Dopamine high-affinity transport site topography in rat brain: major differences between dorsal and ventral striatum

Actions and Consequences of Insulin in the Striatum

Insulin crosses the blood–brain barrier to enter the brain from the periphery. In the brain, insulin has well-established actions in the hypothalamus, as well as at the level of mesolimbic dopamine neurons in the midbrain. Notably, insulin also acts in the striatum, which shows abundant expression of insulin receptors (InsRs) throughout. These receptors are found on interneurons and striatal projections neurons, as well as on glial cells and dopamine axons. A striking functional consequence of insulin elevation in the striatum is promoting an increase in stimulated dopamine release. This boosting of dopamine release involves InsRs on cholinergic interneurons, and requires activation of nicotinic acetylcholine receptors on dopamine axons. Opposing this dopamine-enhancing effect, insulin also increases dopamine uptake through the action of insulin at InsRs on dopamine axons. Insulin acts on other striatal cells as well, including striatal projection neurons and astrocytes that also influence dopaminergic transmission and striatal function. Linking these cellular findings to behavior, striatal insulin signaling is required for the development of flavor–nutrient learning, implicating insulin as a reward signal in the brain. In this review, we discuss these and other actions of insulin in the striatum, including how they are influenced by diet and other physio-logical states.

Up and Down γ-Synuclein Transcription in Dopamine Neurons Translates into Changes in Dopamine Neurotransmission and Behavioral Performance in Mice

The synuclein family consists of α-, β-, and γ-Synuclein (α-Syn, β-Syn, and γ-Syn) expressed in the neurons and concentrated in synaptic terminals. While α-Syn is at the center of interest due to its implication in the pathogenesis of Parkinson's disease (PD) and other synucleinopathies, limited information exists on the other members. The current study aimed at investigating the biological role of γ-Syn controlling the midbrain dopamine (DA) function. We generated two different mouse models with: (i) γ-Syn overexpression induced by an adeno-associated viral vector and (ii) γ-Syn knockdown induced by a ligand-conjugated antisense oligonucleotide, in order to modify the endogenous γ-Syn transcription levels in midbrain DA neurons. The progressive overexpression of γ-Syn decreased DA neurotransmission in the nigrostriatal and mesocortical pathways. In parallel, mice evoked motor deficits in the rotarod and impaired cognitive performance as assessed by novel object recognition, passive avoidance, and Morris water maze tests. Conversely, acute γ-Syn knockdown selectively in DA neurons facilitated forebrain DA neurotransmission. Importantly, modifications in γ-Syn expression did not induce the loss of DA neurons or changes in α-Syn expression. Collectively, our data strongly suggest that DA release/re-uptake processes in the nigrostriatal and mesocortical pathways are partially dependent on substantia nigra pars compacta /ventral tegmental area (SNc/VTA) γ-Syn transcription levels, and are linked to modulation of DA transporter function, similar to α-Syn.

Presynaptic regulation of dopamine release: Role of the DAT and VMAT2 transporters

The signaling dynamics of the neurotransmitter dopamine has been established to have an important role in a variety of behavioural processes including motor control, cognition, and emotional processing. Key regulators of transmitter release and the signaling dynamics of dopamine are the plasma membrane reuptake transporter (DAT) and the vesicular monoamine transporter (VMAT2). These proteins serve to remove dopamine molecules from the extracellular and cytosolic space, respectively and both determine the amount of transmitter released from synaptic vesicles. This review provides an overview of how these transporter proteins are involved in molecular regulation and function together to govern the dynamics of vesicular release with opposing effects on the quantal size and extracellular concentration of dopamine. These transporter proteins are both focal points of convergence for a variety of regulatory molecular cascades as well as targets for many pharmacological agents. The ratio between these transporters is argued to be useful as a molecular marker for delineating dopamine functional subsystems that may differ in transmitter release patterns.

Interactions between insulin and diet on striatal dopamine uptake kinetics in rodent brain slices

Diet influences dopamine transmission in motor- and reward-related basal ganglia circuitry. In part, this reflects diet-dependent regulation of circulating and brain insulin levels. Activation of striatal insulin receptors amplifies axonal dopamine release in brain slices, and regulates food preference in vivo. The effect of insulin on dopamine release is indirect, and requires striatal cholinergic interneurons that express insulin receptors. However, insulin also acts directly on dopamine axons to increase dopamine uptake by promoting dopamine transporter (DAT) surface expression, counteracting enhanced dopamine release. Here, we determined the functional consequences of acute insulin exposure and chronic diet-induced changes in insulin on DAT activity after evoked dopamine release in striatal slices from adult ad-libitum fed (AL) rats and mice, and food-restricted (FR) or high-fat/high-sugar obesogenic (OB) diet rats. Uptake kinetics were assessed by fitting evoked dopamine transients to the Michaelis-Menten equation and extracting C_peak and V_max . Insulin (30 nm) increased both parameters in the caudate putamen and nucleus accumbens core of AL rats in an insulin receptor- and PI3-kinase-dependent manner. A pure effect of insulin on uptake was unmasked using mice lacking striatal acetylcholine, in which increased V_max caused a decrease in C_peak . Diet also influenced V_max , which was lower in FR vs. AL. The effects of insulin on C_peak and V_max were amplified by FR but blunted by OB, consistent with opposite consequences of these diets on insulin levels and insulin receptor sensitivity. Overall, these data reveal acute and chronic effects of insulin and diet on dopamine release and uptake that will influence brain reward pathways.

Regional Heterogeneity of D2-Receptor Signaling in the Dorsal Striatum and Nucleus Accumbens

Dopamine input to the dorsal and ventral striatum originates from separate populations of midbrain neurons. Despite differences in afferent inputs and behavioral output, little is known about how dopamine release is encoded by dopamine receptors on medium spiny neurons (MSNs) across striatal subregions. Here we examined the activation of D2 receptors following the synaptic release of dopamine in the dorsal striatum (DStr) and nucleus accumbens (NAc) shell. We found that D2 receptor-mediated synaptic currents were slower in the NAc and this difference occurred at the level of D2-receptor signaling. As a result of preferential coupling to Gαo, we also found that D2 receptors in MSNs demonstrated higher sensitivity for dopamine in the NAc. The higher sensitivity in the NAc was eliminated following cocaine exposure. These results identify differences in the sensitivity and timing of D2-receptor signaling across the striatum that influence how nigrostriatal and mesolimbic signals are encoded across these circuits.

Effects of diet and insulin on dopamine transporter activity and expression in rat caudate-putamen, nucleus accumbens, and midbrain

Food restriction (FR) and obesogenic (OB) diets are known to alter brain dopamine transmission and exert opposite modulatory effects on behavioral responsiveness to psychostimulant drugs of abuse. Mechanisms underlying these diet effects are not fully understood. In this study, we examined diet effects on expression and function of the dopamine transporter (DAT) in caudate-putamen (CPu), nucleus accumbens (NAc), and midbrain regions. Dopamine (DA) uptake by CPu, NAc or midbrain synapto(neuro)somes was measured in vitro with rotating disk electrode voltammetry or with [³ H]DA uptake and was found to correlate with DAT surface expression, assessed by maximal [³ H](-)-2-β-carbomethoxy-3-β-(4-fluorophenyl)tropane binding and surface biotinylation assays. FR and OB diets were both found to decrease DAT activity in CPu with a corresponding decrease in surface expression but had no effects in the NAc and midbrain. Diet treatments also affected sensitivity to insulin-induced enhancement of DA uptake, with FR producing an increase in CPu and NAc, likely mediated by an observed increase in insulin receptor expression, and OB producing a decrease in NAc. The increased expression of insulin receptor in NAc of FR rats was accompanied by increased DA D₂ receptor expression, and the decreased DAT expression and function in CPu of OB rats was accompanied by decreased DA D₂ receptor expression. These results are discussed as partial mechanistic underpinnings of diet-induced adaptations that contribute to altered behavioral sensitivity to psychostimulants that target the DAT.

