Measurement of metabolites of dopamine and 5-hydroxytryptamine in cerebroventricular perfusates of unanesthetized, freely-moving rats: selective effects of drugs

Central nervous system monoamine metabolite response to alcohol exposure is associated with future alcohol intake in a nonhuman primate model (Macaca mulatta)

It is widely held that the central monoamine neurotransmitters modulate alcohol intake. Few studies, however, directly assess the relationship between baseline and alcohol-induced monoamine turnover, as well as the change from baseline, as predictors of alcohol intake. Using a nonhuman primate model, this study investigates baseline, alcohol-induced and alcohol-induced change in monoamine activity and their relationship with alcohol intake. Alcohol-naïve, adolescent rhesus macaques (Macaca mulatta, N = 114) were administered a standardized intravenous bolus of alcohol solution (16.8%, v/v) on two occasions, approximately 1 month apart. One month prior to and 1 h following each alcohol infusion, cisternal cerebrospinal fluid (CSF) was obtained and assayed for monoamine metabolite concentrations. Approximately 6-7 months later, subjects were allowed unfettered access to an aspartame-sweetened alcohol solution (8.4%, v/v) for 1 h/day, 5 days/week, over 5-7 weeks. Results showed strong positive correlations between baseline and post-infusion CSF monoamine metabolite concentrations, indicating a trait-like response. Low baseline and post-infusion serotonin and dopamine metabolite concentrations and a smaller change in serotonin and dopamine metabolites from one infusion to the next were associated with higher alcohol intake. Low baseline and post-infusion norepinephrine metabolite concentrations predicted high alcohol intake, but unlike the other monoamines, a greater change in norepinephrine metabolite concentrations from one infusion to the next was associated with higher alcohol intake. These findings suggest that individual differences in naturally occurring and alcohol-induced monoamine activity, as well as the change between exposures, are important modulators of initial alcohol consumption and may play a role in the risk for excessive alcohol intake.

Cerebrospinal fluid monoamine metabolite profiles in bipolar disorder, ADHD, and controls

Alterations in monoaminergic signaling are suggested as key aspects of the pathophysiology in bipolar disorder and ADHD, but it is not known if the monoamine metabolic profile differs between these disorders. One method to study monoaminergic systems in humans is to measure monoamine end-point metabolite concentrations in cerebrospinal fluid (CSF). Here, we analyzed CSF monoamine metabolite concentrations in 103 adults with bipolar disorder, 72 adults with ADHD, and 113 controls. Individuals with bipolar disorder had significantly higher homovanillic acid (HVA, 264 ± 112 nmol/L, p < 0.001) and 5-hydroxyindoleacetic acid (5-HIAA, 116 ± 42 nmol/L, p = 0.001) concentration, but lower 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG, 38 ± 8 nmol/L, p < 0.001) concentrations than controls (HVA, 206 ± 70 nmol/L; 5-HIAA, 98 ± 31 nmol/L; and MHPG, 42 ± 7 nmol/L). Higher HVA concentrations were associated with a history of psychosis in the bipolar disorder sample. Subjects with ADHD had higher HVA (240 ± 94 nmol/L, p < 0.001) concentrations compared with controls. In addition, SSRI treatment was associated with lower 5-HIAA concentrations in both patient groups. A power analysis indicated that for within-group comparisons, only large effects would be reliably detectable. Thus, there may be moderate-to-small effects caused by medication that were not detected due to the limited size of the sub-groups in these analyses. In conclusion, the present study suggests disorder-specific alterations of CSF monoamine metabolite concentrations in patients with bipolar disorder and ADHD compared with controls; these differences were independent of acute symptoms and medication effects.

Domperidone induces a probenecid-sensitive rise in immunoreactive prolactin in cerebroventricular perfusates in female rats

The present study examined immunoreactive prolactin (ir-PRL) access into the cerebrospinal fluid (CSF) by monitoring ir-PRL levels in the blood and cerebroventricular perfusates of ovariectomized (ovx) rats treated with the dopamine antagonist, domperidone (DOMP). In Expt. 1 PRL plasma levels were measured in rats treated with DOMP i.p. (2.5, 5.0 and 10.0 mg/kg). All doses of DOMP significantly increased PRL plasma levels. In Expt. 2 animals were treated i.p. with DOMP (10 mg/kg) or DOMP plus the active transport blocking agent, probenecid (PROB; 250 mg/kg). Plasma PRL and ir-PRL in cerebroventricular perfusates were measured in separate sets of animals using catheters and a push-pull perfusion system, respectively. DOMP induced an increase in plasma PRL that was followed 30-40 min later by a rise in ventricular perfusates ir-PRL levels. PROB treatment induced a greater increase in plasma PRL levels in DOMP-treated animals, but delayed the DOMP-induced increase in ir-PRL ventricular perfusate ir-PRL levels. The delay in the rise of ir-PRL in ventricular perfusates observed in rats treated with DOMP plus PROB may be due to a PROB's interference with the transport of PRL from the blood into CSF. These results suggest that under some conditions ir-PRL in CSF originates from the pituitary.

