Changes In Extracellular Dopamine Research Articles

Investigating the Influence of Morphine and Cocaine on the Mesolimbic Pathway Using a Novel Microimaging Platform.

Dopamine (DA)'s relationship with addiction is complex, and the related pathways in the mesocorticolimbic system are used to deliver DA, regulating both behavioral and perceptual actions. Specifically, the mesolimbic pathway connecting the ventral tegmental area (VTA) and the nucleus accumbens (NAc) is crucial in regulating memory, emotion, motivation, and behavior due to its responsibility to modulate dopamine. To better investigate the relationship between DA and addiction, more advanced mapping methods are necessary to monitor its production and propagation accurately and efficiently. In this study, we incorporate dLight1.2 adeno-associated virus (AAV) into our latest CMOS (complementary metal-oxide semiconductor) imaging platform to investigate the effects of two pharmacological substances, morphine and cocaine, in the NAc using adult mice. By implanting our self-fabricated CMOS imaging device into the deep brain, fluorescence imaging of the NAc using the dLight1.2 AAV allows for the visualization of DA molecules delivered from the VTA in real time. Our results suggest that changes in extracellular DA can be observed with this adapted system, showing potential for new applications and methods for approaching addiction studies. Additionally, we can identify the unique characteristic trend of DA release for both morphine and cocaine, further validating the underlying biochemical mechanisms used to modulate dopaminergic activation.

Swiprosin1/EFhd2 is involved in the monoaminergic and locomotor responses of psychostimulant drugs.

Psychostimulants are widely abused drugs that may cause addiction in vulnerable individuals. While the reward circuitry of the brain is involved in addiction establishment, various pathways in the brain may provide protection at the molecular level that limits the acute and chronic effects of drugs. These targets may be used for strategies designed to prevent and treat addiction. Swiprosin-1/EF hand domain 2 (EFhd2) is a Ca2+ -binding cytoskeletal adaptor protein involved in sensation-seeking behaviour, anxiety and alcohol addiction. Here, we tested how EFhd2 contributes to the physiological and behavioural effects of the psychostimulant drugs methamphetamine (METH) and cocaine. An in vivo microdialysis study in EFhd2 knockout mice revealed that EFhd2 controls METH- and cocaine-induced changes in extracellular dopamine, serotonin and noradrenaline levels through different mechanisms in the nucleus accumbens and prefrontal cortex. Electrophysiological recordings in a slice preparation showed that a lack of EFhd2 increases dopaminergic neuronal activity in the ventral tegmental area and increases the sensitivity of neurons to stimulation. We report a role of EFhd2 in METH-induced locomotor activation and in the conditioned locomotor effects. No role, however, was observed in the establishment of METH- or cocaine-induced conditioned place preference. These findings may suggest that EFhd2 modulates the activity of the dopaminergic system and the neurochemical effects of METH and cocaine, which translate into a modulation of the behavioural effects of these drugs at the level of the acute and conditioned locomotor activity.

Dual-mode dopamine increases mediated by 5-HT1B and 5-HT2C receptors inhibition, inducing impulsive behavior in trained rats

Patients with eating disorders exhibit problems with appetitive impulse control. Interactions between dopamine and serotonin (5-HT) neuron in this setting are poorly characterized. Here we examined 5-HT receptor-mediated changes in extracellular dopamine during impulsive appetitive behavior in rats. Rats were trained to perform a cued lever-press (LP) task for a food reward such that they stopped experiencing associated dopamine increases. Trained rats were administered the mixed 5-HT1B/2C-receptor antagonist metergoline, the 5-HT2A/2C-receptor antagonist ketanserin, and p-chlorophenylalanine (PCPA). We measured dopamine changes in the ventral striatum using voltammetry and examined the number of premature LPs, reaction time (RT), and reward acquisition rate (RAR). Compared with controls, metergoline increased premature LPs and shortened RT significantly; ketanserin decreased premature LPs and lengthened RT significantly; and PCPA decreased premature LPs, lengthened RT, and decreased RAR significantly. Following metergoline administration, rats exhibited a fast phasic dopamine increase for 0.25–0.75 s after a correct LP, but only during LP for an incorrect LP. No dopamine increases were detected with ketanserin or PCPA, or in controls. After LP task completion, metergoline also caused dopamine to increase slowly and remain elevated; in contrast, ketanserin caused dopamine to increase slowly and decrease rapidly. No slow dopamine increase occurred with PCPA. Inhibition of 5-HT1B- and 5-HT2C-receptors apparently induced dual modes of extracellular dopamine increase: fast phasic, and slow long-lasting. These increases may be associated with the suppression of acquired prediction learning and retention of high motivation for reward, leading to impulsive excessive premature LPs.