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.

Computational modeling of extracellular dopamine kinetics suggests low probability of neurotransmitter release

Dopamine in the striatum signals the saliency of current environmental input and is involved in learned formation of appropriate responses. The regular baseline-firing rate of dopaminergic neurons suggests that baseline dopamine is essential for proper brain function. The first goal of the study was to estimate the likelihood of full exocytotic dopamine release associated with each firing event under baseline conditions. A computer model of extracellular space associated with a single varicosity was developed using the program MCell to estimate kinetics of extracellular dopamine. Because the literature provides multiple kinetic values for dopamine uptake depending on the system tested, simulations were run using different kinetic parameters. With all sets of kinetic parameters evaluated, at most, 25% of a single vesicle per varicosity would need to be released per firing event to maintain a 5-10 nM extracellular dopamine concentration, the level reported by multiple microdialysis experiments. The second goal was to estimate the fraction of total amount of stored dopamine released during a highly stimulated condition. This was done using the same model system to simulate published measurements of extracellular dopamine following electrical stimulation of striatal slices in vitro. The results suggest the amount of dopamine release induced by a single electrical stimulation may be as large as the contents of two vesicles per varicosity. We conclude that dopamine release probability at any particular varicosity is low. This suggests that factors capable of increasing release probability could have a powerful effect on sculpting dopamine signals.

The dopamine patchwork of the rat nucleus accumbens core

The dopamine (DA) terminal field in the rat dorsal striatum is organized as a patchwork of domains that show distinct DA kinetics. The rate and short-term plasticity of evoked DA release, the rate of DA clearance and the actions of several dopaminergic drugs are all domain-dependent. The patchwork arises in part from local variations in the basal extracellular concentration of DA, which establishes an autoinhibitory tone in slow but not fast domains. The present study addressed the hypothesis that a domain patchwork might also exist in the nucleus accumbens core (NAcc), a DA terminal field that is deeply involved in reward processing and the mechanisms underlying substance abuse. DA recordings in the NAcc by fast-scan voltammetry during electrical stimulation of the medial forebrain bundle confirmed that the NAcc contains a patchwork of fast and slow domains showing significantly different rates of evoked DA release and DA clearance. Moreover, the NAcc domains are substantially different from those in the dorsal striatum. There were no signs in the NAcc of short-term plasticity of DA release during multiple consecutive stimuli, and no signs of a domain-dependent autoinhibitory tone. Thus, the NAcc domains are distinct from each other and from the domains of the dorsal striatum.

Amphetamine elicits opposing actions on readily releasable and reserve pools for dopamine

Amphetamine, a highly addictive drug with therapeutic efficacy, exerts paradoxical effects on the fundamental communication modes employed by dopamine neurons in modulating behavior. While amphetamine elevates tonic dopamine signaling by depleting vesicular stores and driving non-exocytotic release through reverse transport, this psychostimulant also activates phasic dopamine signaling by up-regulating vesicular dopamine release. We hypothesized that these seemingly incongruent effects arise from amphetamine depleting the reserve pool and enhancing the readily releasable pool. This novel hypothesis was tested using in vivo voltammetry and stimulus trains of varying duration to access different vesicular stores. We show that amphetamine actions are stimulus dependent in the dorsal striatum. Specifically, amphetamine up-regulated vesicular dopamine release elicited by a short-duration train, which interrogates the readily releasable pool, but depleted release elicited by a long-duration train, which interrogates the reserve pool. These opposing actions of vesicular dopamine release were associated with concurrent increases in tonic and phasic dopamine responses. A link between vesicular depletion and tonic signaling was supported by results obtained for amphetamine in the ventral striatum and cocaine in both striatal sub-regions, which demonstrated augmented vesicular release and phasic signals only. We submit that amphetamine differentially targeting dopamine stores reconciles the paradoxical activation of tonic and phasic dopamine signaling. Overall, these results further highlight the unique and region-distinct cellular mechanisms of amphetamine and may have important implications for its addictive and therapeutic properties.

Presynaptic dopamine modulation by stimulant self-administration

The mesolimbic dopamine system is an essential participant in the initiation and modulation of various forms of goal-directed behavior, including drug reinforcement and addiction processes. Dopamine neurotransmission is increased by acute administration of all drugs of abuse, including the stimulants cocaine and amphetamine. Chronic exposure to these drugs via voluntary self-administration provides a model of stimulant abuse that is useful in evaluating potential behavioral and neurochemical adaptations that occur during addiction. This review describes commonly used methodologies to measure dopamine and baseline parameters of presynaptic dopamine regulation, including exocytotic release and reuptake through the dopamine transporter in the nucleus accumbens core, as well as dramatic adaptations in dopamine neurotransmission and drug sensitivity that occur with acute non-contingent and chronic, contingent self-administration of cocaine and amphetamine.

Dopamine release in the basal ganglia

Dopamine (DA) is a key transmitter in the basal ganglia, yet DA transmission does not conform to several aspects of the classic synaptic doctrine. Axonal DA release occurs through vesicular exocytosis and is action potential- and Ca²⁺-dependent. However, in addition to axonal release, DA neurons in midbrain exhibit somatodendritic release by an incompletely understood, but apparently exocytotic, mechanism. Even in striatum, axonal release sites are controversial, with evidence for DA varicosities that lack postsynaptic specialization, and largely extrasynaptic DA receptors and transporters. Moreover, DA release is often assumed to reflect a global response to a population of activities in midbrain DA neurons, whether tonic or phasic, with precise timing and specificity of action governed by other basal ganglia circuits. This view has been reinforced by anatomical evidence showing dense axonal DA arbors throughout striatum, and a lattice network formed by DA axons and glutamatergic input from cortex and thalamus. Nonetheless, localized DA transients are seen in vivo using voltammetric methods with high spatial and temporal resolution. Mechanistic studies using similar methods in vitro have revealed local regulation of DA release by other transmitters and modulators, as well as by proteins known to be disrupted in Parkinson's disease and other movement disorders. Notably, the actions of most other striatal transmitters on DA release also do not conform to the synaptic doctrine, with the absence of direct synaptic contacts for glutamate, GABA, and acetylcholine (ACh) on striatal DA axons. Overall, the findings reviewed here indicate that DA signaling in the basal ganglia is sculpted by cooperation between the timing and pattern of DA input and those of local regulatory factors.

Extracellular dopamine levels in striatal subregions track shifts in motivation and response cost during instrumental conditioning

Tonic dopamine (DA) signaling is widely regarded as playing a central role in effort-based decision making and in the motivational control of instrumental performance. The current study used microdialysis to monitor changes in extracellular DA levels across subregions of the nucleus accumbens and dorsal striatum of rats as they lever pressed for food reward on a probabilistic schedule of reinforcement, a procedure that ensured they would experience variation in the amount of effort needed to earn rewards across tests. Each rat was given three tests. Rats were hungry for the first and last test, but were sated on food before the middle test, allowing us to assess the effects of a downshift in motivational state on task performance and conditioning-induced DA efflux. During hungry tests, DA levels rose in both the shell and core of the accumbens and, to a lesser degree, in both the medial and lateral divisions of the dorsal striatum. Interestingly, changes in DA efflux across hungry tests in the accumbens core were negatively correlated with changes in the effort required to obtain rewards. We also found that--across regions--the DA response to instrumental conditioning was attenuated when rats were sated before testing. Furthermore, the effect of satiety on DA efflux in the accumbens shell was positively correlated with its effect on task performance. Together, the results indicate that tonic DA contributes to the control of instrumental performance by conveying information about the costs and benefits of responding to different striatal subregions.

