Increase In Dihydroxyphenylacetic Acid Research Articles

Intraventricular microdialysis: a new method for determining monoamine metabolite concentrations in the cerebrospinal fluid of freely moving rats

A new method is described to estimate the cerebrospinal fluid (CSF) concentrations of monoamine metabolites (dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA)) in the lateral ventricle of freely moving rats by use of in vivo microdialysis. Both the baseline concentrations of these metabolites and the rate of dopamine (DA) turnover (estimated by the accumulation of total DA metabolites after 200 mg/kg probenecid) were within the range reported when other methods were used to sample CSF. A series of preliminary studies were conducted to demonstrate that this method can be used to repeatedly sample CSF, and to show that the method is sensitive to local changes in dopaminergic activity induced by lesions, drugs or grafts. (1) Unilateral 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra produced a significant decrease in the CSF concentrations of DOPAC and HVA ipsilateral to the lesion, relative to the contralateral side or to concentrations in animals without lesions. (2) When left and right lateral ventricles were sampled simultaneously in animals with a unilateral 6-OHDA lesion, haloperidol induced an increase in DOPAC and HVA concentrations in CSF on both sides of the brain. Interestingly, the haloperidol-induced increase in CSF concentrations of DA metabolites was greater adjacent to the intact striatum of rats with unilateral 6-OHDA lesions than in animals with no lesion. (3) Finally, in animals with adrenal medulla tissue grafted into the lateral ventricle there was an increase in the CSF concentration of DOPAC compared to pregraft values or to those of animals with control grafts.

Effects of immobilization stress on dopamine and its metabolites in different brain areas of the mouse: role of genotype and stress duration

Immobilization stress induced, in mice of both C57BL/6 (C57) and DBA/2 (DBA) strains, an increase in dihydroxyphenylacetic acid (DOPAC)/dopamine (DA) and homovanillic acid (HVA)/DA ratios and a reduction of 3-methoxytyramine (3-MT)/DA ratio in the caudatus putamen (CP) and nucleus accumbens septi (NAS). These effects were already evident after 30 min stress in the NAS, while in the CP 120 min were needed in order to show the effects of stress. Immobilization did not produce any effects on dopaminergic metabolism in the frontal cortex (FC) of the C57 strain either after 30 or after 120 min stress while in mice of the DBA strain a time-dependent effect of stress on the HVA/DA ratio was evident. When B6D2F1 hybrids were considered, the effects produced by 120 min immobilization in the CP and the NAS paralleled those observed in parental strains, while in the FC 120 min stress indcued the same increase of HVA observed in DBA mice, thus suggesting that the pattern of response in the FC that characterizes the DBA strain may be inherited through a dominant pattern of inheritance.

Modulation of neurotransmitter metabolism by dihydropyridine calcium channel ligands in mouse brain

The regional concentrations of dopamine, serotonin, dihydroxyphenylacetic acid, homovanillic acid and 5-hydroxyindole acetic acid were measured in mouse brain following administration of the dihydropyridine calcium channle activator BAY K 8644, and antagonist, nifedipine. BAY K 8644 (1–8 mg/kg) produced dose- and time-dependent increases in dihydroxyphenylacetic acid, homovanillic acid and 5-hydroxyindoleacetic acid concentrations in the caudate, without altering dopamine and serotonin levels. No changes in 5-hydroxyindoleacetic acid concentration were observed in the raphe nuclei, hypothalamus, hippocampus and frontal cortex. Nifedipine (4 mg/kg) blocked BAY K 8644- (2 mg/kg) elicited increases in dihydroxyphenylacetic acid in the caudate. Furthermore, a higher dose of nifedipine (8 mg/kg) decreased dihydroxyphenylacetic acid and homovanillic acid, but did not affect dopamine, serotonin or 5-hydroxyindoleacetic acid concentrations, while a lower dose of nifedine (2 mg/kg) significantly increased serotonin, 5-hydroxyindoeleacetic acid and homovanillic acid, but did not affect dopamine and dihydroxyphenylacetic acid concentrations. The findings that both BAY K 8644 and nifedipine affect neurotransmitter metabolism in vivo in a dose- time- and brain region-dependent manner, suggest that high-affinity dihydropyridine calcium channel binding sites play an important role in regulating neurotransmitter turnover in the central nervous system.

Cholinergic mediation in the ventral tegmental area of α-melanotropin induced excessive grooming: Changes of the dopamine activity in the nucleus accumbens and caudate putamen

The effect of α-melanotropin (α-MSH) on the rat mesolimbic dopaminergic activity was estimated by measuring the changes in dihydroxyphenyl acetic acid (DOPAC) and dopamine (DA) endogenous levels in the nucleus accumbens (Ac) and caudate putamen (CP). A marked increase of DOPAC/DA ratios resulting from an increase in DOPAC and decrease in DA levels was found in the Ac 30 and 65 min after bilateral α-MSH-injections (1 ug)into the ventral tegmental area (VTA). Similar changes were observed in the CP 65 min post-injections. These peptide-induced changes were completely inhibited by a previous VTA injection of atropine (lug), at a dose that totally blocked the α-MSH-induced excessive grooming and motor activation. These results confirms that α -MSH affects a cholinergic afferent to the VTA which modifies the mesolimbic dopaminergic system involved in the α -MSH/ACTH-induced behaviors.

