Effects of CGP 28014 on the in vivo release and metabolism of dopamine in the rat striatum assessed by brain microdialysis

The effects on rat striatal dopamine (DA) metabolism of systemic and local administration of CGP 28014, an inhibitor of catechol-O-methyl-transferase (COMT), were studied by in vivo microdialysis. CGP 28014 (30 mg/kg i.p.) significantly reduced the levels of homovanillic acid (HVA), but did not modify DA and 3,4-dihydroxyphenylacetic acid (DOPAC). The intrastriatal administration (via the microdialysis probe) of 5, 7.5, 10, and 20 mM of CGP 28014 elicited a concentration-dependent, several-fold increase in extracellular DA but did not alter the levels of HVA and DOPAC. Thus, the effects of CGP 28014 observed after i.p. injection (decrease in HVA levels) are different from those measured after intrastriatal administration (increase in DA release). Therefore, the inhibition of COMT is likely to be due to the action of a metabolite of CGP 28014 formed in the periphery and not in the brain.     

Characterization and Pharmacological Responsiveness of Dopamine Release Recorded by Microdialysis in the Substantia Nigra of Conscious Rats

The extracellular concentration of dopamine (DA) and 3,4-dihydroxyphenylacetic acid in the substantia nigra (SN) and striatum was estimated by microdialysis. The dialysate content of DA from the SN was recorded during infusion of a DA uptake blocker (nomifensine; 5 mumol/L) dissolved in the perfusion fluid. Perfusion of tetrodotoxin (1 mumol/L) produced a virtually complete disappearance of nigral and striatal DA release. Dendritic as well as terminal release of DA was inhibited for several hours when the nerve impulse flow in dopaminergic neurons was blocked by systemic administration of gamma-butyrolactone (750 mg/kg, i.p.). The systemic administration (0.3 mg/kg, i.p.) as well as infusion (1 mumol/L) of the D2 agonist (-)-N-0437 [2-(n-propyl-N-2-thienylethylamino)-5-hydroxytetralin] produced a significant decrease in the release of DA in both the striatum and the SN. DA levels were recorded in the striatum both with and without addition of nomifensine to the perfusion fluid. The decrease in the striatum after (-)-N-0437 was suppressed in the presence of nomifensine. Infusion (1 mumol/L) as well as systemic administration (40 mg/kg) of sulpiride caused a similar increase in the release of striatal DA; this increase was, in both experiments, potentiated by nomifensine coinfusion. Sulpiride administration induced a small increase in the release of nigral DA. Infusion of (-)-N-0437 or (-)-sulpiride into the nigra caused a moderate decrease and increase, respectively, of striatal DA level.(ABSTRACT TRUNCATED AT 250 WORDS)     

Further Studies on the Nature of Postsynaptic Dopamine Uptake and Metabolism in Rat Striatum: Sodium Dependency and Investigation of a Possible Role for Carrier-Mediated Uptake into Serotonin Neurons

The nature of postsynaptic sites involved in the uptake and metabolism of striatal 3,4-dihydroxyphenylethylamine (dopamine, DA) was investigated. The accumulation of [3H]DA (10(-7) M) into slices of rat striatum was found to be greatly dependent (greater than 99%) on the presence of sodium ion in the incubation medium. However, the formation of the [3H]dihydroxyphenylacetic acid (DOPAC) and [3H]homovanillic acid (HVA) was only partially reduced in the absence of sodium (DOPAC, 27% of control; HVA, 47% of control). Inhibition of carrier-mediated DA neuronal uptake with nomifensine (10(-5) M) significantly decreased DA accumulation (18% of control) and [3H]DOPAC formation (62% of control), but enhanced [3H]HVA production (143% of control). Inhibition of the 5-hydroxytryptamine (5-HT, serotonin) neuronal uptake system with fluoxetine (10(-6) M) or selective 5-HT neuronal lesions with 5,7-dihydroxytryptamine (5,7-DHT) had no effect on [3H]DOPAC or [3H]HVA formed from [3H]DA in the presence or absence of nomifensine. These results demonstrate that the uptake and subsequent metabolism of striatal DA to DOPAC and HVA is only partially dependent on carrier-mediated uptake mechanism(s) requiring sodium ion. These data support our previous findings suggesting a significant role for synaptic glial cell deamination and O-methylation of striatal DA. Further, experiments with fluoxetine or 5,7-DHT suggest that 5-HT neurons do not significantly contribute in the synaptic uptake and metabolism of striatal DA.     

