Increase In Extracellular Concentration Of Dopamine Research Articles

Redefining differential roles of MAO-A in dopamine degradation and MAO-B in tonic GABA synthesis

Monoamine oxidase (MAO) is believed to mediate the degradation of monoamine neurotransmitters, including dopamine, in the brain. Between the two types of MAO, MAO-B has been believed to be involved in dopamine degradation, which supports the idea that the therapeutic efficacy of MAO-B inhibitors in Parkinson’s disease can be attributed to an increase in extracellular dopamine concentration. However, this belief has been controversial. Here, by utilizing in vivo phasic and basal electrochemical monitoring of extracellular dopamine with fast-scan cyclic voltammetry and multiple-cyclic square wave voltammetry and ex vivo fluorescence imaging of dopamine with GRABDA2m, we demonstrate that MAO-A, but not MAO-B, mainly contributes to striatal dopamine degradation. In contrast, our whole-cell patch-clamp results demonstrated that MAO-B, but not MAO-A, was responsible for astrocytic GABA-mediated tonic inhibitory currents in the rat striatum. We conclude that, in contrast to the traditional belief, MAO-A and MAO-B have profoundly different roles: MAO-A regulates dopamine levels, whereas MAO-B controls tonic GABA levels.

Prefrontal Cortex-Driven Dopamine Signals in the Striatum Show Unique Spatial and Pharmacological Properties.

Dopamine (DA) signals in the striatum are critical for a variety of vital processes, including motivation, motor learning, and reinforcement learning. Striatal DA signals can be evoked by direct activation of inputs from midbrain DA neurons (DANs) as well as cortical and thalamic inputs to the striatum. In this study, we show that in vivo optogenetic stimulation of prelimbic (PrL) and infralimbic (IL) cortical afferents to the striatum triggers an increase in extracellular DA concentration, which coincides with elevation of striatal acetylcholine (ACh) levels. This increase is blocked by a nicotinic ACh receptor (nAChR) antagonist. Using single or dual optogenetic stimulation in brain slices from male and female mice, we compared the properties of these PrL/IL-evoked DA signals with those evoked by stimulation from midbrain DAN axonal projections. PrL/IL-evoked DA signals are undistinguishable from DAN evoked DA signals in their amplitudes and electrochemical properties. However, PrL/IL-evoked DA signals are spatially restricted and preferentially recorded in the dorsomedial striatum. PrL/IL-evoked DA signals also differ in their pharmacological properties, requiring activation of glutamate and nicotinic ACh receptors. Thus, both in vivo and in vitro results indicate that cortical evoked DA signals rely on recruitment of cholinergic interneurons, which renders DA signals less able to summate during trains of stimulation and more sensitive to both cholinergic drugs and temperature. In conclusion, cortical and midbrain inputs to the striatum evoke DA signals with unique spatial and pharmacological properties that likely shape their functional roles and behavioral relevance.SIGNIFICANCE STATEMENT Dopamine signals in the striatum play a critical role in basal ganglia function, such as reinforcement and motor learning. Different afferents to the striatum can trigger dopamine signals, but their release properties are not well understood. Further, these input-specific dopamine signals have only been studied in separate animals. Here we show that optogenetic stimulation of cortical glutamatergic afferents to the striatum triggers dopamine signals both in vivo and in vitro These afferents engage cholinergic interneurons, which drive dopamine release from dopamine neuron axons by activation of nicotinic acetylcholine receptors. We also show that cortically evoked dopamine signals have other unique properties, including spatial restriction and sensitivity to temperature changes than dopamine signals evoked by stimulation of midbrain dopamine neuron axons.

