Demonstration of the ascorbate dependence of membrane-bound dopamine beta-monooxygenase in adrenal chromaffin granule ghosts

Chromaffin granule ghosts from bovine adrenal medullae have been used to examine the ability of membrane-bound dopamine beta-monooxygenase to interact directly with intravesicular ascorbate and to investigate vectorial electron transfer from external ascorbate across the ghost membrane. Ghosts prepared by a modification of published procedures were shown to be fully active in both dopamine uptake and norepinephrine production. Dopamine uptake is dependent on the presence of a magnesium and ATP ionic complex, is abolished by reserpine, and reaches a steady-state level in the presence of dopamine beta-monooxygenase, ascorbate, catalase, and fumarate. Omission of ascorbate either inside or outside the ghosts greatly enhances dopamine accumulation, which reaches levels of approximately 30 nmol/mg under these conditions. Correspondingly, in the presence of all components, norepinephrine production reached approximately 100 nmol/mg in 30 min of incubation. Norepinephrine production was strictly magnesium-ATP-dependent, inhibited by either reserpine or dopamine beta-monooxygenase inactivation, and was markedly reduced when ascorbate was omitted from either inside or outside the ghosts. In the presence of limiting amounts of internal ascorbate, rapid norepinephrine production occurred which corresponded to the amount of initial ascorbate present, followed by a much slower endogenous norepinephrine production observable after complete depletion of internal ascorbate. The endogenous rate of norepinephrine production likely represents epinephrine-supported dopamine beta-monooxygenase turnover. Taken together, the data demonstrate that facile norepinephrine production by membrane-bound dopamine beta-monooxygenase occurs only when internal ascorbate is present, terminates upon depletion of internal ascorbate, and can only be sustained at a significant rate when reducing equivalents from external ascorbate are available.     

Depletion of cardiac norepinephrine but not brain catecholamines by MPTP-N-oxide in mice

After a single dose in mice, MPTP-N-oxide caused a dose-dependent depletion of cardiac norepinephrine which was similar although less pronounced than that caused by MPTP itself. After repeated daily doses, MPTP-N-oxide depleted cardiac norepinephrine, but did not deplete norepinephrine in the frontal cortex or dopamine in the striatum of mice. MPTP-N-oxide differed from MPTP, which depleted cardiac norepinephrine, cortical norepinephrine and striatal dopamine after repeated daily doses, but was similar to MPP+, another metabolite of MPTP, which depleted only cardiac norepinephrine. These data suggest that MPTP-N-oxide may contribute to the peripheral catecholamine-depleting effects after MPTP injection.     

Dopamine accumulation after dopamine beta-hydroxylase inhibition in rat heart as an index of norepinephrine turnover

Dopamine concentration in rat heart is normally very low, only a few percent of the concentration of norepinephrine. After treatment of rats with a dopamine beta-hydroxylase inhibitor, 1-cyclohexyl-2-mercapto-imidazole (CHMI), there was a rapid increase in dopamine concentration even before norepinephrine concentration had decreased perceptibility. This accumulation of dopamine was readily measured by liquid chromatography with electrochemical detection. Since the percentage change in dopamine was much greater than the percentage change in norepinephrine, especially at early times, measurement of dopamine accumulation rather than norepinephrine decline was considered as a useful measure of norepinephrine turnover. Drugs that act on noradrenergic receptors and are known to alter norepinephrine turnover were found to alter the rate of dopamine accumulation. Clonidine and guanabenz decreased dopamine accumulation after CHMI, whereas piperoxan (but not prazosin) increased dopamine accumulation after CHMI. Pergolide, a dopamine agonist whose lowering of blood pressure and cardiac rate has been suggested to be due to suppression of neurogenic release or norepinephrine, also decreased dopamine accumulation after CHMI. The results suggest that measuring dopamine accumulation may have advantages over measuring norepinephrine disappearance after dopamine beta-hydroxylase inhibition as an indicator of norepinephrine turnover in heart.     