Propentophylline increases striatal dopamine release but dampens methamphetamine-induced dopamine dynamics: A microdialysis study

While there are currently no medications approved for methamphetamine (METH) addiction, it has been shown that propentofylline (PPF), an atypical methylxanthine, can suppress the rewarding effects of methamphetamine (METH) in mice. This experiment studied the interactions of PPF with METH in striatal dopaminergic transmission. Herein, the impact of PPF (10–40 mM, intrastriatally perfused (80 min) on the effect of METH (5 mg/kg, i.p.) on striatal dopamine (DA) release was evaluated using brain microdialysis in Sprague–Dawley adult rats. METH was injected at the 60 min time point of the 80 min PPF perfusion. The extracellular levels of DA and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were determined using high performance liquid chromatography with electrochemical detection (HPLC-ED). PPF induced a concentration-dependent increase in DA release beginning 30 min after the onset of PPF perfusion. DA peak levels evoked by 40 mM PPF were similar to those induced by 5 mg/kg METH i.p. Only the highest concentration of PPF decreased the METH-induced DA peak (circa 70%). The significant decreases in extracellular levels of DOPAC and HVA evoked by METH were partially blocked by 10 and 20 mM PPF. Although 40 mM of PPF also partially blocked the METH-induced DOPAC decrease, it completely blocked HVA depletion after a transient increase in HVA levels in METH-treated rats. Data indicates for the first time that while PPF increases presynaptic striatal DA dynamics it attenuates METH-induced striatal DA release and metabolism.     

Kinetics of Drug-Induced Changes in Dopamine and Serotonin Metabolite Concentrations in the CSF of the Rat

Cerebrospinal fluid (CSF) was removed at a constant flow rate of 1 microliter/min from the third ventricle of anesthetized rats. Every 15 min, CSF dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) concentrations were determined by direct injection of CSF into a liquid chromatographic system coupled with electrochemical detection. Mean CSF concentrations of DOPAC, HVA, and 5-HIAA were 1.29 microM, 0.88 microM, and 2.00 microM, respectively. In order to determine the turnover rates of dopamine (DA) and serotonin, experiments using monoamine oxidase (MAO) inhibition were performed. Tranylcypromine (20 mg/kg i.p.) induced a sharp exponential decrease of CSF DOPAC, HVA, and 5-HIAA, with respective half-lives of 15.60 min, 16.91 min, and 77.23 min. Their respective turnover rates were 3.74, 2.22, and 1.18 nmol X ml-1 X h-1. m-Hydroxybenzylhydrazine (NSD-1015, 100 mg/kg i.p.) and monofluoromethyl-DOPA (100 mg/kg i.p.), two decarboxylase inhibitors, induced a slow exponential decrease of all three CSF metabolites. alpha-Methyl-p-tyrosine (250 mg/kg i.p.) also induced a slow exponential decrease of DOPAC and HVA. These decreases of CSF DOPAC and HVA induced by DA synthesis inhibitors may reflect the turnover of DA in vivo. Haloperidol (0.5 mg/kg i.p.) considerably enhanced CSF DOPAC and HVA without affecting 5-HIAA, confirming that dopaminergic receptors modulate DA neurotransmission in vivo. Haloperidol administered 1.5 h after NSD-1015 did not increase DOPAC and HVA, in contrast to reserpine (5 mg/kg i.p.) injected under the same conditions.(ABSTRACT TRUNCATED AT 250 WORDS)     

d-Fenfluramine Increases Striatal Extracellular Dopamine In Vivo Independently of Serotonergic Terminals or Dopamine Uptake Sites

