Increase In Dopamine Metabolites Research Articles

PCB95 and PCB153 change dopamine levels and turn-over in PC12 cells

Polychlorinated biphenyls (PCB) exposure at low chronic levels is a significant public health concern. Animal and epidemiological studies indicate that low PCB body burden may cause neurotoxicity and be a risk factor for neurodegenerative diseases. In the current study, we measured the ability of two non-dioxin like PCBs, 2,2′,4,4′,5,5′-hexachlorobiphenyl (PCB153) and 2,2′3,5′,6-pentachlorobiphenyl (PCB95), to alter dopamine (DA) levels and metabolism using the dopaminergic PC12 cell line. Our hypothesis is that treatment of PC12 cells with non-toxic concentrations of PCB153 or PCB95 for 12 and 24 h will have different effects due to different congener structures. Levels of DA and of 3,4-dihydroxyphenylacetaldehyde (DOPAL), 3, 4-dihyroxylphenylethanol (DOPET), and 3,4-dihyroxylphenylacetic acid (DOPAC) metabolite, gene expression of the dopamine synthesis enzyme tyrosine hydroxylase (TH) and the vesicular monoamine transporter (VMAT2), and gene expression of the anti-oxidant enzymes Cu/Zn and Mn superoxide oxidase (Cu/ZnSOD, MnSOD), glutathione peroxidase (GPx) and catalase were determined. PCB153 decreased intracellular and extracellular levels of DA after 12 h exposure and this was consistent with an increase in DA metabolites. After 24 h, the level of DA in medium increased compared to the control. In contrast, PCB95 exposure increased the intracellular DA level and decreased DA in medium consistent with a down-regulation of VMAT2 expression at 12 h. After 24 h exposure, PCB95 increased DA levels in media. Expression of TH mRNA increased slightly following 12 h but not at 24 h exposure. MnSOD mRNA increased up to 6–7 fold and Cu/ZnSOD increased less than two-fold after treatment with both congeners. Catalase expression was up-regulated following 24 h exposure to PCB153 and PCB95, but GPx expression was down-regulated after 12 h exposure to PCB95 only. These results suggest that PCB153 and PCB95 are neurotoxic and affect DA turnover with structure-dependent differences between these two congeners.

α3β4 nicotinic acetylcholine receptors in the medial habenula modulate the mesolimbic dopaminergic response to acute nicotine in vivo

Habenulo-interpeduncular nicotinic receptors, particularly those containing α3, β4 and α5 subunits, have recently been implicated in the reinforcing effects of nicotine. Our laboratory has shown that injection of α3β4 nicotinic receptor antagonists into the medial habenula (MHb) decreases self-administration of multiple abused drugs, including nicotine (Glick et al., 2006, 2008; 2011). However, it is unclear whether blockade of MHb nicotinic receptors has a direct effect on mesolimbic dopamine. Here, we performed in vivo microdialysis in female rats. Microdialysis probes were implanted into the nucleus accumbens (NAcc) and α3β4 nicotinic receptor antagonists (18-methoxycoronaridine; 18-MC or α-conotoxin AuIB; AuIB), were injected into the ipsilateral MHb, just prior to systemic nicotine (0.4 mg/kg, s.c.). Dialysate samples were collected before and after drug administration and levels of extracellular dopamine and its metabolites were measured using HPLC. Acute nicotine administration increased levels of extracellular dopamine and its metabolites in the NAcc. Pre-treatment with intra-habenular AuIB or 18-MC prevented nicotine-induced increases in accumbal dopamine. Neither drug had an effect on nicotine-induced increases in dopamine metabolites, suggesting that α3β4 receptors do not play a role in dopamine metabolism. The effect of intra-habenular blockade of α3β4 receptors on NAcc dopamine was selective for acute nicotine: neither AuIB nor 18-MC prevented increases in NAcc dopamine stimulated by acute d-amphetamine or morphine. These results suggest the mesolimbic response to acute nicotine, but not to acute administration of other drugs of abuse, is directly modulated by α3β4 nicotinic receptors in the MHb, and emphasize a critical role for habenular nicotinic receptors in nicotine's reinforcing effects.