Lateral hypothalamic lesions and striatal dopamine levels

Rats with bilateral lateral hypothalamic lesions were killed on the third day after surgery and their brains were assayed for tel-diencephalic norepinephrine and striatal dopamine. Lesion-induced weight loss was highly correlated with depletion of striatal dopamine but not with tel-diencephalic norepinephrine. In rats with severe dopamine depletions, the degree of weight loss was related more to the striatum with the highest remaining level of dopamine suggesting that a critical level of dopamine in one striatum may be essential for lateral hypothalamic recovery.     

Diethyldithiocarbamate Potentiates the Neurotoxicity of In Vivo l-Methyl-4-Phenyl-1, 2, 3, 6-Tetrahydropyridine and of In Vitro 1-Methyl-4-Phenylpyridinium

Diethyldithiocarbamic acid (DDC) potentiates in vivo neurotoxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and in vitro neurotoxicity of 1-methyl-4-phenylpyridinium (MPP+). Male C57B1/6 mice were given two or five injections of MPTP (30 mg/kg i.p.) preceded 0.5 h by DDC (400 mg/kg i.p.). The mice were tested for catalepsy, akinesia, or motor activity during and after the period of dosing. Striatal and hippocampal tissues were obtained at 2 and 7 days following the last injection and evaluated for dopamine and norepinephrine levels, respectively. These same tissues were also analyzed for the levels of glial fibrillary acidic protein (GFAP), an astrocyte-localized protein known to increase in response to neural injury. Pretreatment with DDC potentiated the effect of MPTP in striatum and resulted in substantially greater dopamine depletion, as well as a more pronounced elevation in GFAP. In hippocampus, the levels of norepinephrine and GFAP were not different from controls in mice receiving only MPTP, but pretreatment with DDC resulted in a sustained depletion of norepinephrine and an elevation of GFAP, suggesting that damage was extended to this brain area by the combined treatment. Mice receiving MPTP preceded by DDC also demonstrated a more profound, but reversible, catalepsy and akinesia compared to those receiving MPTP alone. Systemically administered MPP+ decreased heart norepinephrine, but did not alter the striatal levels of dopamine or GFAP, and pretreatment with DDC did not alter these effects, but did increase lethality. DDC is known to increase brain levels of MPP+ after MPTP, but our data indicate that this is not due to a movement of peripherally generated MPP+ into CNS.(ABSTRACT TRUNCATED AT 250 WORDS)     

Somatostatin Content Increases Following Norepinephrine Depletion in Frontal Cortex of l-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine-Treated Mice

The effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on somatostatin (SS)-containing neurons were examined by measuring dopamine, norepinephrine (NE), SS, and SS mRNA in striatum and frontal cortex of C57/B16 mice at various times following treatment with MPTP-HCl (96 mg/kg i.p.). MPTP caused a 70% depletion of dopamine in striatum by 1 day and a 40% depletion of NE in frontal cortex within 3 days. SS content was increased in frontal cortex 4 days later, but not in striatum; there were no changes in SS mRNA. Maprotiline, a specific NE-uptake blocker, prevented both the depletion of NE and the increase of SS in frontal cortex due to MPTP administration. These results support the possibility that NE can regulate SS in frontal cortex and are discussed in terms of the decrease of SS seen in parkinsonian patients with dementia.     

Differential neurochemical consequences of an escalating dose-binge regimen followed by single-day multiple-dose methamphetamine challenges