Influence of anaesthetics on rat striatal dopamine metabolism in vivo

The effects of 4 anaesthetics, halothane, α-chloralose, chloral hydrate and pentobarbitone, on rat striatal extracellular dihydroxyphenylacetic acid (DOPAC) levels were determined using in vivo voltammetry. Stable baseline levels of DOPAC were maintained under halothane/N 2O and α-chloralose while the extracellular levels of DOPAC gradually declined under chloral hydrate or pentobarbitone anaesthesia. Administration of haloperidol (0.3 mg/kg i.p.) significantly increased DOPAC using halothane/N 2O, α-chloralose or chloral hydrate but not under pentobarbitone anaesthesia. The greatest increase in DOPAC was seen in rats anaesthetized with α-chloralose>halothane/N 2O>chloral hydrate > pentobarbitone. Apomorphine (0.5 mg/kg s.c.) given 2 h after haloperidol partially reversed the increase in DOPAC produced by haloperidol. The results suggest care needs to be exercised in the choice of anaesthetic used for voltammetric studies with pentobarbitone being the least recommended.

In vivo voltammetric and behavioural evidence for somatodendritic autoreceptor control of mesolimbic dopamine neurones

Somatodendritic autoreceptor control of mesolimbic dopamine neurones was assessed using in vivo differential pulse voltammetry to monitor dopamine metabolism in the nucleus accumbens following infusion of haloperidol or dopamine into the ipsilateral ventral tegmental area (VTA) of chloral hydrate anaesthetized rats. Haloperidol (2.5 μg/0.5 μl) produced an increase in dihydroxyphenylacetic acid (DOPAC) levels (45%) whilst dopamine produced a transient decrease (38%). In three experiments this decrease was smaller and was followed by a large increase in DOPAC levels (108–208%). However, the results of radiolabelled drug diffusion studies suggested that the secondary rise in DOPAC in the nucleus accumbens may be due to diffusion of dopamine following infusion into the VTA. In a separate set of experiments rats were injected with haloperidol (2.5 μg/0.5 μl) bilaterally into the VTA and their behaviour monitored using doppler shift radar. Haloperidol infusion produced a phase of increased exploratory behaviour lasting approximately 30 min. Both sets of data support previous electrophysiological studies suggesting the presence of functional dopamine somatodendritic autoreceptors in the mesolimbic system. The results further demonstrate the applicability of in vivo voltammetry for study of the effects of drugs on discrete receptor populations.

Bombesin increases dopamine function in rat brain areas

Bombesin is a tetradecapeptide heterogenously distributed in the mammalian brain. Bombesin (45 μg) given intracisternally (IC) to unanesthetized rats increased the accumulation of dihydroxyphenylalanine (DOPA) in striatum, olfactory tubercles and hypothalamus after DOPA-decarboxylase inhibition, thus indicating an increased dopamine synthesis. A dose-depdendent increase in dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), the principal dopamine metabolites, was seen in several brain areas 1 hr after IC injection of bombesin (0–60 μg). In striatum and olfactory tubercles HVA increased more than DOPAC with a maximal increase after 30–45 μg. In a time-course experiment a biphasic change of dopamine metabolites was observed in the olfactory tubercles with an actual decrease in metabolite levels 4 hr after 60 μg IC bombesin injection. Co-administration of bombesin and naloxone (8 mg/kg IP) or ethanol (2.25 g/kg IP) did not affect the increase in dopamine metabolites seen after bombesin alone. The action of IC administered bombesin on dopamine function was most pronounced in hypothalamus indicating a neuroendocrine regulatory function of the peptide.

Tyrosine administration enhances dopamine synthesis and release in light-activated rat retina.

Exposure of dark-adapted albino rats to light (350 lux) significantly elevated retinal levels of the dopamine metabolite dihydroxyphenyl acetic acid during the next hour; their return to a dark environment caused dihydroxyphenyl acetic acid levels to fall. Retinal dopamine levels were increased slightly by light exposure, suggesting that the increase in dihydroxyphenyl acetic acid reflected accelerated dopamine synthesis. Administration of tyrosine (100 mg/kg, i.p.) further elevated retinal dihydroxyphenyl acetic acid among light-exposed animals, but failed to affect dopamine release among animals in the dark. These observations show that a physiological stimulus--light exposure--can cause catecholaminergic neurons to become tyrosine-dependent; they also suggest that food consumption may affect neurotransmitter release within the retina.