Nalbuphine is effective in decreasing the rewarding effect induced by morphine in rats

Nalbuphine, a κ-opioid agonist and μ-opioid partial agonist, has been widely used as an analgesic or an adjuvant with morphine in clinics. In rats, it attenuates tolerance and physical dependence caused by morphine, when co-administered. In this study, we investigated the effect of nalbuphine on morphine reward. Using the conditioned place preference (CPP) paradigm in rats, we demonstrated that co-administration of nalbuphine (1 mg/kg, i.p.) with morphine (5 mg/kg, i.p.) during conditioning could completely block the CPP induced by morphine. However, in experiments examining locomotor activity in rats, nalbuphine showed no effect on the development of behavioral sensitization induced by reported morphine administration. In microdialysis experiments, morphine induced a significant increase in the dopamine metabolites 3,4-dihydroxyphenylacetic acid and homovanillic acid in the shell region of the nucleus accumbens. Co-administration of nalbuphine blocked the increase in dopamine metabolites induced by morphine. These results may be due to the attenuating effect of nalbuphine on the dopaminergic activity of mesolimbic pathways. All of these results suggest nalbuphine could have a great potential as a pharmacotherapy for opiate abuse.

Halothane enhances dopamine metabolism at presynaptic sites in a calcium-independent manner in rat striatum

We have previously reported that halothane anaesthesia increases the extracellular concentration of dopamine (DA) metabolites in the rat striatum with no change in DA. Although the metabolism of catecholamines is a source of oxidative stress, there is little information about DA metabolism and anaesthesia. We assessed the mechanism(s) of enhanced DA metabolism induced by halothane. Microdialysis probes were implanted into male Sprague-Dawley rats and perfused with artificial cerebrospinal fluid (CSF). The dialysate was injected directly into an HPLC every 20 min. Each group of rats (n=5-7) was administered saline, apomorphine 100 microg kg(-1), pargyline 7.5 or 75 mg kg(-1), reserpine 2 mg kg(-1) or alpha-methyl-p-tyrosine (AMPT) 250 mg kg(-1). Another set of rats was perfused with artificial CSF containing tetrodotoxin (TTX) 1 microM or calcium-free CSF containing 10 mM EGTA. Rats were anaesthetized with halothane 0.5 or 1.5% 1 h after pharmacological treatments. In rats pretreated with apomorphine, despite a decrease in DA concentration, halothane induced a increase in DA metabolites. Pargyline (high dose) and reserpine completely and AMPT partially antagonized the increase in DA metabolites induced by halothane anaesthesia. TTX perfusion reduced the increase in DA, whereas calcium-free CSF perfusion did not. Our data suggest that halothane accelerates DA metabolism at presynaptic sites by releasing DA from reserpine-sensitive storage vesicles to the cytoplasm in a calcium-independent manner. The metabolic oxidative stress of inhalation anaesthesia requires future investigation.

Regional Heterogeneity of Nicotine Effects on Neurotransmitters in Rat Brains in vivo at Low Doses