Deep brain stimulation of the medial forebrain bundle elevates striatal dopamine concentration without affecting spontaneous or reward-induced phasic release

Deep brain stimulation (DBS) of the medial forebrain bundle (MFB) induces rapid improvement of depressive symptoms in patients suffering from treatment-refractory major depressive disorder (MDD). It has been hypothesized that activation of the dopamine (DA) system contributes to this effect. To investigate whether DBS in the MFB affects DA release in the striatum, we combined DBS with fast-scan cyclic voltammetry (FSCV) in freely moving rats. Animals were implanted with a stimulating electrode at the border of the MFB and the ventral tegmental area, and a FSCV microelectrode in the ventromedial striatum to monitor extracellular DA during the acute onset of DBS and subsequent continued stimulation. DBS onset induced a significant increase in extracellular DA concentration in the ventromedial striatum that was sustained for at least 40s. However, continued DBS did not affect amplitude or frequency of so-called spontaneous phasic DA transients, nor phasic DA release in response to the delivery of unexpected food pellets. These findings suggest that effects of DBS in the MFB are mediated by an acute change in extracellular DA concentration, but more research is needed to further explore the potentially sustained duration of this effect. Together, our results provide both support and refinement of the hypothesis that MFB DBS activates the DA system: DBS induces an increase in overall ambient concentration of DA, but spontaneous or reward-associated more rapid, phasic DA dynamics are not enhanced. This knowledge improves our understanding of how DBS affects brain function and may help improve future therapies for depressive symptoms.

Relationship between non‐functional masticatory activity and central dopamine in stressed rats

In humans, diurnal tooth-clenching and other oral stereotyped behaviour are associated with stress/anxiety. In rodents, gnawing/biting of objects is observed during exposure to stress. Both nigrostriatal and mesocortical dopaminergic systems are involved in the development of this coping behaviour. To clarify the relationship between central dopaminergic activity and stress-induced parafunctional masticatory behaviour, using microdialysis in vivo, we assessed the changes in extracellular dopamine concentrations in both prefrontal cortex and striatum of rats subjected to a mild tail pinch. The animals were divided into two groups according to the degree of non-functional masticatory activity (NFMA) displayed during exposure to tail pinch. In prefrontal cortex, rats which displayed severe NFMA showed a greater increase in extracellular dopamine concentration in relation to basal values (Emax=184±26%) than those which did not display this coping behaviour (Emax=139± 23%) (F(NFMA) [1,86]=3·97; P<0·05) (n=17). A positive association was also found between cortical dopamine maximal value from baseline and the degree of NFMA displayed (r=0·36; P<0·05) (n=17). There were no significant differences in the tail-pinch-induced striatal dopamine increase between both groups of rats (Emax=130±10%) (n=17). These results provide further evidence in support of prefrontal dopamine playing a relevant role in the expression of stress-induced masticatory coping behaviour.

Relationship between non-functional masticatory activity and central dopamine in stressed rats

Summary In humans, diurnal tooth-clenching and other oral stereotyped behaviour are associated with stress/anxiety. In rodents, gnawing/biting of objects is observed during exposure to stress. Both nigrostriatal and mesocortical dopaminergic systems are involved in the development of this coping behaviour. To clarify the relationship between central dopaminergic activity and stress-induced parafunctional masticatory behaviour, using microdialysis in vivo, we assessed the changes in extracellular dopamine concentrations in both prefrontal cortex and striatum of rats subjected to a mild tail pinch. The animals were divided into two groups according to the degree of non-functional masticatory activity (NFMA) displayed during exposure to tail pinch. In prefrontal cortex, rats which displayed severe NFMA showed a greater increase in extracellular dopamine concentration in relation to basal values (Emax = 184 ± 26%) than those which did not display this coping behaviour (Emax = 139 ± 23%) (FNFMA[1,86] = 3·97; P < 0·05) (n = 17). A positive association was also found between cortical dopamine maximal value from baseline and the degree of NFMA displayed (r = 0·36; P < 0·05) (n = 17). There were no significant differences in the tail-pinch-induced striatal dopamine increase between both groups of rats (Emax = 130 ± 10%) (n = 17). These results provide further evidence in support of prefrontal dopamine playing a relevant role in the expression of stress-induced masticatory coping behaviour.