Abstract: The effect of various doses of the serotonin (5-HT) release-inducing agent d -fenfluramine ( d -fenf) on extracellular dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), and 5-hydroxyindoleacetic acid (5-HIAA) was studied in vivo in the striatum of halothane-anesthetized rats, following systemic and local administration. At 5 and 10 but not 2.5 mg/kg, d -fenf administered intraperitoneally significantly increased DA extracellular concentration and reduced DOPAC outflow. A concentration-dependent enhancement of DA dialysate content was also found following intrastriatal application (5, 10, 25, and 50 µ M ). The bilateral administration of 5,7-dihydroxytryptamine into the dorsal raphe nucleus, which markedly depleted 5-HT in the striatum, did not modify the effect on extracellular DA concentration of 25 µ M d -fenf locally applied into the striatum. The enhancement of extracellular DA level induced by 25 µ M d -fenf was slightly but significantly reduced by the local application of 25 µ M citalopgram. The blockade of DA uptake sites by nomifensine (0.1, 0.3, and 1 µ M ) did not modify significantly the effect of d -fenf. The rise of DA outflow induced by 25 µ M d -fenf was strongly reduced in the presence of 1 µ M tetrodotoxin (TTX) or by the removal of Ca²⁺ from the perfusion medium. The results obtained show that d -fenf increases the striatal extracellular DA concentration by a Ca²⁺-dependent and TTX-sensitive mechanism that is independent of striatal 5-HT itself or DA uptake sites.     

7-Nitroindazole enhances amphetamine-evoked dopamine release in rat striatum. an in vivo microdialysis and voltammetric study.

The intracellular second messenger nitric oxide (NO) is implicated in a variety of physiological functions, including release and uptake of dopamine (DA). In the described study, in vivo microdialysis and differential pulse voltammetric techniques were used to determine the involvement of NO in release of DA and its metabolites (dihydroxyphenylalanine, DOPAC; homovanillic acid, HVA) in neostriatum of freely moving rats. While the NO donor molsidomine (30.0 mg/kg; MOLS) and neuronal NO synthase- (nNOS-) inhbitor 7-nitroindazole (10.0 mg/kg; 7-NI) had no effect on the basal in vivo microdialysate level of DA, 7-NI specifically enhanced D,L-amphetamine-(1.0 mg/kg i.p.; AMPH) evoked release of DA. Basal or AMPH effects on DOPAC and HVA levels were not influenced by MOLS or 7-NI. Findings indicate that nitrergic systems have an important role in mediating effects of AMPH on dopaminergic systems.     

Extracellular Concentrations of Dopamine and Metabolites in the Rat Caudate After Oral Administration of a Novel Catechol-O-Methyltransferase Inhibitor Ro 40–7592

The effect of the systemic administration of a novel, orally active, catechol-O-methyltransferase (COMT) inhibitor, Ro 40-7592, on the in vivo extracellular concentrations of dopamine (DA) and its metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), was studied by transcerebral microdialysis in the dorsal caudate of freely moving rats. Ro 40-7592 (at doses of 3.0, 7.5, and 30 mg/kg p.o.) elicited a marked and long-lasting reduction of HVA, and at doses of 7.5 and 30 mg/kg, an increase of DOPAC output, but it failed to increase DA output. The administration of L-beta-3,4-dihydroxyphenylalanine (L-DOPA, 20 and 50 mg/kg p.o.) with a DOPA decarboxylase inhibitor (benserazide) increased both HVA and DOPAC output, but failed to modify significantly extracellular DA concentrations in dialysates; in contrast, combined administration of L-DOPA+benserazide with Ro 40-7592 (30 mg/kg p.o.) resulted in a significant increase in DA output. Ro 40-7592 prevented the L-DOPA-induced increase in HVA output and markedly potentiated the increase in DOPAC output. To investigate to what extent the increase in extracellular DA concentrations was related to an exocitotic release, tetrodotoxin (TTX) sensitivity was tested. Addition of TTX to Ringer, although abolishing DA output in the absence of L-DOPA, partially reduced it in the presence of L-DOPA+Ro 40-7592 and even more so after L-DOPA without the COMT inhibitor. The results of the present study suggest that metabolism through COMT regulates extracellular concentrations of DA formed from exogenously administered L-DOPA but not of endogenous DA.(ABSTRACT TRUNCATED AT 250 WORDS)     

