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

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

Dual modes of extracellular serotonin changes in the rat ventral striatum modulate adaptation to a social stress environment, studied with wireless voltammetry.

The role of serotonin in stress and anxiety, particularly in social environments, is not well understood. Selective serotonin reuptake inhibitors are useful for patients that experience social anxiety; however, their mechanism of action has not been fully characterized. Dopamine is known to operate in different temporal modes (fast phasic, intermediate, and tonic changes). We hypothesized that serotonin may also operate in temporal modes in the context of social stress. We used wireless voltammetry (4 Hz) to investigate changes in extracellular ventral striatal serotonin and dopamine during a test of repeated social interactions between two rats. Test rats (electrode-implanted; n = 5) and counter rats (n = 6) were placed in separate sections of a partitioned box. The partitions were raised to allow interactions for 10 min; four sessions were repeated at 10-min intervals. In the first session, serotonin increased gradually, then peaked at approximately the end of the interaction, and decreased rapidly between sessions. This slow phasic increase in serotonin diminished gradually (but significantly) in subsequent interactions. Test rats received active, one-sided contacts (chasing, walking-over, and occasional attacking behavior) from counter rats. Changes in contact times were not correlated with changes in phasic serotonin increases. Dopamine levels did not increase. Citalopram caused significant suppression of slow phasic increases, caused tonic increases in basal serotonin concentrations, and caused active (chasing, all grooming), but not aggressive behavior in test rats. These findings implied that the slow phasic serotonin increase in the ventral striatum induced adaptation to social stress caused by a counter rat; moreover, the tonic increase in serotonin promoted the adaptive change and caused socially dominant behavior.

Impaired in vivo dopamine release in parkin knockout mice.

parkin is the most frequent causative gene among familial Parkinson's disease (PD). Although parkin deficiency induces autosomal recessive juvenile parkinsonism (AR-JP, PARK2) in humans, parkin knockout (PKO) mice consistently show few signs of dopaminergic degeneration. We aimed to directly measure evoked extracellular dopamine (DA) overflow in the striatum with in vivo voltammetry. The amplitude of evoked DA overflow was low in PKO mice. The half-life time of evoked DA overflow was long in PKO mice suggesting lower release and uptake of dopamine. Facilitation of DA overflow by repetitive stimulation enhanced in the older PKO mice. Decreased dopamine release and uptake in young PKO mice suggest early pre-symptomatic changes in dopamine neurotransmission, while the enhanced facilitation in the older PKO mice may reflect a compensatory adaptation in dopamine function during the late pre-symptomatic phase of Parkinson's disease. Our results showed parkin deficiency may affect DA release in PKO mice, although it does not cause massive nigral degeneration or parkinsonian symptoms as in humans.

Wireless voltammetry recording in unanesthetised behaving rats.

In vivo voltammetry is a valuable technique for rapid measurement of dopamine in the brain of freely behaving rats. Using a conventional voltammetry system, however, behavioural freedom is restricted by cables connecting the head assembly to the measurement system. To overcome these difficulties, we developed a wireless voltammetry system utilizing radio waves. This system consisted of a potentiostat and transmitter system that was mounted on the back of the rat, and a receiver and analysis system. A single-step pulse (100-250 mV) was applied at 4 Hz after an activation pulse to a carbon fibre recording electrode (diameter: 7 microm). Measurement of dopamine (detection limit: 2.7 x 10(-7)M) was demonstrated in vitro. In vivo experiment was performed at least 1 week after the recording electrode was implanted in the rat striatum. Administration of 2-phenylethylamine to rats increased dopamine signal current, which was consistent with the result in the microdialysis measurement. During a resident-intruder test, dopamine signal current in a resident rat increased upon introduction of an intruder rat. These results show that the present wireless system is useful for a long-term measurement of dopamine in behaving rats.

Fabrication, characterization, and application of boron-doped diamond microelectrodes for in vivo dopamine detection.

Highly boron-doped diamond (BDD) was deposited on chemically etched micrometer-sized tungsten wires using microwave plasma assisted chemical vapor deposition (MPCVD), and these were used to fabricate BDD microelectrodes. BDD microelectrodes with very small diameter (about 5 microm) and 250 microm in length could be made successfully. In addition to the unique properties of BDD electrodes, such as a very low background current, high stability, and selective oxidation of dopamine (DA) in the presence of ascorbic acid (AA), other superior properties of the microelectrodes, including a constant current response, an increase in the mass transport, and the ability for use in high resistance media were also shown. An application study was conducted for in vivo detection of DA in mouse brain, where the BDD microelectrode was inserted into the corpus striatum of the mouse brain. A clear signal current response following medial forebrain bundle (MFB) stimulation could be obtained with high sensitivity. Excellent stability was achieved, indicating that the BDD microelectrodes are very promising for future in vivo electroanalysis.

