β-endorphin-induced increase in striatal dopamine turnover

Human β-endorphin administered intracisternally in a dose of 15 μg per rat increased striatal concentrations of the dopamine metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) as well as producing catalepsy. These effects were inhibited by naloxone. Pargyline-induced decreases in striatal DOPAC and HVA were greater in endorphin-treated than in saline-treated animals, supporting the concept that β-endorphin increases striatal dopamine turnover. β-endorphin increased the rate of decline in striatal dopamine concentration following synthesis inhibition with α-methyltyrosine, further suggesting that endorphin increases striatal dopamine turnover. β-endorphin and probenecid interacted competitively to decrease the effects of each other to increase striatal HVA. Naloxone prevented the effect of endorphin to decrease the HVA response to probenecid. Thus, probenecid cannot be used to assess the effects of endorphin on striatal dopamine turnover. If β-endorphin acts presynaptically to decrease dopamine release in striatum, the increases in striatal DOPAC and HVA probably represent a compensatory attempt to increase dopamine synthesis. Although turnover of dopamine to its metabolites is increased, dopamine release may be suppressed by β-endorphin.     

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.     

Intracerebral dialysis: direct evidence for the utility of 3-MT measurements as an index of dopamine release

Intracerebral dialysis was used to monitor the in vivo efflux of striatal dopamine (DA), homovanillic acid (HVA), dihydroxyphenylacetic acid (DOPAC) and 3-methoxytyramine (3-MT) in the pentobarbital anesthetized rat. In untreated rats, there were low levels of extra-cellular DA and 3-MT which were increased 15-fold by treatment with amphetamine. Under basal and drug-stimulated conditions, 3-MT concentrations were maintained at approximately 30% of the extracellular DA levels. These data agree with in vivo turnover estimates which indicate that 20 to 30% of DA turnover is through the 3-MT pool in the striatum. In contrast, extracellular DOPAC and HVA levels were reduced only slightly by amphetamine and with a delayed onset. Our data support the hypothesis that striatal DOPAC is an accurate index of intraneuronal DA metabolism and that 3-MT is an index of the extracellular concentration of DA.     

ON THE RELEVANCE OF PREFERENTIAL INCREASES OF MESOLIMBIC VERSUS STRIATAL DOPAMINE TURNOVER FOR THE PREDICTION OF ANTIPSYCHOTIC ACTIVITY OF PSYCHOTROPIC DRUGS

Abstract— Drugs possessing (chlorpromazine, haloperidol, clozapine, thioridazine and sulpiride) or lacking (benzoctamine and perlapine) antipsychotic activity were compared with respect to their ability to enhance x-methyl-p-tyrosine-induced dopamine disappearance from the mesolimbic area and corpus striutum of rat brain. In addition, their effects on the endogenous concentrations of homovanillic (HVA) and 3.4-dihydroxyphenylacetic (DOPAC) acids in these two brain areas were determined. Some of the drugs enhanced dopamine disappearance in the mesolimbic area more than in the striatum. The most active in this respect were sulpiride. perlapine and chlorpromazine. By contrast, haloperidol was slightly more active in the striatum than in the mesolimbic area. None of the drugs was more efficient in elevating HVA levels in the mesolimbic area than in the striatum. However, there were large differences in the relative extent of the HVA increases in the two regions. Benzoctamine, perlapine and chlorpromazine increased HVA concentrations in the mesolimbic area nearly as much as in the striatum. Thioridazine and haloperidol, however, elevated striatal HVA much more effectively. Haloperidol and clozapine increased the DOPAC concentration in both areas to about the same extent. The other drugs were more active in the striatum. The largest difference between both regions was shown by chlorpromazine. Perlapine and benzoctamine, both lacking antipsychotic activity, produced much larger increases of HVA than of DOPAC. This is in contrast to the results obtained with true neuroleptics and may reflect an involvement of release phenomena in the action of these two drugs on dopamine metabolism. These results suggest that a preferential increase of dopamine turnover in the mesolimbic area is not necessarily linked to a better ratio of antipsychotic activity vs. extrapyramidal side effects. Moreover, an antiacetylcholine component of dopamine receptor blocking drugs does not seem to be a prerequisite for preferential activity on dopamine turnover in the mesolimbic system.     

