Formation and clearance of interstitial metabolites of dopamine and serotonin in the rat striatum: an in vivo microdialysis study

Effects of age and sex on photoperiod modulation of nucleus accumbens monoamine content and release in adolescence and adulthood

Day length, or photoperiod, is a reliable environmental cue encoded by the brain's circadian clock that indicates changing seasons and induces seasonal biological processes. In humans, photoperiod, age, and sex have been linked to seasonality in neuropsychiatric disorders, as seen in Seasonal Affective Disorder, Major Depressive Disorder, and Bipolar Disorder. The nucleus accumbens is a key locus for the regulation of motivated behaviors and neuropsychiatric disorders. Using periadolescent and young adult male and female mice, here we assessed photoperiod's effect on serotonin and dopamine tissue content in the nucleus accumbens core, as well as on accumbal synaptic dopamine release and uptake. We found greater serotonin and dopamine tissue content in the nucleus accumbens from young adult mice raised in a Short winter-like photoperiod. In addition, dopamine release and clearance were greater in the nucleus accumbens from young adult mice raised in a Long summer-like photoperiod. Importantly, we found that photoperiod's effects on accumbal dopamine tissue content and release were sex-specific to young adult females. These findings support that in mice there are interactions across age, sex, and photoperiod that impact critical monoamine neuromodulators in the nucleus accumbens which may provide mechanistic insight into the age and sex dependencies in seasonality of neuropsychiatric disorders in humans.

Serotonin Signaling through Lipid Membranes

Serotonin (5-HT) is a vital modulatory neurotransmitter responsible for regulating most behaviors in the brain. An inefficient 5-HT synaptic function is often linked to various mental disorders. Primarily, membrane proteins controlling the expression and activity of 5-HT synthesis, storage, release, receptor activation, and inactivation are critical to 5-HT signaling in synaptic and extra-synaptic sites. Moreover, these signals represent information transmission across membranes. Although the lipid membrane environment is often viewed as fairly stable, emerging research suggests significant functional lipid-protein interactions with many synaptic 5-HT proteins. These protein-lipid interactions extend to almost all the primary lipid classes that form the plasma membrane. Collectively, these lipid classes and lipid-protein interactions affect 5-HT synaptic efficacy at the synapse. The highly dynamic lipid composition of synaptic membranes suggests that these lipids and their interactions with proteins may contribute to the plasticity of the 5-HT synapse. Therefore, this broader protein-lipid model of the 5-HT synapse necessitates a reconsideration of 5-HT's role in various associated mental disorders.

Probenecid, an Old Drug with Potential New Uses for Central Nervous System Disorders and Neuroinflammation

Probenecid is an old uricosuric agent used in clinics to treat gout and reduce the renal excretion of antibiotics. In recent years, probenecid has gained attention due to its ability to interact with membrane proteins such as TRPV2 channels, organic anion transporters, and pannexin 1 hemichannels, which suggests new potential therapeutic utilities in medicine. Some current functions of probenecid include their use as an adjuvant to increase the bioavailability of several drugs in the Central Nervous System (CNS). Numerous studies also suggest that this drug has important neuroprotective, antiepileptic, and anti-inflammatory properties, as evidenced by their effect against neurological and neurodegenerative diseases. In these studies, the use of probenecid as a Panx1 hemichannel blocker to reduce neuroinflammation is highlighted since neuroinflammation is a major trigger for diverse CNS alterations. Although the clinical use of probenecid has declined over the years, advances in its use in preclinical research indicate that it may be useful to improve conventional therapies in the psychiatric field where the drugs used have a low bioavailability, either because of a deficient passage through the blood-brain barrier or a high efflux from the CNS or also a high urinary clearance. This review summarizes the history, pharmacological properties, and recent research uses of probenecid and discusses its future projections as a potential pharmacological strategy to intervene in neurodegeneration as an outcome of neuroinflammation.

Monoamine Oxidase Inhibition by Plant-Derived β-Carbolines; Implications for the Psychopharmacology of Tobacco and Ayahuasca

The monoamine oxidases (MAOs) are flavin-containing amine oxidoreductases responsible for metabolism of many biogenic amine molecules in the brain and peripheral tissues. Whereas serotonin is the preferred substrate of MAO-A, phenylethylamine is metabolized by MAO-B, and dopamine and tyramine are nearly ambivalent with respect to the two isozymes. β-Carboline alkaloids such as harmine, harman(e), and norharman(e) are MAO inhibitors present in many plant materials, including foodstuffs, medicinal plants, and intoxicants, notably in tobacco (Nicotiana spp.) and in Banisteriopsis caapi, a vine used in the Amazonian ayahuasca brew. The β-carbolines present in B. caapi may have effects on neurogenesis and intrinsic antidepressant properties, in addition to potentiating the bioavailability of the hallucinogen N,N-dimethyltryptamine (DMT), which is often present in admixture plants of ayahuasca such as Psychotria viridis. Tobacco also contains physiologically relevant concentrations of β-carbolines, which potentially contribute to its psychopharmacology. However, in both cases, the threshold of MAO inhibition sufficient to interact with biogenic amine neurotransmission remains to be established. An important class of antidepressant medications provoke a complete and irreversible inhibition of MAO-A/B, and such complete inhibition is almost unattainable with reversible and competitive inhibitors such as β-carbolines. However, the preclinical and clinical observations with synthetic MAO inhibitors present a background for obtaining a better understanding of the polypharmacologies of tobacco and ayahuasca. Furthermore, MAO inhibitors of diverse structures are present in a wide variety of medicinal plants, but their pharmacological relevance in many instances remains to be established.

The role of uptake and degradation in the regulation of peripheral serotonin dynamics in Gulf toadfish, Opsanus beta

The neurotransmitter serotonin (5-hyroxytryptamine, 5-HT) is involved in a variety of peripheral processes. Arguably most notable is its role as a circulating vasoconstrictor in the plasma of vertebrates. Plasma 5-HT is maintained at constant levels under normal conditions through the processes of cellular uptake, degradation, and excretion, known collectively as clearance. However, the degree to which each individual component of clearance contributes to this whole animal response remains poorly understood. The goal of this experiment was to determine the extent to which transporter-mediated uptake and intracellular degradation contribute to 5-HT clearance in the model teleost Gulf toadfish (Opsanus beta). Fish that were treated with the 5-HT transport inhibitors fluoxetine, buproprion, and decynium-22 had 1.47-fold higher plasma 5-HT concentrations and a 40% decrease in clearance rate compared to control fish. In contrast, fish treated with the MAO inhibitor clorgyline had a 1.54-fold increase in plasma 5-HT with no change in clearance rate. The results show that transporter-mediated 5-HT uptake plays an important role in controlling circulating 5-HT and whole body 5-HT homeostasis.