Chronic intake of methamphetamine (METH) causes tolerance to its behavioral and subjective effects. To better mimic human patterns of drug abuse, the present study used a rodent model that took into account various facets of human drug administration and measured METH-induced effects on brain monoamine levels. Adult male Sprague–Dawley rats were injected with METH or saline according to an escalating dose schedule for 2 weeks. This was followed by a challenge regimen of either saline or one of two doses of METH (3 × 10 mg/kg every 2 h or 6 × 5 mg/kg given every hour, both given within a single day). Both challenge doses of METH caused significant degrees of depletion of dopamine in the striatum and norepinephrine and serotonin in the striatum, cortex, and hippocampus. Animals pre-treated with METH showed significant attenuation of METH-induced striatal dopamine depletion but not consistent attenuation of norepinephrine and serotonin depletion. Unexpectedly, METH pre-treated animals that received the 3 × 10 mg/kg challenge showed less increases in tympanic temperatures than saline pre-treated rats whereas METH pre-treated animals that received the 6 × 5 mg/kg METH challenge showed comparable increases in temperatures to saline pre-treated rats. Therefore, pre-treatment-induced partial protection against monoamine depletion is probably not because of attenuated METH-induced hyperthermia in those rats.     

Effects of the serotonin agonist, quipazine, on luteinizing hormone and prolactin release: evidence for serotonin-catecholamine interactions

The present study tested whether administration of the serotonin agonist, quipazine maleate, affects the secretion of luteinizing hormone (LH) and prolactin (PRL) and concomitantly, the activity of central noradrenergic and dopaminergic systems. Quipazine (15 mg/kg, ip) significantly reduced LH and increased PRL when administered to ovariectomized rats. Associated with these changes, the depletion of dopamine seen after synthesis inhibition with alpha-methyl tyrosine was reduced by quipazine in the caudate nucleus and median eminence, suggesting a depression of dopaminergic activity. The depletion of norepinephrine in the median eminence was unaffected. In a second experiment, quipazine (1 microM) diminished the potassium-induced release of both norepinephrine and dopamine from fragments of medial basal hypothalamus, in vitro. Release from preoptic area was unaffected. These results suggest that central serotonergic systems may interact with noradrenergic and dopaminergic systems that regulate LH and PRL secretion, respectively.     

Distribution of a catecholamine-binding mechanism in rat brain

Summary The existence of characteristic regional differences in the binding capacity of tissue for catecholamines was demonstrated by incubating sections of rat brain with tritiated norepinephrine or epinephrine or dopamine, and by radioautography. Microscopically, catecholamine binding occurred diffusely in the neuropil, or in pericellular aggregates upon nerve cells, in a pattern highly suggestive of synaptic buttons. A neuroanatomical account of the regional distribution patterns of norepinephrine binding in rat brain showed similarity to the known regional distribution of norepinephrine. However, there were quantitative and qualitative differences between the distribution of the binding mechanism and that of norepinephrine. Minor differences were found between the binding of norepinephrine and epinephrine; little binding occurred for dopamine. The binding mechanism was not facilitated by ATP, and was resistant to reserpine.This investigation was supported by Public Health Grant NB 6239 from the National Institute of Neurological Diseases and Blindness.     

Norepinephrine Depletion in the Spinal Cord Gray Matter of Rats with Experimental Allergic Encephalomyelitis

Norepinephrine and dopamine concentrations were determined by radioenzymatic assay in discrete gray matter regions of the spinal cords of rats with experimental allergic encephalomyelitis (EAE). Norepinephrine was depleted in most spinal cord regions of EAE rats compared with controls, whereas dopamine depletion in EAE rats was restricted to the cervical dorsal horn. There was a rostrocaudal gradient of norepinephrine reduction in the spinal cords of the EAE rats with most severe depletion in the lumbar region. The results of this experiment confirmed recent anatomical observations that suggested that catecholamine-fluorescent axons and terminals were damaged in spinal cords of rats with EAE.     

