The ability of NOR-BNI to enhance the activity of tuberohypophysial DA neurons was blocked by the kappa opioid agonist U-50,488. These results indicate that NOR-BNI, acting on kappa opioid receptors, activates tuberohypophysial DA neurons projecting to the neural and intermediate lobes of the pituitary.     

Exogenous Tryptophan Increases Synthesis, Storage, and Intraneuronal Metabolism of 5-Hydroxytryptamine in the Rat Hypothalamus

The effects of tryptophan administration on neurochemical estimates of synthesis [5-hydroxytryptophan (5-HTP) accumulation following administration of a decarboxylase inhibitor], storage [5-hydroxytryptamine (5-HT) concentrations], and metabolism [5-hydroxyindoleacetic acid (5-HIAA) concentrations] of 5-HT in selected regions of the hypothalamus were determined using HPLC coupled to an electrochemical detector. Tryptophan methyl ester HCl (30-300 mg/kg i.p.) produced a dose-dependent increase in the rate of 5-HTP accumulation throughout the hypothalamus but had no effect on the rate of accumulation of 3,4-dihydroxyphenylalanine. Peak 5-HTP levels were attained by 30 min following administration of tryptophan (100 mg/kg i.p.) and were maintained for an additional 60 min. Tryptophan also produced concomitant dose-dependent increases in 5-HT and 5-HIAA concentrations in these same regions without changes in the 5-HIAA/5-HT ratio. These results indicate that exogenous tryptophan administration selectively increases the synthesis, storage, and metabolism of 5-HT in the hypothalamus without altering the synthesis of catecholamines. Inhibition of 5-HT uptake with chlorimipramine or fluoxetine produced modest (10-40%) reductions in 5-HIAA concentrations throughout the hypothalamus, revealing that only a minor portion of 5-HIAA is derived from released and recaptured 5-HT, whereas the major portion of this metabolite reflects intraneuronal metabolism of unreleased 5-HT. In both chlorimipramine- and fluoxetine-treated rats, 5-HIAA concentrations were significantly increased by tryptophan administration, indicating that the increase in synthesis of 5-HT following precursor loading is accompanied by an increase in the intraneuronal metabolism of 5-HT.     

Turnover of Dopamine and Serotonin and Their Metabolites in the Striatum of Aged Rats

Turnover of dopamine (DA), serotonin [5-hydroxytryptamine (5-HT)], and their metabolites has been measured in adult and aged rats. Turnover rates of 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxy-3-indoleacetic acid (5-HIAA) have been assayed from the disappearance rates after blocking by pargyline inhibition of monoamine oxidase (MAO) and from the accumulation rates by probenecid inhibition of the probenecid-sensitive transport system. DA and 5-HT turnover rates have been measured as accumulation rates of 3,4-dihydroxyphenylalanine and 5-hydroxytryptophan, respectively, after central decarboxylase inhibition by 3-hydroxybenzylhydrazine (NSD-1015) and as accumulation rates of DA and 5-HT after pargyline inhibition of MAO. The DA turnover rate after NSD-1015 was 23.9% lower in aged rats than in adults, whereas after pargyline there was no significant difference between the two age groups. The HVA fractional rate constant and turnover after pargyline were lower in aged rats than in adults, and HVA turnover after probenecid was higher in aged rats than in adults. The DOPAC-HVA pathway seems to be reinforced at the expense of DOPAC conjugation. In aged and adult rats whose 5-HT steady-state levels were not statistically different, the 5-HT turnover rate after pargyline and NSD-1015 treatment was lower in aged rats than in adults. An increase of 5-HIAA levels after pargyline and probenecid treatment in aged rats could be due to the handling stress.     

A Comparison of Biochemical Indices of 5-Hydroxytryptaminergic Neuronal Activity Following Electrical Stimulation of the Dorsal Raphe Nucleus

