In vivo imaging of dopamine D1 receptor and activated microglia in attention-deficit/hyperactivity disorder: a positron emission tomography study

Alterations in the cortical dopamine system and microglial activation have been implicated in the pathophysiology of attention-deficit/hyperactivity disorder (ADHD), one of neurodevelopmental disorders that can be conventionally treated with a dopamine enhancer (methylphenidate) albeit unsatisfactorily. Here, we investigated the contributions of the dopamine D1 receptor (D1R) and activated microglia and their interactions to the clinical severities in ADHD individuals using positron emission tomography (PET). Twenty-four psychotropic-naïve ADHD individuals and 24 age- and sex-matched typically developing (TD) subjects underwent PET measurements with [¹¹C]SCH23390 for the D1R and [¹¹C](R)PK11195 for activated microglia as well as assessments of clinical symptoms and cognitive functions. The ADHD individuals showed decreased D1R in the anterior cingulate cortex (ACC) and increased activated microglia in the dorsolateral prefrontal cortex (DLPFC) and orbitofrontal cortex (OFC) compared with the TD subjects. The decreased D1R in the ACC was associated with severe hyperactivity in the participants with ADHD. Microglial activation in the DLPFC were associated with deficits in processing speed and attentional ability, and that in the OFC was correlated with lower processing speed in the ADHD individuals. Furthermore, positive correlations between the D1R and activated microglia in both the DLPFC and the OFC were found to be significantly specific to the ADHD group and not to the TD group. The current findings suggest that microglial activation and the D1R reduction as well as their aberrant interactions underpin the neurophysiological mechanism of ADHD and indicate these biomolecular changes as a novel therapeutic target.

Effect of Chronic Methylphenidate Treatment in a Female Experimental Model of Parkinsonism

Methylphenidate (MPH) is the most commonly prescribed drug for the treatment of ADHD in males and females. However, a majority of previous studies investigated the effect of MPH in only males, and little is known regarding consequences of female exposure to MPH. This is unfortunate because the few studies that have been conducted indicate that females have a greater sensitivity to MPH. Previous research in male mice has shown that chronic exposure to MPH causes dopaminergic neurons within the nigrostriatal pathway to be more sensitive to the Parkinsonian toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). However, estrogen has been shown to protect dopaminergic neurons from MPTP neurotoxicity. Therefore, in this study, we test the hypothesis that chronic MPH exposure in female mice will render dopaminergic neurons in the nigrostriatal pathway more sensitive to MPTP, and that estrogen may play a protective role. Interestingly, proestrus females exhibited greater sensitivity to MPTP, with significantly reduced dopaminergic neurons in the SN and significant increases in DA quinone production. Chronic MPH exposure contributed to GSH depletion, but surprisingly, it did not increase dopamine quinone levels or dopaminergic cell loss. There were no significant differences in anestrus animals, with the exception of a depletion in GSH seen when animals received chronic high-dose (10 mg/kg) MPH followed by MPTP. Thus, estrogen may actually sensitize neurons to MPTP in this model, and chronic MPH may contribute to GSH depletion within the striatum. This study provides insight into how chronic psychostimulant use may affect males and females differently.

What are the consequences of Methylphenidate exposure for maternally separated rats?

Methylphenidate (MPH) abuse is prevalent among youth. Drug abuse results in pain perception. We sought to investigate whether Maternal separation (MS) prone to MPH addiction. The next question was whether MPH abusers with MS differ in pain perception. We investigated the impact of MS on addiction and drug reward as well as pain perception following 5 days of MPH injection in males and females rats. Initially, rats underwent MS protocol of 3 hr daily for 21 days. Conditioned place preference (CPP) test was an attempt to investigate whether MS rats experience more reward with MPH. The protocol consisted of 10 min habituation on Day 1, conditioning on Day 2-Day 6 (5 mg per kg MPH injection in drug compartment and saline in saline compartment with 4 hr gap between injections) and 10 min test on Day 7. Furthermore, using another group, differences in pain perception were investigated after 5 days of daily MPH injection with 5 mg per kg. Sensory pain sensitivity was tested on PND 39 using tail flick and hotplate in MS and control groups with and without MPH exposure. Results indicated that female rats are equally prone to addiction in CPP. On the other hand, MS males experience a higher reward in CPP. In tail flick test, female MS rats exposed to MPH show a lower sensory pain threshold with similar MPH exposure. Experiencing MPH similarly declined hotplate pain perception in MS and controls in the females. Males, on the other hand, did not show any difference in sensory pain tests. According to results one can argue MS is detrimental. MS males experience more reward with MPH, females are equally addiction prone and MS females experience more pain in tail flick. On the other hand pain threshold can decline in hotplate test for both control and MS females that received MPH.

The new designer drug buphedrone produces rewarding properties via dopamine D1 receptor activation

Substituted cathinones are synthetic analogs of the active components of natural products and are widely abused worldwide. However, the rewarding properties of these agents have not yet been evaluated. In this study, we investigated the abuse potential of buphedrone [2-(methylamino)-1-phenylbutan-1-one, α-methylamino-butyrophenone] and its effects on the mesolimbic dopaminergic system in mice using conditioned place preference (CPP) analysis, a self-administration test, a locomotor activity test, a behavioral sensitization test and Western blot analysis. Treatment with buphedrone supported CPP and self-administration, enhanced locomotor activity and produced behavioral sensitization when mice were challenged with methamphetamine. SCH23390, a D1 dopamine antagonist, prevented buphedrone-induced CPP, whereas raclopride, a D2 dopamine antagonist, had no effect. SCH23390 also blocked locomotor activity increase by buphedrone, while raclopride partially attenuated locomotor activation. Western blot analysis revealed that repeated buphedrone treatment increased D1 dopamine receptor expression in the dorsal striatum and nucleus accumbens in mice. Collectively, these findings suggest the abuse potential of buphedrone and demonstrate the involvement of the dopaminergic system in the establishment of its rewarding properties.