Central and peripheral catecholamine depletion by 1-methyl-4-phenyl-tetrahydropyridine (MPTP) in rodents

1-Methyl-4-phenyl-tetrahydropyridine (MPTP) given in single doses to rats depleted norepinephrine concentration in heart and mesenteric artery but had little effect on catecholamine concentration in brain. MPTP did not share with amphetamine the ability to cause persistent depletion of striatal dopamine in iprindole-treated rats. Administration of MPTP via osmotic minipumps implanted s.c. for 24 hrs after a loading dose of MPTP in rats resulted in depletion of striatal dopamine and its metabolites one week later. MPTP in vitro was a reasonably potent, competitive and reversible inhibitor of MAO-A (monoamine oxidase type A). MPTP appeared to inhibit MAO-A in rat brain in vivo as determined by its antagonism of the inactivation of MAO-A by pargyline and by its antagonism of the increase in dopamine metabolites resulting from the administration of Ro 4-1284, a dopamine releaser. The inhibition of MAO-B by MPTP in vitro was noncompetitive, time-dependent, and not fully reversed by dialysis, consistent with the findings of others that MPTP is acted upon by MAO-B. In mice, four successive daily doses of MPTP is acted upon by MAO-B. In mice, four successive daily doses of MPTP given s.c. resulted in marked depletion of dopamine and its metabolites one week later, and the depletion of dopamine was completely prevented by pretreatment with deprenyl, which inhibited MAO-B but not MAO-A. These and other studies in rodents may help in elucidating the mechanisms involved in the destructive effects of MPTP on striatal dopamine neurons that lead to symptoms of Parkinson's disease in humans and in monkeys.     

Estimation of the cytoplasmic catecholamine concentrations in pheochromocytoma cells

Pheochromocytoma cells contain amine oxidase (flavin-containing), and convert dopamine and norepinephrine to deaminated metabolites. Dihydroxyphenylacetic acid is the major dopamine metabolite produced by the cells, whereas dihydroxyphenylglycol is the predominant metabolite of norepinephrine. Cells incubated under control conditions produce deaminated dopamine metabolites at a rate of about 30 pmol/min per mg protein, and dihydroxyphenylglycol at a rate of approx. 10 pmol/min per mg protein. Activation of tyrosine 3-monooxygenase increases the formation of dihydroxyphenylacetic acid, but does not greatly affect the production of dihydroxyphenylglycol. Inhibition of aromatic-L-amino-acid decarboxylase decreases the production of dihydroxyphenylacetic acid, but does not alter the production of dihydroxyphenylglycol. These results are consistent with the idea that newly synthesized dopamine represents the major source of cytoplasmic dopamine, whereas cytoplasmic norepinephrine is derived largely from catecholamine stores in secretory vesicles. The concentrations of dopamine and of norepinephrine in the cytoplasm of pheochromocytoma cells were estimated by measuring the substrate dependence of amine oxidase activity in extracts of these cells. By this method, the cytoplasmic concentrations of dopamine and of norepinephrine were estimated to be in the range of 0.5 to 1 microM. Incubation of the cells with extracellular norepinephrine or with reserpine results in an increase in the production of dihydroxyphenylglycol, and in inhibition of tyrosine 3-monoxygenase activity. Both of these effects are presumably mediated by a rise in the cytoplasmic norepinephrine concentration. Analysis of the relationship between norepinephrine metabolism and tyrosine 3-monooxygenase activity indicates that the apparent Ki of this enzyme for norepinephrine in intact cells is 10-15-times the basal cytoplasmic concentration of norepinephrine, or approx. 10 microM.     

Ascorbic acid specifically enhances dopamine beta-monooxygenase activity in resting and stimulated chromaffin cells