Striatal dopamine release in the rat during a cued lever-press task for food reward and the development of changes over time measured using high-speed voltammetry.

Substantia nigra dopamine neuronal activity in the primate is thought to be related to the error in predicting reward delivery. Dopamine release in rat nucleus accumbens has been shown to increase in relation to drug/food-seeking behaviour. It is not known how the release of dopamine in the striatum corresponds to the many distinct steps of a rewarded, cued task (e.g. recognizing the cue, executing the behaviour, anticipating the reward, receiving the reward) and how dopamine release then changes over time as task performance improves. To investigate dopamine release during a rewarded, cued task and the development of changes in dopamine release over time, changes in extracellular striatal dopamine concentration during a rewarded, cued lever-press task were measured a few days every week for 5 months using high-speed in vivo voltammetry. Rats were trained to press a lever after a tone to obtain a food reward. The reaction time for the lever press decreased gradually as training continued. Changes in dopamine concentration were measured in the anterior striatum (ventral portion) during the task performance after an initial 6-day familiarization period, in which the animals learned that a lever press yielded food, and a 5-week period for surgery, recovery, and electrode preparation. During the task performance, dopamine concentration started to increase just after the cue, peaked near the time of the lever press, and returned to basal levels 1-2 s after the lever press. This pattern of changes in dopamine concentration was observed over the 5 months of testing, the peak dopamine concentration increasing steadily until peaking at week 7, at which time the task performance had not yet improved significantly from week 2. By week 13, task performance had significantly improved and peak dopamine concentration had begun to subside. Thus, the increase in dopamine concentration after the cue was highest while the task was not yet perfected and subsided toward the end of the learning process. It was concluded that striatal dopamine release during a cued lever-press task is relevant to the novelty of the conditions.

The medial prefrontal cortex mediates 3-methoxytyramine-induced behavioural changes in rat.

L-3,4-Dihydroxyphenylalanine (L-DOPA) remains a common treatment for Parkinson's disease; however, side effects (i.e., dyskinesia and hallucinations) also remain problematic. We recently reported that the dopamine metabolite 3-methoxytyramine causes stereotypy in rats via dopamine receptors, raising the possibility that 3-methoxytyramine is involved in the adverse side effects of chronic L-DOPA treatment. Thus, the present study examined the sites of 3-methoxytyramine action in the rat brain. After intracerebroventricular administration of 3-methoxytyramine, significantly more neurones expressed c-Fos in mesocortico-limbic dopamine areas including frontal cortex, medial prefrontal cortex, parietal cortex, piriform cortex, the nucleus accumbens shell, and ventral tegmental area. 3-Methoxytyramine injection into the medial prefrontal cortex specifically resulted in behavioural changes characteristic of those elicited by the more general intracerebroventricular injection of 3-methoxytyramine. This suggests that the medial prefrontal cortex mediates the 3-methoxytyramine-induced behavioural changes and that a reduction of its action there may alleviate the adverse effects of chronic L-DOPA treatment.

Behavioral activity and stereotypy in rats induced by L-DOPA metabolites: a possible role in the adverse effects of chronic L-DOPA treatment of Parkinson's disease.

L-3,4-Dihydroxyphenylalanine (L-DOPA) is a common and effective treatment for Parkinson's disease, but dyskinesia continues to be a serious adverse effect with chronic use. Evidence suggests that L-DOPA induces increases in dopamine, which then binds to supersensitive dopamine receptors, resulting in dyskinesia. We have shown previously that L-DOPA directly causes stereotypy in rats, suggesting that chronic L-DOPA-induced dyskinesia is also caused by L-DOPA itself. This raises the possibility that other L-DOPA metabolites have a role in dyskinesia. We examined the behavioral effects of five L-DOPA metabolites (3-methoxytyramine, 3-MT; 3,4-dihydroxyphenylalanine, DOPAC; dopamine; homovanillic acid, and 3-o-methyl-DOPA) in rats. A unilateral, intracerebroventricular injection of 3-MT (10-200 microg, 40 microl) over 30 min, dose-dependently increased behavioral activity and stereotypy. This effect was suppressed by the dopamine D1/5-receptor antagonist SCH 23390, but not by the dopamine D2/3/4-receptor antagonist sulpiride. Dopamine denervation resulted in behavioral supersensitivity to 3-MT. Neither dopamine nor DOPAC levels increased in the striatum after 3-MT administration, as measured using in vivo voltammetry. The behavioral changes paralleled a rise in 3-MT in the contralateral striatum. DOPAC also caused behavioral changes and stereotypy, but to a smaller degree than 3-MT. Dopamine-denervated rats did not exhibit a supersensitive response to DOPAC, however. Other L-DOPA metabolites did not cause behavioral effects. These data suggest that 3-MT directly induced dopamine-D1/5-receptor-mediated behavioral changes in rats, and that 3-MT may have a role in dyskinesia due to chronic L-DOPA treatment in Parkinson's disease patients.