In our recent studies on nicotine-induced changes in neurotransmitters in brain areas associated with cognitive function using a nicotine dose of 0.5 mg/kg administered subcutaneously to conscious freely moving rats, we found changes in dopamine, norepinephrine, and serotonin, and their metabolites, in the areas examined. For the present report we examined changes in these neurotransmitters following administration of lower nicotine doses, to test regional differences in nicotine response and possible threshold levels for some effects of nicotine. The doses used were 0.15 mg/kg and 0.03 mg/kg nicotine administered subcutaneously. Nicotine levels in the brain reached peak values in less than 10 min and decreased with a half-life of about 60 min (0.15 mg/kg) or 30 min (0.03 mg/kg) to values below detection limits (1 ng/g), by the later time points of the 0.03 mg/kg experiments. Nicotine-induced dopamine (DA) increase (and increase in DA metabolites) and decrease in 5-HT levels at 0.15 mg/kg were significant in the cortex, less so in the hippocampus. Norepinephrine (NE) increase at 0.15 mg/ kg was much less significant than found previously at 0.5 mg/kg. At a low nicotine dose (0.03 mg/kg), the significant changes observed were a decrease in 5-HT in the hippocampus and small increases of DA and NE in the prefrontal cortex and of NE in the medial temporal cortex. In the nucleus accumbens DA, NE, and 5-HT and their metabolites in the ventral tegmental area, mostly DA and metabolites were increased. We conclude that in areas of cognitive function nicotine-induced DA changes are more concentration dependent than changes in NE or 5-HT, and that there are regional differences in neurotransmitter changes induced by nicotine, with NE changes detectable only in the cortex and 5-HT changes only in the hippocampus at a low nicotine dose, indicating significant regional variation in sensitivity to nicotine-induced neurotransmitter changes in brain areas associated with cognitive function. The decrease in 5-HT shows that nicotine also has indirect effects caused by neurotransmitters released by nicotine. The effects of a low nicotine dose are more significant in areas of reward function, indicating differences in sensitivity between cognitive and reward functions.

An evaluation of the role of 5-HT 2 receptor antagonism during subchronic antipsychotic drug administration in rats

A widely postulated mechanism of action for the atypical profile of many novel antipsychotic drugs (APDs) is their relatively high affinity for 5-HT 2 receptors. The present study investigated motor function and striatal dopamine (DA) efflux and metabolism in rats given 21 daily injections of drugs that differed in 5-HT 2 affinity. These drugs included: risperidone (high 5-HT 2A/2C/high D 2), clozapine (high 5-HT 2A/2C/low D 2), haloperidol (low 5-HT 2A/2C/high D 2), haloperidol+ritanserin (selective 5-HT 2A/2C), or vehicle. Rats injected with haloperidol (0.5 mg/kg) or haloperidol+ritanserin (0.5 mg/kg and 1.0 mg/kg, respectively) showed extreme catalepsy on day 1, but significantly decreased catalepsy when tested again on days 7 and 21. Acute or subchronic risperidone (0.05 or 0.5 mg/kg), clozapine (20 mg/kg), or vehicle did not induce significant catalepsy. Microdialysis performed 24 h after the last injection demonstrated that rats treated with risperidone, clozapine, or vehicle showed similar increases in DA efflux and metabolism following an acute injection of a selective DA D 2/3 antagonist (raclopride, 0.5 mg/kg). DA efflux showed an attenuated response to raclopride in the haloperidol alone group; this effect was less apparent in the haloperidol+ritanserin group. However, both of these groups showed a similar tolerance effect to the raclopride-induced increase in DA metabolites. These results suggest that the profile seen after subchronic risperidone more closely resembles clozapine than haloperidol. While ritanserin reduced the tolerance-like effects of haloperidol on striatal DA efflux, the overall results demonstrate that potent 5-HT 2 blockade alone may not entirely account for the distinctive profile of novel APDs.

Increase in dopamine metabolites in murine striatum after oral exposure to inorganic but not organic form of selenium.