Prolonged Alterations in Canine Striatal Dopamine Metabolism Following Subtoxic Doses of l-Methyl-4-Phenyl- 1,2,3,6-Tetrahydropyridine (MPTP) and 4''-Amino-MPTP Are Linked to the Persistence of Pyridinium Metabolites

Single toxic doses of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP).HCl (2.5 mg/kg i.v.) and 4'-amino-MPTP.2HCl (22.5 mg/kg) induce loss of striatal dopamine (DA) and tyrosine hydroxylase (TH) activity and of nigral DA neurons in the dog. To examine the subacute neurochemical changes induced by low doses of MPTP and 4'-amino-MPTP, dose-response studies of these compounds were carried out in the dog, using 6- and 3-week survival times for these two compounds, respectively. Low single doses of MPTP (1.0, 0.5, and 0.1 mg/kg i.v.) and 4'-amino-MPTP (15, 7.5, and 3.75 mg/kg i.v.) did not cause depletion of canine striatal DA or TH or a loss of nigral neurons. However, levels of the DA metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were decreased in a dose-related fashion, with significant loss of DOPAC being evident 6 weeks after the lowest administered dose of MPTP and 3 weeks after 4'-amino-MPTP. This selective loss of DA metabolites following nontoxic doses of MPTP and 4'-amino-MPTP led to a shift in the ratio of DA to DOPAC or HVA, which was characteristic for each compound. The measurement of striatal 1-methyl-4-phenylpyridinium (MPP+) and 4'-amino-MPP+ levels revealed that high concentrations (up to 150 microM) persist in the striatum for weeks following administration of a single nontoxic dose of MPTP or 4'-amino-MPTP. A causal relationship between the striatal concentration of MPP+ or 4'-amino-MPP+ and the change in DA metabolism as reflected in the DA/DOPAC ratio is suggested by a significant correlation between these measures. It is suggested that presynaptic sequestration and retention of MPP+ and 4'-amino-MPP+ by striatal DA terminals result in the inhibition of the monoamine oxidase contained within these terminals.     

Ketanserin pretreatment attenuates MDMA-induced dopamine release in the striatum as measured by in vivo microdialysis

Systemic administration of the amphetamine analogue, 3,4-methylenedioxymethamphetamine (MDMA) produced a dose-dependent increase in the extracellular concentration of dopamine (DA) in the striatum as measured by in vivo microdialysis in awake, freely-moving rats. The extracellular concentration of the DA metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), was significantly decreased in dialysate samples following the administration of MDMA (10 and 20 mg/kg, i.p.). The serotonin-2 (5-HT2) antagonist ketanserin (3 mg/kg, i.p.) had no effect on the extracellular concentration of DA or DOPAC in the striatum of vehicle- treated rats. The administration of ketanserin (3 mg/kg) 1 hr prior to MDMA (20 mg/kg) significantly attenuated the MDMA- induced increase in the extracellular concentration of DA without affecting the decrease in DOPAC concentrations. These data are suggestive that MDMA administration increases DA release in the striatum of awake, freely-moving rats. In addition, MDMA-induced increase in the extracellular concentration of DA in the striatum is mediated, in part, via 5-HT2 receptor mechanisms.     

