Dopamine agonist-induced inhibition of neurotransmitter release from the awake squirrel monkey putamen as measured by microdialysis

Effects of methylphenidate on the aggressive behavior, serotonin and dopamine levels, and dopamine-related gene transcription in brain of male Nile tilapia (Oreochromis niloticus)

The occurrence of pharmaceuticals in the aquatic environment has increased considerably in the last decades, causing negative biochemical, physiological, and behavioral effects in aquatic organisms. In this study, we evaluated the effects of methylphenidate (MPH) on the aggressive behavior, dopamine-related gene transcript levels, monoamine levels, and carboxylesterase transcript levels and activity in the brain of male Nile tilapia (Oreochromis niloticus). Carboxylesterase activity was also measured in the liver and gills. Fish were exposed for 5 days to MPH at 20 and 100 ng L^-1. Fish exposed to 100 ng L^-1 of MPH showed increased aggressiveness and decreased dopamine (DA) and serotonin (5-HT) levels. No changes were observed in plasma testosterone levels and in the transcript levels of D1 and D2 dopamine receptors, dopamine transporter (DAT), and carboxylesterase 2 (CES2). Exposure to 100 ng L^-1 of MPH caused a decrease in the transcript levels of carboxylesterase 3 (CES3) and an increase in tyrosine hydroxylase (TH), while exposure to 20 ng L^-1 of MPH increased the transcript levels of D5 dopamine receptor. Carboxylesterase activity was unchanged in the brain and liver and increased in the gills of fish exposed to 20 ng L^-1. These results indicate that MPH at 100 ng L^-1 increases aggressiveness in Nile tilapia, possibly due to a decrease in 5-HT levels in the brain and alterations in dopamine levels and dopamine-related genes.

Hyperactivity induced by the dopamine D2/D3 receptor agonist quinpirole is attenuated by inhibitors of endocannabinoid degradation in mice

The present study was designed to investigate the effect of pharmacological inhibition of endocannabinoid degradation on behavioural actions of the dopamine D2/D3 receptor agonist quinpirole in male C57Bl/6J mice. In addition, we studied the effects of endocannabinoid degradation inhibition on both cocaine-induced psychomotor activation and behavioural sensitization. We analysed the effects of inhibition of the two main endocannabinoid degradation enzymes: fatty acid amide hydrolase (FAAH), using inhibitor URB597 (1 mg/kg); monoacylglycerol lipase (MAGL), using inhibitor URB602 (10 mg/kg). Administration of quinpirole (1 mg/kg) caused a temporal biphasic response characterized by a first phase of immobility (0-50 min), followed by enhanced locomotion (next 70 min) that was associated with the introduction of stereotyped behaviours (stereotyped jumping and rearing). Pretreatment with both endocannabinoid degradation inhibitors did not affect the hypoactivity actions of quinpirole. However, this pretreatment resulted in a marked decrease in quinpirole-induced locomotion and stereotyped behaviours. Administration of FAAH or MAGL inhibitors did not attenuate the acute effects of cocaine. Furthermore, these inhibitors did not impair the acquisition of cocaine-induced behavioural sensitization or the expression of cocaine-induced conditioned locomotion. Only MAGL inhibition attenuated the expression of an already acquired cocaine-induced behavioural sensitization. These results suggest that pharmacological inhibition of endocannabinoid degradation might exert a negative feedback on D2/D3 receptor-mediated hyperactivity. This finding might be relevant for therapeutic approaches for either psychomotor disorders (dyskinesia, corea) or disorganized behaviours associated with dopamine-mediated hyperactivity.

Behavioral and neurochemical effects of amphetamine analogs that release monoamines in the squirrel monkey

To date, there are no effective pharmacotherapies for treating psychostimulant abuse. Previous preclinical and clinical studies have shown that continuous treatment with the monoamine releaser amphetamine reduces cocaine self-administration, but amphetamine selectively targets the dopamine system and is reinforcing. In the present study, we examined the consequences of administration of amphetamine and three structurally related analogs that vary in their potencies for releasing dopamine and serotonin on behavioral-stimulant effects and nucleus accumbens dopamine levels in squirrel monkeys. Amphetamine and PAL-353, which have relatively high selectivity for releasing dopamine vs. serotonin, increased accumbens dopamine levels and induced stimulant effects on behavior maintained by a fixed-interval schedule of reinforcement. PAL-313, which has a relatively low selectivity for releasing dopamine vs. serotonin, increased dopamine levels, but did not induce behavioral-stimulant effects. PAL-287, which is relatively nonselective in releasing dopamine and serotonin, did not increase dopamine levels or induce behavioral-stimulant effects. These results demonstrate that increasing serotonergic activity attenuates dopamine release and dopamine-mediated behavioral effects of monoamine releasers. In addition, these results support further investigation of PAL-313 and similar compounds as a potential medication for treating psychostimulant abuse.