Assessing the interplay between multigenic and environmental influences on adolescent to adult pathways of antisocial behaviors

The current investigation utilized a developmental psychopathology approach to test the hypothesis that multigenic (i.e., dopaminergic and serotonergic genes) and multienvironmental factors interactively contribute to developmental pathways of antisocial behavior (ASB). A sample of 8,834 Caucasian individuals from the National Longitudinal Study of Adolescent to Adult Health (Add Health) were used to (a) examine the developmental pathways of ASB from age 13 to 32 using growth mixture modeling, (b) compute weighted multigenic risk scores (Add Health MRS) for ASB from six well-characterized polymorphisms in dopamine and serotonin genes, and (c) test the interaction between the Add Health MRS and a measures of support (incorporating indicators of both positive and negative support from parents and schools). Four pathways of adolescent to adult ASB emerged from the growth mixture models: low, adolescence-peaked, high decline, and persistent. Add Health MRS predicted the persistent ASB pathway, but not other ASB pathways. Males with high Add Health MRS, but not low MRS, had significantly greater odds of being in the adolescence-peaked pathway relative to the low pathway at low levels of school connectedness. Nonfamilial environmental influences during adolescence may have a cumulative impact on the development of ASB, particularly among males with greater underlying genetic risks.

In Vivo Ambient Serotonin Measurements at Carbon-Fiber Microelectrodes

The mechanisms that control extracellular serotonin levels in vivo are not well-defined. This shortcoming makes it very challenging to diagnose and treat the many psychiatric disorders in which serotonin is implicated. Fast-scan cyclic voltammetry (FSCV) can measure rapid serotonin release and reuptake events but cannot report critically important ambient serotonin levels. In this Article, we use fast-scan controlled adsorption voltammetry (FSCAV), to measure serotonin's steady-state, extracellular chemistry. We characterize the "Jackson" voltammetric waveform for FSCAV and show highly stable, selective, and sensitive ambient serotonin measurements in vitro. In vivo, we report basal serotonin levels in the CA2 region of the hippocampus as 64.9 ± 2.3 nM (n = 15 mice, weighted average ± standard error). We electrochemically and pharmacologically verify the selectivity of the serotonin signal. Finally, we develop a statistical model that incorporates the uncertainty in in vivo measurements, in addition to electrode variability, to more critically analyze the time course of pharmacological data. Our novel method is a uniquely powerful analysis tool that can provide deeper insights into the mechanisms that control serotonin's extracellular levels.

Biopsychosocial influence on shoulder pain: Rationale and protocol for a pre-clinical trial

BACKGROUND

Chronic musculoskeletal pain conditions are a prevalent and disabling problem. Preventing chronic musculoskeletal pain requires multifactorial treatment approaches that address its complex etiology. Prior cohort studies identified a high risk subgroup comprised of variation in COMT genotype and pain catastrophizing. This subgroup had increased chance of heightened pain responses (in a pre-clinical model) and higher 12month post-operatives pain intensity ratings (in a clinical model). This pre-clinical trial will test mechanisms and efficacy of personalized pain interventions matched to the genetic and psychological characteristics of the high-risk subgroup.

METHODS

Potential participants will be screened for high risk subgroup membership, appropriateness for exercise-induced muscle injury protocol, and appropriateness for propranolol administration. Eligible participants that consent to the study will then be randomized into one of four treatment groups; 1) personalized pharmaceutical and psychological education; 2) personalized pharmaceutical and general education; 3) placebo pharmaceutical and psychological education; 4) placebo pharmaceutical and psychological education. Over the 5-day study period participants will complete an exercise-induced muscle injury protocol and receive study interventions. Pain and disability assessments will be completed daily, with primary outcomes being duration of shoulder pain (number of days until recovery), peak shoulder pain intensity, and peak shoulder disability. Secondary outcomes include inflammatory markers, psychological mediators, and measures of pain sensitivity regulation.

CONCLUSION

This pre-clinical trial builds on prior cohort studies and its completion will provide foundational data supporting efficacy and mechanisms of personalized interventions for individuals that may be at increased risk for developing chronic shoulder pain.

TRIAL REGISTRATION

ClinicalTrials.gov registry, NCT02620579 (Registered on November 13, 2015).

Allelic variation of the COMT gene in a despotic primate society: A haplotype is related to cortisol excretion in Macaca fuscata

Sequence variations in genes of the monoamine neurotransmitter system and their common function in human and non-human primate species are an ongoing issue of investigation. However, the COMT gene, coding for the catechol-O-methyltransferase, has not yet attracted much scientific attention regarding its functional role in non-human primates. Considering that a polymorphism of the human COMT gene affects the enzyme activity and cortisol level in response to a social stressor, this study investigated the impact of COMT on endocrine stress and behavioural parameters in Japanese macaques (Macaca fuscata). The species exemplifies a despotic hierarchy in which males' social rank positions require an adaptation of behaviour strategies. During the mating period steroid secretion and the frequency of aggressive encounters between males increase. We addressed i) whether this species exhibits potential functional COMT variants, ii) whether these variants are associated with faecal cortisol excretion of males, iii) how they are distributed among different social rank positions and iv) whether they are associated with behavioural strategies during times of mate competition. By genotyping 26 males we identified three COMT haplotypes (HT), including a putative splice mutant (HT3). This variant was associated with increased cortisol excretion. Given the observed inverse correlation between cortisol and physical aggression, we assume that different COMT haplotypes may predispose individuals to pursue more or less aggressive strategies. How these gene-stress effects might favour a specific social role is discussed. Our study of non-invasive genotyping in combination with behavioural and endocrine parameters represents an important step towards the understanding of gene-stress effects in a hierarchically organised primate society.

Specific binding of [(18)F]fluoroethyl-harmol to monoamine oxidase A in rat brain cryostat sections, and compartmental analysis of binding in living brain