Selective enhancement of mesocortical dopaminergic transmission by noradrenergic drugs: therapeutic opportunities in schizophrenia

The superior efficacy of atypical vs. classical antipsychotic drugs to treat negative symptoms and cognitive deficits in schizophrenia appears related to their ability to enhance mesocortical dopamine (DA) function. Given that noradrenergic (NE) transmission contributes to cortical DA output, we assessed the ability of NE-targeting drugs to modulate DA release in medial prefrontal cortex (mPFC) and nucleus accumbens (NAc), with the aim of selectively increasing mesocortical DA. Extracellular DA was measured using brain microdialysis in rat mPFC and NAc after local/systemic drug administration, electrical stimulation and selective brain lesions. Local GBR12909 [a selective DA transporter (DAT) inhibitor] administration increased DA output more in NAc than in mPFC whereas reboxetine [a selective NE transporter (NET) inhibitor] had an opposite regional profile. DA levels increased comparably in both regions of control rats after local nomifensine (DAT+NET inhibitor) infusion, but this effect was much lower in PFC of NE-lesioned rats (DSP-4) and in NAc of 6-OHDA-lesioned rats. Electrical stimulation of the locus coeruleus preferentially enhanced DA output in mPFC. Consistently, the administration of reboxetine+RX821002 (an α2-adrenoceptor antagonist) dramatically enhanced DA output in mPFC (but not NAc). This effect also occurred when reboxetine+RX821002 were co-administered with haloperidol or clozapine. The preferential contribution of the NE system to PFC DA allows selective enhancement of DA transmission by simultaneously blocking NET and α2-adrenoceptors, thus preventing the autoreceptor-mediated negative feedback on NE activity. Our results highlight the importance of NET and α2-adrenoceptors as targets for treating negative/cognitive symptoms in schizophrenia and related psychiatric disorders.

Vulnerability of mesostriatal dopaminergic neurons in Parkinson's disease

The term vulnerability was first associated with the midbrain dopaminergic neurons 85 years ago, before they were identified as monoaminergic neurons, when Foix and Nicolesco (1925) reported the loss of neuromelanin containing neurons in the midbrain of patients with post-encephalitic Parkinson's disease (PD). A few years later, Hassler (1938) showed that degeneration is more intense in the ventral tier of the substantia nigra compacta than in its dorsal tier and the ventral tegmental area (VTA), outlining the concept of differential vulnerability of midbrain dopaminergic (DA-) neurons. Nowadays, we know that other neuronal groups degenerate in PD, but the massive loss of nigral DA-cells is its pathological hallmark, having a pivotal position in the pathophysiology of the disease as it is responsible for the motor symptoms. Data from humans as well as cellular and animal models indicate that DA-cell degeneration is a complex process, probably precipitated by the convergence of different risk factors, mediated by oxidative stress, and involving pathogenic factors arising within the DA-neuron (intrinsic factors), and from its environment and distant interconnected brain regions (extrinsic factors). In light of current data, intrinsic factors seem to be preferentially involved in the first steps of the degenerative process, and extrinsic factors in its progression. A controversial issue is the relative weight of the impairment of common cell functions, such as energy metabolism and proteostasis, and specific dopaminergic functions, such as pacemaking activity and DA handling, in the pathogenesis of DA-cell degeneration. Here we will review the current knowledge about the relevance of these factors at the beginning and during the progression of PD, and in the differential vulnerability of midbrain DA-cells.

Altered dopamine transporter function and phosphorylation following chronic cocaine self-administration and extinction in rats

Cocaine binds with the dopamine transporter (DAT), an effect that has been extensively implicated in its reinforcing effects. However, persisting adaptations in DAT regulation after cocaine self-administration have not been extensively investigated. Here, we determined the changes in molecular mechanisms of DAT regulation in the caudate-putamen (CPu) and nucleus accumbens (NAcc) of rats with a history of cocaine self-administration, followed by 3weeks of withdrawal under extinction conditions (i.e., no cocaine available). DA uptake was significantly higher in the CPu of cocaine-experienced animals as compared to saline-yoked controls. DAT V(max) was elevated in the CPu without changes in apparent affinity for DA. In spite of elevated CPu DAT activity, total and surface DAT density and DAT-PP2Ac (protein phosphatase 2A catalytic subunit) interaction remained unaltered, although p-Ser- DAT phosphorylation was elevated. In contrast to the CPu, there were no differences between cocaine and saline rats in the levels of DA uptake, DAT V(max) and K(m) values, total and surface DAT, p-Ser-DAT phosphorylation, or DAT-PP2Ac interactions in the NAcc. These results show that chronic cocaine self-administration leads to lasting, regionally specific alterations in striatal DA uptake and DAT-Ser phosphorylation. Such changes may be related to habitual patterns of cocaine-seeking observed during relapse.

Rapid substrate-induced down-regulation in function and surface localization of dopamine transporters: rat dorsal striatum versus nucleus accumbens

The dopamine transporter (DAT) substrates dopamine, d-amphetamine (AMPH), and methamphetamine are known to rapidly and transiently reduce DAT activity and/or surface expression in dorsal striatum and heterologous expression systems. We sought to determine if similar substrate-induced regulation of DATs occurs in rat nucleus accumbens. In dorsal striatum synaptosomes, brief (15-min) in vitro substrate pre-exposure markedly decreased maximal [(3)H]dopamine uptake velocity whereas identical substrate pre-exposure in nucleus accumbens synaptosomes produced a smaller, non-significant reduction. However, 45 min after systemic AMPH administration, maximal ex vivo [(3)H]dopamine uptake velocity was significantly reduced in both brain regions. Protein kinase C inhibition blocked AMPH's down-regulation of DAT activity. DAT synaptosomal surface expression was not modified following either the brief in vitro or in vivo AMPH pre-exposure but was reduced after a longer (1-h) in vitro pre-exposure in both brain regions. Together, our findings suggest that relatively brief substrate exposure results in greater down-regulation of DAT activity in dorsal striatum than in nucleus accumbens. Moreover, exposure to AMPH appears to regulate striatal DATs in a biphasic manner, with an initial protein kinase C-dependent decrease in DAT-mediated uptake velocity and then, with longer exposure, a reduction in DAT surface expression.

A small dopamine permeability of storage vesicle membranes and end product inhibition of tyrosine hydroxylase are sufficient to explain changes occurring in dopamine synthesis and storage after inhibition of neuron firing

Computer simulations of dopamine (DA) regulation at a striatal varicosity were developed to determine basic principles that explain the pattern of changes in level of neurotransmitter and its rate of synthesis and metabolism when DA neuron firing is inhibited. The models suggest that DA synthesis is normally at a slower rate because of end-product inhibition of tyrosine hydroxylase (TH) by cytosolic DA. The vast majority of DA in the cytosol arrives there via "recycling"--DA that was released during an exocytotic event is moved into the cytosol via the dopamine transporter (DAT). When neuronal firing is inhibited, the amount of cytosolic DA markedly decreases as there is no recycling. The rate of DA synthesis then increases because of the loss of end-product inhibition of TH. The newly synthesized DA is stored in vesicles, thus increasing the total amount of DA in the vesicles. A small amount of DA is continually leaking out of vesicles, and the amount leaking out increases proportionally to the amount of DA in vesicles. When the amount of DA leaking out balances the amount being stored by the vesicular monoamine transporter, DA accumulates in the cytosol. The accumulating DA inhibits TH activity, and the system enters a steady state condition characterized by approximately double the normal amount of DA in vesicles and approximately normal rate of DA synthesis and metabolism.