The activity of 5-hydroxytryptaminergic neurons has been estimated from measurements of: concentrations of 5-hydroxyindoleacetic acid; the ratio of the concentrations of 5-hydroxyindoleacetic acid to 5-hydroxytryptamine; the rate of accumulation of 5-hydroxytryptophan following the administration of an aromatic L-amino acid decarboxylase inhibitor (e.g., NSD 1015); the rate of accumulation of 5-hydroxytryptamine, and the rate of decline of 5-hydroxyindoleacetic acid following the administration of a monoamine oxidase inhibitor (e.g., pargyline). The purpose of the present study was to compare these different methods under conditions of changing neuronal impulse traffic produced by electrical stimulation of 5-hydroxytryptaminergic neurons. Male rats anesthetized with chloral hydrate were killed following 0, 15, or 30 min of electrical stimulation of the dorsal raphe nucleus at a frequency of 0, 5, or 10 Hz. The concentrations of 5-hydroxytryptamine, 5-hydroxyindoleacetic acid, and 5-hydroxytryptophan in nucleus accumbens, amygdala, suprachiasmatic nucleus, and dorsomedial nucleus were measured using HPLC coupled to an electrochemical detector. In each brain region, stimulation elicited an increase in the concentration of 5-hydroxyindoleacetic acid and the 5-hydroxyindoleacetic acid/5-hydroxytryptamine concentration ratio in saline-treated animals and an increase in 5-hydroxytryptophan accumulation in NSD 1015-treated animals, but did not alter the concentration of 5-hydroxytryptamine or 5-hydroxyindoleacetic acid in pargyline-treated rats. The results o f this study indicate that although the first three methods serve as valid indices of 5-hydroxytryptaminergic neuronal activity, the pargyline-dependent techniques are not responsive to changes in the rate of 5-hydroxytryptamine nerve firing.     

Effects of pyridoxine deficiency and dl-p-chlorophenylalanine administration to rats on 5-hydroxytryptamine and noradrenaline concentrations in brain and 5-hydroxytryptamine concentration in blood

Pyridoxine deficiency in post-weanling rats caused a marked decrease in body weight and a small but significant decrease in brain weight. Although the concentration of circulating 5-hydroxytryptamine was markedly decreased, the concentrations of 5-hydroxytryptamine and noradrenaline in the brain were not affected. p-Chlorophenylalanine, an inhibitor of 5-hydroxytryptamine synthesis, decreased the 5-hydroxytryptamine content of brain to very low values in both the deficient and control animals, whereas the noradrenaline contents were not appreciably affected. The concentration of 5-hydroxytryptamine in blood, the origin of which is primarily gastrointestinal, was decreased only in the controls but not in the deficient animals after p-chlorophenylalanine treatment. These results suggest that whereas l-tryptophan hydroxylase (EC 1.14.3.2) is rate-limiting in the brain as has been reported by others, the pyridoxal 5'-phosphate-dependent enzyme 5-hydroxytryptophan decarboxylase (EC 4.1.1.28) may be more important in the gastrointestinal tract in the regulation of 5-hydroxytryptamine synthesis.     

A method for the direct detection of aromatic amino acid decarboxylase in electrophoretic media

Aromatic amino acid decarboxylase is an enzyme of broad specificity which decarboxylates a range of aromatic amino acids including 3,4-dihydroxyphenylalanine and 5-hydroxytryptophan, which yield dopamine and serotonin, respectively. The name aromatic amino acid decarboxylase was applied to the enzyme by workers (1–3) who found that, on purification, a single enzyme appeared to be responsible for activities previously ascribed to two separate enzymes. The separate enzymes had been named 5-hydroxy-l-tryptophan carboxylyase (EC 4.1.1.28) and 3,4-dihydroxy-l-phenylalanine carboxylyase (EC 4.1.1.26). More recent papers (4–7) still leave unresolved the question of whether one enzyme or two is responsible for the two activities.     

Aromatic L-Amino Acid Decarboxylase Activity of Mouse Striatum Is Modulated via Dopamine Receptors

Abstract: Aromatic L-amino acid decarboxylase (AAAD) activity is enhanced in the striatum of control and MPTP-treated mice after administration of a single dose of the dopamine receptor antagonists haloperidol, sulpiride, and SCH 23390. MPTP-treated mice appear more sensitive to the antagonists, i.e., respond earlier and to lower doses of antagonists than control mice. The rise of AAAD activity induced by the antagonists is prevented by pretreatment with cycloheximide. The apparent K _m values for L-3,4-dihydroxyphenylalanine (L-DOPA) and pyridoxal 5-phosphate appear unchanged after treatment with the antagonists. Increased AAAD activity was observed also after subchronic administration of dopamine receptor antagonists or treatment with reserpine. A single dose of a selective dopamine receptor agonists had no effect on AAAD activity. In contrast, administration of L-DOPA, quinpirole, or SKF 23390 for 7 days lowers AAAD activity in the striatum. We conclude that AAAD is modulated in striatum via dopaminergic receptors.     