Single administration of 1-benzyl-1,2,3,4-tetrahydroisoquinoline increases the extracellular concentration of dopamine in rat striatum

We performed a combined neurochemical and behavioral study to determine the effects of 1-benzyl-1,2,3,4-tetrahydroisoquinoline (1-BnTIQ) on the extracellular dopamine concentrations in the striatum. Single dose administration of 1-BnTIQ (20, 40, and 80 mg/kg i.p.) increased striatal dopamine extracellular levels in a dose-dependent manner when an in vivo microdialysis technique was used to assess dopamine levels in the striatum of rats. Enhancement of striatal dopamine levels by systemic administration of a single dose of 1-BnTIQ was suppressed by perfusion of tetrodotoxin and a calcium ion-free solution into the striatum. This 1-BnTIQ-induced increase in extracellular dopamine concentration was also inhibited by pre-treatment with a dopamine uptake inhibitor, GBR12909 (1-(2-[bis(4-Fluorophenyl)-4-(3-phenylpropyl)piperazine dihydrochloride). Local application of 1-BnTIQ into the striatum via a dialysis probe failed to enhance the extracellular concentration of dopamine. However, microinjection of 1-BnTIQ into the substantia nigra pars compacta increased the extracellular dopamine levels in the striatum. Locomotor activity was increased by systemic administration of a single dose of 1-BnTIQ in a dose-dependent manner. This 1-BnTIQ-induced locomotor activity was attenuated by pre-treatment with SCH23390 (R(+)-7-Chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochlodride) and raclopride, D 1 and D 2 dopaminergic receptor antagonists, respectively. Moreover, 1-BnTIQ induced ipsilateral rotational behavior in 6-hydroxydopamine–lesioned rats. These results suggest that systemic administration of a single dose of 1-BnTIQ increases striatal extracellular dopamine concentration through activation of dopaminergic nigra striatal neurons via the dopamine transporter.

Sub-chronic nandrolone treatment modifies neurochemical and behavioral effects of amphetamine and 3,4-methylenedioxymethamphetamine (MDMA) in rats

Misuse of anabolic-androgenic steroids (AASs) is increasing, and appears to have much in common with the use of substances known to induce drug dependence. Moreover, persons who abuse AASs also tend to abuse other psychotropic drugs such as amphetamine or 3,4-methylenedioxymethamphetamine (MDMA, “ecstasy”). The aim of this study was to investigate whether nandrolone (5 × 5 or 5 × 20 mg/kg) pre-exposure modulates the acute neurochemical and behavioral effects of amphetamine (1 mg/kg) and MDMA (5 mg/kg) in rats. Dopamine (DA), 5-hydroxytryptamine (5-HT) and their metabolites were measured from samples collected from the nucleus accumbens (NAc) by in vivo microdialysis. The behavior of the animals was recorded on videotapes, from which it was later rated. Our results demonstrate that sub-chronic treatments with supraphysiological doses of nandrolone attenuate dose-dependently the increase in extracellular DA concentration evoked by amphetamine or MDMA. The lower dose of nandrolone attenuated MDMA-induced increase in 5-HT-levels, while the higher dose potentiated it. Analysis of the behavioral data suggests that effects of the amphetamine and MDMA are dose-dependently attenuated by AAS-treatment, paralleling DA results. In conclusion, the results of this study show that AAS-pre-treatment is able to modulate the reward-related neurochemical and behavioral effects of amphetamine and MDMA.

Animal models of attention-deficit hyperactivity disorder

Although animals cannot be used to study complex human behaviour such as language, they do have similar basic functions. In fact, human disorders that have animal models are better understood than disorders that do not. ADHD is a heterogeneous disorder. The relatively simple nervous systems of rodent models have enabled identification of neurobiological changes that underlie certain aspects of ADHD behaviour. Several animal models of ADHD suggest that the dopaminergic system is functionally impaired. Some animal models have decreased extracellular dopamine concentrations and upregulated postsynaptic dopamine D1 receptors (DRD1) while others have increased extracellular dopamine concentrations. In the latter case, dopamine pathways are suggested to be hyperactive. However, stimulus-evoked release of dopamine is often decreased in these models, which is consistent with impaired dopamine transmission. It is possible that the behavioural characteristics of ADHD result from impaired dopamine modulation of neurotransmission in cortico-striato-thalamo-cortical circuits. There is considerable evidence to suggest that the noradrenergic system is poorly controlled by hypofunctional α2-autoreceptors in some models, giving rise to inappropriately increased release of norepinephrine. Aspects of ADHD behaviour may result from an imbalance between increased noradrenergic and decreased dopaminergic regulation of neural circuits that involve the prefrontal cortex. Animal models of ADHD also suggest that neural circuits may be altered in the brains of children with ADHD. It is therefore of particular importance to study animal models of the disorder and not normal animals. Evidence obtained from animal models suggests that psychostimulants may not be acting on the dopamine transporter to produce the expected increase in extracellular dopamine concentration in ADHD. There is evidence to suggest that psychostimulants may decrease motor activity by increasing serotonin levels. In addition to providing unique insights into the neurobiology of ADHD, animal models are also being used to test new drugs that can be used to alleviate the symptoms of ADHD.