Selenium (Se) is an essential as well as a toxic trace element. Se intoxication has been reported in both livestock and humans. The central nervous system is sensitive to Se poisoning; exposure to Se causes blind staggers in cattle, poliomyelomalacia in pigs, and nervous system disorders in humans. Differences in neurotoxicity between inorganic and organic Se have been demonstrated. In this study, groups of five male BALB/c mice each were administered sodium selenite or selenomethionine in drinking water ad libitum at 0, 1, 3, and 9 ppm as Se for 14 days. At the end of Se exposure, their brains were removed and dissected into different regions. The concentration of norepinephrine (NE), dopamine (DA), dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), serotonin (5-HT), and 5-hydroxyindolacetic acid (5-HIAA) were determined in each brain region. Food and water consumption and body weight gain were significantly decreased in the group treated with the highest concentration of sodium selenite. In mice administered sodium selenite at 3 and 9 ppm, DOPAC was significantly higher in the striatum than in the control group. The striatal HVA was also increased in the group treated with 3 ppm Se; the DA showed a similar pattern, but the increase was not statistically significant. No alterations of NE, 5-HT, or 5-HIAA levels were detected in any brain region of mice treated with sodium selenite. No significant differences in any parameter among the groups treated with selenomethionine were observed indicating that inorganic Se was more neurotoxic than organic Se via drinking water. The alterations of DA metabolites by inorganic Se in DA-rich striatum suggested a Se-specific increased neural activity of dopaminergic pathways. Results may be useful in further elucidation of neurotoxicity of Se and in establishing a safe level of intake for this element.

(+/-)Cyclazocine blocks the dopamine response to nicotine.

(+/-)Cyclazocine, synthesized by Archer in 1962, was originally tested as a treatment for heroin addiction. (+/-)Cyclazocine is a mu opioid antagonist and kappa opioid agonist, and because of these actions, would be expected to modulate dopamine release in the nucleus accumbens as well as the reinforcing effects of drugs of abuse. In a recent study (+/-)cyclazocine was reported to decrease cocaine self-administration in rats. The aim of the present study was to determine whether (+/-)cyclazocine would alter the dopaminergic effects of nicotine that are thought to mediate its rewarding effects. Using in vivo microdialysis in awake and freely moving rats, we investigated the effect of (+/-)cyclazocine (0.5 mg/kg, i.p.) on the acute dopamine response to nicotine (0.32 mg/kg, i.v. over a 5 min period, infused 30 min later) in the nucleus accumbens. (+/-)Cyclazocine significantly attenuated the increase in extracellular dopamine levels induced by the nicotine infusion and enhanced nicotine-induced increases in dopamine metabolites. (+/-)Cyclazocine alone did not significantly affect extracellular dopamine levels. However, both the (+) and (-) enantiomers of cyclazocine did alter basal dopamine levels and these effects made it difficult to assess their individual interactions with nicotine. The results suggest that the effects of both enantiomers contribute to the effects of the racemate; (+/-)cyclazocine may decrease the rewarding effect of nicotine and may be the prototype of a potentially novel treatment for smoking.

EFFECTS OF ALLOPURINOL ON STRIATAL DOPAMINE, ASCORBATE AND URIC ACID DURING AN ACUTE MORPHINE CHALLENGE: EX VIVO AND IN VIVO STUDIES

In the present study in vivoand ex vivoexperiments were combined to evaluate the effects of allopurinol on the neurochemical changes induced by an acute morphine challenge (2 mg kg −1, s.c.). In samples from rat striatum, levels of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), 3-methoxytyramine (3-MT), ascorbate (AA), dehydroascorbate (DHAA), hypoxanthine, xanthine and uric acid (UA) were measured. Brain microdialysis experiments were carried out in freely moving rats. Striatal dialysate levels were assayed for DA, DOPAC+HVA, AA and UA using liquid chromatography followed by electrochemical detection. Morphine administration increased the striatal levels of DA metabolites, UA and DHAA and the extracellular concentrations of DA, DOPAC+HVA, UA and AA. Allopurinol (50 mg kg −1by gavage), an inhibitor of xanthine oxidase which catalyses oxidation of xanthine to UA, decreased basal UA and AA concentrations and the morphine-induced increase in DA metabolites and AA oxidation. Since oxidation of DA and xanthines generates reactive oxygen species (ROS) and AA and UA are the main cellular antioxidants, these findings suggest that: (a) single morphine administration increases DA and xanthine oxidative metabolism with a consequent increase in ROS production, which may account for changes in concentrations of extracellular AA and tissue DHAA; (b) allopurinol decreases morphine-induced DA and xanthine oxidation; (c) UA and AA may act in concert to regulate levels of ROS in the brain.