Ascorbic acid enhancement of norepinephrine formation from tyrosine in cultured bovine chromaffin cells was characterized in detail as a model system for determining ascorbate requirements. In resting cells, ascorbic acid increased dopamine beta-monooxygenase activity without changing tyrosine 3-monooxygenase activity. [14C]Norepinephrine specific activity was increased by ascorbic acid, while [14C]dopamine specific activity was unchanged. Dopamine content, dopamine biosynthesis, tyrosine content, and tyrosine uptake were also unaffected by ascorbic acid. Furthermore, increased norepinephrine formation could not be attributed to changes in norepinephrine catabolism. Enhancement of dopamine beta-monooxygenase activity was specific for ascorbic acid, since other reducing agents with higher redox potentials were unable to increase norepinephrine formation. The specific effect of ascorbic acid on enhancement of norepinephrine formation was also observed in chromaffin cells stimulated to secrete with carbachol, acetylcholine, veratridine, and potassium chloride. In stimulated cells with and without ascorbate, there were no differences in dopamine content, tyrosine uptake, dopamine specific activity, and norepinephrine catabolism. These data indicate that, under a wide variety of conditions, only one catecholamine biosynthetic enzyme activity, dopamine beta-monooxygenase, is specifically stimulated by ascorbic acid alone in cultured chromaffin cells. This model system exemplifies a new approach for determining ascorbic acid requirements in cells and animals.     

Differential Interactions of Desipramine with Amphetamine and Methamphetamine: Evidence that Amphetamine Releases Dopamine from Noradrenergic Neurons in the Medial Prefrontal Cortex

Amphetamine is more effective than methamphetamine at raising dopamine levels in the prefrontal cortex. The current study tested the hypothesis that norepinephrine transporters are involved in this difference. Using microdialysis, dopamine, norepinephrine, and serotonin were measured in the rat prefrontal cortex after administration of methamphetamine or amphetamine, with and without perfusion of desipramine. Amphetamine raised norepinephrine levels more than methamphetamine did. Desipramine raised dopamine and serotonin levels but did not alter metabolite levels. Desipramine attenuated the increase in dopamine by amphetamine while increasing the dopamine released by methamphetamine. These data suggest that methamphetamine and amphetamine differ in altering prefrontal cortical dopamine levels and in interacting with norepinephrine transporters. It is proposed that amphetamine releases dopamine in the prefrontal cortex primarily through norepinephrine transporters, whereas methamphetamine interacts minimally with norepinephrine transporters.     

Persistent depletion of striatal dopamine and cortical norepinephrine in mice by 1-methyl-4-phenyl-1,2,3,6-tetrahydro-3-pyridinol (MPTP-3-OL), an analog of MPTP.

MPTP-3-ol injected s.c. once daily for 4 days resulted in a dose-dependent depletion of striatal dopamine and cortical norepinephrine one week after the last dose. MPTP-3-ol was approximately one-fourth as potent as MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) in causing these effects. MPTP-3-ol was oxidized by monoamine oxidase in mouse brain in vitro and resulted in MPP+ (1-methyl-4-phenylpyridinium) formation in brain in vivo, both at about one-fourth the rates with MPTP. The in vitro metabolism of MPTP-3-ol was inhibited by deprenyl, a selective inhibitor of monoamine oxidase type B, and deprenyl pretreatment also blocked the depletion of striatal dopamine and cortical norepinephrine in vivo. Pretreatment with EXP 561, an inhibitor of catecholamine uptake, also prevented the dopamine- and norepinephrine-depleting effects of MPTP-3-ol. Thus, substitution of a hydroxy group on the 3-position of MPTP retains its neurotoxic potential toward catecholamine neurons but reduces potency by decreasing the rate of oxidation via monoamine oxidase type B.     

Effects of dopamine beta-monooxygenase substrate analogs on ascorbate levels and norepinephrine synthesis in adrenal chromaffin granule ghosts