Comparison between male and female rats in a model of self-administration of a chocolate-flavored beverage: Behavioral and neurochemical studies

The existence of sex differences was studied in a rat model of operant self-administration of a chocolate-flavored beverage (CFB), which possesses strong reinforcing properties and is avidly consumed by rats. Whether these differences occurred concomitantly to changes in extracellular dopamine in the dialysate obtained from the nucleus accumbens, was assessed by intracerebral microdialysis. Male, ovariectomized and intact female rats showed similar self-administration profiles, with minor differences in both acquisition and maintenance phases. Intact females self-administered larger amounts of CFB, when expressed per body weight, than males and ovariectomized females, in spite of similar values of lever-responding, latency to the first lever-response and consumption efficiency (a measure of rat’s licking effectiveness) in males, ovariectomized and intact females and no difference in breakpoint value and number of lever-responses emerged when males, ovariectomized and intact females were exposed to a progressive ratio schedule of reinforcement. Intracerebral microdialysis revealed a slight but significant increase in dopamine activity in the shell of the nucleus accumbens of male rats when compared to intact female rats during CFB self-administration. The above differences may be caused by the hormonal (mainly estradiol) fluctuations that occur during the estrus cycle in intact females. Accordingly, in intact females CFB self-administration and dopamine activity were found to fluctuate across the estrus cycle, with lower parameters of CFB self-administration and lower dopamine activity in the Proestrus and Estrus phases vs. the Metestrus and Diestrus phases of the cycle.

Modafinil Activates Phasic Dopamine Signaling in Dorsal and Ventral Striata.

Modafinil (MOD) exhibits therapeutic efficacy for treating sleep and psychiatric disorders; however, its mechanism is not completely understood. Compared with other psychostimulants inhibiting dopamine (DA) uptake, MOD weakly interacts with the dopamine transporter (DAT) and modestly elevates striatal dialysate DA, suggesting additional targets besides DAT. However, the ability of MOD to induce wakefulness is abolished with DAT knockout, conversely suggesting that DAT is necessary for MOD action. Another psychostimulant target, but one not established for MOD, is activation of phasic DA signaling. This communication mode during which burst firing of DA neurons generates rapid changes in extracellular DA, the so-called DA transients, is critically implicated in reward learning. Here, we investigate MOD effects on phasic DA signaling in the striatum of urethane-anesthetized rats with fast-scan cyclic voltammetry. We found that MOD (30-300 mg/kg i.p.) robustly increases the amplitude of electrically evoked phasic-like DA signals in a time- and dose-dependent fashion, with greater effects in dorsal versus ventral striata. MOD-induced enhancement of these electrically evoked amplitudes was mediated preferentially by increased DA release compared with decreased DA uptake. Principal component regression of nonelectrically evoked recordings revealed negligible changes in basal DA with high-dose MOD (300 mg/kg i.p.). Finally, in the presence of the D2 DA antagonist, raclopride, low-dose MOD (30 mg/kg i.p.) robustly elicited DA transients in dorsal and ventral striata. Taken together, these results suggest that activation of phasic DA signaling is an important mechanism underlying the clinical efficacy of MOD.

Mesocorticolimbic monoamine correlates of methamphetamine sensitization and motivation.