Transport of dopamine at the blood-brain barrier of the guinea pig: inhibition by psychotropic drugs and nicotine.

Transport of dopamine (DA) across the blood-brain barrier (BBB) was examined in guinea pigs. In situ brain perfusion (1-10 min), capillary depletion, and high pressure liquid chromatography (HPLC) were used. There was a saturable DA influx into the brain with a KM of 389 +/- 55 nM, and a VMAX of 1.95 +/- 0.25 pmol/min/g of brain. The diffusion constant, KD, was not significantly different from zero. About 0.5% of DA remained tightly bound to cerebral microvessels isolated from the perfused brain. DA influx into the brain was not altered by the monoamine oxidase-B (MAO-B) inhibitor pargyline (50 microM). HPLC analysis of perfused brain confirmed transport of intact DA, and no detectable increases in DA metabolites were observed. At perfusate concentrations of 500 nM, several dopaminergic receptor antagonists inhibited [3H]-DA (21 nM) influx; the percent inhibitions for the mixed D1 and D2 antagonists haloperidol and chlorpromazine, the D1 antagonist SCH-23390, and the D2 antagonist spiperone were 90%, 68%, 77%, and 50%, respectively. Brain perfusion with nicotine (500 nM) inhibited DA uptake by 86%. This nicotine effect was not altered by mecamylamine, but was partially prevented by the nicotinic receptor antagonist hexamethonium. (a) A significant cerebrovascular permeability to intact DA is mediated by a MAO-B independent specific transport system at the BBB, (b) this system could be inhibited by D1 and D2 DA receptor antagonists, and (c) DA blood-to-brain transport was inhibited by nicotine.

Synaptophysin: A test of the etiological hypotheses

Developing and adult Sprague-Dawley rats were tested after acute or repeated haloperidol administration. Although 8-day-old rat pups showed a form of immobility in response to a single injection of haloperidol (1 mg/kg), 14-day-old rats did not show any behavioral response to the neuroleptic. By 21 days of age, an acute dose of haloperidol induced a cataleptic response similar to that described for adult animals. Following 7 days of repeated haloperidol administration, the cataleptogenic effects of haloperidol were attenuated in animals aged 21 days and older, but not in 8- and 14-day-old rats. Subjects were sacrificed 70 min after the injection of the test dose of haloperidol or saline and the corpus striatum and olfactory tubercles were dissected for HPLC determination of dopamine (DA) and its metabolites, homovanillic acid (HVA) and 3,4-dihydroxyphenylacetic acid (DOPAC). In the corpus striatum, an area believed to be important for DA-related catalepsy, acute and repeated haloperidol induced a slight increase in concentrations of HVA in 8-day-old rats, and an increase in both DOPAC and HVA concentrations in animals aged 14 days and older. Tolerance after repeated haloperidol administration, in the form of an attenuation of the haloperidol-induced increase in DA metabolites, was not apparent until 35 days of age. These data contrast with the behavioral data, which indicate that the ability to develop a tolerance to the cataleptogenic effects of haloperidol matures by 21 days of age. The pattern of responses in the olfactory tubercles differed from those observed in the striatum. Following acute haloperidol, subjects did not show any increase in HVA until 14 days of age, and in both HVA and DOPAC until 21 days of age. At no age, including adults, was one week of repeated administration of haloperidol sufficient to induce tolerance to the effects of haloperidol on DA metabolites in the olfactory tubercles. In addition to providing information about the development of certain aspects of DA systems in rats these studies suggest that an attenuation of the haloperidol-induced increase in DA metabolites is not necessary for the development of tolerance to haloperidol-induced catalepsy.