We investigated [(18)F]fluoroethyl-harmol ([(18)F]FEH) as a reversible and selective ligand for positron emission tomography (PET) studies of monoamine oxidase A (MAO-A). Binding of [(18)F]FEH in rat brain cryostat sections indicated high affinity (KD = 3 nM), and density (Bmax; 600 pmol/g). The plasma free fraction was 45%, and untransformed parent constituted only 13% of plasma radioactivity at 10 min after injection. Compartmental analysis of PET recordings in pargyline-treated rats showed high permeability to brain (K1; 0.32 mL/g/min) and slow washout (k2; 0.024/min), resulting in a uniformly high equilibrium distribution volume (VD; 20 mL/g). Using this VD to estimate unbound ligand in brain of untreated rats, the binding potential ranged from 4.2 in cerebellum to 7.2 in thalamus. We also calculated maps of rats receiving [(18)F]FEH at a range of specific activities, and then estimated saturation binding parameters in the living brain. In thalamus, striatum and frontal cortex KD was globally close to 300 nM and Bmax was close to 1600 pmol/g; the 100-fold discrepancy in affinity suggests a very low free fraction for [(18)F]FEH in the living brain. Based on a synthesis of findings, we calculate the endogenous dopamine concentration to be 0.4 μM in the striatal compartment containing MAO-A, thus unlikely to exert competition against [(18)F]FEH binding in vivo. In summary, [(18)F]FEH has good properties for the detection of MAO-A in the rat brain by PET, and may present logistic advantages for clinical research at centers lacking a medical cyclotron. We made a compartmental analysis of [(18)F]fluoroethylharmol ([(18)F]FEH) binding to monoamine oxidase A (MAO-A) in living rat brain and estimated the saturation binding parameters from the binding potential (BPND). The Bmax was of comparable magnitude to that in vitro, but with apparent affinity (300 nM), it was 100-fold lower in vivo. PET imaging with [(18) F]FEH is well suited for quantitation of MAO-A in living brain.

Citalopram protects against cold-restraint stress-induced activation of brain-derived neurotrophic factor and expression of nuclear factor kappa-light-chain-enhancer of activated B cells in rats

The present study evaluates the protective effect of citalopram against cold-restraint stress (CRS) paradigm. Rats were pretreated with citalopram (0.1, 1.0, and 10.0 mg/kg) acutely and repeatedly for 21 days before exposure to the CRS procedure. None of the doses of citalopram attenuated CRS-induced gastric ulcers in the acute study. In contrast, repeated pretreatment of citalopram at a dose level of 0.1 mg/kg attenuated the CRS-induced gastric ulcers. Citalopram (0.1 mg/kg) diminished CRS-induced increase in plasma corticosterone, but not plasma norepinephrine level in the chronic study indicating its effect on hypothalamic-pituitary-adrenal axis function. Repeated citalopram (0.1 mg/kg) pretreatment attenuated CRS-induced changes in serotonin turnover in the hippocampus and amygdala. Moreover, repeated pretreatment with citalopram (0.1 mg/kg) mitigated the CRS-induced increase in the expression of brain-derived neurotrophic factor (BDNF) and nuclear factor kappa-light-chain-enhancer of activated B cells (NFκB) in the hippocampus and amygdala. These results suggest that there is a region- and a dose-specific effect of citalopram on CRS-induced BDNF-NFκB activation. Therefore, citalopram showed antistress activity in the CRS model through changes in the stress-responsive pathways such as hypothalamic-pituitary-adrenal-axis and brain serotonergic system apart from decreasing the expression of BDNF and NFκB.

Dopac distribution and regulation in striatal dopaminergic varicosities and extracellular space

DOPAC, the major intermediate metabolite of dopamine, is found in the cytosolic compartment of dopaminergic terminals/varicosities and in the extracellular space. It has been proposed that extracellular DOPAC is derived from newly synthesized dopamine rather than from dopamine in the signaling pool. On the basis of literature data supporting such a concept, we hypothesize a DOPAC synthesis/secretory complex producing extracellular DOPAC and use a computational simulation model of dopaminergic varicosities to estimate the distribution of DOPAC between cytosolic and extracellular compartments, amount of newly synthesized dopamine entering the DOPAC synthesis/secretory complex, and potential regulatory processes in the complex. Results suggest that about two-thirds of DOPAC is in the extracellular space. Approximately one-third of newly synthesized dopamine is immediately processed to DOPAC, which is then secreted into extracellular space. Extracellular DOPAC concentration is approximately 300 times higher than extracellular dopamine, and cytosolic DOPAC is ∼18-fold higher than cytosolic dopamine. We suggest that the high levels of extracellular DOPAC coupled with evidence for its production from newly synthesized dopamine imply the existence of an as yet undiscovered regulatory/signaling role for DOPAC.

The COMT and DRD3 genes interacted in bipolar I but not bipolar II disorder

OBJECTIVES. Clarifying the association between bipolar I and bipolar II disorders at the genetic level is essential for improving our understanding of them. In this study, we evaluated the hypothesis that the dopaminergic polymorphisms are risk factors for bipolar disorders. We examined the association between the catechol-O-methyltransferase (COMT) Val158Met and dopamine D3 receptor (DRD3) Ser9Gly polymorphisms and bipolar I and II disorders, as well as possible interactions between these genes. METHODS. Seven hundred and eleven participants were recruited: 205 with bipolar I, 270 with bipolar II, and 236 healthy controls. The genotypes of the COMT Val158Met and DRD3 Ser9Gly polymorphisms were determined using polymerase chain reactions plus restriction fragment length polymorphism analysis. RESULTS. Logistic regression analyses showed a statistically significant main effect for the Met/Met genotype of the COMT Val158Met polymorphism (P=0.032) and a significant interaction effect for the Met/Met genotype of the COMT Val158Met and Ser/Ser genotypes of the DRD3 Ser9Gly polymorphism (P=0.001) predicted bipolar I patients. However, there was no association between the COMT Val158Met or DRD3 Ser9Gly and bipolar II. CONCLUSIONS. We provide initial evidence that the COMT Val158Met and DRD3 Ser9Gly genotypes interact in bipolar I and bipolar II disorders and that bipolar I and bipolar II are genetically distinct.

Age-dependent decline of steady state dopamine storage capacity of human brain: an FDOPA PET study

Conventional indices of the utilization of FDOPA in living human brain have not consistently revealed important declines in dopamine function with normal aging. However, most methods of kinetic analysis have assumed irreversible trapping of decarboxylated FDOPA metabolites in brain, an assumption that is violated even in PET recordings of short duration. Therefore, we have developed methods for the calculation of steady-state storage of FDOPA together with its decarboxylated metabolites (V(d), mlg(-1)), based upon improved kinetic analysis of 120-min emission recordings. In a group of 28 normal male subjects, of age ranging from 23 to 73 years, the magnitude of V(d) in the striatum and in extrastriatal regions declined by approximately 10% with each decade. The utilization of FDOPA was also calculated by several conventional methods assuming irreversible trapping, i.e. the net blood brain clearance (K(in)(app), mlg(-1)min(-1)), the DOPA decarboxylase activity relative to a reference tissue input (k(3)(S), min(-1)), and relative to the arterial input (k(3)(D), min(-1)). None of these methods revealed an age-related decline in FDOPA utilization in the extended striatum, although the magnitude of K(in)(app) did decline in cerebral cortex. Thus, the capacity to synthesize [(18)F]fluorodopamine remained largely intact in striatum of the elderly subjects, but in the presence of a substantially increased rate of washout (k(loss)), which was evident in all brain regions examined. Consequently, the magnitude of V(d) declined with healthy aging, possibly reflecting impaired vesicular storage capacity, resulting in enhanced exposure of cytosolic [(18)F]fluorodopamine to monoamine oxidase.