Methamphetamine (MA) is a highly addictive psychomotor stimulant, with life-time prevalence rates of abuse ranging from 5–10% world-wide. Yet, a paucity of research exists regarding MA addiction vulnerability/resiliency and neurobiological mediators of the transition to addiction that might occur upon repeated low-dose MA exposure, more characteristic of early drug use. As stimulant-elicited neuroplasticity within dopamine neurons innervating the nucleus accumbens (NAC) and prefrontal cortex (PFC) is theorized as central for addiction-related behavioral anomalies, we used a multi-disciplinary research approach in mice to examine the interactions between sub-toxic MA dosing, motivation for MA and mesocorticolimbic monoamines. Biochemical studies of C57BL/6J (B6) mice revealed short- (1 day), as well as longer-term (21 days), changes in extracellular dopamine, DAT and/or D2 receptors during withdrawal from 10, once daily, 2 mg/kg MA injections. Follow-up biochemical studies conducted in mice selectively bred for high vs. low MA drinking (respectively, MAHDR vs. MALDR mice), provided novel support for anomalies in mesocorticolimbic dopamine as a correlate of genetic vulnerability to high MA intake. Finally, neuropharmacological targeting of NAC dopamine in MA-treated B6 mice demonstrated a bi-directional regulation of MA-induced place-conditioning. These results extend extant literature for MA neurotoxicity by demonstrating that even subchronic exposure to relatively low MA doses are sufficient to elicit relatively long-lasting changes in mesocorticolimbic dopamine and that drug-induced or idiopathic anomalies in mesocorticolimbic dopamine may underpin vulnerability/resiliency to MA addiction.

Contrasting changes in extracellular dopamine and glutamate along the rostrocaudal axis of the anterior cingulate cortex of the rat following an acute d-amphetamine or dopamine challenge

There is evidence for functional specificity of subregions along the rostrocaudal axis of the anterior cingulate cortex (ACC). The subregion-specific distribution of dopaminergic afferents and glutamatergic efferents along the ACC make these obvious candidates for coding such regional responses. We investigated this possibility using microdialysis in freely-moving rats to compare changes in extracellular dopamine and glutamate in the rostral (‘rACC’: Cg1 and Cg3 (prelimbic area)) and caudal (‘cACC’: Cg1 and Cg2) ACC induced by systemic or local administration of d-amphetamine. Systemic administration of d-amphetamine (3 mg/kg, i.p.) caused a transient increase in extracellular dopamine in the rACC, but an apparent increase in the cACC of the same animals was less clearly defined. Local infusion of d-amphetamine increased dopamine efflux in the rACC, only. Glutamate efflux in the rACC was increased by local infusion of dopamine (5–50 μM), which had negligible effect in the cACC, but only systemic administration of d-amphetamine increased glutamate efflux and only in the cACC. The asymmetry in the neurochemical responses within the rACC and cACC, to the same experimental challenges, could help explain why different subregions are recruited in the response to specific environmental and somatosensory stimuli and should be taken into account when studying the regulation of neurotransmission in the ACC.This article is part of the Special Issue entitled ‘CNS Stimulants’.

Effect of risperidone on the fluoxetine-induced changes in extracellular dopamine, serotonin and noradrenaline in the rat frontal cortex

BackgroundSeveral clinical reports have documented a beneficial effect of the addition of a low dose of risperidone to the ongoing treatment with antidepressants, in particular selective serotonin reuptake inhibitors, in the treatment of drug resistant depression. The aim of our study was to understand the mechanism of the clinical efficacy of a combination of fluoxetine (FLU) and risperidone (RIS) in drug-resistant depression.We studied the effect of FLU and RIS, given separately or jointly on the extracellular levels of dopamine (DA), serotonin (5-HT) and noradrenaline (NA) in the rat frontal cortex. MethodsAnimals were given single intraperitoneal injections of RIS at a doses of 0.1 or 1mg/kg and FLU at a dose of 10mg/kg. The release of DA, 5-HT and NAin the rat frontal cortex was investigated using microdialysis in freely moving animals. The extracellular level of DA, 5-HT and NA was assayed by HPLC with coulochemical detection. ResultsRIS (0.1 and 1mg/kg) and FLU (10mg/kg) increased the extracellular level of cortical DA, 5-HT and NA. Co-treatment of both drugs was more effective in increasing DA release than administration of each of the drugs alone at doses of RIS 1mg/kg and FLU 10mg/kg. Co-treatment of FLU and RIS 0.1mg/kg was more potent than FLU alone, while the effect of joint injection of FLU and RIS 1mg/kg was stronger than RIS 1mg/kg alone on 5-HT release. The combination of FLU with both doses of RIS was not effective in increasing NA release as compared to drugs given alone. ConclusionsOur data indicate that the effect of the combined administration of RIS and FLU on DA and 5-HT release in the rat frontal cortex may be of crucial importance to the pharmacotherapy of drug resistant depression.