Behavioral recovery in MPTP-treated monkeys: neurochemical mechanisms studied by intrastriatal microdialysis

Parkinson's disease (PD) patients express motor symptoms only after 60-80% striatal dopamine (DA) depletion. The presymptomatic phase of the disease may be sustained by biochemical modifications within the striatum. We used an appropriate specific 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) monkey model (Mounayar et al., 2007) to study the compensatory mechanisms operating in recovery from PD motor symptoms. We assessed the levels of DA and its metabolites (DOPAC, homovanillic acid), GABA, glutamate (Glu), serotonin (5-HT) and its metabolite (5HIAA) by repeated intracerebral microdialysis in awake animals before exposure to MPTP during full expression of the motor symptoms induced by MPTP and after recovery from these symptoms. Measurements were obtained from two functionally and anatomically different striatal areas: the associative-limbic territory and sensorimotor territory. Animals with motor symptoms displayed an extremely large decrease in levels of DA and its metabolites and an increase in Glu and GABA levels, as reported by other studies. However, we show here for the first time that serotonin levels increased in these animals. We found that increases in DA levels in the sensorimotor and/or associative-limbic territory and high levels of 5-HT and of its metabolite, 5HIAA, were associated with recovery from motor symptoms in this model. Determining whether similar changes in DA and 5-HT levels are involved in the compensatory mechanisms delaying the appearance of motor symptoms in the early stages of PD might make it possible to develop new treatment strategies for the disease.

Reciprocal effects of response contingent and noncontingent intravenous heroin on in vivo nucleus accumbens shell versus core dopamine in the rat: a repeated sampling microdialysis study

RATIONALE

Although passive administration of heroin to drug-naive rats increases extracellular dopamine (DA) in the nucleus accumbens (NAc), its ability to do so also after active drug exposure (self-administration) is debated.

OBJECTIVES

This study investigated by repeated microdialysis sampling the inter- and intrasession changes in the responsiveness of the NAc shell and core DA and the behavioral effects of active and passive heroin exposure in the intravenous self-administration/yoked paradigm.

MATERIALS AND METHODS

Rats were implanted with jugular catheters and bilateral intracerebral chronic guide cannulae. Nose poking in the active hole by master rats resulted in heroin administration to the same subjects and to their yoked mates. Concentric microdialysis probes were inserted daily in the guide cannulae, and changes in dialysate DA in response to heroin exposure (0.05 mg/kg) were monitored in the same subject for 90 min for 4 weeks. Behavior associated with heroin exposure, distinguished into nonstereotyped and stereotyped, was also recorded.

RESULTS

Dialysate DA increased preferentially in the shell of master rats from the first session (+112%) and throughout the 4 weeks of self-administration (+130-140%). In yoked rats, a preferential but lesser increase in DA in the shell was observed only on the first session (+60%), as the DA response in the NAc core increased progressively (+25-118%), so that within a week, the shell/core ratio was reversed, and this pattern was maintained for the following 2 weeks. Yoked rats showed a progressive and larger increase in stereotyped behaviors than master rats.

CONCLUSIONS

Chronic heroin self-administration increases extracellular DA preferentially in the NAc shell. Response-noncontingent heroin administration is particularly prone, compared to response-contingent administration, to induce behavioral and biochemical sensitization.

In vivo comparison of the reinforcing and dopamine transporter effects of local anesthetics in rhesus monkeys

Dopaminergic mechanisms are thought to play a central role in the reinforcing effects of cocaine. Similar to cocaine, other local anesthetics bind to the dopamine transporter (DAT) and inhibit DA uptake in rodent and monkey brain. Additionally, local anesthetics are self-administered in rhesus monkeys, indicative of abuse liability. The present study examined the reinforcing and DAT effects of the local anesthetics dimethocaine, procaine and cocaine using in vivo techniques. Monkeys were trained to respond under a second-order schedule for i.v. cocaine administration (0.10 or 0.30 mg/kg/infusion). When responding was stable, dimethocaine (0.030-1.7 mg/kg/ infusion) or procaine (0.10-10 mg/kg/ infusion) was substituted for the cocaine training dose. Dimethocaine administration produced higher response rates compared with that of procaine, and was a more potent reinforcer. Drug effects on behavior were related to DAT occupancy in monkey striatum during neuroimaging with positron emission tomography (PET). DAT occupancy was determined by displacement of 8-(2-[(18)F]fluroethyl)2beta-carbomethoxy-3beta-(4-chlorophenyl)nortropane (FECNT). DAT occupancy was between 66 and 82% and <10-41% for doses of dimethocaine and procaine that maintained maximum response rates, respectively. Finally, in vivo microdialysis in awake subjects determined drug-induced changes in extracellular DA in the caudate nucleus. There was close correspondence between peak increases in DA and DAT occupancy. Overall, reinforcing effects were consistent with DAT effects determined with in vivo techniques. The results further support a role for the DAT in the abuse liability of local anesthetics.