Determination of cerebrospinal fluid production rate using a push-pull perfusion procedure in the conscious rat

The inulin dilution technique was used to determine the cerebrospinal fluid (CSF) production rate by means of a push-pull cannula implanted into a lateral ventricle. Artificial CSF containing trace amount of 3H-inulin was perfused for 2 h in conscious rats. 3H-inulin in the effluent reached a plateau level depending on the CSF production rate. The control lateroventricular CSF production was 0.98 microliters/min. Production was reduced to 0.34 microliters/min during a perfusion with acetazolamide (1 mM), a carbonic anhydrase inhibitor.

In vivo changes in brain catecholamine release from rat hypothalamus following olfactory bulbectomy

The mechanism eliciting mouse-killing behavior (muricide), induced by bilateral olfactory bulbectomy, has been shown to involve the brain noradrenergic system; this is because muricide is specifically inhibited by the drugs which potentiate the activity of catecholaminergic neurons such as tricyclic antidepressants. Our previous reports also demonstrated that the hypothalamic noradrenaline (NA) contents increased in the rats which exhibited muricide. To further examine the hypothalamic noradrenergic function in muricide, a push-pull perfusion technique was applied for direct measurement of NA release from the lateral (LH) and ventromedial (VMH) hypothalamus in freely moving rats. Subsequently, the perfusates, including catecholamines and their metabolites were measured by means of high-performance liquid chromatography with electrochemical detection (HPLC-ECD). Three days after olfactory bulbectomy, 67% of the rats elicited muricide and NA release from LH tended to decrease. Moreover, 7 days after olfactory bulbectomy, most of the rats elicited muricide and NA release from LH was significantly decreased, but not from VMH. On the other hand, dopamine (DA) release from VMH without LH conversely increased on the 7th day after olfactory bulbectomy. These results suggest that the dysfunction of the noradrenergic system caused by the decrease in NA release from LH played an important role for the incidence of muricide.

Dopamine D2 receptors exert tonic regulation over discrete neurotensin systems of the rat brain

Blockade of dopamine D2 receptors with either the selective antagonist, sulpiride, or the non-selective antagonist, haloperidol, induces 2- to 3-fold increases in the content of neurotensin-like immunoreactivity in the striatum and the nucleus accumbens of the rat brain. Quantitatively similar increases were also observed (a) in the striatum following selective degeneration of more than 85% of the nigrostriatal dopamine pathway with 6-hydroxydopamine and (b) in both the striatum and the nucleus accumbens after non-selective depletion of brain dopamine using reserpine plus alpha-methyl-p-tyrosine. Interestingly, treatment of animals with sulpiride or haloperidol, following the depletion of dopamine by either 6-hydroxydopamine or reserpine plus alpha-methyl-p-tyrosine, did not add to the elevation in neurotensin content of either structure caused by the dopamine depletion alone. These data suggest that an intact dopamine system is required for the neuroleptics to exert their effects on individual neurotensin systems. In addition, the same mechanism appears to underlie the responses of the neurotensin pathways to treatments with the neuroleptics or dopamine-depleting drugs. A likely explanation for the effects of neuroleptics and dopamine-depleting drugs is that they eliminate tonic activity on D2 receptors by basally released dopamine in the striatum and the nucleus accumbens. Supportive evidence for this hypothesis is that concurrent administration of the D2 receptor agonist, LY 171555, with reserpine, completely blocked the effects of reserpine-induced dopamine depletion on neurotensin systems of the striatum and the nucleus accumbens.