Effects of VMAT2 inhibitors lobeline and GZ-793A on methamphetamine-induced changes in dopamine release, metabolism and synthesis in vivo.

Vesicular monoamine transporter-2 (VMAT2) inhibitors reduce methamphetamine (METH) reward in rats. The current study determined the effects of VMAT2 inhibitors lobeline (LOB; 1 or 3 mg/kg) and N-(1,2R-dihydroxylpropyl)-2,6-cis-di(4-methoxyphenethyl)piperidine hydrochloride (GZ-793A; 15 or 30 mg/kg) on METH-induced (0.5 mg/kg, SC) changes in extracellular dopamine (DA) and its metabolite dihydroxyphenylacetic acid (DOPAC) in the reward-relevant nucleus accumbens (NAc) shell using in vivo microdialysis. The effect of GZ-793A (15 mg/kg) on DA synthesis in tissue also was investigated in NAc, striatum, medial prefrontal cortex and orbitofrontal cortex. In NAc shell, METH produced a time-dependent increase in extracellular DA and decrease in DOPAC. Neither LOB nor GZ-793A alone altered extracellular DA; however, both drugs increased extracellular DOPAC. In combination with METH, LOB did not alter the effects of METH on DA; however, GZ-793A, which has greater selectivity than LOB for inhibiting VMAT2, reduced the duration of the METH-induced increase in extracellular DA. Both LOB and GZ-793A enhanced the duration of the METH-induced decrease in extracellular DOPAC. METH also increased tissue DA synthesis in NAc and striatum, whereas GZ-793A decreased synthesis; no effect of METH or GZ-793A on DA synthesis was found in medial prefrontal cortex or orbitofrontal cortex. These results suggest that selective inhibition of VMAT2 produces a time-dependent decrease in DA release in NAc shell as a result of alterations in tyrosine hydroxylase activity, which may play a role in the ability of GZ-793A to decrease METH reward.

The cystine–glutamate transporter enhancer N-acetyl-l-cysteine attenuates cocaine-induced changes in striatal dopamine but not self-administration in squirrel monkeys

Extrasynaptic glutamate has been shown to regulate dopamine function in the mesocorticolimbic pathway, which plays an important role in the behavioral pharmacology of psychostimulants. Basal levels of glutamate are primarily regulated by the cystine–glutamate transporter and provide glutamatergic tone on extrasynaptic glutamate receptors. The present study examined the effects of a cystine–glutamate transporter enhancer on the neurochemical and behavioral effects of cocaine and amphetamine in nonhuman primates. It was hypothesized that augmenting extrasynaptic glutamate release with N-acetyl-l-cysteine (NAC), a cystine prodrug, would attenuate cocaine- or amphetamine-induced increases in extracellular dopamine and their corresponding behavioral-stimulant and reinforcing effects. In vivo microdialysis was used to evaluate cocaine-induced changes in extracellular dopamine (DA) in the caudate nucleus (n=3). NAC significantly attenuated cocaine-induced increases in dopamine but had inconsistent effects on amphetamine-induced increases in dopamine (n=4). Separate groups of subjects were either trained on a fixed-interval schedule of stimulus termination (n=6) or on a second-order schedule of self-administration (n=5) to characterize the behavioral-stimulant and reinforcing effects of psychostimulants, respectively. Systemic administration of NAC did not alter the behavioral-stimulant effects of either cocaine or amphetamine. Furthermore, cocaine self-administration and reinstatement of previously extinguished cocaine self-administration were not altered by pretreatment with NAC. Hence, drug interactions on caudate neurochemistry in vivo were not reflected in behavioral measures in squirrel monkeys. The present results in nonhuman primates do not support the use of NAC as a pharmacotherapy for cocaine abuse, although rodent and clinical studies suggest otherwise.