Differential neurochemical and behavioral adaptation to cocaine after response contingent and noncontingent exposure in the rat

RATIONALE

In naive rats, passive administration of drugs of abuse preferentially increases extracellular dopamine (DA) in the nucleus accumbens (NAc) shell as compared to the core. Repeated exposure to the same drugs results in behavioral and biochemical sensitization characterized by stereotyped activity and reduction of the shell/core DA response ratio.

OBJECTIVES

The aim of this work is to study the neurochemical and behavioral effects of response-contingent vs response-noncontingent drug administration in rats, who were bilaterally implanted with chronic intracerebral guide cannulae and trained to self-administer cocaine by nose poking in daily 1-h sessions for 3 weeks (5 days/week). Nose poking in the active hole by master rats resulted in intravenous injection of cocaine (0.25 mg/kg) in master rats and in rats yoked to them. Dialysate DA was monitored before, during, and for 30 min after cocaine availability on alternate days by inserting the probe into the NAc shell and core. Stereotyped and non-stereotyped behavior was recorded during the sessions.

RESULTS

In master rats, dialysate DA increased preferentially in the NAc shell during cocaine self-administration throughout the 3 weeks of cocaine exposure. In yoked rats, DA increased preferentially in the shell but to a lesser extent than in master rats. With continued exposure to cocaine, the shell/core ratio of DA changes decreased progressively and, on the third week, was reversed so that DA increased more in the core than in the shell. Yoked rats showed a progressive and faster increase in stereotyped behaviors than master rats.

CONCLUSIONS

Response-noncontingent cocaine administration is particularly prone, compared to response-contingent administration, to induce behavioral and biochemical sensitization.

Differential nicotinic regulation of the nigrostriatal and mesolimbic dopaminergic pathways: implications for drug development

Neuronal nicotinic acetylcholine receptors (nAChRs) modulate dopaminergic function. Discovery of their multiplicity has lead to the search for subtype-selective nAChR agonists that might be therapeutically beneficial in diseases linked to brain dopaminergic pathways. The regulation and responses of the nigrostriatal and mesolimbic dopaminergic pathways are often similar, but some differences do exist. The cerebral distribution and characteristics of various nAChR subtypes differ between nigrostriatal and mesolimbic dopaminergic pathways. Comparison of nicotine and epibatidine, two nAChR agonists whose relative affinities for various nAChR subtypes differ, revealed differences in the nAChR-mediated regulation of dopaminergic activation between these dopamine systems. Nicotine preferentially stimulates the mesolimbic pathway, whereas epibatidine's stimulatory effect falls on the nigrostriatal pathway. Thus, it may be possible to stimulate the nigrostriatal pathway with selective nAChR agonists that do not significantly affect the mesolimbic pathway, and thus lack addictive properties. Furthermore, dopamine uptake inhibition revealed a novel inhibitory effect of epibatidine on accumbal dopamine release, which could form a basis for novel antipsychotics that could alleviate the elevated accumbal dopaminergic tone found in schizophrenia during the active psychotic state. Different regulation of nigrostriatal and mesolimbic dopaminergic pathways by nAChRs could be an important basis for developing novel drugs for treatment of Parkinson's disease and schizophrenia.

Catecholamine mapping within nucleus accumbens: differences in basal and amphetamine-stimulated efflux of norepinephrine and dopamine in shell and core

The nucleus accumbens is believed to play a critical role in mediating the behavioral responses to rewarding stimuli. Although most studies of the accumbens focus on dopamine, it receives afferents from many other nuclei, including noradrenergic cell groups in the brainstem. We used in vivo microdialysis to measure extracellular levels of both norepinephrine and dopamine in the accumbens shell and core. Regional analysis of shell and core and border regions demonstrated that norepinephrine was high in shell and decreased from medial shell to lateral core, where baseline levels were low or undetectable. Conversely, extracellular dopamine in core was twice the level seen in shell. Both catecholamines increased following a single injection of amphetamine (2 mg/kg, i.p.). The norepinephrine response was greater and long-lasting in shell compared with core. The maximal dopamine response was higher in core than in shell, but the duration of the effect was comparable in both regions. The distinct neurochemical characteristics of shell and core are likely to contribute to the functional heterogeneity of the two subregions. Furthermore, norepinephrine may be involved in many of the functions generally attributed to the accumbens, either directly or indirectly via modulation of extracellular dopamine.

Nicotine and epibatidine alter differently nomifensine-elevated dopamine output in the rat dorsal and ventral striatum

We studied the effects of nicotine and epibatidine given in combination with dopamine uptake inhibitor, nomifensine, on striatal extracellular dopamine and its metabolites by using brain microdialysis in freely moving rats. Nomifensine (3 mg/kg) elevated extracellular dopamine in the caudate-putamen, and clearly more in the nucleus accumbens. In the caudate-putamen, nicotine (0.5 mg/kg) and epibatidine (0.6 microg/kg but not 3.0 microg/kg) enhanced nomifensine's effect on dopamine. The effect of nomifensine on accumbal dopamine was enhanced by nicotine, but inhibited by epibatidine at 0.6 microg/kg. The larger dose of epibatidine had no effect. Thus, the effects of the smaller epibatidine dose (0.6 microg/kg) on the dopamine output in the caudate-putamen but not in the accumbens resemble those of nicotine 0.5 mg/kg. Discrepancies in the effects of epibatidine and nicotine are most probably due to differences in their affinities to nicotinic receptor subtypes regulating dopamine release. Further, different responses to low concentrations of epibatidine between the brain areas suggest that there are differences in the nicotinic regulation of nigrostriatal and mesolimbic dopaminergic pathways.

Vulnerabilities of ventral mesencephalic neurons projecting to the nucleus accumbens following infusions of 6-hydroxydopamine into the medial forebrain bundle in the rat

The terminal arbors of dopaminergic projections in the nucleus accumbens (Acb) core degenerate more rapidly, completely and permanently in a variety of neurotoxic circumstances than do those in the medial shell. It is unknown if this always reflects purely losses of the distal parts of axons from the core (as proposed in methamphetamine intoxication), or whether, in some circumstances, the disproportionate loss of core axons may also stem from an intrinsic vulnerability to degeneration of core-projecting neuronal perikarya. Experiments described here addressed this issue in the following manner. Three days after Fluoro-Gold (FG), a retrogradely transported tracer, had been iontophoresed selectively into the core or medial shell of male Sprague-Dawley rats, each received an infusion of saline vehicle containing or lacking 6-hydroxydopamine (6-OHDA) in the ipsilateral medial forebrain bundle (MFB). Twenty-one days later the brains were processed to exhibit ventral mesencephalic neurons containing FG. Application of an unbiased sampling method revealed substantially greater losses of FG labeled neurons relative to controls in rats that had received 6-OHDA lesions and deposition of FG in the Acb core as compared to the medial shell. Of the few core-projecting neurons that remained in the ventral mesencephalon after these lesions, 54% did not co-localize tyrosine hydroxylase immunoreactivity (TH-ir) and, thus, were not expected to degenerate. The capacity to selectively remove core-projecting dopaminergic neurons may be useful in the determination of molecular correlates of vulnerability and resistance to neurotoxicity and to possibly test the role of the core in reinforcement paradigms.