The reward system and maternal behavior in an animal model of depression: a microdialysis study

RATIONALE AND OBJECTIVES

Flinders sensitive line (FSL) rats, an animal model of depression, display a different pattern of maternal behavior compared to Sprague-Dawley (SD) controls. In this study, we examined the rewarding value of mother-infant interaction for FSL dams.

MATERIALS AND METHODS

In the main study, we measured monoamine levels in the nucleus accumbens (NAc) of early postpartum FSL and SD dams during an interaction with pups, using the microdialysis technique. In addition, we compared the preference patterns of FSL and SD rats using the conditioned place preference paradigm, with pups as the unconditioned stimuli.

RESULTS

Dopamine (DA) levels in dialysates from the NAc of SD dams but not FSL dams were elevated while interacting with pups but the metabolism of DA to dihydroxyphenylacetic acid was greater in FSL than in SD dams. While SD dams showed a conditioned preference for a region that was associated with SD pups, FSL dams did not show a preference for regions associated either with SD or FSL pups, but water deprived FSL rats demonstrated a preference to a region associated with water, eliminating an alternative explanation of learning deficit in FSL rats.

CONCLUSIONS

Taken together, these results suggest that FSL dams are less rewarded by pups, compared to control dams.

Genetics and neuroimaging of attention and hypnotizability may elucidate placebo

Attention binds psychology to the techniques of neuroscience and exemplifies the links between brain and behavior. Associated with attentional networks, at least 3 brain modules govern control processes by drawing on disparate functional neuroanatomy, neuromodulators, and psychological substrates. Guided by data-driven brain theories, researchers have related specific genetic polymorphisms to well-defined phenotypes, including those associated with different attentional efficiencies and hypnosis. Because attention can modulate both cognitive and affective processes, genetic assays together with neuroimaging data have begun to elucidate individual differences. Findings from genetic assays of both attention and hypnotizability pave the way to answering questions such as how high hypnotizable individuals may differ from less-hypnotizable persons. These exploratory findings may extend to the identification of placebo responders.

Mapping the amphetamine-evoked changes in [11C]raclopride binding in living rat using small animal PET: Modulation by MAO-inhibition

The performance of small animal PET for neuroreceptor studies in a psychopharmacological challenge paradigm is not yet well-described. Therefore, we used microPET and [(11)C]raclopride to map the availability of dopamine D(2/3) receptors in brain of anesthetized rats, first in a baseline condition, and again after challenge with saline or d-amphetamine. Parametric maps of the specific binding (binding potential, pB) were calculated using a reference tissue input from cerebellum, and spatially normalized to a digitized stereotaxic coordinate system for rat brain. In volumes of interest (VOIs), the mean baseline pB (n=6) was 2.05 in dorsal striatum (caudate-putamen), and 1.34 in ventral striatum (nucleus accumbens), and did not significantly differ upon retest 2 h later. The availability of [(11)C]raclopride binding sites at baseline was 8% higher in the right striatum. Challenge with amphetamine sulfate (1 mg/kg, i.v., n=4) decreased pB by 19% in both ventral and dorsal striatum. We have earlier predicted that blockade of monoamine oxidase (MAO) should potentiate the amphetamine-evoked dopamine release, thus enhancing the displacement of [(11)C]raclopride binding in vivo. However, pretreatment of rats with pargyline hydrochloride (4 mg/kg, n=4; 20 mg/kg, n=4) 1 day prior to PET did not potentiate the amphetamine-evoked reduction in dopamine receptor availability within the extended striatum. We conclude that small animal PET can be used to investigate stimulant-induced dopamine release, but that the spatial resolution is insufficient to detect differences between relative changes in dorsal vs. ventral divisions of the rat striatum. Furthermore, the present results do not reveal potentiation of the amphetamine-evoked release of dopamine in rats with MAO inhibition.

Effect of monoamine oxidase inhibition on amphetamine-evoked changes in dopamine receptor availability in the living pig: A dual tracer PET study with [11C]harmine and [11C]raclopride

The activity of both isozymes of monoamine oxidase (MAO) is reduced by 50% in the brain of human smokers. We hypothesized that this is not an epiphenomenon, but should bring about potentiation of the action of psychostimulant drugs. To test this hypothesis, we carried out serial positron emission tomography (PET) studies in Göttingen miniature pigs to measure the binding of the MAO-A ligand [11C]harmine and to measure the changes in [11C]raclopride binding evoked by a low dose of amphetamine (0.7 mg/kg as free base, i.v.), first in a baseline condition, and, one month later, after acute treatment with pargyline (2 x 3 mg/kg as free base, i.m.). In the baseline, the distribution volume of [11C]harmine relative to the arterial input (V(d), ml g(-1)) ranged from 74 ml g(-1) in cerebellum to 139 ml g(-1) in the medial hypothalamus. Pargyline treatment reduced the magnitude of V(d) globally to 34-54 ml g(-1). Nearly complete displacement of [11C]harmine binding was detected in neocortex and striatum, but there was evidence for pargyline-resistant binding in the pituitary gland and diencephalon. In the baseline condition, the low dose of amphetamine evoked a 14% decline in the binding potential (BP) (pB) of [11C]raclopride in striatum (P = 0.026). After pargyline treatment, the amphetamine effect was of similar magnitude (-11%), although not statistically significant (P = 0.054). However, the second amphetamine challenge evoked a 24% reduction in [11C]raclopride pB relative to the original baseline condition (P = 0.018). Present results do not strongly support our hypothesis that MAO inhibition should potentiate the amphetamine-evoked dopamine release as measured in the [11C]raclopride competition paradigm.

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

BACKGROUND

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Poor evidence for depolarization block but uncoupling of nigral from striatal dopamine metabolism after chronic haloperidol treatment in the rat

Chronic haloperidol treatment induces depolarization block in midbrain dopamine neuronal systems. We studied the effect of this treatment on nigrostriatal dopamine catabolism using microwave fixation in situ of the brain to prevent post-mortem changes. Male Sprague-Dawley rats were given haloperidol (0.4 mg/kg/day, i.p.) or vehicle for 21 days. On day 22, some rats in each group received a haloperidol challenge (0.4 mg/kg, i.p.), and the remaining rats were given the vehicle. Dopamine metabolite levels 60 min after the challenge were assayed by combined gas chromatography-mass fragmentography. Haloperidol pretreatment significantly modified haloperidol challenge effect on regional dopamine metabolite contents. The challenge elevated all striatal metabolites studied similarly in the chronic vehicle- or chronic haloperidol-pretreated rats. In contrast, it did not significantly affect nigral dopamine metabolites except it elevated 3,4-dihydroxyphenylacetic acid in the haloperidol-pretreated rats. A linear correlation between the nigral and striatal contents of 3-methoxytyramine (R = 0.72, p = 0.03), and a trend for correlation (R = 0.65, p = 0.06) between the respective 3,4-dihydroxyphenylacetic acid contents were found after the haloperidol challenge in the vehicle-pretreated rats only. These results suggest that chronic haloperidol treatment uncouples somatodendritic dopamine turnover and release from those in the axon terminals of nigrostriatal dopamine neurons.

