Presynaptic control of dopamine metabolism in the nucleus accumbens. Lack of effect of buspirone as demonstrated using in vivo voltammetry

Buspirone is a non-benzodiazepine drug with anxiolytic properties. It has been reported to induce a marked increase in the metabolism of dopamine in the striatum and the nucleus accumbens which is similar to that induced by neuroleptics. It has been suggested that the effect observed in the striatum reflects an action of buspirone on dopaminergic autoreceptors in both terminals and cell bodies. In the present study, presynaptic effects of buspirone on dopaminergic metabolism in the nucleus accumbens were investigated, and they were compared to the effects of the classical neuroleptic, haloperidol. Dopaminergic terminals were isolated by infusion of tetrodotoxin into the median forebrain bundle in order to evaluate the effects of buspirone and haloperidol on presynaptic receptors. Changes in dopamine metabolism were determined by in vivo voltammetry. Buspirone administered after interruption of the impulse flow did not affect dopamine metabolism. In contrast haloperidol treatment led to an increase in metabolism of dopamine. It is concluded that buspirone did not act at the presynaptic level and furthermore on dopaminergic autoreceptors.     

Differential Effects of Forced Locomotion, Tail-Pinch, Immobilization, and Methyl-β-Carboline Carboxylate on Extracellular 3,4-Dihydroxyphenylacetic Acid Levels in the Rat Striatum, Nucleus Accumbens, and Prefrontal Cortex: An In Vivo Voltammetric Study

In vivo voltammetry with carbon fiber electrodes was used to assess extracellular 3,4-dihydroxyphenylacetic acid (DOPAC) levels in striatum, nucleus accumbens, and anteromedial prefrontal cortex of freely moving rats subjected to altered motor activity or anxiogenic stimuli. Forced locomotion on a rotarod for 40 min caused an increase in extracellular DOPAC levels in the striatum and to a lesser extent in the nucleus accumbens but not in the prefrontal cortex. Subcutaneous injection of the anxiogenic agent methyl-beta-carboline carboxylate (10 mg/kg) increased extracellular DOPAC levels to a similar extent in prefrontal cortex and nucleus accumbens. Immobilization for 4 min augmented dopamine (DA) metabolism preferentially in the nucleus accumbens and to a lesser extent in the prefrontal cortex. Tail-pinch caused a selective activation of DA metabolism in the nucleus accumbens. None of these stimuli altered extracellular striatal DOPAC levels. These results confirm the involvement of dopaminergic systems projecting to the striatum and nucleus accumbens in motor function and suggest that mesolimbic and mesocortical dopaminergic systems can be specifically activated by certain kinds of anxiogenic stimuli; the relative activation of either of these latter systems could depend primarily on the nature (sensory modality, intensity) of the acute stressor.     

Difference in the effects of the antidepressant tianeptine on dopaminergic metabolism in the prefrontal cortex and the nucleus accumbens of the rat. A voltammetric study

The effects of the new tricyclic antidepressant tianeptine were investigated on dopaminergic (DAergic) metabolism in the anteromedian prefrontal cortex and the nucleus accumbens of the rat. DAergic metabolism was assessed by the measurement of DOPAC, the main presynaptic metabolite of dopamine, using in vivo voltammetry in rats ventilated with halothane (0.5-0.75% in air). Acute treatment with tianeptine (10 mg/kg, 20 mg/kg) only increased significantly DOPAC levels in the anteromedian prefrontal cortex. After chronic treatment with tianeptine (15 days, 2 times/day) the increases in DOPAC levels in this structure were altered and less pronounced with the 20 mg/kg dose. Previous studies led to suggest that both acute and chronic effects on DAergic terminals in the anteromedian prefrontal cortex may be involved in the therapeutic action of this new antidepressant.     

Different Effects of Opiate Withdrawal on Dopamine Turnover,Uptake, and Release in the Striatum and Nucleus Accumbens

The purpose of these experiments was to further characterize changes in dopaminergic function that follow withdrawal from chronic opiate treatment. Withdrawal after treatment to a maximum dose of 120 mg/kg of morphine did not alter dopamine concentrations in the substantia nigra, ventral tegmental area, striatum, or nucleus accumbens; but did decrease concentrations of DOPAC and the ratio of DOPAC to dopamine in the lateral striatum and nucleus accumbens. Uptake of tritiated dopamine was diminished for withdrawn slices obtained from the striatum with no effect observed for tissue from the nucleus accumbens. Deficits of in vitro release of tritiated dopamine also occurred following withdrawal, with the nucleus accumbens being sensitive to dependence produced by a lower dose of morphine. In conclusion, opiate withdrawal produces a complex pattern of effects on dopaminergic function that is specific for the striatum and nucleus accumbens.     