Effects of dopaminergic and serotonergic receptor blockade on neurochemical changes induced by acute administration of methamphetamine and 3,4-methylenedioxymethamphetamine

As dopamine (DA) causes neurochemical changes in the central serotonergic system after an acute injection of methamphetamine, the present study examined the possibility that this response is mediated through dopaminergic receptors. Pretreatment with the DA receptor antagonist, haloperidol, failed to prevent the decreases in the activity of tryptophan hydroxylase and the concentration of serotonin (5-HT) in the frontal cortex, hippocampus and neostriatum 1 hr after a single administration of methamphetamine. Because methamphetamine is also a potent releaser of 5-HT, the possibility that 5-HT receptors mediate the effects of methamphetamine was evaluated. Pretreatment with methiothepin an antagonist of both DA and 5-HT receptors, failed to prevent the decline in activity of tryptophan hydroxylase but did attenuate the decreases in concentrations of 5-HT measured in the frontal cortex and hippocampus. This attenuation is not mediated through 5-HT2 receptors, as ritanserin failed to interfere with the changes induced by methamphetamine. In addition, DA or 5-HT receptors were apparently not involved in the changes in activity of tryptophan hydroxylase and concentrations of 5-HT induced by another analogue of amphetamine, 3,4-methylenedioxymethamphetamine (MDMA). This study suggests different mechanisms are responsible for the acute and long-term changes observed in the central serotonergic system following a single or multiple doses of methamphetamine.

Effects of N-ethyl-3,4-methylenedioxyamphetamine (MDE) on central serotonergic and dopaminergic systems of the rat

The influence of N-ethyl-3,4-methylenedioxyamphetamine (MDE) on the central serotonergic and dopaminergic systems of the rat after a single or multiple injections was studied. MDE (10 mg/kg) produced a significant decrease in the concentration of 5-hydroxytryptamine (5-HT) 1 hr later in the frontal cortex and the hippocampus without affecting the concentration of 5-hydroxyindoleacetic acid (5-HIAA) or tryptophan hydroxylase (TPH) activity. Hypothalamic and neostriatal concentrations of 5-HT, 5-HIAA, dopamine (DA), dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) remained unaffected, as well as the neostriatal TPH and tyrosine hydroxylase (TH) activities. However, 3 hr after the MDE injection, the serotonergic variables including TPH activity were decreased in most of the brain areas examined. The dopaminergic system remained unaffected, except for a significant reduction in neostriatal DOPAC concentrations. The changes in transmitter concentrations after a single injection were dose dependent; the maximum depletion in TPH activity was reached with a 10 mg/kg dose. The administration of multiple doses of MDE caused greater decreases in TPH activity and 5-HT concentrations 3 hr after the treatment than did a single injection; in addition, a partial recovery from multiple administrations occurred by 18 hr. The effects of MDE on DA and its metabolites were transient, and neostriatal TH activity was not altered. This study demonstrates that MDE primarily affects the central serotonergic system, as reported for its congeners 3,4-methylenedioxyamphetamine and 3,4-methylenedioxymethamphetamine. It does, however, produce less neurotoxicity as judged by its lower potency on the dopaminergic and the serotonergic systems as well as the recovery occurring in these systems.

Electrochemistry in vivo: monitoring dopamine release in the brain of the conscious, freely moving rat

Changes in electrochemical responses at stearate-modified and unmodified graphite paste electrodes were compared simultaneously by intrastriatal chronoamperometry in freely moving rats. Responses at the modified electrodes were unaffected by ascorbate administration, decreased by pergolide and increased by pargyline and haloperidol. The effects of haloperidol were reversed by gamma-butyrolactone but not by pargyline. In contrast, responses at the unmodified electrodes were increased by ascorbate or pergolide and decreased by pargyline. Haloperidol-induced increases at these electrodes were rapidly reversed by pargyline. Responses of the two electrodes differed significantly in both magnitude and temporal characteristics after amphetamine administration. The results demonstrate that the modified electrodes can selectively monitor released dopamine in the freely moving animal, even when there are simultaneous, large changes in ascorbate and 3,4-dihydroxyphenylacetic acid.

In vivo release of endogenous dopamine, 5-hydroxytryptamine and some of their metabolites from rat caudate nucleus by phenylethylamine

The in vivo release of endogenous 3,4-dihydroxyphenylethylamine (DA) and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 3-methoxytyramine (3-MT), and of 5-hydroxytryptamine (5-HT) and its metabolite, 5-hydroxyindoleacetic acid (5-HIAA), has been measured in the caudate nucleus of the anesthetized rat. A push-pull cannula was implanted into the brain, and the tissue perfused with artificial CSF or artificial CSF containing 5 X 10(-4) M phenylethylamine. The perfusate was collected and analyzed for DA, 5-HT and their metabolites by high performance liquid chromatography with electrochemical detection (HPLC-ECD). DA was released by phenylethylamine at rates significantly greater than its basal rate. 3-MT and 5-HT were undetectable in perfusates collected under basal conditions, but could be detected readily during phenylethylamine stimulation. DOPAC, HVA and 5-HIAA concentrations were not significantly affected by phenylethylamine. The results suggest (1) that phenylethylamine may exert its behavioural effects through increased release of both DA and 5-HT, and (2) that in vivo measurements of the acid metabolites alone may not be indicative of the release of the amines.