Changes in extracellular dopamine in the rat globus pallidus induced by typical and atypical antipsychotic drugs

Typical antipsychotic drugs with a high extrapyramidal motor side-effects liability markedly increase extracellular dopamine in the caudate-putamen, while atypical antipsychotic drugs with a low incidence of extrapyramidal motor side-effects have less pronounced stimulating actions on striatal dopamine. Therefore, it has been suggested that the extrapyramidal motor side-effects liability of antipsychotic drugs (APD) is correlated with their ability to increase extracellular dopamine in the caudate-putamen. The globus pallidus (GP) is another basal ganglia structure probably mediating extrapyramidal motor side-effects of typical antipsychotic drugs. Therefore, the present study sought to determine whether extracellular dopamine in the globus pallidus might be a further indicator to differentiate neurochemical actions of typical and atypical antipsychotic drugs. Using in vivo microdialysis we compared effects on pallidal dopamine induced by typical and atypical antipsychotic drugs in rats. Experiment I demonstrated that systemic administration of haloperidol (1 mg/kg; i.p.) and clozapine (20 mg/kg; i.p.) induced a significant pallidal dopamine release to about 160 and 180% of baseline, respectively. Experiment II revealed that reverse microdialysis of raclopride and clozapine using a cumulative dosing regimen did not stimulate extracellular dopamine in the globus pallidus if low (1microM) or intermediate (10 and 100 microM) concentrations were used. Only at a high concentration (1,000 microM), raclopride and clozapine induced a significant pallidal dopamine release to about 130 and 300% of baseline values, respectively. Thus, effects of typical and atypical antipsychotic drugs on pallidal dopamine were similar and thus, may not be related to their differential extrapyramidal motor side-effects liability. Furthermore, the finding that reverse microdialysis of raclopride over a wide range of concentrations did not stimulate pallidal dopamine concentrations tentatively suggests that pallidal dopamine release under basal conditions is not regulated by D2 autoreceptors.

Nucleus accumbens serotonin1A receptors control cocaine-induced hyperactivity but not local serotonin increase: an in vivo microdialysis study

The nucleus accumbens (Nac) is an important structure for cocaine-induced hyperactivity and receives a dense serotonergic (5-HT) innervation. Previous studies showed that a systemic activation of 5-HT(1A) receptors potentiates cocaine-induced hyperlocomotion, but attenuates the cocaine-induced 5-HT increase in the Nac. In order to address the role of Nac 5-HT(1A) receptors in the control of cocaine-induced and spontaneous behavioural activity and local 5-HT release, we used in vivo microdialysis in freely moving rats. The 5-HT(1A)-receptor agonist, 8-OH-DPAT (0, 1 and 10 microM), was applied locally into the Nac by reverse dialysis followed by a cocaine (10 mg/kg) or saline i.p. injection. The Nac 5-HT(1A)-receptor activation potentiated cocaine-induced hyperlocomotion, but attenuated rearing behaviour dose-dependently. Parallel to that, the cocaine-induced increase in Nac 5-HT dialysate level was unaffected, as were the decreases in 5-HIAA and DOPAC dialysate levels after cocaine. In saline treated rats, the local application of 8-OH-DPAT into the Nac affected neither spontaneous behavioural activity nor 5-HT, 5-HIAA or DOPAC dialysate levels in the Nac. These data suggest that Nac 5-HT(1A) receptors exert a bi-directional control of cocaine-induced hyperactivity, while not affecting spontaneous behaviour. Furthermore, accumbal 5-HT(1A) receptors do not appear to be directly involved in the acute effects of cocaine on 5-HT, 5-HIAA or DOPAC levels in the Nac.

Hippocampus 5-HT1A-receptors attenuate cocaine-induced hyperlocomotion and the increase in hippocampal but not nucleus accumbens 5-HT

Cocaine induces an increase in hippocampal and nucleus accumbens (Nac) serotonin (5-HT) concentration parallel to locomotor activation. Both effects can be modulated by systemic 5-HT(1A)-receptor agonism/antagonism. Given the contribution of the hippocampus to spontaneous behavioral activity, these observations suggest a role for hippocampal 5-HT as well in the modulation of cocaine effects on behavior. To determine the role of hippocampal 5-HT(1A)-receptors in cocaine effects on behavior and hippocampal 5-HT release, we used in vivo microdialysis in freely moving rats. The 5-HT(1A)-receptor agonist, 8-OH-DPAT (0, 0.1, 1 and 10 microM), was applied locally into the hippocampus by reversed dialysis followed by a cocaine (10 mg/kg) or saline i.p. injection. The hippocampal 5-HT(1A)-receptor activation attenuated cocaine-induced hyperlocomotion and rearing behavior dose-dependently. Parallel to that, the cocaine-induced 5-HT increase was attenuated dose-dependently in the hippocampus but was left unaffected in the Nac. The intra-hippocampal application of 8-OH-DPAT affected neither behavioral activity nor 5-HT concentration in the hippocampus and in the Nac. In accord with these findings, hippocampal 5-HT(1A)-receptors may not be directly involved in the regulation of spontaneous behavior or basal 5-HT concentration in the hippocampus and Nac. However, the results indicate an inhibitory role of hippocampal 5-HT(1A)-receptors in cocaine-induced hyperactivity and in the 5-HT increase evoked by cocaine in the hippocampus but not in the Nac.