Different effects of intracerebral and systemic administration of buspirone in the forced swimming test: involvement of a metabolite

Buspirone, a drug with high affinity for serotonin1A receptors, was studied for its ability to reduce rats' immobility in the forced swimming test when injected systemically or into the nucleus raphe dorsalis (DR). Between 0.1 and 10 mg/kg buspirone had no effect on rats' immobility when injected systemically as a single dose or as a 3-injection course during 24 hours. Direct injection of 1 and 5 mu/0.5 microliter buspirone in the DR significantly reduced the duration of immobility without changing rats' activity in an open field. The anti-immobility effect of 1 microgram/0.5 microliter buspirone in the DR was completely prevented by injecting 2.5 micrograms (-)-propranolol in the same area. Oral administration of 0.3-1.0 mg/kg 1-(2-pyrimidinyl)piperazine (1-PP), one of the main metabolites of buspirone, and 0.3-3.0 mg/kg s.c. idazoxan, two substances with alpha 2 adrenergic blocking properties, completely antagonized the effect of 0.25 mg/kg s.c. 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), an agent with selective affinity for serotonin1A receptors. The anti-immobility effect of an infusion of 1 microgram/0.5 microliter buspirone or 8-OH-DPAT in the DR was also antagonized by 1 mg/kg p.o. 1-PP. The results suggest that buspirone possesses potential antidepressant properties but its effects may be masked in certain tests by its metabolite, 1PP, through its alpha 2 adrenergic blocking activity.     

3-Methoxytyramine Is the Major Metabolite of Released Dopamine in the Rat Frontal Cortex: Reassessment of the Effects of Antipsychotics on the Dynamics of Dopamine Release and Metabolism in the Frontal Cortex, Nucleus Accumbens, and Striatum by a Simple Two Pool Model

Abstract: 3-Methoxytyramine (3-MT) and 3,4-dihydroxyphenylacetic acid (DOPAC) rates of formation were used, respectively, to assess the dynamics of dopamine (DA) release and turnover in the rat frontal cortex, nucleus accumbens, and striatum. Assuming total (re)uptake and metabolism of released DA are relatively uniform among the three brain regions, a simplified two pool model was used to assess the metabolic fate of released DA. Under basal conditions, 3-MT formation was found to comprise >60% of total DA turnover (sum of 3-MT plus DOPAC rates of formation) in the frontal cortex, and not more than 15% in the nucleus accumbens and striatum. Haloperidol increased the 3-MT rate of formation to a greater extent in the frontal cortex than in the two other regions. Clozapine increased the 3-MT rate of formation in the frontal cortex and decreased it in the striatum. Both drugs increased DOPAC rate of formation in the frontal cortex and nucleus accumbens. It was elevated by haloperidol but not clozapine in the striatum. It is concluded that (1) O -methylation is a prominent step in the catabolism of DA in the frontal cortex under both physiological conditions and after acute treatment with antipsychotics, (2) 3-MT is the major metabolite of released DA in the frontal cortex and possibly also in the nucleus accumbens and striatum, (3) in contrast to the frontal cortex, most of the DOPAC in the nucleus accumbens and striatum appear to originate from intraneuronal deamination of DA that has not been released, (4) because presynaptic uptake and metabolism of DA give rise to DOPAC, whereas postsynaptic uptake and metabolism produced both DOPAC and 3-MT, the ratio of 3-MT to DOPAC rates of formation can be a useful index of reuptake inhibition.     

Effect of stress and ACTH on dopamine metabolism in the nucleus accumbens and frontal cortex

The effect of ACTH and electric foot shock stress on DOPAC content were determined in the frontal cortex and nucleus accumbens. Twenty min of stress enhanced DOPAC levels in the frontal cortex and in the nucleus accumbens by about 80% and 35%, respectively. On the other hand, a single dose of ACTH failed to change DOPAC concentration in the above brain areas. The present results show that the activation of mesolimbic and mesocortical dopaminergic systems is not mediated by ACTH secretion.     