Study of Dopamine Turnover by Monitoring the Decline of Dopamine Metabolites in Rat CSF After α-Methyl-p-Tyrosine

CSF was continuously withdrawn from the third ventricle of anesthetized rats. CSF 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid concentrations were determined every 15 min by liquid chromatography coupled with electrochemical detection. Acute tyrosine hydroxylase inhibition [with alpha-methyl-p-tyrosine (alpha-MPT)] induced an exponential decline in levels of DOPAC and HVA in CSF. The decline in DOPAC and HVA concentrations was identical in CSF and forebrain but was much slower in the striatum, suggesting that CSF metabolites of 3,4-dihydroxyphenylethylamine (dopamine) reflect whole forebrain metabolites. The decay in CSF DOPAC and HVA levels after dopamine synthesis inhibition was also used as an in vivo index of forebrain dopamine turnover after various pharmacological treatments. Haloperidol pretreatment accelerated this decay, confirming the increase in brain dopamine turnover induced by neuroleptics. After reserpine pretreatment (15 h before), alpha-MPT produced a very sharp decay in levels of DOPAC and HVA. This result indicates that the residual dopamine that cannot be stored after reserpine treatment is very rapidly renewed and metabolized. Nomifensine strongly diminished the slope of DOPAC and HVA level decreases after alpha-MPT, a result which can be explained either by a slower dopamine turnover or by the involvement of storage dopamine pools. These results exemplify the use of monitoring the decay of dopamine metabolites after alpha-MPT administration in the study of the pharmacological action of drugs on the central nervous system of the rat.     

Cholinergic effect on the dopamine turnover in the rat corpus striatum]

The peripheral administration of oxotremorine caused a significant increase in dihydroxyphenylacetic acid (DOPAC) in the striatum of rats, dopamine (DA) level was unaffected. Injection of oxotremorine into the substantia nigra failed to change the content of dopamine and its acid metabolites homovanillic acid (HVA) and DOPAC in striatum. Injection of oxotremorine or carbachol into the substantia nigra or into the caudate nucleus did not significantly influence the DA-turnover. The partly inconsistent results are discussed in connection with literature data in regard to the existence of excitatory as well as inhibitory cholinergic systems, which are located differently and are involved in the regulation of DA-turnover.     

Halothane Anesthesia Affects NMDA-Stimulated Cholinergic and GABAergic Modulation of Striatal Dopamine Efflux and Metabolism in the Rat In Vivo

Microdialysis of the striatum of halothane-anesthetized rats was used to study the participation of local cholinergic and GABAergic neurotransmission in NMDA receptor-modulated striatal dopamine release and metabolism. Reverse dialysis.of NMDA (1 mM) evoked a 10-fold increase in dopamine efflux and reduced DOPAC and HVA to > 20% of basal values. The effect of NMDA on dopamine efflux was abolished by atropine (10 microM) but unaffected by (+)-bicuculline (50 microM). NMDA-induced decrease in DOPAC (but not HVA) efflux was potentiated by atropine, whereas (+)-bicuculline attenuated the decrease in DOPAC and HVA. Compared to our previous studies in unanesthetised rats, our data suggest that halothane anesthesia alters the balance between NMDA-stimulated cholinergic and GABAergic influences on striatal dopamine release and metabolism. Differential sensitivity to halothane of NMDA receptors expressed by the neurones mediating these modulatory influences, or loss of specific NMDA receptor populations through voltage-dependent Mg2+ block under anesthesia, could underlie these observations.     

The role of dopamine in behavioral supersensitivity to muscarinic antagonists following cholinesterase inhibition

The role of brain dopamine (DA) in the enhancement of muscarinic antagonist-induced hyperactivity was investigated. The effects of atropine and scopolamine on the concentrations of DA and its metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), following DFP administration were determined. In control animals, atropine and scopolamine decreased the concentration of DA and increased the ratios of DOPAC/DA and HVA/DA in the striatum, but not in the N. accumbens - T. olfactorium (mesolimbic) area. Following a single dose of DFP, the two antimuscarinic drugs caused decreases of DA and further increases of the above ratios in both brain regions. However, following repeated DFP treatment for 2 weeks, these antimuscarinic drug-induced changes were observed only in the mesolimbic area, but not in the striatum. It is suggested that an increased DA turnover, indicated by elevated DOPAC/DA and HVA/DA ratios, underlies the muscarinic antagonist-induced hyperactivity. The well-known occurrence of muscarinic receptor down-regulation after DFP administration, could be responsible for the enhancement of the actions of muscarinic antagonists in DFP-treated animals. The observed differential effect on DA turnover in the two broad areas may involve both muscarinic and DA receptors.     