Cerebrospinal fluid monoamine and metabolite concentrations and aggression in rats

In humans and other primates low cerebrospinal fluid (CSF) levels of the major serotonin (5-HT) metabolite 5-hydroxyindoleacetic acid (5-HIAA) have been correlated to high aggressiveness. This finding forms the basis of the 5-HT deficiency hypothesis of aggression. Surprisingly, this correlation has not been confirmed in rodents so far, while manipulation studies aimed to investigate the link between 5-HT and aggressive behaviour are mostly carried out in rodents. In this study the relation between aggression and CSF monoamine and metabolite concentrations was investigated in male Wildtype Groningen rats. In sharp contrast to the hypothesis and our expectation, a clear positive correlation was found between the individual level of trait-like aggressiveness and CSF concentrations of 5-HT, 5-HIAA, norepinephrine (NE), dopamine (DA), and 3,4-dihydroxyphenylacetic acid (DOPAC). Shortly after the acute display of aggressive behaviour (as a state-like phenomenon), decreased 5-HT levels and an increase in 5-HIAA/5-HT ratio and NE concentrations were found. Surprisingly, pharmacological challenges known to influence 5-HT transmission and aggressive behaviour did not affect CSF 5-HT and 5-HIAA concentrations, only the NE level was increased. Lesioning 5-HT terminals by 5,7-dihydroxytryptamine (5,7-DHT) administration caused a decrease in CSF 5-HT and 5-HIAA, but without affecting aggressive behaviour. The observed positive correlation between CSF 5-HIAA and trait aggressiveness makes it questionable whether a direct extrapolation of neurobiological mechanisms of aggression between species is justified. Interpretation of CSF metabolite levels in terms of activity of neural substrates requires a far more detailed knowledge of the dynamics and kinetics of a neurotransmitter after its release.

Serotonin1A-receptor agonism attenuates the cocaine-induced increase in serotonin levels in the hippocampus and nucleus accumbens but potentiates hyperlocomotion: an in vivo microdialysis study

The hippocampus and the nucleus accumbens (Nac) are important structures for the modulation of spontaneous locomotor activity. Both structures receive a serotonergic (5-HT) innervation. We have previously reported that the 5-HT(1A)-receptor antagonist WAY 100635 blocked cocaine-induced hyperactivity, while potentiating cocaine-induced 5-HT increases in the hippocampus and the Nac. In order to further investigate the relationship between extracellular 5-HT concentration and cocaine-induced behaviour, we used in vivo microdialysis to measure the effects of the 5-HT(1A)-receptor agonist 8-OH-DPAT on cocaine-induced changes in the extracellular 5-HT concentration in the hippocampus and the Nac and on behavioural activity. Following a pilot pretest in which we determined the lowest effective dose of 8-OH-DPAT for potentiating cocaine-induced hyperlocomotion, four groups of rats were given one of the following drug treatments: 8-OH-DPAT (0.2 mg/kg) and cocaine (10 mg/kg), saline and cocaine (10 mg/kg), 8-OH-DPAT (0.2 mg/kg) and saline, or saline and saline. The injections were administered i.p. and spaced 30 min apart. We found that the 5-HT(1A)-receptor agonist 8-OH-DPAT attenuated the cocaine-induced increases in 5-HT in the hippocampus and the Nac, but potentiated cocaine-induced hyperlocomotion. 5-HT metabolite measurements revealed a complex role for the 5-HT(1A)-receptor in the broad spectrum of cocaine's neurochemical effects. Altogether, these observations support an important role of the 5-HT(1A)-receptor in the hippocampus and Nac in the modulation of cocaine stimulant effects.

COMT val158met genotype affects mu-opioid neurotransmitter responses to a pain stressor

Responses to pain and other stressors are regulated by interactions between multiple brain areas and neurochemical systems. We examined the influence of a common functional genetic polymorphism affecting the metabolism of catecholamines on the modulation of responses to sustained pain in humans. Individuals homozygous for the met158 allele of the catechol-O-methyltransferase (COMT) polymorphism (val158met) showed diminished regional mu-opioid system responses to pain compared with heterozygotes. These effects were accompanied by higher sensory and affective ratings of pain and a more negative internal affective state. Opposite effects were observed in val158 homozygotes. The COMT val158met polymorphism thus influences the human experience of pain and may underlie interindividual differences in the adaptation and responses to pain and other stressful stimuli.

Transcranial magnetic stimulation induces increases in extracellular levels of dopamine and glutamate in the nucleus accumbens

Transcranial magnetic stimulation (TMS) is a non-invasive approach used for stimulating the human brain. Repetitive stimulation over the prefrontal cortex has proven effective in the treatment of major depression, however the mechanism of the antidepressant action is unknown. Since the nucleus accumbens is a major region implicated in reward circuitry and depressive disorders, we used the microdialysis technique to study some of the neurochemical changes induced in that region during and after acute TMS. Magnetic stimulation was applied over the frontal or the caudal cortex of the rat brain using a special coil design and microdialysis samples were collected before, during and after the stimulation session. The extracellular levels of both dopamine and glutamate in the nucleus accumbens were increased during the stimulation while the extracellular levels of acetylcholine were not affected. Stimulation over the caudal cortex caused a greater increase in dopamine levels than the stimulation over the frontal cortex, while such difference was not observed for glutamate levels. The changes in dopamine and glutamate extracellular levels in the nucleus accumbens may play a role in the antidepressant effect of TMS and it is therefore suggested that the effect of stimulation over caudal cortical sites on depressive patients will be examined.

Short- and long-term effects of p-ethynylphenylalanine on brain serotonin levels

Changes in tissue and extracellular serotonin (5-HT) in raphe dorsalis, raphe medialis and in their main projections areas (hippocampus, striatum and frontal cortex) were investigated at short and long-term times after single injection (5 mg/kg ip) of a novel tryptophan hydroxylase inhibitor, p-ethynylphenylalanine (p-EPA). The 5-HT tissue concentration decreased significantly in raphe nuclei, 30 min post-injection and for 4 days, whereas it decreased from 24 hours post-injection in the 5-HT projections. Normal 5-HT levels reappeared after 12 days post-injection in all areas. Moreover, in the projection areas, the extracellular 5-HT levels decreased rapidly, 90, 40 and 30 min after p-EPA injection, in hippocampus, striatum and frontal cortex, respectively. Decreased accumulation of 5-hydroxytryptophan (5-HTP) under NSD-101 perfusion in the serotoninergic projections after p-EPA injection, confirmed the direct inhibitory effect of the drug on the tryptophan hydroxylase activity. These results demonstrated that p-EPA is a useful pharmacological tool which powerfully, acutely and irreversibly reduces the 5-HT levels.

Oxygen inhalation enhances striatal dopamine metabolism and monoamineoxidase enzyme inhibition prevents it: a microdialysis study

In order to explore the effect of normobaric oxygen on the extracellular level of dopamine and its metabolites, oxygen (30, 60 and 90%) was administered to freely moving rats after the animals had been pretreated with either monoamineoxidase (MAO)-A and -B inhibitors (0.1 or 1 mg kg(-1) of clorgyline, 1 or 10 mg kg(-1) of selegiline and 75 mg kg(-1) pargyline) or control solution. The levels of dopamine and its metabolites were monitored in microdialysis samples collected every 20 min and directly applied to an on-line high-performance liquid chromatograph combined with electrochemical detection. Normobaric oxygen inhalation decreased the level of extracellular dopamine and increased that of 3,4-dihydroxyphenylacetic acid (DOPAC) in a concentration-dependent manner. These changes were partly prevented by pre-treatment with low doses of selegiline or clorgyline, i.e. by conditions in which monoamineoxidase-A or -B was inhibited. When both isoforms of monoamineoxidase were inhibited, there was a drastic increase in extracellular concentrations of dopamine and 3-methoxytyramine, and the levels of DOPAC and homovanilic acid (HVA) were very low. These results indicate that the intracellular metabolism of cytoplasmic dopamine is enhanced by normobaric hyperoxia in rat striatum.