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

The effect of alpha-melanotropin (alpha-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 alpha-MSH-injections (1 microgram) 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 (1 microgram), at a dose that totally blocked the alpha-MSH-induced excessive grooming and motor activation. These results confirms that alpha-MSH affects a cholinergic afferent to the VTA which modifies the mesolimbic dopaminergic system involved in the alpha-MSH/ACTH-induced behaviors.     

Formation and Metabolism of Dopamine in Nine Areas of the Rat Brain: Modifications by Haloperidol

The rate of removal of 3,4-dihydroxyphenylacetic acid (DOPAC) in nine rat brain areas (striatum, nucleus accumbens, tuberculum olfactorium, hypothalamus, lateral hippocampus, occipital cortex, brain stem, cerebellum, and retina) was calculated from its exponential decline after monoamine oxidase inhibition by pargyline. The experiments were carried out with rats pretreated with either saline or haloperidol. It appeared that the efficiency with which DOPAC was removed from the brain (expressed by the fractional rate constant k) varied considerably throughout the brain. Haloperidol dramatically decreased the k values, and in addition these effects differed widely in the various brain areas. Similarly to DOPAC, haloperidol had a pronounced retarding effect on the efflux of homovanillic acid (HVA) from the brain. These findings strongly suggest that great care should be taken when drug-induced alterations in DOPAC and HVA concentrations are interpreted as changes in dopaminergic activity. The dopamine (DA) concentrations were measured in the same experiments, but it appeared that the pargyline-induced rise in DA was of limited use for the estimation of the synthesis rate of the amine. We calculated the rate of catecholamine synthesis in the nine brain areas from the rise of 3,4-dihydroxyphenylalanine (DOPA) during decarboxylase inhibition. In saline- as well as in haloperidol-pretreated rats it was found that the total catecholamine synthesis rate in the typical dopaminergic areas (striatum, nucleus accumbens, and tuberculum olfactorium) was of the same order of magnitude as the DOPAC rate of removal. This confirms that DOPAC formation is quantitatively the main route of degradation in these brain areas.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of (-)-cathinone, a khat leaf constituent, on dopaminergic firing and dopamine metabolism in the rat brain

The effect of (-)-cathinone (CAT), an alkaloid from khat leaves, on brain dopamine (DA) metabolism and on the firing rate of nigral DA neurons was studied in rats, in comparison with that of d-amphetamine. Like d-amphetamine, CAT (8-40 mg/kg i.p.) decreased DOPAC levels in the caudate nucleus, nucleus accumbens and frontal cortex, without modifying DA concentrations. CAT showed approximately one fifth of the potency of d-amphetamine in this effect. CAT, injected i.v. to unanesthetized, paralyzed rats, inhibited the firing rate of DA neurons in the substantia nigra, pars compacta, showing a similar potency to that of d-amphetamine in this respect. CAT-induced inhibition of dopaminergic firing was reversed by haloperidol.     

Effect of Excitotoxin Lesions in the Medial Prefrontal Cortex on Cortical and Subcortical Catecholamine Turnover in the Rat

Catecholamine turnover in brain areas innervated by dopaminergic neurons was examined 2, 6, and 12 days after bilateral, N-methyl-D-aspartate lesions confined to the rat medial prefrontal cortex. The lesion produced a significant regional increase in the concentration of 3,4-dihydroxyphenylethylamine (DA, dopamine) in both the medial prefrontal cortex and the ventral tegmental area. DA concentrations were increased in the nucleus accumbens on day 6 (128% of control), in the ventral tegmental area on day 2 (130% of control), and in the medial prefrontal cortex on days 2 (145% of control) and 6 (127% of control). The only significant changes in the concentration of 3,4-dihydroxyphenylacetic acid (DOPAC) (197% of control), and in the ratio DOPAC/DA (163% of control) were found in the medial prefrontal cortex on day 6 post-lesion. All parameters had returned to control levels by day 12. DA depletion after the administration of alpha-methyl-p-tyrosine (AMPT) was not significantly different between excitotoxin-lesioned and sham animals on day 6 in all brain regions. Noradrenaline (NA) and 3,4-dihydroxyphenylethyleneglycol concentrations and their ratios, and the depletion of noradrenaline after AMPT were also determined, and the lesion resulted in a significant regional increase in NA in both the nucleus accumbens and the ventral tegmental area. An elevation of NA (147% of control) in the nucleus accumbens was found on day 12. Since the excitotoxin lesion destroys corticofugal efferents from medial prefrontal cortex to the nucleus accumbens, the anterior corpus striatum and the ventral tegmental area, our results provide no evidence for a role of these cortical projections in the regulation of subcortical DA metabolism.(ABSTRACT TRUNCATED AT 250 WORDS)     