Inhibition of Ischemia-Induced Dopamine Release by ω-Conotoxin, a Calcium Channel Blocker, in the Striatum of Spontaneously Hypertensive Rats: In Vivo Brain Dialysis Study

The effect of omega-conotoxin GVIA (CgTX), an N-and L-type voltage-sensitive calcium channel (VSCC) blocker, on the release of dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) in the striatum before and during transient cerebral ischemia in spontaneously hypertensive rats was studied using an in vivo brain dialysis technique. Continuous perfusion of CgTX in the striatum was started 20 min before ischemia and concentrations of dopamine and DOPAC in the dialysate were measured using HPLC with an electro-chemical detector. Before ischemia, both 10 and 100 microM CgTX significantly lowered the concentration of dopamine, to 49% of the basal values. DOPAC concentrations also decreased significantly, by 28 and 17%, respectively. Forebrain ischemia, produced by bilateral carotid artery occlusion, reduced striatal blood flow to less than 6% of the resting value in each group. During 20 min of ischemia, the vehicle group showed a marked increase in dopamine (175 times the basal concentration). In the 10 or 100 microM CgTX perfusion group, in contrast, dopamine release was significantly attenuated, to 38 or 29% of the vehicle group, respectively. DOPAC concentrations decreased during ischemia to 58% of the basal value in the vehicle group and 49% in both CgTX groups. These results indicate that the massive release of striatal dopamine during ischemia depends largely on the influx of extracellular calcium via CgTX-sensitive VSCCs.     

Effects of an N-methyl-d-aspartate receptor agonist and its antagonist CPP on the levels of dopamine and serotonin metabolites in rat striatum collected in vivo by using a brain dialysis technique

3-((±)-2-Carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) is an antagonist at the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor. In the present study, levels of dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindolacetic acid (5-HIAA) were measured after intracerebroventricular injection of NMDA, CPP or both in rat striatum using a brain dialysis method. The injection of NMDA produced a significant increase in DOPAC level. HVA level was also increased by NMDA injection. The level of 5-HIAA was not affected by NMDA injection. The injection of CPP had no effect on DOPAC, HVA and 5-HIAA levels. The injection of CPP restrained the increase of DOPAC and HVA levels induced by NMDA injection. The results suggest that intracerebral injection of NMDA may increase dopamine release from rat striatum, but have no effect on serotonin release. Furthermore, CPP inhibits NMDA induced release of dopamine.     

Different effects of L-, N- and T-type calcium channel blockers on striatal dopamine release measured by microdialysis in freely moving rats.

Using a microdialysis method, we have investigated effects of the voltage-dependent calcium channel blockers, verapamil, nicardipine, omega-conotoxin and flunarizine on the dopamine release and metabolism in the striatum of freely moving rat. Perfusion of verapamil (1-300 microM) and nicardipine (1-100 microM), an L-type calcium channel blocker, into the striatum through the dialysis membrane showed a dose-dependent decrease of dopamine release in the dialysate and slight increase of 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) levels. Treatment of omega-conotoxin (0.1, 1 microM), an N-type channel blocker, decreased about 50% basal dopamine release and slightly decreased DOPAC and HVA levels. Treatment with flunarizine (10 microM), an T-type channel blocker, did not affect the dopamine release and metabolism. From these data, it appears that treatments of the L- and N-type voltage-dependent calcium channel blockers in rat striatum suppress basal dopamine release, but T-type blocker does not suppress it, suggesting that L-, N- and T-type calcium channels regulate in vivo dopamine release in a different mechanism.     

A possible role of nitric oxide in the regulation of dopamine transporter function in the striatum.

Brain microdialysis and high-performance liquid chromatography with electrochemical detection were used to study the effect of the nitric oxide synthase inhibitor Nomega-nitro-L-arginine methyl ester (L-NAME) on striatal dopamine (DA) release in the anesthetized rat. Systemic administration of L-NAME (10 mg/kg, i.p.) significantly decreased the resting release of DA. The peak effect (23% decrease) was reached 45 min after injection. The inactive enantiomer D-NAME (10 mg/kg, i.p.) or the vehicle (saline, 5 ml/kg i.p.) had no effect on the striatal DA level. Neither treatment altered significantly the concentration of dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). To investigate the possible involvement of the DA uptake system L-NAME was injected also in the presence of the DA uptake inhibitor nomifensine. Local application of nomifensine (10 microM in the dialysate medium) increased the extracellular concentration of DA to about eight-fold of the basal value and stabilized it at this higher level. Under these conditions L-NAME (10 mg/kg, i.p.) was not able to alter the striatal DA level. Neither nomifensine nor L-NAME caused any change in the level of DOPAC and HVA. Our data suggest that endogenously produced nitric oxide may influence the activity of the DA transporter which effect may have special importance in the regulation of extracellular transmitter concentration in the striatum.     