Parkin-deficient Mice Exhibit Nigrostriatal Deficits but Not Loss of Dopaminergic Neurons

Loss-of-function mutations in parkin are the major cause of early-onset familial Parkinson's disease. To investigate the pathogenic mechanism by which loss of parkin function causes Parkinson's disease, we generated a mouse model bearing a germline disruption in parkin. Parkin-/- mice are viable and exhibit grossly normal brain morphology. Quantitative in vivo microdialysis revealed an increase in extracellular dopamine concentration in the striatum of parkin-/- mice. Intracellular recordings of medium-sized striatal spiny neurons showed that greater currents are required to induce synaptic responses, suggesting a reduction in synaptic excitability in the absence of parkin. Furthermore, parkin-/- mice exhibit deficits in behavioral paradigms sensitive to dysfunction of the nigrostriatal pathway. The number of dopaminergic neurons in the substantia nigra of parkin-/- mice, however, is normal up to the age of 24 months, in contrast to the substantial loss of nigral neurons characteristic of Parkinson's disease. Steady-state levels of CDCrel-1, synphilin-1, and alpha-synuclein, which were identified previously as substrates of the E3 ubiquitin ligase activity of parkin, are unaltered in parkin-/- brains. Together these findings provide the first evidence for a novel role of parkin in dopamine regulation and nigrostriatal function, and a non-essential role of parkin in the survival of nigral neurons in mice.

Baclofen antagonizes nicotine-, cocaine-, and morphine-induced dopamine release in the nucleus accumbens of rat.

Evidence recently provided has suggested a specific involvement of the GABAergic system in modulating positive reinforcing properties of several drugs of abuse through an action on mesolimbic dopaminergic neurons. The GABA(B) receptor agonist baclofen has been proposed as a potential therapeutic agent for the clinical treatment of several forms of drug addiction. In the present study, using the in vivo microdialysis technique, we investigated the effect of baclofen on nicotine, cocaine, and morphine-induced increase in extracellular dopamine (DA) levels in the shell of the nucleus accumbens, a brain area supposedly involved in the modulation of the central effects of several drugs of abuse, of freely moving rats. As expected, nicotine (0.6 mg/kg s.c.), morphine (5 mg/kg s.c.), and cocaine (7.5 mg/kg i.p.) administration in rats induced a marked increase in extracellular DA concentrations in the nucleus accumbens, reaching a maximum value of +205 +/- 8.4%, +300 +/- 22.2%, and +370 +/- 30.7%, respectively. Pretreatment with baclofen (1.25 and 2.5 mg/kg i.p.) dose-dependently reduced the nicotine-, morphine-, and cocaine-evoked DA release in the shell of the nucleus accumbens. Furthermore, baclofen alone did not elicit changes in basal DA extracellular levels up to 180 min. Taken together, our data are in line with previous reports demonstrating the ability of baclofen to modulate the mesolimbic DAergic transmission and indicate baclofen as a putative candidate in the pharmacotherapy of polydrug abuse.