Amperozide, a Novel Antipsychotic Drug, Inhibits the Ability of d-Amphetamine to Increase Dopamine Release In Vivo in Rat Striatum and Nucleus Accumbens

The in vivo effects of amperozide, a novel atypical antipsychotic drug, on the release of dopamine (DA) and the output of its metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), were investigated in the striatum and the nucleus accumbens of awake, freely moving rats using microdialysis. Amperozide (2-10 mg/kg, s.c.) significantly increased extracellular levels of DA in both the striatum and nucleus accumbens in a dose-dependent manner. It had a similar but lesser effect on extracellular DOPAC levels in both regions. d-Amphetamine (2 mg/kg, s.c.) alone produced a very large (43-fold) increase in DA release, together with a 70% decrease in DOPAC levels in both the striatum and the nucleus accumbens. Amperozide (1-5 mg/kg, s.c.) 30 min before d-amphetamine (2 mg/kg) dose-dependently attenuated d-amphetamine-induced DA release but had no effect on the d-amphetamine-induced decrease in extracellular DOPAC levels in both regions. The effect of amperozide on d-amphetamine-induced DA release in the nucleus accumbens may explain the inhibitory effect of amperozide on amphetamine-induced locomotor activity. However, the failure of amperozide to block amphetamine-induced stereotypy, despite marked inhibition of striatal DA release, suggests the need to reexamine the importance of striatal DA for amphetamine-induced stereotypy.     

Modification of dopamine neurotransmission in the nucleus accumbens of rats deficient in n-3 polyunsaturated fatty acids

We studied the effects of a diet chronically deficient in alpha-linolenic acid, the precursor of long-chain n-3 polyunsaturated fatty acids, on dopaminergic neurotransmission in the shell region of the nucleus accumbens of rats. In vivo microdialysis experiments showed increased basal levels of dopamine and decreased basal levels of metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), in awake rats from the deficient group compared to controls. The release of dopamine under KCl stimulation was similar in both dietary groups. By contrast, the release of dopamine from the vesicular storage pool under tyramine stimulation was 90% lower in the deficient than in the control rats. Autoradiographic studies in the same cerebral region revealed a 60% reduction in the vesicular monoamine transporter sites in the deficient group. Dopamine D(2) receptors were 35% increased in these rats compared to controls, whereas no change occurred for D(1) receptors and membrane dopamine transporters. These results demonstrated that chronic n-3 polyunsaturated fatty acid deficiency modifies several factors of dopaminergic neurotransmission in the nucleus accumbens. These findings are in agreement with the changes in dopaminergic neurotransmission already observed in the frontal cortex, and with the behavioral disturbances described in these deficient rats.     

Electrophysiological and microiontophoretic studies with buspirone: influence on the firing rate of central monoaminergic neurons and their responsiveness to dopamine, clonidine or GABA

The influence of an i.v. perfusion of buspirone on the firing rate of central monoaminergic neurons was studied in rats anaesthetized with chloral hydrate. Buspirone increased the firing rate of A10 dopaminergic neurons and blocked the inhibitory effect of iontophoretically applied dopamine on these neurons. A slight attenuation of the inhibitory effect of iontophoretically applied GABA was also observed. Buspirone increased the firing rate of locus coeruleus (LC) noradrenergic neurons and induced an attenuation of the inhibitory effect of iontophoretically applied clonidine. A slight attenuation of the inhibitory effect of iontophoretically applied GABA was also observed. Furthermore buspirone was a very potent inhibitor of the firing rate of dorsal raphe (DR) serotonergic neurons. It is concluded that activation of A10 neurons by buspirone is due to blockade of dopaminergic autoreceptors and that activation of LC neurons is related to blockade of alpha-2 autoreceptors. The significance of the interaction with gabaergic inhibition is unclear. The mechanisms involved in the inhibition of DR neurons remain to be investigated.     