"Passive stabilization" of striatal extracellular dopamine across the lesion spectrum encompassing the presymptomatic phase of Parkinson's disease: a voltammetric study in the 6-OHDA-lesioned rat

Symptoms of Parkinson's disease do not present until the degeneration of nigrostriatal dopaminergic neurons is nearly complete. Maintenance of dopaminergic tone governing striatal efferents is postulated to preserve motor control during the presymptomatic phase, but the neuroadaptation responsible for normalization is not completely understood. In particular, the prevailing view that surviving dopaminergic neurons compensate by up-regulating release has been difficult to demonstrate directly. Here we investigate dopaminergic neurotransmission in the hemiparkinsonian rat using fast-scan cyclic voltammetry at carbon-fiber microelectrodes. Electrical stimulation was used to elicit extracellular dopamine levels mimicking the steady-state dynamics of tonic dopaminergic signaling. In agreement with microdialysis studies, evoked steady-state dopamine levels remained constant over the entire lesion spectrum (0 to approximately 85%) observed during the presymptomatic stage. Kinetic analysis of the voltammetric recordings demonstrated that evoked dopamine concentrations were normalized without plasticity of dopamine release and uptake, suggesting that the primary mechanisms controlling ambient levels of extracellular dopamine were not actively altered. In the present study, we formalize this neuroadaptation as "passive stabilization" . We further propose that passive stabilization is mediated by the simple physical principles of diffusion and steady state, is predicated on extrasynaptic transmission, and forms the basis for a new compensation model of preclinical parkinsonism.

Dopamine responsiveness to drugs of abuse: A shell-core investigation in the nucleus accumbens of the mouse

The existence of subterritories within the nucleus accumbens has now been widely supported by histochemical, neurochemical, electrophysiological, as well as morphological and ultrastructural studies and suggest specific afferent and efferent systems involved in different behavioral aspects. Microdialysis studies in the rat have consistently shown that most drugs of abuse increase extracellular dopamine levels preferentially in the shell subregion of the nucleus accumbens. The study of the relative roles of NAc subregions may considerably help our understanding of the neurobiological basis of drug addiction. Accordingly, the aim of the present work was to extend the outcome of rat studies to the mouse species. Five major drugs of abuse were systemically and acutely administered to mice with a microdialysis probe implanted in either the shell or the core. A statistical comparison was performed on data transformed as percentage values of baseline dopamine vs. logarithmic values with baseline dopamine as a covariate. Results show a significant increase in dopamine levels in both the shell and core subregions following cocaine, amphetamine, nicotine, ethanol, and morphine treatments. A difference between shell and core after cocaine, nicotine, and morphine was evident when data were analyzed as percent values of baseline. However, such a shell-core dichotomy became no longer significant when ANOVA was applied on the statistically more appropriate logarithmic transformation of data with baseline as a covariate. The significant baseline differences among groups of mice (dopamine levels in the shell significantly lower compared with dopamine levels in the core) may have compromised, at least in part, the statistical procedure usually applied in microdialysis studies. These findings suggest that a careful evaluation of the data is required when subtle changes in extracellular levels of DA are measured.

Rapid regulation of dopamine transporter function by substrates, blockers and presynaptic receptor ligands

The extracellular actions of dopamine are terminated primarily through its binding to dopamine transporters and translocation back into dopamine neurons. The transporter thereby serves as an optimal target to regulate dopamine neurotransmission. Although acute pharmacological blockade of dopamine transporters is known to reversibly inhibit transporter function by preventing the binding of its endogenous substrate dopamine, it recently has become clear that dopamine transporter substrates, such as amphetamines, and blockers, such as cocaine, also have the ability to rapidly and persistently regulate transporter function after their direct pharmacological effect has subsided. Presynaptic receptor ligands can also regulate dopamine transporter function. This has been investigated most extensively for dopamine D2 receptors, but there is also evidence for regulation by gamma-aminobutyric acid (GABA) GABAB receptors, metabotropic glutamate, nicotinic acetylcholine, serotonin, sigma2- and kappa-opioid receptors. The focus of this review is the rapid, typically reversible, regulation of dopamine transporter velocity by substrates, blockers and presynaptic receptor ligands. The research discussed here suggests that a common mechanism through which these different classes of compounds regulate transporter activity is by altering the cell surface expression of dopamine transporters.

Identification of dopamine transporter in bovine pineal gland using [3H]GBR 12935

The mammalian pineal gland contains several neurotransmitters and receptors for amino acids, biogenic amines, and peptides. Some of these, such as D1 and D2 dopamine receptors, have been previously identified and characterized in the bovine pineal gland by our group. As a matter of fact, the density of D1 dopamine receptors in the pineal gland is higher than that of corpus striatum, suggesting that this organ must possess a high affinity dopamine transporter, which has been identified in this study by using [3H]GBR 12935 as a radiological ligand and nomifensine to determine non-specific binding. The association rate of [3H]GBR 12935 binding to the pineal membrane was examined as a function of time. The binding reached equilibrium within 45 min of incubation at 25 degrees C. The specific binding was reversible and saturable. The dissociation time course of the specific [3H]GBR 12935 binding from the bovine pineal membrane was also studied. A half-life (t1/2) of 14-min was obtained. The saturation analysis of the [3H]GBR 12935 binding revealed a dissociation equilibrium constant (Kd) of 6.0 +/- 0.9 nm and a receptor density (Bmax) of 6.9 +/- 0.3 pmol/mg protein, which were comparable with those values obtained from bovine striatum and frontal cortex. In competitive experiments, the concentrations of drugs required to inhibit 50% of the binding (IC50) were in descending order GBR 12909 > GBR 12935 > trans-flupenthixol > nomifensine > cis-flupenthixol > amitriptyline > imipramine > desipramine > dopamine > fluoxetine > fuvoxamine > d-amphetamine. However, nisoxetine, SCH 23390, norepinephrine, and serotonin were unable to displace [3H]GBR binding. These results show that drugs capable of blocking dopamine transporters were effective in displacing [3H]GBR binding; whereas specific norepinephrine and serotonin transporter inhibitors were less effective or ineffective. In addition, the dopamine transporter is ion-dependent as sodium increased [3H]GBR binding in a concentration related manner. These results indicate that a high affinity dopamine transporter exists in the bovine pineal, which may exhibit circadian periodicity, and whose physiological functions need to be delineated and characterized in future investigations.

Modeling fast dopamine neurotransmission in the nucleus accumbens during behavior

Recent advances in electrophysiology and voltammetry permit monitoring of dopamine (DA) neuronal activity in real time in the brain of awake animals. Studies using these approaches demonstrate that behaviorally relevant events elicit characteristic patterns of electrical activity in midbrain DA neurons as well as large, transient changes in extracellular DA in the nucleus accumbens (NAc). In addition to providing insight into the role of the DA system in the processing of motor, motivational, and sensory information, the new findings also shed light on fast DA neurotransmission in a behavioral context. This report, (1). summarizes the information obtained by electrophysiological and real-time voltammetric approaches and (2). describes a general model of phasic DA signaling in the NAc that links the observed changes in DA electrical activity and extracellular dynamics. The analysis demonstrates that the behaviorally evoked DA transients are governed by similar mechanisms as those produced by short trains of electrical stimulation. Thus, action potential-dependent release and presynaptic uptake are primary determinants of functional DA levels in the brain during behavior. Interestingly, the model predicts that the same burst of electrical activity generated at DA cell bodies produces markedly different DA dynamics in forebrain projection fields. The distinct changes result from heterogeneous release and uptake rates and may underlie region-specific effects of DA. Auto- and heteroreceptors, as well as other sites of presynaptic control, could further modulate the DA transients.