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.     

Effects of Single and Repeated Footshock on Dopamine Release and Metabolism in the Brains of Fischer Rats

Abstract: Changes in the tissue levels of 3-methoxytyramine (3-MT), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and dopamine in the frontal cortex, hypothalamus, nucleus accumbens, and striatum were evaluated after 0.5-4 h of footshock (2 mA, for 3 s every 30 ± 5 s) in Fischer rats. 3-MT, DOPAC, and HVA levels in the four brain areas peaked at 0.5 h and in most cases returned to baseline values within 4 h. No changes were found in dopamine levels. Repeated footshock stress was evaluated by administering 10 footshock sessions (0.5 h, two per day for 5 days). At the end of the 10th footshock session, 3-MT levels were higher than at the end of the first footshock session in three of the four brain regions, indicating sensitization of dopamine release. No differences were found between the first and 10th footshock sessions in DOPAC and HVA levels. Fourteen days after the 10th footshock session, the levels of 3-MT, DOPAC, and HVA were the same as in control rats in all four brain regions. A 0.5-h footshock challenge presented 14 days after the 10th footshock session attenuated DOPAC levels in the hypothalamus and nucleus accumbens. In contrast, DOPAC and HVA levels in the frontal cortex showed sensitization after footshock challenge, and a similar trend was apparent for 3-MT levels. These results indicate that repeated footshock stress induces generalized sensitization of dopamine release and turnover in some areas of the brain of Fischer rats. This sensitization may persist in the cortical but not subcortical dopamine neurons after discontinuation of the treatment.     

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.     

Time Course of Adaptations in Dopamine Biosynthesis, Metabolism, and Release Following Nigrostriatal Lesions: Implications for Behavioral Recovery from Brain Injury

Alterations in neostriatal dopamine metabolism, release, and biosynthesis were determined 3, 5, or 18 days following partial, unilateral destruction of the rat nigrostriatal dopamine projection. Concentrations of dopamine and each of its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 3-methoxytyramine (3-MT) were markedly decreased in the lesioned striata at 3, 5, or 18 days postoperation. The decline in striatal high-affinity [3H]dopamine uptake closely matched the depletion of dopamine at 3 and 18 days postoperation. However, neither DOPAC, HVA, nor 3-MT concentrations were decreased to as great an extent as dopamine at any time following lesions that depleted the dopamine innervation of the striatum by greater than 80%. In these more severely lesioned animals, dopamine metabolism, estimated from the ratio of DOPAC or HVA to dopamine, was increased two- to four-fold in the injured hemisphere compared with the intact hemisphere. Dopamine release, estimated by the ratio of 3-MT to dopamine, was more increased, by five- to sixfold. Importantly, the HVA/dopamine, DOPAC/dopamine, and 3-MT/dopamine ratios did not differ between 3 and 18 days postlesioning. The rate of in vivo dopamine biosynthesis, as estimated by striatal DOPA accumulation following 3,4-dihydroxyphenylalanine (DOPA) decarboxylase inhibition with NSD 1015, was increased by 2.6- to 2.7-fold in the surviving dopamine terminals but again equally at 3 and 18 days postoperation. Thus, maximal increases in dopamine metabolism, release, and biosynthesis occur rapidly within neostriatal terminals that survive a lesion. This mobilization of dopaminergic function could contribute to the recovery from the behavioral deficits of partial denervation by increasing the availability of dopamine to neostriatal dopamine receptors. However, these presynaptic compensations are not sufficient to account for the protracted (at least 3-week) time course of sensorimotor recovery that has been observed following partial nigrostriatal lesion.     