Striatal 3-methoxytyramine as an index of dopamine release: Effects of electrical stimulation

Electrical stimulation of the substantia nigra elicited frequency-dependent increases in striatal 3-methoxytramine (3-MT). Subsequently, successive increases in dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were measured. Inhibition of nigrostriatal cell firing with HA-966 resulted in decreased striatal 3-MT, but in this case elevated DOPAC and HVA were also observed. These data indicate that 3-MT is a reliable index of dopamine (DA) release but that concurrent measurements of DOPAC, HVA and DA provide the most reliable interpretation of drug effects on dopaminergic function.

Pharmacological analysis of the functional ontogeny of the nigrostriatal dopaminergic neurons

Functional development of the dopaminergic nigrostriatal projection was studied by determining the age at which the biochemical responses of these neurons to physical or pharmacological manipulation are similar to those of adult neurons. Transection of the pathway acutely elevates striatal dopamine in adult and 8- or 10-day-old rats, but not in the 4- or 6-day-old animal. This axotomy-induced increase in striatal dopamine is believed to be a response of the dopaminergic terminals to cessation of impulse traffic and is secondary to a decrease in dopamine release and a concomitant increase in dopamine synthesis resulting from tyrosine hydroxylase activation. Therefore, this response to axotomy acts as an indicator of (1) the presence of impulse traffic in the pathway, and (2) the ability of tyrosine hydroxylase to be activated in response to a reduction in such impulse traffic. In vivo estimation of tyrosine hydroxylase activity showed that axotomy activates the enzyme at 10 days, but not at 4 days, whereas γ-hydroxybutyrate is effective at both ages, The fact that the enzyme can be activated by γ-hydroxybutyrate at 4 days indicates that the lack of effect of axotomy at this age is due to the absence of impulse traffic in the system. This conclusion is supported by the finding that the AMT-induced depletion of striatal dopamine is not related to impulse conduction at 4 days since transection of the pathway has no effect on the rate of dopamine loss whereas such transection blocks the AMT-induced depletion at 10 days of age. Nevertheless, despite the absence of neuronal activity at 4 days of age, these neurons are capable of generating and conducting impulse traffic since both 4- and 10-day-old rats showed increased striatal dihydroxyphenylacetic acid (DOPAC) levels when treated with haloperidol indicating increased dopamine release; such increases in DOPAC being dependent on an intact pathway. Given this data, the most parsimonious explanation for the abrupt development of the response to axotomy after the 6th day of age is that an event occurs which physiologically initiates impulse traffic. This event may be activation of afferent neuronal inputs to the cell bodies of the nigrostriatal projection.

Stimulation of Dopamine Synthesis in Caudate Nucleus by Intrastriatal Enkephalins and Antagonism by Naloxone

The intraventricular injection of methionine-enkephalin (50 to 100 micrograms) or [d-Ala2]-methionine-enkephalinamide (1.5 to 12 micrograms), a synthetic enkephalin analog resistant to enzyme degradation, caused a marked dose-dependent increase in dihydroxyphenylacetic acid and homovanillic acid concentrations in the rat striatum. The [d-Ala2] analog increased the accumulation of dopa in the striatum after aromatic amino acid decarboxylase inhibition, indicating that it increased dopamine synthesis. At the highest doses used both enkephalins failed to modify brain serotonin metabolism. The monolateral microinjection of the [d-Ala2]] analog (3 to 6 micrograms) into the caudate nucleus increased the concentration of dihydroxyphenylacetic acid in the injected side, whereas bilateral injection increased the concentration of this compound in both caudate nuclei and caused catalepsy. The stimulant effect of the [d-Ala2] analog on dopamine synthesis in the striatum persisted after destruction of striatal postsynaptic dopamine receptors with kainic acid. The biochemical and behavioral effects of enkephalins were prevented by naloxone, a specific narcotic antagonist. The results indicate that enkephalins stimulate dopamine synthesis by an action on opioid receptors localized on dopaminergic nerve terminals.

Conjugated 3,4 dihydroxy phenyl acetic acid (DOPAC) in human and monkey cerebrospinal fluid and rat brain and the effects of probenecid treatment

Gas chromatography-mass spectrometry (GC-MS) was used to measure 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in cerebrospinal fluid from humans and monkeys and in rat caudate nuclei. DOPAC was found to be present mainly in conjugated form. In human lumbar CSF the average concentration of total DOPAC before probenecid treatment was 1.48 ± 0.31 ng/ml; after probenecid it increased to 15.06 ± 3.17 ng/ml. This increase was mainly due to conjugated DOPAC but increases in free DOPAC also occurred. There is a relatively greater accumulation of DOPAC than of HVA, suggesting that in human CSF conjugated DOPAC may have a faster turnover rate than HVA. In monkey, ventricular CSF contained higher concentrations of DOPAC and HVA than did lumbar CSF. In rat brain, treatment with probenecid caused increases in DOPAC, HVA and their conjugates. These results suggest that DOPAC is conjugated in brain and that both compounds are removed from brain and CSF by a probenecid-sensitive acid transport system in the same manner as is HVA.