Simultaneous measurement of tyrosine, tryptophan and related monoamines for determination of neurotransmitter turnover in discrete rat brain regions by liquid chromatography with electrochemical detection

Neurochemical mechanism of action of drugs which modify feeding via the serotoninergic system

The neurochemical mechanisms by which drugs acting on central serotoninergic system modify feeding were reviewed. Fenfluramine, a clinically effective appetite suppressant, releases serotonin from nerve terminals and inhibits its reuptake, and considerable evidence suggests that these effects mediate its anorectic activity. The D isomer of fenfluramine is particularly specific in affecting serotonin mechanisms and causing anorexia. Transmitters other than serotonin such as acetylcholine, catecholamines and GABA are also affected by systemic administration of fenfluramine, but some of these effects are secondary to fenfluramine's action on serotoninergic mechanisms. Moreover, there is no evidence that these brain substances are involved in fenfluramine's ability to cause anorexia. Several studies with drugs affecting different serotonin mechanisms such as release and uptake or mimicking the action of serotonin at post-synaptic receptors suggest that increase serotonin release and direct stimulation of postsynaptic receptors are the most effective mechanisms for causing depression of food intake, although inhibition of serotonin uptake may also contribute in appropriate conditions. Development of serotonin receptor hyposensitivity and, in some instances, decreased serotonin levels may lead to tolerance to the anorectic activity of drugs enhancing serotonin transmission, the degree of this depending critically on the type of effect on serotonin mechanisms and intensity and duration of serotonin receptor activation. Recent evidence suggests that a decrease in serotonin function causes stimulation of feeding. This may lead to development of new strategies for the treatment of clinical anorexias.

Effect of repeated dietary exposure of aflatoxin B1 on brain biogenic amines and metabolites in the rat

Male Sprague-Dawley rats were treated po twice weekly for 3 weeks with a low (32.8 micrograms/kg) and high dose (327.9 micrograms/kg) of aflatoxin B1 (AFB1) in corn oil. A control group received corn oil only. At the end of the experiment the rats were killed, and the concentrations of the brain catecholamines, norepinephrine (NE) and dopamine (DA), catecholamine metabolites, 3-methoxy-4-hydroxymandelic acid (VMA), homovanillic acid (HVA), and dihydroxyphenylacetic acid (DOPAC), and the indoleamine serotonin (5-HT) and its metabolite, 5-hydroxyindoleacetic acid (5-HIAA), were determined by high-pressure liquid chromatography in five brain regions. The major effects were found in striatal dopamine and serotonin concentrations, with decreases of 37 and 29%, respectively. A corresponding decline was observed in the dopamine metabolites, homovanillic acid (44%) and dihydroxyphenylacetic acid (30%). Concentrations of these neurotransmitters and metabolites were only marginally altered in cerebral cortex, cerebellum, hypothalamus, and medulla oblongata. It appears that a major effect of AFB1 is on dopaminergic pathways, possible by selectively perturbing the conversion of tyrosine to biogenic catecholamine neurotransmitters.

An in vivo steady-state method for the determination of catecholamine biosynthesis in the rat brain using high-performance liquid chromatography with electrochemical detection

A technique is described for the measurement of steady-state catecholamine (CA) synthesis in the rat brain in vivo, using [3H]tyrosine incorporation with high-performance liquid chromatography (HPLC) and electrochemical detection. Adult male rats chronically implanted with lateral intracerebroventricular (i.c.v.) cannulas, were injected i.c.v. with [3H]tyrosine. CA and tyrosine content and specific activity were measured in mediobasal hypothalamus, anterior hypothalamus and striatum. A time-dependent increase in CA synthesis occurred in all tissues over 20 min post-i.c.v. injection. The technique described may prove to be useful in the assessment of central neurotransmitter turnover in various physiological and pharmacological settings.

Altered central monoamine response to D-amphetamine in rats chronically exposed to inorganic lead

Investigations of the mechanisms involved in the neurotoxicity resulting from chronic inorganic lead (Pb) exposure have centered on CNS biogenic amine function on the basis of behavioral and neurochemical findings. The following study examined the time course of the response of dopamine (DA) and 5-hydroxytryptamine (5-HT) neurons to d-amphetamine (AMPH) in rats chronically exposed to Pb from birth in order to further examine neurochemical mechanisms implicated by previous work. Offspring were exposed to 0.2% Pb acetate via the lactating dam and then weaned to the same drinking solution. At 120-140 days animals were injected with 1.0 mg/kg s.c. of the drug or with saline and sacrificed after various intervals. DA content in nucleus accumbens and corpus striatum in Pb-exposed animals was significantly higher than corresponding levels in controls at 20 minutes post-drug and remained significantly higher than baseline values at 80 minutes after the drug when DA concentrations in controls had returned to normal. These data suggest enhanced AMPH-induced DA synthesis in exposed rats. 5-Hydroxyindoleacetic acid (5-HIAA) content was significantly increased in three brain regions in exposed rats given AMPH compared to values in saline-injected exposed animals, indicating a compensation in these areas for the decreases in 5-HIAA values produced by Pb exposure alone. The results of this study reinforce the hypothesis that DA and 5-HT neurons are sensitive to relatively low levels of Pb exposure.