Monitoring 5HT metabolism in the brain of the freely moving rat

The techniques of repeated CSF withdrawal and intracerebral dialysis are described. They were used to monitor 5HT metabolism in the brains of freely moving rats. Evidence indicates that results obtained proportionately reflect changes of 5HT metabolism in whole brain (CSF method) and in the striatum (intracerebral dialysis). The potential value of the methods is illustrated by two behavioral studies. The first suggested that 5HT turnover values (determined two weeks prior to behavioral testing by the CSF method) were predictive for neck + body biting in a social interaction test. The second study showed a consistent pattern of change of hypothalamic dialysate 5HIAA concentration associated with feeding in rats on a restricted food intake schedule.

Measurement of endogenous noradrenaline release in the rat cerebral cortex in vivo by transcortical dialysis: effects of drugs affecting noradrenergic transmission

The release of endogenous noradrenaline was measured in the cerebral cortex of the halothane-anesthetized rat by using the technique of brain dialysis coupled to a radioenzymatic assay. A thin dialysis tube was inserted transversally in the cerebral cortex (transcortical dialysis) and perfused with Ringer medium (2 microliter min-1). Under basal conditions, the cortical output of noradrenaline was stable over a period of at least 6 h and amounted to 8.7 pg/20 min (not corrected for recovery). Histological control of the perfused area revealed very little damage and normal morphology in the vicinity of the dialysis tube. Omission of calcium from the perfusion medium caused a marked drop in cortical noradrenaline output. Bilateral electrical stimulation (for 10 min) of the ascending noradrenergic pathways in the medial forebrain bundle caused a frequency-dependent increase in cortical noradrenaline output over the range 5-20 Hz. Stimulation at a higher frequency (50 Hz) resulted in a levelling off of the increase in cortical noradrenaline release. Systemic administration of the dopamine-beta-hydroxylase inhibitor bis-(4-methyl-1-homopiperazinylthiocarbonyl) disulfide (FLA 63) (25 mg/kg i.p.) markedly reduced, whereas injection of the monoamine oxidase inhibitor pargyline (75 mg/kg i.p.) resulted in a progressive increase in, cortical noradrenaline output. d-Amphetamine (2 mg/kg i.p.) provoked a sharp increase in cortical noradrenaline release (+450% over basal values within 40 min). Desmethylimipramine (10 mg/kg i.p.) produced a twofold increase of cortical noradrenaline release. Finally, idazoxan (20 mg/kg i.p.) and clonidine (0.3 mg/kg i.p.), respectively, increased and decreased the release of noradrenaline from the cerebral cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of neuroleptic drugs on striatal dopamine release and metabolism in the awake rat studied by intracerebral dialysis

This study investigated the effect of three neuroleptic drugs, (+/-)-sulpiride, haloperidol and cis-flupenthixol, on dopamine release and metabolism in the striatum of the awake rat. Endogenous extracellular dopamine and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), as well as the 5-hydroxytryptamine (5HT) metabolite 5-hydroxyindoleacetic acid (5HIAA), were determined in striatal perfusates in awake rats by using intracerebral dialysis together with high performance liquid chromatography with electrochemical detection. Sulpiride (10, 50 and 250 mg/kg), cis-flupenthixol (0.5 and 2 mg/kg) and haloperidol (2 mg/kg) all increased the levels of dopamine in striatal perfusates. However, the time course and magnitude of these effects differed markedly depending upon the neuroleptic used. Sulpiride (10, 50 and 250 mg/kg), cis-flupenthixol (0.05, 0.5 and 2 mg/kg) and haloperidol (0.05, 0.5 and 2 mg/kg) increased extracellular levels of DOPAC and HVA while having little effect on 5HIAA. In contrast to the effect on dopamine levels the changes in DOPAC and HVA followed similar time courses and were of similar magnitude independent of the neuroleptic used. The response of the dopamine metabolites seemed to occur at lower doses of the neuroleptics than the response of dopamine release itself. Furthermore, there was no close relationship between changes in dopamine as compared to changes in DOPAC and HVA. Finally, there was no correlation between any of the neurochemical changes measured and the occurrence of catalepsy. These data suggest that neuroleptic drugs have two separate actions on the dopamine neuron in vivo, one causing an increase in dopamine release and another producing an increase in dopamine metabolism, which is probably a consequence of increased dopamine synthesis. Furthermore neither of these effects are related to catalepsy.