Raclopride studies of dopamine release: dependence on presynaptic integrity

Background Raclopride and dopamine (DA) compete for in vivo binding to the D 2 receptors. Thus, measurements of raclopride binding provide a method to evaluate endogenous release in a variety of conditions. Amphetamine elicits DA release, provoking a rapid increase in synaptic DA and leading to a reduction in raclopride binding, which outlasts the temporary increase in extracellular DA concentrations by several hours. The mechanism responsible for the decrease in raclopride binding is still unclear. Methods We used a multiple ligand concentration receptor assay method in normal monkeys and in monkeys with varying degrees of lesion of the DA nigrostriatal terminals to measure the density and affinity of D 2 receptors after methamphetamine challenge. Results The reduced raclopride binding can be accounted for by a decreased affinity of the ligand to the receptors. There is a direct, nonlinear relationship between the presynaptic storage capacity and the change in raclopride binding after methamphetamine. Conclusions This observation may bear important implications for the understanding of diseases such as schizophrenia in which the marked increase in amphetamine-induced displacement of raclopride compared with normal control subjects suggests increased release of DA from presynaptic stores and potential abnormalities in presynaptic DA function.

Changes in basal and cocaine-evoked extracellular dopamine uptake and release in the rat nucleus accumbens during early abstinence from cocaine: quantitative determination under transient conditions

Despite an abundance of studies on mechanisms of behavioral sensitization, considerable uncertainty exists as to whether alterations in dopamine neurotransmission underlie the exacerbated behavioral effects of cocaine observed during the early stages of abstinence. One of the factors contributing to the uncertainty and controversy may be the limitations in utilized measurement techniques (mostly conventional microdialysis). The techniques of quantitative microdialysis under transient conditions and rotating disk electrode voltammetry were used to characterize basal dopamine dynamics as well as time-related changes in extracellular dopamine concentrations and dopamine uptake that occur in response to an acute drug challenge in control animals and animals with previous history of cocaine. Basal extracellular dopamine concentrations were unaltered on abstinence day 3 from repeated cocaine administration (5 days, 20 mg/kg, i.p.). The extraction fraction of dopamine, an indirect measure of dopamine uptake, was significantly lower in cocaine-sensitized animals relative to controls. These two facts, taken together, suggest that basal dopamine release is depressed in cocaine-sensitized animals on abstinence day 3. At the same time, a cocaine challenge decreased the extraction fraction and increased the extracellular dopamine concentration in both experimental groups. The magnitude of the increase in extracellular dopamine concentration was greater in cocaine-sensitized animals, while the ability of cocaine to decrease the extraction fraction was unaltered, suggesting that the increase in extracellular dopamine concentration reflects an increase in drug-evoked dopamine release. Moreover, cocaine-pretreated rats demonstrated greater depolarization-induced dopamine release and the ability of dopamine D 2 receptor agonist, quinpirole, to inhibit release was decreased in these animals. These data demonstrate that a cocaine treatment regimen resulting in behavioral sensitization is associated with a reduction in basal dopamine release, an enhancement in both cocaine and K +-evoked dopamine release, and a subsensitivity of dopamine D 2 autoreceptors that regulate dopamine release in the nucleus accumbens.

Effect of paroxetine on extracellular serotonin and dopamine levels in the prefrontal cortex.

Abstract. Paroxetine, a selective serotonin (5-HT) reuptake inhibitor, increased extracellular 5-HT and dopamine levels, as determined by microdialysis, in the medial prefrontal cortex (mPFC) of freely moving rats. There was a difference in the time course of the maximum response between the 5-HT and dopamine levels after paroxetine administration. The extracellular dopamine concentration reached its maximum 20 min after the peak effect of 5-HT had appeared. The paroxetine-induced increase in extracellular dopamine concentration, but not 5-HT concentration, was inhibited by the 5-HT(3)-receptor antagonist granisetron. These results suggest that the increase in extracellular dopamine concentration in the mPFC elicited by paroxetine is the result of stimulation of 5-HT(3) receptors by the extracellular accumulation of 5-HT in the mPFC.