Chromaffin granule ghosts from bovine adrenal medullae have been used to investigate the effects of prototypic dopamine beta-monooxygenase substrate analogs of two distinct classes on intravesicular reduced ascorbic acid (AscH2) levels and on norepinephrine synthesis. Phenyl-2-aminoethyl sulfide (PAES), a sulfur-containing substrate, was shown to concentrate within ghosts, a process that was time and ATP dependent, but reserpine insensitive. Dopamine beta-monooxygenase oxygenation of PAES resulted in accumulation of the oxygenation product, PAESO, without affecting intravesicular levels of AscH2. Similarly, incubations of ghosts with phenyl-2-aminoethyl selenide (PAESe) also resulted in rapid, time- and ATP-dependent, but reserpine-insensitive uptake. However, oxygenation of PAESe by dopamine beta-monooxygenase within ghosts was found to cause a marked decrease in intravesicular AscH2, without buildup of the oxygenated product, phenyl 2-aminoethyl selenoxide. These results illustrate two basic differences between the consequences of PAES and PAESe turnover: while PAES accumulation proceeds concomitant with PAESO production and without AscH2 depletion, PAESe accumulation proceeds with a marked lowering of internal AscH2 but without observable product formation. Both PAES and PAESe were capable of competing with dopamine, the physiological substrate, for enzymatic oxygenation and/or vesicular uptake, and were capable of significantly reducing norepinephrine synthesis. In experiments where ghosts were preincubated with either PAES or PAESe with delayed addition of dopamine, it was clear that neither compound nor their oxygenated products interfered with electron transport via cytochrome b561. These results are consistent with the hypothesis that the physiological activity observed with both PAES and PAESe may be related to their ability to gain entrance to adrenergic neurons and decrease norepinephrine synthesis within neurotransmitter storage vesicles.     

Different effects of monoamine oxidase inhibition on MPTP depletion of heart and brain catecholamines in mice

Pargyline, an inhibitor of monoamine oxidase type B (MAO-B), did not prevent the depletion of heart norepinephrine 24 hr after a single dose of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) in mice. In mice killed 24 hr after the last of 4 daily doses of MPTP, the depletion of dopamine in the striatum and of norepinephrine in the frontal cortex was completely prevented by pargyline, but the depletion of heart norepinephrine was not prevented. These results with pargyline are the same as results obtained earlier with deprenyl, another selective inhibitor of MAO-B. The doses of pargyline and of deprenyl that were used resulted in almost complete inhibition of MAO-B activity (phenylethylamine as substrate) in brain, heart and liver of mice. Deprenyl did not inhibit MAO-A activity (serotonin as substrate) in brain, but pargyline caused some inhibition of MAO-A in brain. In heart and liver, serotonin was oxidized only at about 1/10 the rate of phenylethylamine oxidation, suggesting that MAO-B predominates in these tissues. Both pargyline and deprenyl caused some inhibition of serotonin deamination in heart and liver, suggesting that the oxidation may have been due partly to MAO-B. Experiments with selective MAO inhibitors in vitro showed that only about 20% of the oxidation of serotonin was occurring via MAO-B in heart and liver. The in vitro oxidation of MPTP by MAO in mouse brain, heart and liver was almost completely inhibited by pretreatment with either pargyline or deprenyl. Neither pargyline nor deprenyl had any significant effect on the concentrations of MPTP in brain or heart one-half hr after injection of MPTP into mice. The concentrations of the metabolite, MPP+ (1-methyl-4-phenyl-pyridinium), were markedly reduced in brain and in heart by pretreatment with either pargyline or deprenyl. The data suggest that MPP+ formation, which is necessary for the depletion of brain catecholamines after MPTP injection, may not be necessary for depletion of norepinephrine in heart. Since the oxidation of MPTP in vitro was inhibited more by pargyline or deprenyl pretreatment than was the appearance of MPP+ in vivo, the possibility exists that some MPP+ formation might occur by an enzyme other than MAO.     

Stimulatory Effect of Ascorbic Acid on Norepinephrine Biosynthesis in Digitonin-Permeabilized Adrenal Medullary Chromaffin Cells