Nucleus accumbens shell and core dopamine: differential role in behavior and addiction

Drug addiction can be conceptualized as a disturbance of behavior motivated by drug-conditioned incentives. This abnormality has been explained by Incentive-Sensitization and Allostatic-Counteradaptive theories as the result of non-associative mechanisms acting at the stage of the expression of incentive motivation and responding for drug reinforcement. Each one of these theories, however, does not account per se for two basic properties of the motivational disturbance of drug addiction: (1). focussing on drug- at the expenses of non-drug-incentives; (2). virtual irreversibility. To account for the above aspects we have proposed an associative learning hypothesis. According to this hypothesis the basic disturbance of drug addiction takes place at the stage of acquisition of motivation and in particular of Pavlovian incentive learning. Drugs share with non-drug rewards the property of stimulating dopamine (DA) transmission in the nucleus accumbens shell but this effect does not undergo habituation upon repeated drug exposure, as instead is the case of non-drug rewards. Repetitive, non-decremental stimulation of DA transmission by drugs in the nucleus accumbens septi (NAc) shell abnormally strengthens stimulus-drug associations. Thus, stimuli contingent upon drug reward acquire powerful incentive properties after a relatively limited number of predictive associations with the drug and become particularly resistant to extinction. Non-contingent occurrence of drug-conditioned incentive cues or contexts strongly facilitates and eventually reinstates drug self-administration. Repeated drug exposure also induces a process of sensitization of drug-induced stimulation of DA transmission in the NAc core. The precise significance of this adaptive change for the mechanism of drug addiction is unclear given the complexity and uncertainties surrounding the role of NAc core DA in responding but might be more directly related to instrumental performance.

Brief, repeated exposure to substrates down-regulates dopamine transporter function in Xenopus oocytes in vitro and rat dorsal striatum in vivo

In heterologous expression systems, dopamine transporter (DAT) cell-surface localization is reduced after relatively prolonged exposure to d-amphetamine (AMPH) or dopamine (DA), suggesting a role for substrate-mediated regulation of transporter function. Here, we investigated whether brief, repeated periods of substrate exposure modulated transporter function, first, in an in vitro model system and, second, in intact rat brain. In human DAT-expressing Xenopus laevis oocytes, repeated exposure to low micromolar concentrations of DA, AMPH or tyramine markedly reduced transport-mediated currents. This functional down-regulation was attenuated by inclusion of a protein kinase C (PKC) inhibitor and probably reflects DAT redistribution, as cell-surface [3H]WIN 35 428 binding was significantly lower following DA exposure. High-speed chronoamperometry was used to measure clearance of exogenously applied DA in dorsal striatum (STR) and nucleus accumbens (NAc) of anesthetized rats. In STR, frequent (every 2 min) applications of DA altered DA clearance parameters in a manner consistent with profound down-regulation of DAT function. Similar changes were not observed in NAc or after repeated vehicle (ascorbic acid) application. Together, our results suggest that brief, repeated periods of substrate exposure lead to rapid down-regulation of DAT activity and that this type of regulation can occur in vivo in STR, but not NAc.

Dopamine release in the dorsal striatum during cocaine-seeking behavior under the control of a drug-associated cue

Compulsive drug use is characterized by a pattern of drug seeking and consumption that becomes progressively habitual and less and less modifiable by external and internal factors. Although traditional views would posit that nigrostriatal dopamine (DA) neurons originating in the substantia nigra and innervating the dorsal striatum are primarily concerned with motor functions, recent studies have implicated the dorsal striatum in mediating stimulus-response (habit) learning. In this study, in vivo microdialysis in combination with a second-order schedule of cocaine reinforcement was used to investigate the role of the dorsal striatal dopamine innervation in well established drug-seeking behavior under the control of a drug-associated cue [light conditioned stimulus (CS+)]. Rats were initially trained to self-administer cocaine under a continuous reinforcement schedule where a response on one of two identical levers led to a 20 sec presentation of a light CS+ and an intravenous cocaine infusion (0.75 mg/kg). The response requirement for the CS+ and cocaine was then progressively increased until stable responding was established under a second-order schedule of reinforcement. During microdialysis, rats were presented with the cocaine-associated CS+ either noncontingently or contingent on responding during a session of cocaine-seeking behavior. The results showed a marked increase in DA release in the dorsal striatum during drug-seeking, when cocaine cues were presented contingently, but not when the same cue was presented noncontingently. These data indicate a possible involvement of the dopaminergic innervation of the dorsal striatum in well established, or habitual, cocaine-seeking behavior.

High-resolution phosphor imaging: validation for use with human brain tissue sections to determine the affinity and density of radioligand binding

This study investigated the suitability of high-resolution storage phosphor imaging for the quantitative analysis of radioligand binding to human brain tissue. Hence, the binding of [(3)H]mazindol to the dopamine transporter in caudate-putamen tissue homogenates or frozen tissue sections apposed to either autoradiographic film or phosphor imaging plates was measured. Estimates of binding affinity were similar for homogenate studies and phosphor imaging plates (Kd=6.44+/-0.14 and 6.91+/-0.47 nM, respectively), but higher values were obtained with film autoradiography (Kd=11.31+/-0.82 nM). The density of binding was similar for both autoradiographic techniques (Bmax=371.9+/-30.8 fmol/mg estimated tissue equivalent, ETE (imaging plate) and 425+/-13.77 fmol/mg ETE (film)), although lower values were obtained from tissue homogenates (Bmax=64.27+/-6.74 fmol/mg wet weight). These results suggest that high resolution phosphor imaging can be used to analyse radioligand binding parameters in human brain tissue. Moreover, the reduced exposure time of phosphor imaging plates (e.g. 7 days vs 5 weeks) allows results to be obtained more rapidly than with conventional film autoradiography.

Determination of release and uptake parameters from electrically evoked dopamine dynamics measured by real-time voltammetry

Quantifying mechanisms underlying extracellular signaling by the neurotransmitter dopamine (DA) is a difficult task, particularly in the complex extracellular microenvironment of the intact brain. In this study, two methods for evaluating release and uptake from DA dynamics monitored by real-time voltammetry are described. Both are based on a neurochemical model characterizing electrically evoked levels of DA as a balance between these opposing mechanisms. The theoretical basis of what is called here nonlinear regression and single curve analyses is given. Fitting simulated data tests the reliability of the methods. The two analyses are also compared with an experimental data set describing the effects of pharmacologically inhibiting the DA transporter in the caudate-putamen (CP) and nucleus accumbens (NAc). The results indicate that nonlinear regression and single curve analyses are suitable for quantifying release and uptake mechanisms underlying DA neurotransmission. Additionally, the most important experimental finding of this technical study was the independent confirmation of high affinity (approximately 0.2 microM) DA uptake in the intact striatum.

Involvement of the rat caudate nucleus in the immunostimulatory effect of DAGO

The involvement of the caudate nucleus, i.e., the terminal zone of the nigrostriatal dopaminergic system, in neuroimmunostimulation during the activation of mu opioid receptors by the highly specific agonist DAGO. Single doses of DAGO (100 microg/kg) in sham-operated control Wistar rats induced significant increases in the numbers of direct IgM-antibody-forming and total rosette-forming cells at the peak of the immune response after immunization with sheet red blood cells. The experiments showed that bilateral electrolytic lesioning of the caudate nucleus in rats suppressed the immune response, demonstrating its involvement in neuroimmunomodulation. Since the effect of immunostimulation induced by DAGO disappeared when given to animals with caudate nucleus lesions, it was concluded that this structure is involved in activatory immunogenesis via mu opioid mechanisms.