Nicotinic acetylcholine receptors and neurodegenerative disease

Nicotinic acetylcholine receptor (nAChR) in the central nervous system represents a new potential therapeutic target in neurodegenerative diseases, and this is driven by new findings in the molecular biology of nicotinic ion channel. Results of epidemiological studies have revealed that the incidence of Alzheimer's disease is lower among smokers compared with findings for nonsmokers, which seems to indicate that nicotine may influence cortical functions. We observed an increase in extracellular dopamine concentrations after administration of nicotine through a microdialysis tube. Nicotine might inhibit the uptake of dopamine through the nAChR, which could serve as a preventive factor against neurodegenerative diseases. We evaluated the ability of nicotine to protect neuronal cells from death by using the model system of serum- and nerve growth factor (NGF)–free cultures of PC12 cells. Serum and NGF deprivation induced rapid and massive death of these cells, which was inhibited by the addition of nicotine. These results suggest to us that nicotine may be involved in the protection of neuronal cells from death by means of nAChR. The effect of NGF and nicotine on the expression of nAChR subunits in PC12 cells was examined by using Northern blot analysis. Nerve growth factor increased the transcription of α5 and β4 subunits, whereas nicotine increased mRNA level encoding α5 and β2 subunits. These results suggest to us that NGF changes the expression of nAChR in a subtype-specific manner over the course of differentiation, and disproportionate subunit expressions might be related to the neuroprotective effect exerted by nicotine.

Nitric oxide-mediated regulation of dopamine release in the hippocampus in vivo

Infusion of N-methyl- d-aspartate (NMDA) into the hippocampus of freely moving rats produced a concentration-dependent decrease in the extracellular levels of dopamine, an effect which was reversed by D-2-amino-5-phosphonovaleric acid (D-AP5). To determine the involvement of nitric oxide (NO) in this response, two nitric oxide synthase (NOS) inhibitors, N-nitro- l-arginine methyl ester (L-NAME) and 7-nitroindazole (7-NI), were examined for their ability to modify both basal and NMDA-inhibited dopamine release. When infused alone both NOS inhibitors elicited an increase in extracellular dopamine concentration, moreover, when administered prior to the application of NMDA, the agonist failed to elicit a decrease in dopamine levels. Infusion of the NO donor S-nitroso- N-acetylpenicillamine (SNAP) over a 30 min period caused either an increase or a decrease in dopamine release depending upon the concentration used. At the lower concentration (0.5 mM) SNAP promoted dopamine release whilst at the higher concentration (5 mM), the donor elicited a long lasting reduction in basal dopamine levels. The effect of the lower concentration of SNAP was reversed by the prior application of D-AP5, but that of the higher concentration was unaffected by the antagonist.

Effect of selegiline on dopamine concentration in the striatum of a primate

Monoamine oxidase (MAO) has two subtypes, A and B, that have different distributions between the rodent and the human. In the striatum, dopamine (DA) of the rat seems to be metabolized by MAO A, and DA of the human is largely deaminated by MAO B. MAO in the striatum of common marmosets is also type B. Using in vivo microdialysis, we investigated the pharmacological activity of selegiline, a selective irreversible inhibitor of MAO B, in the striatum of marmosets. Intraperitoneal co-administration of selegiline (1 mg kg −1, i.p.) with levodopa/carbidopa (10/2.5 mg kg −1, i.p.) did not significantly increase extracellular concentration of DA in the striatum of common marmosets compared with control animals receiving levodopa/carbidopa alone. Daily pretreatment with 0.1 mg kg −1 (i.p.) selegiline for two weeks, however, dramatically increased extracellular concentration of DA to about seven times that of control animals treated with levodopa/carbidopa alone in marmosets. Such an increase in extracellular concentrations of DA could not be observed in a similar study with Wistar rats. This study showed that chronic administration of a small dose of selegiline caused a marked increase in extracellular DA concentration in the striatum of primates, but not in the rodents.