Effects of postnatal stress on dopamine mesolimbic system responses to aversive experiences in adult life

The effects of postnatal stress on mesolimbic dopamine (DA) functioning in 90-day-old mice were investigated. Postnatal stress consisted of 15 min daily exposure to clean bedding (CB) in the absence of the mother for the first two weeks of life. Controls were daily exposed to home cage bedding (HCB) in the absence of the mother. A single brief (5–10 min) exposure to restraint produced a clear-cut increase in DA metabolites (3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 3-methoxytyramine (3-MT)) in the nucleus accumbens septi (NAS) of adult HCB but not CB mice. Moreover, when tested in an elevated plus maze, CB mice showed more exploration and reduced fearfulness in comparison with HCB mice. Taken together, these results indicate reduced emotional reactivity in adult mice repeatedly stressed during postnatal development. Moreover, HCB mice but not CB mice showed altered behavioral responsiveness to apomorphine following repeated restraint stress (10 daily 120 min) in adult life, although no difference in the behavioral response to either a low or a high dose of apomorphine was observed in adult unstressed mice of the CB and HCB groups. These results indicate that the effects of early experiences on brain DA functioning may not be evident in basal conditions and be revealed only under environmental pressure. Finally, a single exposure to 120 min of restraint had similar effects on DA metabolism (decrease of 3-MT and increase of DOPAC and HVA levels) in the NAS of adult HCB and CB mice, suggesting that postnatal stress experiences do not affect the ability of the mesolimbic DA system to respond to unavoidable aversive experiences.

Mechanisms of Veratramine-Induced 5-HT Syndrome in Mice

Regional monoamine assays revealed that during veratramine-induced myoclonic movements, the contents of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-H1AA) in the cerebral cortex were reduced with a slight increase in dopamine metabolites in the midbrain and brainstem. A similar tendency to decrease 5-HT and 5-HIAA contents was observed in the hypothalamus and hippocampus without increase in the contents of dopamine and its metabolites. Norepinephrine levels were not modified in any brain region at any time after the administration of the veratrum alkaloid. It was found that the veratramine evoked 3H-5-HT release from the frontal cortical slices was Ca++-independent and persistent, and it continued approximately 20 min after the 2-min exposure to veratramine. The uptake of 3H-5-HT into the frontal cortical slices was inhibited competitively by veratramine. These results suggest that veratramine is both a releaser and uptake inhibitor of 5-HT and that the veratramine-induced involuntary movements may be mediated by serotonergic hyperfunction.

Mechanisms of veratramine-induced 5-HT syndrome in mice.

Regional monoamine assays revealed that during veratramine-induced myoclonic movements, the contents of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in the cerebral cortex were reduced with a slight increase in dopamine metabolites in the midbrain and brainstem. A similar tendency to decrease 5-HT and 5-HIAA contents was observed in the hypothalamus and hippocampus without increase in the contents of dopamine and its metabolites. Norepinephrine levels were not modified in any brain region at any time after the administration of the veratrum alkaloid. It was found that the veratramine evoked 3H-5-HT release from the frontal cortical slices was Ca+(+)-independent and persistent, and it continued approximately 20 min after the 2-min exposure to veratramine. The uptake of 3H-5-HT into the frontal cortical slices was inhibited competitively by veratramine. These results suggest that veratramine is both a releaser and uptake inhibitor of 5-HT and that the veratramine-induced involuntary movements may be mediated by serotonergic hyperfunction.

Comparative study of sulpiride and haloperidol on dopamine turnover in the rat brain.