Enhanced Striatal Dopamine Release During Food Stimulation in Binge Eating Disorder

Subjects with binge eating disorder (BED) regularly consume large amounts of food in short time periods. The neurobiology of BED is poorly understood. Brain dopamine, which regulates motivation for food intake, is likely to be involved. We assessed the involvement of brain dopamine in the motivation for food consumption in binge eaters. Positron emission tomography (PET) scans with [(11)C]raclopride were done in 10 obese BED and 8 obese subjects without BED. Changes in extracellular dopamine in the striatum in response to food stimulation in food-deprived subjects were evaluated after placebo and after oral methylphenidate (MPH), a drug that blocks the dopamine reuptake transporter and thus amplifies dopamine signals. Neither the neutral stimuli (with or without MPH) nor the food stimuli when given with placebo increased extracellular dopamine. The food stimuli when given with MPH significantly increased dopamine in the caudate and putamen in the binge eaters but not in the nonbinge eaters. Dopamine increases in the caudate were significantly correlated with the binge eating scores but not with BMI. These results identify dopamine neurotransmission in the caudate as being of relevance to the neurobiology of BED. The lack of correlation between BMI and dopamine changes suggests that dopamine release per se does not predict BMI within a group of obese individuals but that it predicts binge eating.

Effect of metyrapone on the fluoxetine-induced change in extracellular dopamine, serotonin and their metabolites in the rat frontal cortex

Major depression is frequently associated with the hyperactivity of the hypothalamic-pimitary-adrenocortical axis, and glucocorticoid synthesis inhibitors have been shown to exert antidepressant action. Metyrapone (an inhibitor of the enzyme 11-β-hydroxylase) has been found to be effective as an adjunctive therapy in combination with other antidepressants (ADs) in both treatment-resistant depression and animal models. To understand the mechanism of the clinical efficacy of a combination of an AD and metyrapone in treatment-resistant depression, the present study was aimed at determining the influence of fluoxetine (FLU; a selective serotonin reuptake inhibitor) and metyrapone, given separately or jointly, on the extracellular level of dopamine (DA), serotonin (5-HT) and their metabolites in rat frontal cortex of freely moving rats using microdialysis and high performance liquid chromatography (HPLC) with electrochemical detection. FLU (10mg/kg) given alone increased the extracellular level of DA and 5-HT in the rat frontal cortex. Metyrapone (100mg/kg) alone did not change the level of monoamines. A combination of FLU and metyrapone produced the same change in the efflux of both DA and 5-HT as did FLU alone. However, the latter combination (FLU and metyrapone) produced significantly bigger increases in the levels of extracellullar DA metabolites (3, 4-dihydroxyphenylacetic acid, homovanillic acid) and a 5-HT metabolite (5-hydroxyindoleacetic acid) than did FLU alone. The above findings suggest that – among other mechanisms – increases in the levels of extracellullar DA and 5-HT metabolites may play a role in the enhancement of FLU efficacy by metyrapone, and may be of crucial importance to the pharmacotherapy of drug-resistant depression.

A Positron Emission Tomography Study Examining The Dopaminergic Activity of Armodafinil in Adults Using [ 11C]Altropane and [ 11C]Raclopride

Armodafinil, prescribed principally to treat narcolepsy, is undergoing assessment of therapeutic potential for other neuropsychiatric disorders and medical conditions. The neurochemical substrates and mechanisms of armodafinil are unresolved. We investigated the hypothesis that armodafinil enhances wakefulness by modulating the activities of the dopamine transporter (DAT). With positron emission tomography imaging, we determined DAT occupancy and changes in extracellular dopamine by armodafinil in vivo. Twelve subjects were enrolled. Plasma armodafinil levels were obtained. In vivo armodafinil occupancy of the DAT in striatum was detected by [¹¹C]altropane and changes in extracellular dopamine were detected by indirect displacement of [¹¹C]raclopride in human subjects at different times after drug administration. Armodafinil (100 mg by mouth [PO]) occupied striatal DAT (34.0 ± 9.0% at 1 hour, 40.4 ± 9.5% at 2.5 hours, n = 6) and 250 mg occupied striatal DAT (60.5 ± 7.4% at 1 hour, 65.2 ± 6.1% at 2.5 hours, n = 6). In addition, armodafinil was associated with changes in extracellular dopamine (17.8 ± 30.1% [100 mg PO] and 7.0 ± 8.6% [250 mg PO] at 2.5 hours, n = 6). Occupancy of the DAT and changes in extracellular dopamine in vivo further implicates the actions of armodafinil on DAT as a potential candidate for its therapeutic improvement of wakefulness and other conditions.

Effect of metyrapone on the fluoxetine-induced change in extracellular dopamine, serotonin and their metabolites in rat prefrontal cortex

Effect of metyrapone on the fluoxetine-induced change in extracellular dopamine, serotonin and their metabolites in rat prefrontal cortex

Detection of endogenous dopamine changes in Drosophila melanogaster using fast-scan cyclic voltammetry.