The nitric oxide synthase inhibitor. L-NAME, blocks certain phencyclidine-induced but not amphetamine-induced effects on behaviour and brain biochemistry in the rat

1. 1. The present experiments examined the effects of the nitric oxide synthase (NOS) inhibitor, N G-nitro-L-arginine methyl ester (L-NAME), on some behavioural and biochemical effects of phencyclidine (PCP) and d-amphetamine (AMPH) in rats. Observation of behaviour was performed using a subjective scoring system. 2. 2. PCP (4 mg/kg) increased locomotor activity, rearing, sniffing, grooming and stereotyped behaviour, and decreased stillness. PCP also increased forepaw myoclonus, forepaw treading, head weaving, licking and chewing behaviour. Most of these behaviours were significantly suppressed by pretreatment with L-NAME (10 mg/kg). 3. 3. AMPH (1 mg/kg) exerted different effects on behaviour. It increased locomotor activity, rearing, sniffing, and stereotyped behaviour, and decreased stillness and grooming, but failed to affect the other behavioural items observed. Pretreatment with L-NAME did not counteract these effects. 4. 4. Ex vivo biochemical analysis indicated that PCP increased the tissue concentration of the dopamine (DA) metabolites, dihydroxyphenyl acetic acid (DOPAC) and homovanillic acid (HVA), in the ventral striatum (i.e., the nucleus accumbens and olfactory tubercles) and frontal cortex but not in the dorsal striatum. DA was not significantly affected in any of these regions. Furthermore, PCP increased the tissue concentration of the serotonin (5-HT) metabolite, 5-hydroxyindole acetic acid (5-HIAA), in a similar manner, while 5-HT was not affected. These biochemical effects were significantly counteracted by pretreatment with L-NAME. 5. 5. AMPH decreased tissue DOPAC concentration in the dorsal striatum, an effect which was not sensitive to pretreatment with L-NAME. However, the combined treatment with L-NAME and AMPH increased tissue DA concentration in all three regions investigated. 6. 6. The neurochemical and behavioural effects of PCP and AMPH were further investigated in an experimental model which allowed measurement of prepulse inhibition (PPI) of acoustic startle in parallel with in vivo microdialysis sampling of extracellular DA concentration in the brain of awake, freely moving animals. 7. 7. Both PCP (2 mg/kg) and AMPH (2 mg/kg) caused a significant decrease in PPI of similar magnitude and duration. These behavioural effects were accompanied by a significant increase in extracellular DA concentration in the nucleus accumbens (PCP: 213 ± 21% of basal concentration; AMPH: 563±117%). 8. 8. The decrease in PPI caused by PCP was blocked by pretreatment with L-NAME, but the change in DA concentration was not. Neither of these measures were significantly affected by L-NAME in AMPH-treated rats. 9. 9. In conclusion, this study shows that several behavioural and biochemical effects of PCP are prevented by pretreatment with the NOS inhibitor, L-NAME, while the effects of AMPH are less sensitive to this pretreatment. These observations emphasise the involvement of nitric oxide in the pharmacological effects of PCP.

Endothelin induces dopamine release from rat striatum via endothelin-B receptors

The aim of the present study was to determine whether local administration of endothelin induces the release of dopamine in the rat striatum and to characterize and localize endothelin receptors in this brain region. Local injection of endothelin-1 (10 pmol) into the ventral striatum of urethane-anaesthetized rats caused an increase of 8 μM in the extracellular concentration of dopamine as measured by in vivo chronoamperometry. The peak increase in dopamine concentration occurred within 5 min of endothelin injection. Injection of the selective endothelin-B receptor agonist [Ala 1,3,11,15]endothelin-1 (10 pmol) also caused an increase in extracellular dopamine concentration, suggesting that endothelin is acting at the endothelin-B receptor to elicit its effect. In rats with unilateral 6-hydroxydopamine lesions of the nigrostriatal pathway, the response to local injection of endothelin-1 (10 pmol) was significantly inhibited on the lesioned side as compared to the non-lesioned side. In contrast, pretreatment of the rats with the N-methyl- d-aspartate receptor antagonist dizocilpine maleate (5 mg/kg, i.p.) or the nitric oxide synthase inhibitor N G-nitro- l-arginine (3 mg/kg, i.p.) did not alter the endothelin-induced release of dopamine. In binding studies, addition of endothelin-1 displaced [ 125I]endothelin-1 with a K i of 220 pM. The endothelin-B receptor antagonist BQ788 displaced [ 125I]endothelin-1 with a K i of 120 nM, whereas the endothelin-A receptor antagonist BQ123 produced only a 25% displacement at 10 μM, suggesting that endothelin receptors in the striatum are of the endothelin-B subtype. In rats with unilateral 6-hydroxydopamine lesions of the nigrostriatal dopamine system, [ 125I]endothelin-1 binding was reduced by 53% in lesioned striatum compared to non-lesioned striatum, with no difference in the K d . These data provide evidence that endothelin acts on a homogeneous population of endothelin-B receptors within the striatum to cause the release of dopamine and that a significant proportion of these receptors is located on dopaminergic neurons.