The effects of the neuroleptics, sulpiride and haloperidol, on dopamine (DA) turnover were compared following the acute and chronic administration of these drugs alone or in combination with levodopa or apomorphine. In the acute treatment, the increase in DA metabolites in the striatum and nucleus accumbens was more marked in the haloperidol-treated rats than in the sulpiride-treated rats. Following the additional administration of levodopa, however, the potency of the neuroleptics in elevating DA metabolites was reversed. A low dose of apomorphine induced a marked reduction in the striatal DA metabolite levels by approximately 50%. When rats were pretreated with the neuroleptics, haloperidol was more effective in preventing an apomorphine-induced reduction in DA metabolites. On repeated administration of the neuroleptics, a tolerance occurred in the striatum and nucleus accumbens, but not in the prefrontal cortex. This differential development of tolerance was observed in the different brain regions and with the different drugs administered. These results suggest that the pharmacological mechanism of sulpiride on DA turnover differs from that of haloperidol.

Sensitivity of regional brain dopamine systems to haloperidol

As the extrapyramidal side effects of antipsychotic drugs tend to di~nish with time, while the ~tipsychotic effects do not (Coleman and Hayes 1975), it seems plausible that the sources of these two phenomena lie in different regions of the brain. With the demonstration of dopaminergic terminals in the rat frontal cortex (Thierry et al. 1973), these neurons have become the focus of much research describing their functional properties and implicating them in the therapeutic action of neuroleptics. For example, it has been shown that rat pmfrontal cortex shows a smaller increase in dopamine metabolites upon acute drug administration (Bannon et al. 1983) and less tolerance to this effect upon chronic administration (Bannon et al. 1982) than other brain regions. More recently, however, certain authors (Laduron et al. 1977; Matsumoto et al. 1983; Chang et al. 1986) have suggested that the prefrontal cortex is preferentially sensitive to haloperidol at very low doses and that this unique propetty lends support to the assertion that this region is the site of action of neuro-

D-DOPA and l-DOPA similarly elevate brain dopamine and produce turning behavior in rats

In the intact rat, intragastric administration of d-dihydroxyphenylalanine ( d-DOPA) together with carbidopa (alpha-methyldopahydrazine, a peripheral dopadecarboxylase inhibitor) increased striatal dopamine concentration to the same extent as a similar treatment with l-DOPA plus carbidopa. In rats with unilateral 6-hydroxydopamine-induced lesions of their substantia nigra, both stereoisomers of DOPA produced significant increases in dopamine and its metabolites in the intact striata. Although dopamine concentrations in the lesioned striata did not change, a significant increase in dopamine metabolites was observed, indicating some extraneuronal formation of dopamine. These results suggest that d-DOPA can be converted to dopamine in the normal striatum as well as in the striatum devoid of dopamine nerve terminals. d- and l-DOPA produced turning behavior in unilaterally lesioned rats with a similar efficacy. The onset of turning after d-DOPA was delayed compared with l-DOPA. Turning behavior elicited by these amino acids was attributed to stimulation of supersensitive dopamine receptors in the lesioned striata by the extraneuronally formed dopamine. Preliminary results suggest that d-DOPA is converted to dopamine via transamination and/or d-amino acid oxidation to 3,4-dihydroxyphenylpyruvic acid which upon further transamination gives rise to l-DOPA and hence dopamine. The relatively fast and slow onset of stimulation of dopamine receptors by l-DOPA and d-DOPA respectively suggests that the use of the racemic mixture of DOPA combined with a peripheral dopadecarboxylase inhibitor may prove useful in the treatment of parkinsonism.

Chapter 59 Effect of haloperidol on transplants of fetal substantia nigra: evidence for feedback regulation of dopamine turnover in the graft and its projections

Publisher Summary This chapter describes the technique of transplantation of solid grafts placed in resection cavities in the cortex overlying the striatum to evaluate the effect of haloperidol on dopamine (DA) metabolism within the graft itself and in the host striatum. The data presented in the chapter indicate that the solid grafts of fetal mesencephalon and the terminal region of these grafts in the host striatum contain ratios of dihydroxyphenylacetic acid (DOPAC)/DA and homovanillic acid (HVA)/DA that are comparable to those found in the normal intact substantia nigra of adult animals. In response to administration of the DA receptor antagonist haloperidol, both the transplant and its terminal area in the host striatum showed marked increases in DA metabolites. The significant elevation of HVA/DA and DOPAC/DA ratios in these tissues following haloperidol treatment indicates that the DA neurons of the transplant increase their DA turnover under conditions of DA receptor blockade. The chapter suggests that the existence of regulatory mechanisms via which, the host target tissue could influence the output and metabolism of transplant-derived neurotransmitters would have highly significant implications for the therapeutic potential of transplantation in the central nervous system.