Study by the intracerebral microdialysis method of the effects of atypical neuroleptics and anxiolytics on striatal release and metabolism of dopamine in awake rats]

Using brain microdialysis in awake rats effects of risperidone, ritanserin, buspirone, sulpiride and 5-methoxy-N,N-dimethyltryptamine (MeODMT) on striatal dopamine (DA) release and metabolism were studied. Risperidone, sulpiride and buspirone increased levels of DA, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). Ritanserin failed to affect DA release, while increased DOPAC and HVA levels. MeODMT had no effect on striatal DA release and metabolism. Possible interaction between DA and serotonin systems is discussed.     

Enhanced dopamine metabolism after small lesions in the midbrain of the rat

The effect of midbrain lesions on the metabolism of dopamine (DA) in various regions of the rat brain was investigated. Small midbrain lesions produced an acute increase in the levels of the acidic metabolites homovanillic acid (HVA) and 3,4-dihydroxyphenylacetic acid (DOPAC) in the striatum. Elevated levels of HVA were also found in the nucleus accumbens, tuberculum olfactorium and the cerebral cortex. The levels of HVA in the substantia nigra remained unaffected. The acute effect in the striatum of a complete transection of the ascending DA-pathway consists in an initial decrease of the levels of the metabolites followed by gradual increase. The results indicate that dopaminergic neurons do not act in an uncoordinated fashion, and that rapidly acting compensatory mechanisms are able to modify the output of this system.     

An analysis of the hypothermic action of 8-hydroxy-2-(di-N-propylamino)tetralin, 1-(2-pyrimidinyl)piperazine and its derivatives

The hypothermic activity of 8-OH-DRAT, 1-(2-pyrimidinyl) piperazine (1-PP), buspirone and its structure analogues tightly correlates with the degree of inhibition of 3H-serotonine release by electrically stimulated dorsal raphe slices (r = 0.84), but not cerebral cortex slices (r = 0.66). The pretreatment of mice with p-chlorphenylalanine decreased the hypothermic effects evoked by 8-OH-DRAT, buspirone, campirone (but not 1-PP) only in first 30 min after there injection. It was concluded that hypothermic effects of 8-OH-DRAT, buspirone and its structure analogues were bound with preferable influence as partial agonists on the postsynaptic serotonine 1A receptors. Ipsapirone as a partial agonist has maximal antagonistic activity.     

Effects of acute and subacute cocaine administration on the CNS dopaminergic system in Wistar-Kyoto and spontaneously hypertensive rats: I. Levels of dopamine and metabolites

Effects of acute and subacute cocaine administration on dopamine (DA) and its metabolites in striata and nucleus accumbens of nine week-old Wistar-Kyoto and spontaneously hypertensive rats were studied. Levels of DA,3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were determined by HPLC-EC. There were no differences in DA levels in striata and nucleus accumbens between control WKY and SHR. Levels of DA in two brain regions were unaffected in groups treated acutely with cocaine. Both strains showed a significant increase in striatal HVA 2 hr after cocaine injection. Seven day treatment declined DA levels in striatum of WKY and in nucleus accumbens of SHR. However, only WKY treated subacutely with cocaine showed significantly increased HVA either with or without changes in DOPAC in nucleus accumbens and striatum, respectively. Increased DOPAC/DA and HVA/DA ratios appeared only in striatum of WKY and in nucleus accumbens of SHR following subacute treatment. These results suggest that subacute cocaine administration affects DA levels in striata and nucleus accumbens differently between WKY and SHR.     

Determination of dopamine, homovanillic acid and 3,4-dihydroxyphenylacetic acid in rat brain striatum by high-performance liquid chromatography with electrochemical detection

Two procedures using liquid chromatography with electrochemical detection are described for the determination of dopamine (DA) and its two acidic metabolites, homovanillic acid (HVA) and 3,4-dihydroxyphenylacetic acid (DOPAC), in subregions of rat striatum and nucleus accumbens. A strong cation-exchange column was used for DA analysis and a C₁ reversed-phase column was used for the analysis of the metabolites. Effects of pH, temperature and percentage of methanol on the retention time of HVA and DOPAC were studied. Levels of these compounds in the subregions of rat striatum and nucleus accumbens are reported.