Inhibition of basal and stress-induced dopamine release in the cerebral cortex and nucleus accumbens of freely moving rats by the neurosteroid allopregnanolone

The neurosteroid allopregnanolone is a potent and efficacious modulator of γ-aminobutyric acid (GABA) type A receptors. The effects of intracerebroventricular injection of allopregnanolone (5 to 15 μg in 5 μl) on basal and stress-induced changes in the extracellular concentrations of dopamine were investigated by microdialysis in various brain areas of freely moving rats and compared with those of the benzodiazepine midazolam (1 to 10 μg in 5 μl). Allopregnanolone reduced (by a maximum of 65 to 75%) basal dopamine content in the prefrontal cortex and nucleus accumbens in a dose-dependent manner, but had no effect on dopamine output in the striatum. Allopregnanolone (10 to 15 μg) also completely prevented the increase in extracellular dopamine concentrations in the nucleus accumbens and cerebral cortex induced by foot-shock stress. Midazolam reduced basal dopamine content in all three brain regions studied as well as the stress- induced increase in dopamine content in the nucleus accumbens and cerebral cortex with a greater potency than allopregnanolone. These results suggest that endogenous neurosteroids may participate in the GABAergic modulation of dopaminergic transmission in the rat cerebral cortex and nucleus accumbens, two brain areas which are important in the regulation of emotional processes. These agents do not appear to affect striatal dopaminergic transmission which modulates motor function.

Comparison of changes in extracellular dopamine concentrations in the nucleus accumbens during intravenous self-administration of cocaine or d-amphetamine

Changes in extracellular concentrations of dopamine (DA) were measured in vivo in the nucleus accumbens of the rat during intravenous self-administration of either cocaine (0.25, 0.5, 1.0mg/infusion) or d-amphetamine (0.05, 0.1, 0.2mg/infusion). Drug intake was limited to 12 self-administered infusions per session for each drug/dose combination. Changes in extra-cellular DA concentrations were measured by two different techniques: chronoamperometry in conjunction with chronically-implanted stearate-modified carbon paste electrodes, or intracerebral microdialysis with off-line analyses using high performance liquid chromatography with electrochemical detection (HPLC-ED). Significant increases in extracellular DA concentrations were observed with both in vivo techniques during self-administration of each dose of cocaine or d-amphetamine. For each drug, the magnitude of change during the first hour of the test session was comparable across doses. However, the change observed over the first 2h period, as measured by microdialysis and HPLC-ED, revealed a dose effect for cocaine, but no dose-response effect for d-amphetamine. The duration of the drug-induced elevation was increased significantly as a function of dose with both cocaine and d-amphetamine. Data from the microdialysis experiments indicated that the high dose of d-amphetamine (0.2mg/infusion) produced a significantly greater increase in extracellular DA concentrations in the nucleus accumbens than did the high dose of cocaine (1.0mg/infusion), but that comparable changes were induced by doses of 0.1mg/infusion of d-amphetamine and 1.0mg/infusion of cocaine, respectively. Each dose of both psychostimulant drugs also produced a significant decrease in dihydroxyphenylacetic acid (DOPAC) levels. The latter finding indicated that the electrochemical signal measured in these studies was not due to the oxidation of DOPAC. These results confirm that self-administration of cocaine or d-amphetamine by the rat is accompanied by a significant increase in extracellular DA concentrations in the nucleus accumbens. The fact that two different psychomotor stimulant drugs of abuse have qualitatively similar neurochemical correlates when self-administered, adds credence to the hypothesis that their reinforcing properties are related to dynamic changes in DA concentrations in the ventral striatal region of the brain.