Changes in monoamine metabolites measured by simultaneous in vivo differential pulse voltammetry and intracerebral dialysis

A direct comparison has been made of the drug-induced changes in extracellular levels of 3,4-dihydroxyphenylacetic acid and 5-hydroxyindoleacetic acid measured using intracerebral dialysis and differential pulse voltammetry with carbon fibre electrodes. The comparison was carried out in chloral hydrate anaesthetized rats with a pretreated carbon fibre electrode implanted in one striatum and an intracerebral dialysis loop in the contralateral striatum. 3,4-Dihydroxyphenylacetic acid and 5-hydroxyindoleacetic acid in the dialysis perfusates were assayed by high pressure liquid chromatography with electrochemical detection. d-Amphetamine (2 mg/kg) decreased extracellular 3,4-dihydroxyphenylacetic acid and the height of the 3,4-dihydroxyphenylacetic acid oxidation peak, while haloperidol (0.5 mg/kg) increased 3,4-dihydroxyphenylacetic acid levels measured in the perfusates and the height of the 3,4-dihydroxyphenylacetic acid oxidation peak. In these experiments there were parallel changes in 3,4-dihydroxyphenylacetic acid levels and peak height and a close correlation between these changes. Tranylcypromine (10 mg/kg) produced an almost parallel decrease in extracellular 5-hydroxyindoleacetic acid (dialysis) and the height of the 5-hydroxyindoleacetic acid oxidation peak, with similar percentage changes and good correlation values being obtained. However, while 5-hydroxy-L-tryptophan (25 mg/kg) increased both the 5-hydroxyindoleacetic acid levels and the height of the 5-hydroxyindoleacetic acid oxidation peak, 5-hydroxyindoleacetic acid in the dialysis perfusates showed a greater increase than the oxidation peak. The results show a close correlation between changes in extracellular 3,4-dihydroxyphenylacetic acid and its respective voltammetric peak and strongly support the use of in vivo differential pulse voltammetry for monitoring dopamine metabolism.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyrotropin-releasing hormone and its analog MK-771 increase the cerebroventricular perfusate content of dihydroxyphenylacetic acid

The effects of thyrotropin-releasing hormone (TRH) and its synthetic analog, pyro-2-aminoadipyl-histidyl-thiazolidine-4-carboxamide (MK-771), were determined on the efflux of dihydroxyphenylacetic acid (DOPAC) collected from push-pull cannulae chronically implanted into the lateral cerebral ventricles of rats. Intracerebroventricular and intraperitoneal injections of both peptides increased the efflux of DOPAC. These results suggest that TRH and MK-771 increase the activity of dopaminergic neurons that terminate in periventricular regions.

Differential effects of d-amphetamine, beta-phenylethylamine, cocaine and methylphenidate on the rate of dopamine synthesis in terminals of nigrostriatal and mesolimbic neurons and on the efflux of dopamine metabolites into cerebroventricular perfusates of rats

The in vivo effects of four psychomotor stimulants (d-amphetamine, beta-phenylethylamine, cocaine and methylphenidate) were determined on: 1) the rate of dopamine (DA) synthesis, as measured by the accumulation of dihydroxyphenylalanine (DOPA) after aromatic L-amino acid decarboxylase inhibition, in the striatum (terminals of nigrostriatal neurons) and in the nucleus accumbens and olfactory tubercle (terminals of mesolimbic neurons) and 2) the efflux of the DA metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) into cerebroventricular perfusates of conscious, freely-moving rats. d-Amphetamine and beta-phenylethylamine produced biphasic responses with lower doses of each drug increasing both the efflux of DOPAC and the rate of DA synthesis in the striatum. Higher doses of each drug either had no effect or actually decreased the efflux of DOPAC and also decreased the rate of DA synthesis in the striatum. On the other hand, cocaine and methylphenidate only decreased the efflux of DOPAC and the rate of DA synthesis in the striatum. The effects of the drugs on the rate of DA synthesis in the nucleus accumbens and olfactory tubercle were similar to, but less pronounced than those seen in the striatum. These results are consistent with the following suggestions: 1) low doses of d-amphetamine and beta-phenylethylamine facilitate the neuronal release of DA while higher doses of both drugs facilitate release and inhibit neuronal reuptake of the amine, and 2) cocaine and methylphenidate preferentially block the neuronal reuptake of DA.