Limbic dopaminergic adaptation to a stressful stimulus in a rat model of depression

The dopaminergic mesolimbic system has a key role in motivation and reward, and stressful stimuli appear to alter its functionality. Since stress is considered to be one of the primary factors that mediate the expression of depressive behavior, dopamine and its metabolites in the nucleus accumbens of control and Flinders Sensitive Line rats, an animal model of depression, were examined prior to and after a forced swim test. In both types of rats, the levels of dopamine metabolites markedly decreased after the forced swimming, albeit to different extents. In contrast, 60 min after the swim test, dopamine levels were elevated only in the control rats. The accumbal dopaminergic activity is discussed in relation to the behavior of 'depressed' and normal rat lines subjected to a stressful event.

Pharmacological analysis of extracellular dopamine and metabolites in the striatum of conscious as/agu rats, mutants with locomotor disorder

The as/agu rat is a spontaneously occurring mutation which exhibits locomotor abnormalities, reduced tyrosine hydroxylase levels in the substantia nigra and lower extracellular levels of dopamine. The animal could represent a model of some human locomotor disorders. High-potassium medium evoked a 460% rise of dopamine levels in control rats but double this in mutants. Amphetamine increased extracellular dopamine by 710% in controls and 1480% in mutants. Clorgyline produced a small increase of dopamine levels in controls but an 1170% increase in mutants. The uptake inhibitor nomifensine increased dopamine levels by 910% in controls but only 270% in mutants. After treatment with benserazide plus L-DOPA, an acute injection of L-DOPA evoked a release of dopamine which was twice as large in the as/agu rats compared with controls. The results show reduced extracellular dopamine in as/agu rats when the locomotor disorder is apparent, but there has been little loss of tyrosine hydroxylase. The responses to drugs are qualitatively different from those obtained using 6-hydroxydopamine.Overall, the effects of compounds affecting aminergic neurons suggest that one possible mechanism for the neuronal abnormality in as/agu rats is a defective regulation of dopamine release from striatal terminals.

Dopamine release in the nucleus accumbens and latent inhibition in the rat following microinjections of a 5-HT1B agonist into the dorsal subiculum: implications for schizophrenia

Microinjection of a serotonergic 5-HT1B agonist (S-CM-GTNH2, 3 microg/l) into the dorsal subiculum (DS) induced long-lasting increases in dopamine (DA; +58%), dihydroxyphenylacetic acid (DOPAC; +15%) and homovanillic acid (HVA; +31%), without changing extracellular levels of the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA), measured by microdialysis in freely moving rats in the shell area of the nucleus accumbens (n. acc). Perfusion of a glutamate-N-methyl-D-aspartate (NMDA) receptor antagonist (MK 801, dizocilpine, 10 microM) through the dialysis probe in the n. acc induced similar long-lasting increases in DA and DOPAC, whereas the glutamate-quisqualate/kainate receptor antagonist (CNQX, 50 microM) had no effect. In the presence of dizocilpine in the n. acc, microinjection of S-CM-GTNH2 into the DS could still increase DOPAC and HVA, but DA levels were not further changed, whereas in the presence of CNQX, microinjection of S-CM-GTNH2 into the DS still increased not only DOPAC and HVA, but also DA levels in a way similar to that in the absence of glutamate antagonist. Therefore, activation of 5-HT1B receptors located in the DS increases the release of DA in the n. acc, presumably via the glutamatergic projection to this structure and acting through NMDA receptors in it. This implies either the suppression of a tonic indirect inhibitory influence and/or stimulation of a phasic excitatory effect of glutamate. Disruption of latent inhibition (LI) has been suggested as a model for a cognitive deficit in schizophrenia (hyperattention to irrelevant stimuli) and is usually associated with an increase in DA release in the n. acc. However, s.c. injection of RU 24 969 (0.5 mg/kg), a mixed 5-HT1A-5-HT1B agonist, which was previously shown to increase DA release in the n. acc, left LI unchanged. Moreover, bilateral microinjections of S-CM-GTNH2 into the rat DS tended to potentiate LI, in spite of the increase in DA in n. acc demonstrated here. It is concluded that not all increases in DA release in the n. acc are functionally equivalent. Sensitization of receptors or impulse-dependent increase in DA release might be necessary to disrupt LI. The possible role of altered serotonergic transmission, through h5-HT1B receptors (human homologue of the rat 5-HT1B receptors) located in the DS, in acute schizophrenia needs to be further investigated.

Increased septal 5-HIAA efflux in rats that do not develop learned helplessness after inescapable stress

Learned helplessness is a behavioral deficit that can be induced by exposure to inescapable stress. Previous studies have implicated the lateral septum in mediating this phenomenon, and in this brain region, serotonin plays an important role in the development, maintenance, prevention, and reversal of learned helplessness behavior. Using the technique of in vivo microdialysis, we measured the efflux of serotonin (5-HT), dopamine (DA), and their respective metabolites, 5-hydroxyindoleacetic acid (5-HIAA) and 3, 4-dihydroxyphenylacetic acid (DOPAC), from the lateral septum of rats that either developed or did not develop learned helplessness. During the microdialysis session all rats were subjected to restraint stress. Control groups included naïve, home cage rats as well as tested control rats that were subjected to the identical handling, restraint, and shuttlebox testing as the rats that received inescapable shock. Overall, levels of 5-HIAA were significantly higher in non-helpless rats. There were no significant effects of restraint or differences in levels of 5-HT, DA, or DOPAC. We propose that this increase in 5-HIAA is indicative of an overall increase in serotonin metabolism in the lateral septum of rats that do not become helpless after inescapable stress. This increased serotonin metabolism in the lateral septum may protect the animal from adverse behavioral consequences of inescapable stress. J. Neurosci. Res. 61:101-106, 2000. Published 2000 Wiley-Liss, Inc.