Partial, graded losses of dopamine terminals in the rat caudate-putamen: an animal model for the study of compensatory adaptation in preclinical parkinsonism

Procedures to lesion dopamine (DA) neurons innervating the rat caudate-putamen (CP) in a partial, graded fashion are described in this study. The goal is to provide a lesion model that supports intra-animal comparisons of voltammetric recordings used to investigate compensatory adaptation of DA neurotransmission. Lesions exploited the topography of mesostriatal DA neurons, microinjections of the neurotoxin 6-hydroxydopamine (6-OHDA) into the medial and lateral edges of the ventral mesencephalon containing DA cell bodies and microdissection of the CP into six regions. Analysis of tissue DA content in these regions by HPLC-EC demonstrated that 6-OHDA injected into the lateral substantia nigra results in a significantly greater loss of DA in lateral versus medial regions of the CP. The direction of the graded loss of DA was reversed (i.e. a medial to lateral lesion gradient) by the injection of 6-OHDA into the ventral tegmental area near the medial SN. Extracellular concentrations of electrically evoked DA could be measured across the mediolateral axis of the CP in a single animal using the technique of in vivo voltammetry. More importantly, graded decreases in the amplitude of evoked DA levels generally followed the direction of the tissue DA gradient in lesioned animals. These results suggest that the graded loss of DA terminals in the CP, coupled to a spatially and temporally resolved technique for monitoring extracellular DA, is a viable tool for investigating compensatory adaptation in the mesostriatal DA system.

Haloperidol downregulates phospholipase A(2) signaling in rat basal ganglia circuits

Our laboratory has developed an in vivo method to quantitatively evaluate phospholipase A(2) (PLA(2))-mediated signal transduction in brain regions of rodents. In this method, quantitative autoradiography is used to identify brain uptake of intravenously injected, radiolabeled arachidonic acid ([3H]AA). Dopamine D(2) receptors are coupled to G-proteins that activate PLA(2), releasing AA from the stereospecifically numbered (sn) 2 position of phospholipids, and regional [3H]AA uptake is proportional to the rate of release. In the present experiment, the D(2) antagonist haloperidol (1.0 mg/kg i.p.) or the drug vehicle was administered to male adult rats for 21 days. Rats were infused 3 days later with 1.75 mCi/kg [3H]AA (i.v.), anesthetized and decapitated 20 min after infusion onset, and brains were processed for quantitative autoradiography. Chronic haloperidol significantly decreased [3H]AA incorporation in two primary dopaminergic basal ganglia-frontal cortex circuits, the mesocorticolimbic and nigrostriatal systems, while insignificant changes in AA incorporation were noted in other brain regions. These results suggest that one mechanism by which haloperidol exerts its effect is by downregulating D(2)-mediated PLA(2) signaling involving AA release in basal ganglia-frontal cortex circuitry.

Dopamine release and uptake dynamics within nonhuman primate striatum in vitro

The putamen of the human striatum is a heterogeneous nucleus that contains the primary site of loss of dopamine (DA) in Parkinson's disease (PD). Furthermore, different functional domains of the putamen are heterogeneously susceptible to DA loss, and yet the dynamic regulation of extracellular DA concentration ([DA](o)) and comparison between domains has not been explored in the primate brain. In these studies, DA was measured in real time using fast-scan cyclic voltammetry at a carbon-fiber microelectrode in vitro in striatal sections from the common marmoset (Callithrix jacchus). [DA](o) released by a single stimulus pulse varied threefold along a ventromedial-dorsolateral axis. DA uptake was via the DA transporter (GBR12909 sensitive, desipramine insensitive). On the basis of data modeling with simulations of Michaelis-Menten kinetics, rate maximum, V(max), varied with region: both [DA](o) and V(max) were greatest in regions most vulnerable in PD. These differences were reflected in part by regional variation in DA content. [DA](o), V(max), and regional variation were two- to threefold greater than in rodent caudatoputamen. In addition, steady-state [DA](o) at physiological firing rates in primate striatum was controlled by depolarization frequency, uptake, and presynaptic autoreceptors. Furthermore, regulation of [DA](o) by these mechanisms differed significantly between limbic- and motor-associated domains. These data indicate interspecies heterogeneity in striatal DA dynamics that must be considered when extrapolating behavioral and drug responses from rodent to the primate brain. Moreover, the heterogeneity demonstrated within the primate putamen in the availability and dynamic regulation of DA may be central to understanding DA function in health, cocaine abuse, and disease.

Differential behavioural and neurochemical effects of para-methoxyamphetamine and 3,4-methylenedioxymethamphetamine in the rat

1. This study was prompted by recent deaths that have occurred after recreational administration of the substituted amphetamine para-methoxyamphetamine (PMA). Because relatively little is known regarding its mechanism(s) of action, its effects on physiological, behavioural and neurochemical parameters were compared with the well known effects of 3,4-methylenedioxymethamphetamine (MDMA). 2. Equivalent doses of PMA (5-20 mg/kg) produced greater hypothermia than MDMA at an ambient temperature of 20 degrees C. At 30 degrees C, PMA continued to evoke hypothermia except the highest dose where hyperthermia ensued. MDMA altered body temperature only at the highest dose where hyperthermia also resulted. 3. At both 20 and 30 degrees C, MDMA stimulated locomotor activity whereas PMA had modest effects and then, only at high doses. 4. In vivo chronoamperometry was used to measure the effect of MDMA and PMA on release, and inhibition of uptake, of serotonin (5-HT) and dopamine (DA) in the dorsal striatum of anaesthetised rats. As expected, MDMA evoked release of DA and inhibited uptake of both DA and 5-HT. By contrast, PMA was a relatively weak releasing agent and did not inhibit DA uptake. However, PMA potently inhibited uptake of 5-HT. 5. Taken together these data suggest that the acute adverse effects of PMA are more likely to be associated with alterations in serotonergic rather than dopaminergic neurotransmission.

Region-specific enhancement of basal extracellular and cocaine-evoked dopamine levels following constitutive deletion of the Serotonin(1B) receptor

The behavioral effects of cocaine are enhanced following constitutive deletion of the serotonin(1B) receptor. The neural substrates mediating the enhanced response to cocaine are unknown. The present studies determined whether basal dopamine dynamics or cocaine-evoked dopamine levels are altered in projection areas of mesostriatal or mesoaccumbens dopamine neurons following serotonin(1B) receptor deletion. Male wild-type and serotonin(1B) knockout mice were implanted with microdialysis guide cannulas aimed at the dorsal striatum or nucleus accumbens. The zero net flux method of quantitative microdialysis was used to quantify basal extracellular dopamine concentrations (DA(ext)) and the extraction fraction of dopamine (E(d)), which provides an index of dopamine uptake. Conventional microdialysis techniques were used to quantify cocaine (0, 5.0, and 20.0 mg/kg)-evoked dopamine overflow. Basal DA(ext) and E(d) did not differ in striatum of wild-type and knockout mice. Similarly, cocaine-stimulated dopamine overflow did not differ between genotype. The basal E(d) did not differ in the nucleus accumbens of wild-type and knockout mice. However, DA(ext) was significantly elevated in the nucleus accumbens of knockout mice. Cocaine-evoked dopamine overflow (nM) was also enhanced in the nucleus accumbens of knockout mice. However, the cocaine-induced increase in dopamine levels, relative to basal values, did not differ between genotype. These data demonstrate that deletion of the serotonin(1B) receptor is associated with increases in basal DA(ext) in the nucleus accumbens. This increase is not associated with an alteration in E(d), suggesting increased basal dopamine release in these animals. It is hypothesized that these alterations in presynaptic neuronal activity are a compensatory response to constitutive deletion of the serotonin(1B) receptor and may contribute to the enhanced behavioral effects of psychostimulants observed in knockout mice.