Changes in rat brain monoamine turnover following chronic antidepressant administration

Changes in rat brain monoamine turnover were studied following the chronic administration of five agents which markedly differ in their patterns of monoamine uptake inhibition. Compounds (10 mg/kg, i.p.) were injected once daily for 14 days and experiments undertaken 24 h after the last injection. Chronic administration of desipramine or mianserin elevated brain MOPEG-SO₄ content and the α-MT-induced reduction in brain NA levels was enhanced by chronic desipramine. either antidepressant altered turnover of brain DA or 5-HT. Steady state levels of brain 5-HIAA or striatal levels of DOPAC or HVA were also unchanged. Chronically administered Org 6582, a selective inhibitor of 5-HT uptake, decreased basal and attenuated the probenecid-induced increase iin brain 5-HIAA levels. Chronic Org 6582 had no effect on NA or DA turnover and on the levels of MOPEG-SO₄, DOPAC or HVA. Neither maprotiline nor chlorimipramine altered turnover of NA, DA or 5-HT or levels of metabolites. Thus, in contrast to the acute situation, chronically administered desipramine increases rat brain NA turnover. Conversely, acute and chronic Org 6582 administration yield similar findings, viz. a decrease in turnover. These observations suggest that rat brain 5-HT systems are more resistant than NA systems to adaptive changes following a prolonged inhibition of monoamine uptake.     

Effects of Catechol-O-Methyltransferase Inhibitors and L-3,4-Dihydroxyphenylalanine With or Without Carbidopa on Extracellular Dopamine in Rat Striatum

Abstract: The effects of two new catechol- O -methyltransferase (COMT) inhibitors, OR-611 and Ro 40-7592, in combination with L-3,4-dihydroxyphenylalanine (L-dopa) with or without carbidopa on extracellular levels of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), 3- O -methyldopa (3-OMD), and 5-hydroxyindoleacetic acid in rat striatum were studied. A dose of 10 mg/kg i.p. of Ro 40-7592 alone, in contrast to the same dose of OR-611, decreased the dialysate level of HVA and increased that of DOPAC; this dose was thus used to differentiate between the effects of central and peripheral COMT inhibition. L-Dopa (50 mg/kg i.p.) alone slightly increased extracellular levels of DA, DOPAC, and HVA. The effects of L-dopa were potentiated by carbidopa (50 mg/kg i.p.), and even 3-OMD levels in dialysate samples became detectable. Both OR-611 and Ro 40-7592 significantly further increased the DA and DOPAC efflux from striatum produced by L-dopa. This increase was more pronounced when carbidopa was added to the treatment. OR-611 did not modify the effect of L-dopa or carbidopa/L-dopa on dialysate HVA levels, whereas Ro 40-7592 markedly reduced those levels. Both OR-611 and Ro 40-7592 very clearly suppressed dialysate 3-OMD levels produced by carbidopa/L-dopa. Ro 40-7592 was more effective than OR-611 in potentiating the effects of L-dopa or carbidopa/L-dopa. These in vivo data show that the new COMT inhibitors markedly inhibit the O -methylation of L-dopa and increase its availability to brain, which is reflected as increased DA formation. A significant effect can be achieved even by inhibiting only the peripheral COMT activity. The data suggest that COMT inhibitors may be of clinical importance as adjuncts in the treatment of Parkinson's disease.     

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.     

Systemic administration of d-amphetamine induces long-lasting oxidative stress in the rat striatum

The long-term effect of d-amphetamine (AMPH) on the induction of oxidative stress was examined in vivo in the rat brain. In this study, 2,3-dihydroxybenzoic acid (2,3-DHBA) and malonaldehyde (MDA) were used as the index of the hydroxyl radical and lipid peroxidation, respectively. The levels of 2,3-DHBA, MDA and dopamine (DA) in striatal homogenates were examined 7 days following injection of a single large dose of AMPH (7.5 mg/kg, i.p.) in rats pretreated with desipramine (10 mg/kg, i.p.), an agent that inhibits the metabolism of AMPH. Our results showed that 2,3-DHBA and MDA levels were significantly increased by AMPH, whereas DA and its metabolites, DOPAC and HVA were depleted in the striatum. Pretreatment with the glutamate NMDA receptor subtype antagonist MK-801 (1 mg/kg, i.p.) attenuated the increases of 2,3-DHBA and MDA, and provided partial protection against the long-lasting loss of DA produced by AMPH. Overall, the results demonstrate that AMPH could induce sustained production of free radical and oxidative damage, and lead to DA terminal degeneration in the striatum of the rat.     