Interaction of cocaine and dopamine transporter inhibitors on behavior and neurochemistry in monkeys

Drugs that target the dopamine transporter (DAT) have been proposed as pharmacotherapies to treat cocaine abuse. Accordingly, it is paramount to understand pharmacological interactions between cocaine and DAT inhibitors. The present study characterized acute interactions between cocaine and several DAT inhibitors (RTI-177, FECNT, RTI-112) that differed in selectivity for monoamine transporters on operant behavior and in vivo neurochemistry in squirrel monkeys. RTI-177 and FECNT, two DAT inhibitors with low affinity at norepinephrine transporters (NET), produced dose-dependent stimulant effects on behavior maintained by a fixed-interval schedule of stimulus termination. Compared to cocaine, RTI-177 and FECNT had a slower onset and longer duration of action. In vivo microdialysis in the caudate nucleus of awake monkeys confirmed dose-dependent increases in extracellular dopamine that corresponded to behavioral effects. Among the drugs characterized, RTI-112 is reportedly the least selective for binding to DAT, NET, and serotonin transporters (SERT). Interestingly, RTI-112 failed to produce significant behavioral-stimulant effects, and its effects on extracellular dopamine were highly variable across subjects. The results indicate that the pharmacological profile of DAT inhibitors may be influenced by actions at multiple monoamine transporters. Importantly, there was little evidence of additivity on behavioral or neurochemical measures when cocaine was administered in combination with behavioral-stimulant doses of the DAT inhibitors.

Methylphenidate elevates resting dopamine which lowers the impulse-triggered release of dopamine: a hypothesis

How do 'stimulants' reduce hyperactivity in children and adults? How can drugs which raise extracellular dopamine result in psychomotor slowing of hyperactive children when dopamine is known to enhance motor activity, such as in Parkinson's disease? In summary, the hypothesis for the anti-hyperactivity effects of the stimulants is as follows: during normal nerve activity, extracellular dopamine levels transiently rise 60-fold. At low therapeutic doses (0.2-0.5 mg/kg) to treat attention-deficit hyperactivity disorder, stimulant drugs such as methylphenidate and dextroamphetamine reduce locomotion in both humans and animals. The drugs raise resting extracellular levels of dopamine several-fold, but reduce the extent to which dopamine is released with nerve impulses, compared to the impulse-associated release in the absence of the drug. This relatively reduced amplitude of impulse-associated dopamine would result in less activation of post-synaptic dopamine receptors which drive psychomotor activity. At higher doses, stimulants produce generalized stimulation of the nervous system, as a result of the very high concentrations of extracellular dopamine at rest, and the markedly increased release of dopamine with nerve impulses. These high levels of resting and pulsatile dopamine cause widespread stimulation of post-synaptic dopamine receptors, overcoming any concomitant presynaptic inhibition of dopamine release.

Delivering drugs, via microdialysis, into the environment of extracellularly recorded hippocampal neurons in behaving primates

Hippocampal neurons in primates have been extensively studied with electrophysiological and neuroanatomical methods. Much less effort has been devoted to examining these cells with contemporary pharmacological techniques. Therefore, we modified a recently developed integrative technique (N. Ludvig, P.E. Potter, S.E. Fox, Simultaneous single-cell recording and microdialysis within the same brain site in freely behaving rats: a novel neurobiological method, J. Neurosci. Methods 55 (1994) 31-40 [9] ) for cellular neuropharmacological studies in behaving monkeys. A driveable microelectrode-microdialysis probe guide assembly was implanted stereotaxically into the left hippocampus of squirrel monkeys (Saimiri sciureus) under isoflurane anesthesia. The assembly was covered with a protective cap. After 3 weeks of postsurgical recovery and behavioral training, the experimental subject was seated in a primate chair. For 4-5 h, single-cell recording and microdialysis were simultaneously performed in the hippocampal implantation site. The technique allowed the recording of both complex-spike cells and fast-firing neurons without the use of head restraint. The control microdialysis solution, artificial cerebrospinal fluid (ACSF), was replaced with either 1 M ethanol or 500 microM N-methyl-D-aspartate (NMDA) for 10-30 min intervals. The ethanol perfusions principally suppressed the firing of the neurons in the dialysis area. The NMDA perfusions initially increased the firing of local neurons, then caused electrical silence. These drug delivery/cell recording sessions were performed with 1-4 day intersession intervals over a 1-month period. The described method provides a tool to elaborate the pharmacology of primate hippocampal neurons during behavior and without the confounding effects of systemic drug administrations.

The effect of selenium on the central dopaminergic system: a microdialysis study

The effects of Selenium (Se) on central dopaminergic function were examined in male Sprague-Dawley rats. In this experiment, animals were implanted with microdialysis probes and dialysates were analyzed for dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). After reaching baseline values, sodium selenite was either injected intraperitoneally (i.p.) or directly infused into the striatum (ST) or nucleus accumbens (NA). Se administration of 3.0 mg/kg (i.p.) significantly increased (70%) DA overflow in the ST. Meanwhile direct Se perfusion (10 mM) also caused a significant elevation of synaptic DA concentrations in the ST and NA. Levels of DOPAC and HVA were minimally affected in all studies. In order to test for the effects of DA receptor activation, animals were pretreated with quinpirole (0.5 mg/kg, s.c.), an hour prior to Se (10 mM) infusion through the probe. It was found that quinpirole pre-treatment reduced Se-induced changes in DA concentrations. It was concluded from the present study that Se's central action might be related to its ability to potentiate DA function.