The regulatory role of ascorbic acid in norepinephrine biosynthesis was studied using digitonin-permeabilized chromaffin cells. When permeabilized chromaffin cells were incubated with [3H]3,4-dihydroxyphenylethylamine ([3H]dopamine) in calcium-free medium, the amounts of radioactive dopamine and norepinephrine measured in the cell fraction were increased as a function of incubation time and dopamine concentration. Both the accumulation of dopamine and the formation of norepinephrine were shown to require the presence of Mg-ATP in the medium. These results indicate that the permeabilization of chromaffin cells by digitonin treatment does not disrupt the functions of chromaffin granules, including dopamine uptake, norepinephrine formation, and storage of these amines. Using this permeabilized cell system, the effect of ascorbic acid on the rates of dopamine uptake and hydroxylation was investigated. The formation of norepinephrine was stimulated by ascorbic acid at concentrations of 0.5-2 mM in the presence of Mg-ATP. By contrast, dopamine uptake was not affected by the presence or absence of ascorbic acid in the medium. These findings provide evidence that ascorbic acid may stimulate the conversion of dopamine to norepinephrine by increasing dopamine beta monooxygenase activity rather than by increasing the substrate supply of dopamine. These observations also suggest that the rate of norepinephrine biosynthesis in adrenal medullary cells may be regulated by the concentration of ascorbic acid within the cell cytoplasm.     

Atomoxetine-Induced Increases in Monoamine Release in the Prefrontal Cortex are Similar in Spontaneously Hypertensive Rats and Wistar-Kyoto Rats

Spontaneously hypertensive rats (SHRs) are used as a model for attention-deficit/hyperactivity disorder (ADHD), since SHRs are hyperactive and show defective sustained attention in behavioral tasks. The psychostimulants amphetamine and methylphenidate and the selective norepinephrine reuptake inhibitor atomoxetine are used as ADHD medications. The effects of high K⁺ stimulation or psychostimulants on brain norepinephrine or dopamine release in SHRs have been previously studied both in vitro and in vivo, but the effects of atomoxetine on these neurotransmitters have not. The present study examined the effects of administration of atomoxetine on extracellular norepinephrine, dopamine, and serotonin levels in the prefrontal cortex of juvenile SHRs and Wistar-Kyoto (WKY) rats. Baseline levels of prefrontal norepinephrine, dopamine, and serotonin were similar in SHRs and WKY rats. Systemic administration of atomoxetine (3 mg/kg) induced similar increases in prefrontal norepinephrine and dopamine, but not serotonin, levels in both strains. Furthermore, there was no difference in high K⁺-induced increases in extracellular norepinephrine, dopamine, and serotonin levels in the prefrontal cortex between SHRs and WKY rats. These findings indicate that monoamine systems in the prefrontal cortex are similar between SHRs and WKY rats.     

Methamphetamine Self-Administration Is Associated with Persistent Biochemical Alterations in Striatal and Cortical Dopaminergic Terminals in the Rat

Methamphetamine (meth) is an illicit psychostimulant that is abused throughout the world. Repeated passive injections of the drug given in a single day or over a few days cause significant and long-term depletion of dopamine and serotonin in the mammalian brain. Because meth self-administration may better mimic some aspects of human drug-taking behaviors, we examined to what extent this pattern of drug treatment might also result in damage to monoaminergic systems in the brain. Rats were allowed to intravenously self-administer meth (yoked control rats received vehicle) 15 hours per day for 8 days before being euthanized at either 24 hours or at 7 and 14 days after cessation of drug taking. Meth self-administration by the rats was associated with a progressive escalation of daily drug intake to 14 mg/kg per day. Animals that self-administered meth exhibited dose-dependent decreases in striatal dopamine levels during the period of observation. In addition, there were significant reductions in the levels of striatal dopamine transporter and tyrosine hydroxylase proteins. There were also significant decreases in the levels of dopamine, dopamine transporter, and tyrosine hydroxylase in the cortex. In contrast, meth self-administration caused only transient decreases in norepinephrine and serotonin levels in the two brain regions, with these values returning to normal at seven days after cessation of drug taking. Importantly, meth self-administration was associated with significant dose-dependent increases in glial fibrillary acidic protein in both striatum and cortex, with these changes being of greater magnitude in the striatum. These results suggest that meth self-administration by rats is associated with long-term biochemical changes that are reminiscent of those observed in post-mortem brain tissues of chronic meth abusers.