Cross-tolerance of dopamine metabolism to baclofen, γ-butyrolactone and HA-966 in the striatum and olfactory tubercle of the rat

Rats received 7 daily injections with baclofen (40 mg/kg), GBL (750 mg/kg) or HA-966 (100 mg/kg). Dopamine (DA) was measured in the striatum and olfactory tubercle (OT) of rats, sacrificed 0.5 h or 1 h after the last injection. Marked tolerance and cross-tolerance for the DA-elevating effect of these drugs was seen in the striatum, but not in OT. When on day 7 a unilateral lesion of the nigrostriatal pathway was made, also some tolerance to the DA increase in the striatum on the lesioned side was seen in HA-966-pretreated rats, but it was small compared to the tolerance after an additional drug administration in non-lesioned animals. A low dose of apomorphine (0.25 mg/kg, i.p.) had no effect on DA, dihydroxyphenylacetic acid DOPAC) or homovanillic acid (HVA) levels in the lesioned striata, whether the rats had been pretreated for 6 days with HA-966 or not. However, this dose of apomorphine had a significantly more lowering effect on striatal DOPAC and HVA levels on the unlesioned side of HA-966 pretreated rats. The results show that tolerance develops to the increase of DA synthesis, which is possibly receptor-mediated. This tolerance develops more readily in the striatum than in the olfactory tubercle.     

An In Vivo Study of Dopamine Release and Metabolism in Rat Brain Regions Using Intracerebral Dialysis

Intracerebral dialysis was used with a specifically designed HPLC with electrochemical detection assay to monitor extracellular levels of endogenous 3,4-dihydroxyphenylethylamine (dopamine, DA) and its major metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), in brain regions of the halothane-anesthetized rat. Significant amounts of DA, DOPAC, and HVA were detected in control perfusates collected from striatum and n. accumbens whereas the medial prefrontal cortex showed lower monoamine levels. The ratio of DA in perfusate to DA in whole tissue suggests that in f. cortex, compared to n. accumbens and striatum, there is a greater amount of DA in the extracellular space relative to the intraneuronal DA content. The DOPAC/HVA ratio in control perfusates varied between regions in accordance with whole tissue measurements. This ratio was highest in n. accumbens and lowest in f. cortex. The monoamine oxidase inhibitor pargyline (100 mg/kg i.p.) caused an exponential decline in DOPAC, but not of HVA, in regional perfusates, an effect that was associated with an increase in DA. The data indicated a higher turnover of extracellular DOPAC in n. accumbens than in striatum and the lowest DOPAC turnover in f. cortex. The rate of decline in extracellular DA metabolite levels was slow compared to whole tissue measurements. In the perfusates there was no statistical correlation between basal amounts of DA in the perfusates and DOPAC and HVA levels or DOPAC turnover for any of the areas, indicating that measurement of DA metabolism in the brain under basal conditions does not provide a good index of DA release. In summary, this study shows clear regional differences in basal DA release and metabolite levels, metabolite patterns, and DOPAC turnover rates in rat brain in vivo.     

Development of Haloperidol-Induced Dopamine Release in the Rat Striatum Using Intracerebral Dialysis

Haloperidol-induced dopamine (DA) release and metabolism were studied in the rat striatum at 10-11, 21-22, and 35-36 days of age using intracerebral dialysis and HPLC with electrochemical detection. There was an age-related increase in basal DA release and extracellular levels of 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), with the greatest increases occurring between 10-11 and 21-22 days of age. Haloperidol (0.1 mg/kg, i.p.) significantly increased DA release at each age compared to control. Also, haloperidol produced a significantly greater increase in DA release at 10-11 days than at 21-22 or 35-36 days of age when expressed as percentage of predrug release. Haloperidol increased DA release over 60 min to 235%, 138%, and 158% above baseline at 10-11, 21-22, and 35-36 days of age, respectively, after which time the levels remained relatively constant. Haloperidol significantly increased extracellular DOPAC and HVA levels at each age compared to controls, but there were no significant differences in DOPAC or HVA levels between ages in response to haloperidol. The results indicate that, at 10 days of age, DA release in the striatum is physiologically functional and that the regulatory feedback control of DA release and metabolism in the striatum develops prior to 10 days of age.     