Drosophila melanogaster, the fruit fly, is a commonly used model organism because of its homology to mammals and facile genetic manipulations. However, the size of the nervous system is very small. We report a method to evoke and detect rapid changes in extracellular dopamine in a single nerve cord isolated from a Drosophila larva. Flies were genetically modified to express Channelrhodopsin-2, a blue-light activated cation channel, in only dopaminergic neurons. Extracellular dopamine changes were measured with fast-scan cyclic voltammetry at an implanted carbon-fiber microelectrode. Stimulations of 7 s with blue light result in an average peak dopamine concentration of 810 +/- 60 nM, similar to electrically-stimulated release in mammals. Stimulations repeated at 15 min intervals are stable for 65 min, allowing pharmacological experiments in the same sample. Peak duration is extended after cocaine or nisoxetine, inhibitors of the dopamine transporter (DAT). Release was reduced upon exposure to reserpine, which inhibits vesicular packaging. Chronic administration of NSD-1015, a dopamine synthesis inhibitor, decreased dopamine release and inhibited pupation, showing a link between neurotransmission and physiology. This is the first method to measure endogenous dopamine in an intact larval Drosophila nervous system and will allow studies of genetic and pharmacological manipulations of dopamine release and uptake.

Administration of MDMA to ethanol-deprived rats increases ethanol operant self-administration and dopamine release during reinstatement

Recreational use of (±)-3,4-methylenedioxymethamphetamine (MDMA, ecstasy) is often associated with other drugs, among which ethanol (EtOH) is one of the most common. However, little is known about how neurochemical sensitization produced by MDMA can modulate EtOH abuse. In this study we used EtOH operant self-administration tasks to investigate the effect of several low doses (0.33, 1.0 and 3.0 mg/kg) of MDMA in Dark Agouti rats. Motor activity was recorded after each MDMA administration. Changes in extracellular dopamine in the nucleus accumbens following a single EtOH injection (1.5 g/kg i.p.) were measured using intracerebral microdialysis in vivo after 1 wk of abstinence from EtOH, in order to mimic the dopaminergic response associated with reinstatement into EtOH consumption. Animals exposed to higher doses of MDMA (1.0 and 3.0 mg/kg) showed significantly enhanced EtOH self-administration during reinstatement and an increased EtOH-induced dopamine efflux. MDMA treatment acutely elevated motor activity after each administration in a dose-dependent manner. These findings suggest that repeated administration of MDMA, a relatively common drug of abuse, even at low doses, can alter subsequent vulnerability to EtOH consumption.

Agmatine inhibits morphine-induced locomotion sensitization and morphine-induced changes in striatal dopamine and metabolites in rats

The effects of agmatine on morphine-induced locomotion sensitization and morphine-induced changes in extracellular striatal dopamine (DA) and DA metabolites were studied. The locomotor response to morphine challenge (3 mg/kg, s.c.) was enhanced in rats 3 days after repeated morphine administration, indicating development of locomotion sensitization. In vivo microdialysis demonstrated a significant increase in striatal basal levels of the DA metabolites DOPAC and HVA, but not in DA itself, and an increase in DA response to morphine challenge in rats 3 days after withdrawal. Agmatine (1, 10, 80 mg/kg) inhibited morphine-induced locomotion sensitization and the changes in DA noted above. Idazoxan attenuated the effects of agmatine on locomotion, suggesting that the effects are mediated by imidazoline receptors. In addition, repeated morphine also increased the expression of tyrosine hydroxylase mRNA in the VTA after 4 days of morphine pretreatment, while decreasing the expression of dynorphin mRNA at 3 days after withdrawal. Agmatine inhibited morphine-induced changes in dynorphin, but not in tyrosine hydroxylase mRNA expression. These data suggest that agmatine, likely by activating imidazoline receptors, inhibits morphine-induced locomotion sensitization and morphine-induced changes in extracellular DA and in dynorphin expression. Thus, agmatine deserves further study as an anti-opioid medication.