The concentration of dopamine, 5-hydroxytryptamine, and some of their acid metabolites in the brain of genetically diabetic rats

This is a report of the concentrations of DA, DOPAC, HVA, 5-HT, and 5-HIAA in the brain of spontaneously diabetic male Wistar rats. These rats showed a marked increase in blood and urine glucose, polydipsia, polyuria, and weight loss that had an onset 11-23 days earlier. Controls were litter mates with no hyperglycemia, glucosuria or weight reduction. The spontaneously diabetic rats showed a significant reduction of DOPAC in the striatum, and DOPAC, HVA, and 5HIAA in the olfactory tubercles (to 69, 61, 62, and 65% of their respective controls). No changes were found in the concentrations of DA or 5-HT. Thus the spontaneously diabetic rats showed a marked reduction in striatal and mesolimbic DA and mesolimbic 5-HT metabolism. This reduction in metabolism could be the consequence of a reduction in the formation of DA and 5-HT.

Determination of tyrosine, tryptophan and their metabolic derivatives by liquid chromatography-electrochemical detection: application to post mortem samples from patients with Parkinson's and Alzheimer's disease

A procedure is described for the rapid determination of the major indoles and catechols. Analysis with picogram detection limits was done by high-pressure liquid chromatography on a C18 reverse-phase column using electrochemical detection (LCEC). This method provides a comprehensive list of compounds which can be simultaneously determined in brain samples and for which there is no necessity of derivatization or pre-column purification. The regional distribution of 9 neurochemicals from rat brain and the levels of 10 neurochemicals from human brain are presented. DOPA, TYR, NE, MHPG, DOPAC, 5-HIAA, TRP, DA, HVA, 3-MT and 5-HT were detected in the caudate nucleus and putamen. The levels of neurochemicals from the caudate and putamen of a demented patient with Parkinson's disease were variably decreased; catechol and indole losses were greatest in the putamen. The levels of neurochemicals in the caudate and putamen of patients with Alzheimer's disease (SDAT) were also variably decreased; loss of NE was seen only in putamen and losses of DA, HVA and 5-HT were uniform across both caudate and putamen. The CSF of SDAT patients showed changes in NE only.

Influence of chronic inorganic lead exposure on regional dopamine and 5-hydroxytryptamine turnover in rat brain

The results of previous behavioral studies utilizing chronic exposure to low amounts of inorganic lead (Pb) have suggested alterations in the function of biogenic amine neuronal systems. The following study was performed to provide evidence for the possible bases of these changes in pharmacological responsiveness in exposed animals. Dams were administered 0.2% Pb acetate in drinking water to expose their offspring to Pb via the maternal milk. Males were weaned to the same drinking solution. At 120-140 days a tracer dose of 1.0 mCi L-[3H]2,6-tyrosine (3H-TYR) and 0.5 mCi L-[3H(G)]tryptophan (3H-TRP) was injected through an indwelling jugular catheter, and norepinephrine (NE), dopamine (DA), 5-hydroxytryptamine (5-HT) and their respective precursors and metabolites were quantified by liquid chromatography with electrochemical detection with column eluate collected for liquid scintillation counting. At this level of exposure (blood lead (PbB) at day 90 in exposed animals = 43.1 +/- 1.7 micrograms/dl) no changes were observed in concentration of NE or DA or DA metabolites in any brain region. However, DA turnover was decreased in Pb-exposed animals in nucleus accumbens and frontal cortex. No changes in 5-HT content and turnover were observed in any brain region, but 5-hydroxyindoleacetic acid (5-HIAA) levels were decreased in 6 of the 9 brain regions examined. These findings are consistent with observations of an attenuated behavioral responsiveness to d-amphetamine (AMPH) in exposed animals, and suggest that the changes in DA and 5-HT neurons noted by other workers at higher levels of exposure persist when PbBs are in the range of 40 micrograms/dl.

Determination of gamma-aminobutyric and glutamic acids in rat brain by liquid chromatography with electrochemical detection

The determination of glutamic and gamma-aminobutyric acids in rat brain regions by derivatization with o-phthaldialdehyde-thiol, isocratic separation by liquid chromatography, and quantification by electrochemical detection provides a simple and precise method for assessing changes in glutamatergic and GABAergic neuronal systems. Gamma-aminobutyric acid was eluted in 30-35 minutes followed by a washout step with 90% methanol to remove all amino acid derivatives with longer retention times. Homoserine was used as an internal standard. Significant increases in gamma-aminobutyric acid content in nucleus accumbens and substantia nigra could be detected 20 minutes after injection of 400 mg/kg valproic acid.