Tolerance to haloperidol-induced increases in dopamine metabolites: fact or artifact?

Haloperidol increased 3,4-dihydroxyphenylacetic acid and homovanillic acid concentrations in the striatum, nucleus accumbens and olfactory tubercle of both drug-naive and rats pretreated with haloperidol (10 injections). The increases in metabolite concentrations were greater in all brain regions of the naive rats, suggesting that haloperidol pretreatment resulted in a decreased responsiveness to the drug (tolerance). However, subchronic haloperidol injections also resulted in decreased basal metabolite concentrations in rats killed 48 h after the last injection. While the response of drug-experienced rats to haloperidol was attenuated relative to that of drug-naive rats, this difference could be accounted for entirely by the decreased basal metabolite concentrations that occur after repeated haloperidol injections.

Effects of cholecystokinin peptides and neurotensin on dopamine release and metabolism in the rostral and caudal part of the nucleus accumbens using intracerebral dialysis in the anaesthetized rat

By means of intracerebral microdialysis effects of cholecystokinin peptides and neurotensin administered via the microdialysis probe have been studied on dopamine release and metabolism in the nucleus accumbens and neostriatum of the halothane anaesthetized male rat. Levels of extra cellular dopamine (DA) and its metabolites 3,4 dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were assessed in nuc. accumbens (rostral and caudal part) using high performance liquid chromatography in combination with electrochemical detection. (1) In the rostral part of the nuc. accumbens CCK-8 (10 and 100 ?M), CCK-33 (100 ?M) but not CCK-4 (10 and 100 ?M) increased the levels of DA in the perfusate without increasing the extracellular levels of DOPAC and HVA. (2) In the caudal nuc. accumbens CCK-8 and CCK-4 in concentrations of 10 ?M and 100 ? M of CCK-33 had no effect on DA release and metabolism, since the extracellular levels of DA, DOPAC and HVA were not changed. (3) In the rostral nuc. accumbens perfusion with 10 ?M of neurotensin but not with any other concentration of neurotensin (0.01, 0.1, 1 and 100 ?M) increased the levels of DA in the extracellular fluid. (4) In the caudal nuc. accumbens a 40 min perfusion with neutrotensin produced a concentration dependent increase of the levels of DA in the perfusate (peak action at 10 ? M) which in this case was associated with increases in the extracellular levels of DOPAC and HVA. (5) By means of receptor autoradiography using (3-[(125)I]iodotyrosyl(3)) neurotensin it was found that a 40 min perfusion with this radioligand in the rostral nuc. accumbens reached a total volume of 0.051 mm(3). The diffusion of the radioligand was limited to the rostral or caudal part of the nuc. accumbens depending upon the site of placement of the dialysis probe. The results indicate the existence of cholecystokinin (CCK) receptors in the rostral nuc. accumbens, which are sensitive to CCK-8 and CCK-33 but not to CCK-4, and which facilitate DA release without producing any detectable increase in DA metabolites. In contrast, such receptors do not appear to play a similar role in the regulation of DA release in the caudal nuc. accumbens, where DA terminals contain CCK-like immunoreactivity. Furthermore, the results indicate that neurotensin receptors exist both in the rostral and caudal nuc. accumbens, where they inter alia enhance the release of DA. In the caudal nuc. accumbens these effects of neurotensin are also associated with an increase of DA metabolites, possibly suggesting that in this region neurotensin receptors may also control DA synthesis.