Chronic Ethanol Exposure During Adolescence Increases Basal Dopamine in the Nucleus Accumbens Septi During Adulthood

In humans, adolescent exposure to alcohol is associated with the onset of adult alcohol dependency and suggests that early use potentiates vulnerability to addiction. The aim of the present study was to address whether chronic ethanol exposure during adolescence would alter nucleus accumbens septi (NAcc) dopamine (DA) levels in the adult brain. Rats were injected daily from postnatal day (PND) 30 to 50 with either 0.75 g/kg/i.p. ethanol or saline followed by an ethanol-abstinent period from PND 51 to 65. Changes in extracellular DA levels in the anterior NAcc shell were measured via the no net flux (NNF) paradigm. Extracellular DA levels were greater in rats chronically treated with ethanol during adolescence (6.5 nM DA) in comparison with saline-exposed controls (3.6 nM DA). There were no differences in extraction fraction (E(d)), an indirect measure of DA reuptake, between ethanol-treated (87%) and nontreated (68%) rats. Together these findings suggest that changes in extracellular DA may be an underlying physiological mechanism in adolescent vulnerability to the rewarding properties of ethanol.

Stress-induced changes in extracellular dopamine and serotonin in the medial prefrontal cortex and dorsal hippocampus of prenatally malnourished rats

Prenatal protein malnutrition continues to be a significant problem in the world today. Exposure to prenatal protein malnutrition increases the risk of a number of neuropsychiatric disorders in adulthood including depression, schizophrenia and attentional deficit disorder. In the present experiment, we have examined the effects of stress on extracellular serotonin (5-HT) and dopamine in the medial prefrontal cortex and dorsal hippocampus of rats exposed in utero to protein malnutrition. The medial prefrontal cortex and dorsal hippocampus were chosen as two limbic forebrain regions involved in learning and memory, attention and the stress response. Extracellular 5-HT and dopamine were determined in the medial prefrontal cortex and dorsal hippocampus of adult male Sprague–Dawley rats using dual probe in vivo microdialysis. Basal extracellular 5-HT did not differ between malnourished and well-nourished controls in either the medial prefrontal cortex or the dorsal hippocampus. Basal extracellular dopamine was significantly decreased in the medial prefrontal cortex of malnourished animals. Restraint stress (20 m) produced a significant rise in extracellular dopamine in the medial prefrontal cortex of well-nourished rats but did not alter release in malnourished rats. In malnourished rats, stress produced an increase in 5-HT in the hippocampus, whereas stress produced a decrease in 5-HT in the hippocampus of well-nourished rats. These data demonstrate that prenatal protein malnutrition alters dopaminergic neurotransmission in the medial prefrontal cortex as well as alters the dopaminergic and serotonergic response to stress. These changes may provide part of the bases for alterations in malnourished animals' response to stress.

Effects of amantadine and budipine on antidepressant drug-evoked changes in extracellular dopamine in the frontal cortex of freely moving rats

NMDA receptors play a role in the aetiology of depression with non-competitive NMDA receptor antagonists such as amantadine showing synergy with conventional antidepressants. To advance a neurochemical rational for these findings, we have studied the effects of administration of amantadine and budipine with the antidepressants reboxetine (REB), paroxetine (PAROX) and clomipramine (CLOM) on extracellular DA in rats using microdialysis. Acutely, amantadine (40 mg/kg) or budipine (10 mg/kg) did not significantly alter extracellular DA. REB (10 mg/kg), PAROX (10 mg/kg) both increased cortical DA while CLOM (10 mg/kg) produced a decrease. When amantadine or budipine was administered 30 min before the antidepressants, DA increases were markedly greater than following the antidepressants alone. Chronically drug effects were studied at 4, 7, 14 and 21 days. Amantadine and budipine did not significantly alter extracellular DA at any time. The three antidepressants elicited a gradual increase in DA which became significant after 7 days and tended to plateau thereafter. When amantadine (20 mg/kg) or budipine (5 mg/kg) was co-administered with the three antidepressants, two differences were seen compared with the antidepressants alone. Firstly, the time required for significant increases in cortical DA was reduced with elevated levels now being observed by 4 days. Secondly, the increase in extracellular DA was greater in these rats throughout the experiment. If increased extracellular DA represents a step in the mechanism of action of antidepressants, these data suggest that combined treatment with clinically tolerated NMDA antagonists such as amantadine could reduce the delay in therapeutic onset of antidepressants and possibly enhance their efficacy.