Regulation of quantal size by presynaptic mechanisms

Quantal size is often modeled as invariant, although it is now well established that the number of transmitter molecules released per synaptic vesicle during exocytosis can be modulated in central and peripheral synapses. In this review, we suggest why presynaptically altered quantal size would be important at social synapses that provide extrasynaptic neurotransmitter. Current techniques used to measure quantal size are reviewed with particular attention to amperometry, the first approach to provide direct measurement of the number of molecules and kinetics of presynaptic quantal release, and to CNS dopamine neuronal terminals. The known interventions that alter quantal size at the presynaptic locus are reviewed and categorized as (1) alteration of transvesicular free energy gradients, (2) modulation of vesicle transmitter transporter activity, (3) modulation of fusion pore kinetics, (4) altered transmitter degranulation, and (5) changes in synaptic vesicle volume. Modulation of the number of molecules released per quantum underlies mechanisms of drug action of L-DOPA and the amphetamines, and seems likely to be involved in both normal synaptic modification and disease states. Statistical analysis for examining quantal size and data presentation is discussed. We include detailed information on performing nonparametric resampling statistical analysis, the Kolmogorov-Smirnov test for two populations, and random walk simulations using spreadsheet programs.

Region-specific changes of NOS-IR cells in the basal ganglia of the aged rat

Nitric oxide (NO) is a free radical postulated to act as a neurotransmitter, neuromodulator, or second messenger molecule in the central nervous system. Several findings suggest that NO production may be decreased in the aged rats. In the present study, we investigated regional discrepancies in changes with aging in the number of nitric oxide synthase-immunoreactive (NOS-IR) cells in the basal ganglia of the aged rat by immunocytochemistry. The number of NOS-IR neurons in the striatum and substantia innominata of the aged rat decreased. In contrast, the number of NOS-IR neurons in the subthalamic nucleus increased in the aged rat. On the other hand, the number of NOS-IR neurons in the nucleus accumbens and olfactory tubercle did not change. Taken together, important functional changes can be caused by the region-specific changes of NOS-IR neurons in the basal ganglia with aging.

Functional heterogeneity in dopamine release and in the expression of Fos-like proteins within the rat striatal complex

The dorsolateral striatum, and the core and shell of the nucleus accumbens are three major anatomical regions of the striatal complex. The shell is considered as a part of the extended amygdala, and is involved in the control of motivation and reward. The core and the striatum are considered central to sensory motor integration. In this study we compared the responses of these three regions to mild stress and drugs of abuse by measuring extracellular dopamine (DA) concentrations and Fos-like immunoreactivity (Fos-LI). The results are summarrized as follows. (i) In unchallenged conditions, extracellular DA concentrations were highest in the dorsolateral striatum and lowest in the core, whereas Fos-LI was highest in the shell and lowest in the dorsolateral striatum. (ii) After challenges that increase DA by depolarizing DAergic neurons (injection stress or 2 mg/kg morphine), the shell presented the largest increase in DA levels and Fos-LI. (iii) After the administration of a DA-uptake blocker (15 mg/kg cocaine), the percentage increase in DA was still largest in the shell. However, the absolute increase in DA and Fos-LI in the shell and the dorsolateral striatum were similar. (iv) After a full D1 agonist (SKF82958), Fos-LI was highest in the shell and lowest in the dorsolateral striatum. In conclusion, the nucleus accumbens shell seems to be the area of the striatal complex most functionally reactive to stress and drugs of abuse. However, the dorsolateral striatum and the core appear functionally distinct, as for most of the parameters studied these two regions differed.

Motor activation by amphetamine infusion into nucleus accumbens core and shell subregions of rats differentially sensitive to dopaminergic drugs

Selective breeding based on activity in a swim test has been used to produce lines of rats that show a high level of activity in the swim test (Swim High-active (SwHi) rats) and a low level of activity in the swim test (Swim Low-active (SwLo) rats). Previous studies have indicated that dopamine (DA) function is enhanced in SwHi rats and reduced in SwLo rats; a principal finding was that SwLo rats showed much smaller increases in ambulatory activity after systemic administration of amphetamine than did SwHi or non-selected rats. In light of the importance of the nucleus accumbens (NAC) in amphetamine-induced activity, the present study investigated whether DA function in NAC differs in SwHi and SwLo rats. Amphetamine was infused bilaterally into either the core or shell subregion of NAC, and ambulation or swim test activity was then measured. In SwLo rats, infusion of amphetamine (0.2-2.0 microg) into either NAC core or shell produced moderate increases in ambulation. In SwHi rats, infusion of amphetamine into NAC shell produced similar moderate increases in ambulation, but infusion into the core produced markedly larger dose-related increases in ambulation. In the swim test, infusion of amphetamine (1.0 microg) increased activity by affecting the dominant behavior of each line; i.e. struggling increased in SwHi rats and floating decreased in SwLo rats, with large effects seen in both lines with infusion into either NAC core or shell. These results support the idea that the distinct behavioral characteristics of SwHi and SwLo rats are mediated in part by differences in NAC-DA function.

Behavioral neurochemistry reveals a new functional dichotomy in the shell subregion of the nucleus accumbens

1. The behavioral and neurochemical effects produced by the direct infusion of amphetamine by reverse microdialysis into either the core or shell of the nucleus accumbens were studied across the anteroposterior axis of this nucleus. 2. Amphetamine (0.05; 0.10; 0.50; 1.00 microM) produced a dose-dependent increase in locomotor activity after microinfusion into either the rostral shell, caudal shell or core of the nucleus accumbens. However, the amphetamine-induced locomotor activating effect, was significantly higher in the rostral shell of the nucleus accumbens compared with both the caudal shell and core. 3. The lowest concentrations of amphetamine produced an equipotent decrease in dialysate dopamine in either the rostral shell, caudal shell, or core. At 1.0 microM, however, amphetamine selectively increased dopamine in the rostral shell. In contrast, the highest dose of amphetamine significantly increased dialysate serotonin levels over baseline only in the caudal shell of the nucleus accumbens. 4. These results demonstrate the preferential effect of amphetamine on dopamine in the rostral shell and serotonin in the caudal shell subterritory of the nucleus accumbens.

Differential alterations in basal and D-amphetamine-induced behavioural pattern following 6-OHDA or ibotenic acid lesions into the dorsal striatum

It is well known that the corpus striatum is related to the sterotyped activation induced by several psychostimulants. In this study we analyzed the effects of 6-OHDA, in comparison with those of ibotenic acid lesions, into the dorsal striatum, on the behavioural pattern induced by saline or D-amphetamine treatment. A computerized technique for recording the animal motor activity was developed in order to define in a detailed way the behavioural profile in lesioned and sham-operated rats induced by the saline or D-amphetamine treatment. A 6-OHDA lesion into the dorsal striatum modified the basal behavioural pattern which was mainly characterized by reduced motor activation while ibotenic acid lesion affected the structure of the basal behavioral pattern. D-Amphetamine administration in 6-OHDA lesioned rats induced a behavioural stimulation, but a decreased motor and stereotyped activation was observed compared to the sham-operated animals treated with D-amphetamine. In contrast, D-amphetamine administration in the ibotenic acid-lesioned rats induced a motor and stereotyped activity which was not reduced compared to that seen after D-amphetamine treatment in sham-operated rats. These results suggest that these two types of lesion induced differential effects on the behavioural pattern either after saline or after D-amphetamine administration. Dopaminergic neurotransmission in the dorsal striatum plays a permissive role on the emergence of the behavioural responses, while the dorsal striatum circuitry plays a crucial role on the organization of the behavioural pattern. In addition, dopaminergic activity in this structure serves a primary control in the D-amphetamine-elicited motor activation or stereotypy, while the striatal structure is involved in the shaping of the D-amphetamine behavioural pattern.