Stress induced spontaneous recurrence of methamphetamine psychosis: the relation between stressful experiences and sensitivity to stress

We examined increased sensitivity to stress in relation to spontaneous recurrences of methamphetamine (MAP) psychosis (i.e., flashbacks). Plasma monoamine metabolite levels were assayed in: 26 flashbackers, of whom 11 were on neuroleptics before and during the study, and the other 15 received neuroleptics in the course of the study; 18 non-flashbackers with a history of MAP psychosis; eight subjects with persistent MAP psychosis; and 23 MAP user and 11 non-user controls. The 26 flashbackers had experienced stressful events and/or MAP-induced fear-related psychotic symptoms during previous MAP use. Mild psychosocial stressors then triggered flashbacks. During flashbacks plasma norepinephrine levels increased markedly; among the flashbackers, those with a history of stressful events, whether or not they had experienced fear-related symptoms, showed a further increase in 3-methoxytyramine levels. Stressful experiences, together with MAP use, may therefore induce sensitization to stress associated with noradrenergic hyperactivity, involving increased dopamine release, and so triggering flashbacks.

Astrocyte line SVG-TH grafted in a rat model of Parkinson's disease

The present review describes gene transfer into the brain using extraneuronal cells with an ex vivo approach. The mild immunological reactions in the central nervous system to grafts provided the rationale and empirical basis for brain-transplantation, to replace dying cells, of potential clinical relevance. Fetal human astrocytes were genetically engineered to express tyrosine hydroxylase, the rate-limiting enzyme for the synthesis of catecholamines. These cells were also found to produce constitutively and secrete GDNF and interleukins. Therefore, these cells may prove as a drug-delivery system for the treatment of neurological degenerative conditions such as Parkinson's disease (PD). The field of neuronal reconstruction has reached a critical threshold and there is a need to evaluate the variables that will become critical as the field matures. One of the needs is to characterize the neurochemical alterations in the microenvironment in the context of grafted-host connectivity. This review discusses the functional effects of the pharmacologically-active construct, which consists of astrocytes producing L-DOPA and GDNF. The striatum in PD that lacks the dopaminergic projection from the substantia nigra metabolizes and releases dopamine differently from normal tissue and may react to different factors released by the grafted cells. Moreover, neurochemicals of the host tissue may effect grafted cells as well. An understanding of the way in which these neurochemicals are abnormal in PD and their role in the grafted brain is critical to the improvement of reconstructive strategies using cellular therapeutic strategies.

Local striatal infusion of MPP+ does not result in increased hydroxylation after systemic administration of 4-hydroxybenzoate

In vivo bilateral microdialysis in the rat striatum was used to investigate hydroxyl radical formation under basal conditions and after intrastriatal administration of the neurotoxin, 1-methyl-4-phenylpyridinium (MPP+). After a short equilibration period, 4-hydroxybenzoate (4HBZ), which scavenges hydroxyl radicals to produce 3,4-dihydroxybenzoate (34DHB), was injected intraperitoneally 15 min before infusion of MPP+. To evaluate the enzymatic contribution to hydroxyl radical formation, two other series of microdialyses were performed following administration of monoamine oxidase B inhibitors, either 1-deprenyl (selegiline) or MDL 72,974A [(E)-2-(4-fluorophenethyl)-3-fluoroallylamine hydrochloride]. Microdialysate samples were analyzed by high-performance liquid chromatography for catecholamines, 3,4-dihydroxyphenylacetate (DOPAC), homovanillate (HVA), along with the hydroxyl radical adduct, 34DHB and its precursor, 4HBZ. MPP+ administration resulted in a massive release of dopamine along with a decrease in DOPAC and HVA in all three groups. A striking effect in all three groups was noted in which MPP+ resulted in a decrease in interstitial 4HBZ to < 50% of the non-MPP+ -treated side. In absolute terms, the amount of 34DHB produced was low but similar in all three groups, even after unilateral MPP+ infusion. When 34DHB was normalized to 4HBZ release to account for differences in precursor availability, there were no significant differences in the 34DHB/4HBZ ratios either with or without MAO inhibitor treatment or after local MPP+ infusion. Systemic 4HBZ administration appears to result predominantly in intra-cellular sampling of hydroxyl radicals which produces different results from local infusion of trapping agents such as salicylate.

The effects of paradoxical sleep deprivation and valine on spatial learning and brain 5-HT metabolism

We have previously reported that rapid eye movement sleep deprivation (REMSD), induced by the flower pot technique, causes a deficit in reference spatial memory and increases rates of serotonin (5-HT) metabolism in the brain. In this study we used increased concentrations of dietary valine to inhibit tryptophan (TRP) transport across the blood-brain barrier in an attempt to modify the REMSD-induced increase of 5-HT metabolism. Rats were fed either a control diet or the same diet supplemented to 2% by weight valine, and were allocated to one of three experimental groups: cage control (CC), stress tank control (TC), or REMSD. Reference and working spatial memory of all rats was tested in a Morris water maze on Days 2, 3, and 4. REMSD produced a significant decrement in reference memory on Days 2 and 4, independent of dietary condition. The valine diet had a detrimental effect on the reference memory of TC rats on Day 2 but not Day 4. Measurements made on Day 4 indicated that the valine diet decreased brain TRP only in the CC rats. In contrast, the valine diet did not prevent increases in brain TRP or 5-HT metabolism in REMSD rats, and increased hypothalamic and brain stem TRP concentrations and the hippocampal 5-HIAA/5-HT ratio in TC rats. These results indicate that dietary valine does not prevent REMSD-induced changes in spatial memory or serotonin metabolism, although it does reduce brain TRP in nonstressed rats.

Recovery of presynaptic dopaminergic functioning in rats treated with neurotoxic doses of methamphetamine

Repeated administration of methamphetamine (METH) to animals can result in long-lasting decreases in striatal dopamine (DA) content. In addition, the evoked overflow of striatal DA is reduced in rats 1 week after neurotoxic doses of METH. However, whether these functional changes in DA release are permanent or tend to recover over time has not been established. In the present study we used in vivo electrochemistry and microdialysis to examine evoked overflow of DA in the striatum of METH-treated rats at several time points after treatment to determine if DA overflow would spontaneously recover. Male Fischer-344 rats were administered METH (5 mg/kg, s.c. ) or saline four times in one day at 2 hr intervals. In vivo electrochemistry experiments in anesthetized rats, and in vivo microdialysis studies in awake rats, were carried out 1 week, 1 month, 6 months, and 12 months after treatment. At 1 week after treatment there were significant decreases in potassium- and amphetamine-evoked overflow of DA, and in clearance of DA, in the striatum of the METH-treated animals. Basal extracellular levels of DA and its metabolites were also decreased. Evoked overflow had partially recovered by 1 month. By 6 months evoked overflow of DA appeared to be normal in the METH-treated rats. However, whole tissue levels of striatal DA were still significantly decreased. All parameters were back to control values by 12 months. These results suggest that presynaptic dopaminergic functioning can recover to normal levels in the striatum of METH-treated rats by 12 months after treatment.