Enhancement of Brain Dopamine Metabolism by Tyrosine During Immobilisation: An In Vivo Study Using Repeated Cerebrospinal Fluid Sampling in Conscious Rats

Central dopamine (DA) and 5-hydroxytryptamine (5-HT) metabolism was monitored in conscious, freely moving rats by determination of levels of the DA metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) and the 5-HT metabolite 5-hydroxyindoleacetic acid (5-HIAA) in CSF samples withdrawn repeatedly from the cisterna magna and treated with acid to hydrolyse DOPAC and HVA conjugates. The effect of tyrosine on DA metabolism was investigated. Time courses of metabolite concentrations in individual rats in a quiet room showed that tyrosine (20, 50, or 200 mg/kg i.p.) was without significant effect; brain changes were essentially in agreement. However, the increases of CSF DOPAC and HVA levels that occurred on immobilisation for 2 h were further enhanced by tyrosine (200 mg/kg). The associated increases of 5-HIAA level were unaffected. The corresponding increases of DA metabolite concentrations in the brains of immobilised rats given tyrosine were less marked than the CSF changes and only reached significance for "rest of brain" DOPAC. The CSF studies revealed large interindividual variation in the magnitude and duration of the effects of immobilisation on transmitter amine metabolism. These results may help toward the elucidation of possible relationships between the neurochemical and behavioural effects of stress.     

Effects of nootropic drugs on brain cholinergic and dopaminergic transmission

High affinity choline uptake (HACU) in the hippocampus and striatal concentration of dopamine (DA) and homovanillic acid (HVA) as measures of the in vivo acetylcholine and DA turnover, respectively, were estimated in male rats, Long-Evans, following 6-day administration of various nootropics in clinically relevant doses: piracetam and its derivatives pramiracetam and oxiracetam (100 mg/kg/day), pyritinol (50 mg/kg/day). Piracetam treatment was without effect on HACU, but induced significant increase of HVA in the striatum leaving striatal DA concentration unchanged. On the contrary, pyritinol, pramiracetam and oxiracetam increased HACU, but did not change striatal DA and HVA levels.     

TURNOVER OF FREE AND CONJUGATED (SULPHONYLOXY) DIHYDROXYPHENYLACETIC ACID AND HOMOVANILLIC ACID IN RAT STRIATUM

Abstract— Conjugated (sulphonyloxy) dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were synthesized from free DOPAC and HVA and used as reference compounds in their fluorimetric determination in rat brain (detection limit 0.2 nmol/g). The conjugated DOPAC and HVA form 29 and 36% of the total DOPAC and HVA found in rat striatum, respectively. Dopamine (DA) metabolism was studied in the rat striatum by following the decline of both free and conjugated DOPAC and HVA after treatment with pargyline (100mg/kg. i.p.) either alone or in combination with tropolone (100 mg/kg, i.p.). or from the accumulation of the free and conjugated acids after treatment with probenecid (100-500mg/kg. i.p.). The rates of decline were analysed by a non-linear curve fitting method using a simple model of DA metabolism that postulates the formation of the conjugates exclusively from the free acids, and HVA from DOPAC, with first order kinetics and single open compartments only. The curves computed all passed through the s.e.m. of every experimental point. The rate constants thus found indicate that DOPAC turnover is about 23nmol/g/h. Of this about 16 nmol/g/h are O -methylated to HVA, about 6 nmol/g/h are conjugated and less than 1 nmol/g/h is eliminated as free DOPAC. Of the HVA formed, about 8.5nmol/g/h are conjugated and about 7.5 nmol/g/h eliminated as free HVA. The conjugates accumulated after treatment with probenecid (1 h) faster than the free acids. The maximal accumulation of all four metabolites found (21 nmol/g/h) approximates the total turnover of DOPAC.