6-Hydroxydopamine and 5,7-dihydroxytryptamine selectively reduce dopamine and 5-hydroxytryptamine metabolites in cerebroventricular perfusates of rats

The efflux into the lateral cerebral ventricles of metabolites of dopamine (DA) and 5-hydroxytryptamine (5HT) was determined in unanesthetized rats bearing chronically implanted push-pull cannulae. Pretreatment with 6-hydroxydopamine (6-OHDA) reduced the basal efflux of 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), but not of 5-hydroxyindoleacetic acid (5HIAA). The haloperidol-induced increase in the efflux of DOPAC and HVA was markedly attenuated in the 6-OHDA-pretreated rats. In rats treated with 5,7-hydroxytryptamine (5,7-DHT) the basal efflux of DOPAC and HVA was unaffected, while that of 5HIAA was markedly reduced; in these animals the ability of L-tryptophan to increase the perfusate content of 5HIAA was abolished. These results indicate that metabolites of DA and 5HT appearing in cerebroventricular perfusates of rats originate from DA and 5HT neuronal terminals in the brain.

Chemically modified electrode for in vivo monitoring of brain catecholamines

A chemically modified graphite paste electrode was developed to allow catecholamines to be electrochemically distinguished from ascorbic acid and the dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC). Interference from 5-hydroxyindoles is eliminated through appropriate choice of electrode potential. The electrode gives linear current responses with increasing concentrations of catecholamines unaffected by the presence of ascorbic acid or DOPAC and exhibits long-term response stability in brain tissue. Examples are provided demonstrating the selectivity of the electrode to changes in extracellular dopamine in the rat striatum.

Probenecid sensitive pathway of elimination of dopamine and serotonin metabolites in CSF of the rat

CSF was removed at a constant flow rate of 1 microliter/min from the third ventricle of anesthetized rats. Five microliter CSF samples were directly injected every 15 min into a liquid chromatographic system coupled with an amperometric detector. Mean CSF values for free dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindolacetic acid (5-HIAA) were 1.4, 0.9, and 2.6 X 10(-6)M respectively. High doses of probenecid resulted in a linear increase of acidic metabolite concentrations which gave an index of the fractional turnover rates related to the resorption by the weak organic acid carrier. Accumulation rates were 0.24, 0.87, and 1.58 mumol/l/h for DOPAC, HVA and 5-HIAA respectively. This route of elimination was predominant for 5-HIAA while it represented only a small part of total turnover for DOPAC. A high elimination rate constant for HVA validates the use of control levels of this metabolite as an indication of fractional HVA turnover dependent upon probenecid-sensitive carrier.

Tolerance develops to the haloperidol-induced increase in the efflux of dopamine metabolites from the brains of unanesthetized, freely-moving rats

The lateral cerebral ventricles of freely moving rats were perfused by means of chronically implanted push-pull cannulae every second day for 2 weeks. Perfusates were analyzed for metabolites of dopamine [dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA)] and of 5-hydroxytryptamine [5-hydroxyindoleacetic acid (5HIAA)] using high performance liquid chromatography and an amperometric detector. Rats received daily subcutaneous injections of haloperidol (1 mg/kg) or its vehicle. After the first injection of haloperidol the concentrations of DOPAC and HVA were markedly increased while that of 5HIAA was unchanged. Complete tolerance developed to the haloperidol-induced increased efflux of dopamine metabolites by day 9, although a higher dose of haloperidol (2 mgf/kg) on day 15 was still capable of eliciting a modest increase in the efflux of DOPAC and HVA.

Effects of chloral hydrate, gamma-butyrolactone, and equithesin on the efflux of dopamine and 5-hydroxytryptamine metabolites into cerebroventricular perfusates of rats

Chloral hydrate, gamma-butyrolactone, and Equithesin are general anesthetics which have been employed in a variety of neurochemical and electrophysiological experiments. The effects of these anesthetics on the efflux of dopamine and 5-hydroxytryptamine metabolites into lateral cerebroventricular perfusates were determined in rats bearing chronically implanted cannulas. Chloral hydrate and Equithesin increased, whereas gamma-butyrolactone decreased, the efflux of the dopamine metabolites 3,4-dihydroxyphenylacetic acid and homovanillic acid. None of the anesthetics had a consistent effect on the efflux of the 5-hydroxytryptamine metabolite 5-hydroxyindoleacetic acid.