Effects of Manganese on Extracellular Levels of Dopamine in Rat Striatum: An Analysis <Emphasis Type="Italic">In vivo</Emphasis> by Brain Microdialysis

The aim of this study is to determine the effects of intrastriatal administration of MnCl_2, on the extracellular levels of dopamine (DA) and metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in basal conditions and stimulated by depolarization with KCl and pargyline administration. Also, we studied the effect of MnCl₂ on extracellular levels of l-Dopa in the presence of aromatic amino acid decarboxylase (AADC) inhibitor 3-hydroxybencilhydracine-HCl (NSD 1015). This study concluded that MnCl₂, reduced the basal and K⁺-stimulated DA-release in striatum, without notably affecting the DOPAC and HVA levels. Intraperitoneal injection of pargyline increased striatal DA levels, decreasing DOPAC and HVA levels. The infusion of MnCl₂ removed the increase in DA levels, without affecting DOPAC and HVA levels. Perfusion of NSD 1015 increased the extracellular levels of l-DOPA in striatum, and MnCl₂ increased the effect of NSD1015 on l-Dopa.     

Relationships of Dopamine, Cortical Oxygen Pressure, and Hydroxyl Radicals in Brain of Newborn Piglets During Hypoxia and Posthypoxic Recovery

Abstract: The present study describes the relationships of extracellular striatal dopamine, cortical oxygen pressure, and striatal hydroxyl radicals in brain of newborn piglets during hypoxia and posthypoxic reoxygenation. Hypoxia was induced by reducing the fraction of inspired oxygen (F_iO₂) from 22% (control) to 7% for 1 h. The F_iO₂ was then returned to the control value and measurements were continued for 2 h. Cerebral oxygen pressure was measured by the oxygen dependent quenching of phosphorescence and extracellular levels of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and hydroxy radicals in the striatum were determined by in vivo microdialysis. Hypoxia decreased the cortical oxygen pressure from 47 ± 2 to 9 ± 1.3 torr (p < 0.001); the levels of extracellular dopamine in the striatum increased to 16,000 ± 3,270% of control (p < 0.01), whereas the levels of DOPAC and HVA decreased to 25.3 ± 6% (p < 0.001) and 36 ± 5% (p < 0.01) of control, respectively. Compared with control, the hydroxyl radical levels at each time point were not significantly increased during hypoxia, although the sum of the measured values was significantly increased (p < 0.05). During the first 5 min after F_iO₂ was returned to 22%, the cortical oxygen pressure increased to control values and stayed at this level for the remainder of the measurement period. The extracellular level of dopamine declined to values not statistically different from control during 40 min of reoxygenation. During the first 10 min of reoxygenation, DOPAC and HVA further decreased and then began to slowly increase. By 70 min of reoxygenation, the values were not significantly different from control. Hydroxyl radicals were above control during the entire period of reoxygenation, with maximal values observed after 100 min of reoxygenation. This increase was largely abolished by injecting the animals with α-methyl-p-tyrosine 5 h before hypoxia, a procedure that depleted the brain of dopamine. Our results suggest that oxidation of striatal dopamine during posthypoxic reoxygenation is at least partly responsible for the observed increase in striatal level of hydroxyl radicals that may exacerbate posthypoxic cerebral injury.     

Formation and Clearance of Interstitial Metabolites of Dopamine and Serotonin in the Rat Striatum: An In Vivo Microdialysis Study

In vivo microdialysis was employed in order to characterize the steady-state kinetics of the turnover of specific dopamine and serotonin metabolites in the rat striatum 48 h after surgery. Inhibitors of monoamine oxidase (MAO; pargyline) and catechol-O-methyltransferase (COMT; Ro 40-7592) were administered, either separately or in conjunction, at doses sufficient to block these enzymes in the CNS. In some experiments, the acid metabolite carrier was blocked with probenecid. Temporal changes were then observed in the efflux of interstitial dopamine, 3-methoxytyramine (3-MT), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA). The fractional rate constants for the accumulation or disappearance of the metabolites could be determined after pharmacological blockade of catabolic enzymes or the acid metabolite carrier. Interstitial 5-HIAA was found to be cleared with a half-life of approximately 2 h. After blockade of either MAO or COMT, HVA disappeared with a half-life of 17 min. Experiments employing probenecid suggested that some of the interstitial HVA was cleared by the acid metabolite carrier, the remainder being cleared by a probenecid-insensitive process, possibly conjugation. After MAO inhibition, DOPAC disappeared with an apparent half-life of 11.3 min. The rate of 3-MT accumulation after pargyline indicated that the majority of interstitial HVA (> 95%) is formed from DOPAC rather than 3-MT. The formation of 3-MT from interstitial dopamine, calculated from the accumulation rate of 3-MT after pargyline, appeared to follow first-order kinetics (k = 0.1 min-1).