Phasic dopamine (DA) signaling, during which burst firing by DA neurons generates short-lived elevations in extracellular DA in terminal fields called DA transients, is implicated in reinforcement learning. Disrupted phasic DA signaling is proposed to link DA depletions and cognitive-behavioral impairment in methamphetamine (METH)-induced neurotoxicity. Here, we further investigated this disruption by assessing effects of METH pretreatment on DA transients elicited by a drug cocktail of raclopride, a D2 DA receptor antagonist, and nomifensine, an inhibitor of the dopamine transporter (DAT). One advantage of this approach is that pharmacological activation provides a large, high-quality data set of transients elicited by endogenous burst firing of DA neurons for analysis of regional differences and neurotoxicity. These pharmacologically evoked DA transients were measured in the dorsomedial (DM) and dorsolateral (DL) striatum of urethane-anesthetized rats by fast-scan cyclic voltammetry. Electrically evoked DA levels were also recorded to quantify DA release and uptake, and DAT binding was determined by means of autoradiography to index DA denervation. Pharmacologically evoked DA transients in intact animals exhibited a greater amplitude and frequency and shorter duration in the DM compared to the DL striatum, despite similar pre- and post-drug assessments of DA release and uptake in both sub-regions as determined from the electrically evoked DA signals. METH pretreatment reduced transient activity. The most prominent effect of METH pretreatment on transients across striatal sub-region was decreased amplitude, which mirrored decreased DAT binding and was accompanied by decreased DA release. Overall, these results identify marked intrastriatal differences in the activity of DA transients that appear independent of presynaptic mechanisms for DA release and uptake and further support disrupted phasic DA signaling mediated by decreased DA release in rats with METH-induced neurotoxicity.
[Show abstract][Hide abstract] ABSTRACT: The dopamine-containing projection from the ventral tegmental area of the midbrain to the nucleus accumbens is critically involved in mediating the reinforcing properties of cocaine. Although neurons in this area respond to rewards on a subsecond timescale, neurochemical studies have only addressed the role of dopamine in drug addiction by examining changes in the tonic (minute-to-minute) levels of extracellular dopamine. To investigate the role of phasic (subsecond) dopamine signalling, we measured dopamine every 100 ms in the nucleus accumbens using electrochemical technology. Rapid changes in extracellular dopamine concentration were observed at key aspects of drug-taking behaviour in rats. Before lever presses for cocaine, there was an increase in dopamine that coincided with the initiation of drug-seeking behaviours. Notably, these behaviours could be reproduced by electrically evoking dopamine release on this timescale. After lever presses, there were further increases in dopamine concentration at the concurrent presentation of cocaine-related cues. These cues alone also elicited similar, rapid dopamine signalling, but only in animals where they had previously been paired to cocaine delivery. These findings reveal an unprecedented role for dopamine in the regulation of drug taking in real time.
[Show abstract][Hide abstract] ABSTRACT: 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.
Journal of Neurochemistry 01/2004; 87(5):1224-36. DOI:10.1046/j.1471-4159.2003.02104.x · 4.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The fundamental process that underlies volume transmission in the brain is the extracellular diffusion of neurotransmitters from release sites to distal target cells. Dopaminergic neurons display a range of activity states, from low-frequency tonic firing to bursts of high-frequency action potentials (phasic firing). However, it is not clear how this activity affects volume transmission on a subsecond time scale. To evaluate this, we developed a finite-difference model that predicts the lifetime and diffusion of dopamine in brain tissue. We first used this model to decode in vivo amperometric measurements of electrically evoked dopamine, and obtained rate constants for release and uptake as well as the extent of diffusion. Accurate predictions were made under a variety of conditions including different regions, different stimulation parameters and with uptake inhibited. Second, we used the decoded rate constants to predict how heterogeneity of dopamine release and uptake sites would affect dopamine concentration fluctuations during different activity states in the absence of an electrode. These simulations show that synchronous phasic firing can produce spatially and temporally heterogeneous concentration profiles whereas asynchronous tonic firing elicits uniform, steady-state dopamine concentrations.
Journal of Neurochemistry 04/2004; 87(5):1284-95. DOI:10.1046/j.1471-4159.2003.02109.x · 4.28 Impact Factor
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