Monoamine Synthesis and Concentration in Neonatal Rat Brain During Hypercapnia and Recovery

One-day-old rats were exposed to a gas mixture of 15% CO2-21% O2-64% N2 for a 30-min period. Monoamine synthesis in whole brain was measured during, and at various intervals after, hypercapnia by estimating the accumulation of dihydroxyphenylalanine (DOPA) and 5-hydroxytryptophan (5-HTP) after inhibition of aromatic L-amino-acid decarboxylase with NSD 1015. Endogenous concentrations of tyrosine, dopamine (DA), noradrenaline (NA), tryptophan, 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were measured at the same intervals. Exposure to CO2 induced an increased synthesis of catecholamines and 5-HT. Further, an increase in DA concentration was seen during hypercapnia, while NA and 5-HT were unchanged. After the CO2 exposure the increased in vivo synthesis rates of catecholamines and 5-HT were rapidly normalized, as was the endogenous DA concentration. A slight increase in 5-HT and 5-HIAA concentrations was seen immediately after CO2 exposure. These results indicate that in neonatal animals, hypercapnia induces changes in central monoamine neurons, primarily an increased synthesis. These alterations may be relevant to some physiological changes seen during CO2 exposure, such as the alteration in central respiratory performance.     

Sexual differences in the activity of periventricular-hypophysial dopaminergic neurons in rats.

The activities of periventricular-hypophysial dopaminergic (DA) neurons were compared in male and female rats by measuring dopamine synthesis (accumulation of 3,4-dihydroxyphenylalanine [DOPA] after inhibition of L-aromatic amino acid decarboxylase) and metabolism (concentrations of 3,4-dihydroxyphenylacetic acid [DOPAC]) in terminals of these neurons in the intermediate lobe of the pituitary. For comparison, the synthesis and metabolism of dopamine in the neural lobe of the pituitary and median eminence were also determined. The concentrations of DOPAC and accumulation of DOPA were higher in females than in males in both the intermediate lobe and median eminence, revealing a sexual difference in the basal activity of periventricular-hypophysial and tuberoinfundibular DA neurons. In contrast, there were no differences between male and female rats in activity of DA neurons terminating in the neural lobe. One week following gonadectomy, DOPA accumulation in the median eminence was decreased in females and increased in males, but remained unchanged in the intermediate lobe. These results indicate that sexual differences in the activity of periventricular-hypophysial DA neurons terminating in the intermediate lobe are not dependent upon the presence of circulating gonadal steroids, and in this respect, these neurons differ from tuberoinfundibular DA neurons.     

6R-L-erythro-5,6,7,8-tetrahydrobiopterin as a regulator of dopamine and serotonin biosynthesis in the rat brain

6R-L-Erythro-tetrahydrobiopterin (6R-BH4), the natural isomer of tetrahydrobiopterin, was synthesized from 7,8-dihydrobiopterin using dihydrofolate reductase. The effects of intracerebroventricular injection of 6R-BH4 on the biosyntheses of neurotransmitter monoamines in the rat brain were investigated by measuring accumulation of 3,4-dihydroxyphenylalanine (DOPA) and 5-hydroxytryptophan (5-HTP) after the inhibition of aromatic L-amino acid decarboxylase by NSD 1015 and the contents of metabolites of dopamine (DA) and 5-hydroxytryptamine (5-HT). The formation of DOPA and 5-HTP increased after the injection, reached a maximum level at about 1 h, then leveled off and reached a plateau over 2 h up to 8 h. The formation of DOPA and 5-HTP increased dose-dependently in the whole brain after the injection of 6R-BH4, and reached a plateau when the dose was more than 300 micrograms/rat. The enhancement was 100 and 70% for DOPA and 5-HTP, respectively. The formation of DOPA and 5-HTP increased in four brain regions, but the degree of stimulation was different among the brain regions. The contents of DA and 5-HT metabolites increased after the injection of 6R-BH4 in all brain regions tested, especially in the diencephalon and brain stem. The contents of DA and 5-HT increased slightly after the injection of 6R-BH4. These results suggest that 6R-BH4 concentration may be submaximal within DA and 5-HT neurons, and that an increase in 6R-BH4 in the brain enhances the biosyntheses of DA and 5-HT.     

Structural insight into Parkinson's disease treatment from drug-inhibited DOPA decarboxylase.

DOPA decarboxylase (DDC) is responsible for the synthesis of the key neurotransmitters dopamine and serotonin via decarboxylation of L-3,4-dihydroxyphenylalanine (L-DOPA) and L-5-hydroxytryptophan, respectively. DDC has been implicated in a number of clinic disorders, including Parkinson's disease and hypertension. Peripheral inhibitors of DDC are currently used to treat these diseases. We present the crystal structures of ligand-free DDC and its complex with the anti-Parkinson drug carbiDOPA. The inhibitor is bound to the enzyme by forming a hydrazone linkage with the cofactor, and its catechol ring is deeply buried in the active site cleft. The structures provide the molecular basis for the development of new inhibitors of DDC with better pharmacological characteristics.