Learning and memory effects of neonatal methamphetamine exposure in rats: Role of reactive oxygen species and age at assessment

In utero methamphetamine (MA) exposure leads to a range of adverse effects, such as decreased attention, reduced working-memory capability, behavioral dysregulation, and spatial memory impairments in exposed children. In the current experiment, preweaning Sprague-Dawley rats-as a model of third trimester human exposure-were administered the spin trapping agent, N-tert-butyl-α-phenylnitrone (PBN), daily prior to MA. Rats were given 0 (SAL) or 40 mg/kg PBN prior to each MA dose (10 mg/kg, 4× per day) from postnatal day (P) 6-15. Littermates underwent Cincinnati water maze, Morris water maze, and radial water maze assessment beginning on P30 (males) or P60 (females). Males were also tested for conditioned contextual and cued freezing, while females were trained in passive avoidance. Findings show that, regardless of age/sex, neonatal MA induced deficits in all tests, except passive avoidance. PBN did not ameliorate these effects, but had a few minor effects. Taken together, MA induced learning deficits emerge early and persist, but the mechanism remains unknown.

Acute and long-term effects of adolescent methylphenidate on decision-making and dopamine receptor mRNA expression in the orbitofrontal cortex

Though commonly used as a treatment for ADHD, the psychostimulant methylphenidate (MPH) is also misused and abused in adolescence in both clinical and general populations. Although MPH acts via pathways activated by other drugs of abuse, the short- and long-term effects of MPH on reward processing in learning and decision-making are not clearly understood. We examined the effect of adolescent MPH treatment on a battery of reward-directed behaviors both in adolescence during its administration and in adulthood after its discontinuation. We further measured whether MPH had lasting effects on dopamine receptor mRNA expression in orbitofrontal cortex (OFC) that may correspond with behavior. Long-Evans rats were injected with MPH (0, 1, 2.5, or 5mg/kg IP) twice daily from middle to late adolescence (PD38-57). During adolescence, the high dose of MPH reduced preference for large rewards in a Reward Magnitude Discrimination task, but did not affect preference for smaller-sooner rewards in a Delay Discounting task. In adulthood, after discontinuation of MPH, animals previously treated with the moderate dose of MPH showed improved acquisition, but not reversal, in a Reversal Learning task. MPH exposure did not increase preference for large-risky rewards in a Risk task in adulthood. We then quantified mRNA expression of D1, D2, and D3 receptors in the OFC using qPCR. MPH increased mRNA expression of dopamine D3 receptor subtype, but not D1 or D2. Overall, these results indicate that MPH has both immediate and lasting effects on reward-dependent learning and decisions, as well as dopaminergic function in rodents.

The effects of adolescent methylphenidate exposure on the behavioral and brain-derived neurotrophic factor response to nicotine

This study analyzed the interaction of adolescent methylphenidate on the behavioral response to nicotine and the effects of these drug treatments on brain-derived neurotrophic factor in the nucleus accumbens and hippocampus in male and female Sprague-Dawley rats. Animals were intraperitoneal administered 1 mg/kg methylphenidate or saline using a "school day" regimen (five days on, two days off) beginning on postnatal day (P)28 and throughout behavioral testing. In Experiment 1, animals were intraperitoneal administered 0.5 mg/kg (free base) nicotine or saline every second day for 10 days from P45-P63 and tested after a three-day drug washout on the forced swim stress task on P67-P68. Results revealed that adolescent methylphenidate blunted nicotine behavioral sensitization. However, methylphenidate-treated rats given saline during sensitization demonstrated decreased latency to immobility and increased immobility time on the forced swim stress task in males that was reduced by nicotine. In Experiment 2, a different set of animals were conditioned to nicotine (0.6 mg/kg free base) or saline using the conditioned place preference behavioral paradigm from P44-P51, and given a preference test on P52. On P53, the nucleus accumbens and hippocampus were analyzed for brain-derived neurotrophic factor. Methylphenidate enhanced nicotine-conditioned place preference in females and nicotine produced conditioned place preference in males and females pre-exposed to saline in adolescence. In addition, methylphenidate and nicotine increased nucleus accumbens brain-derived neurotrophic factor in females and methylphenidate enhanced hippocampus brain-derived neurotrophic factor in males and females. Methylphenidate adolescent exposure using a clinically relevant dose and regimen results in changes in the behavioral and brain-derived neurotrophic factor responses to nicotine in adolescence that are sex-dependent.

The effect of a dopamine antagonist on conditioning of sexual arousal in women

RATIONALE

Dopamine (DA) plays a key role in reward-seeking behaviours. Accumulating evidence from animal and human studies suggests that human sexual reward learning may also depend on DA transmission. However, research on the role of DA in human sexual reward learning is completely lacking.

OBJECTIVES

To investigate whether DA antagonism attenuates classical conditioning of sexual response in humans.

METHODS

Healthy women were randomly allocated to one of two treatment conditions: haloperidol (n = 29) or placebo (n = 29). A differential conditioning paradigm was applied with genital vibrostimulation as unconditional stimulus (US) and neutral pictures as conditional stimuli (CSs). Genital arousal was assessed, and ratings of affective value and subjective sexual arousal were obtained.

RESULTS

Haloperidol administration affected unconditional genital responding. However, no significant effects of medication were found for conditioned responding.

CONCLUSIONS

No firm conclusions can be drawn about whether female sexual reward learning implicates DA transmission since the results do not lend themselves to unambiguous interpretation.

Single exposure of dopamine D1 antagonist prevents and D2 antagonist attenuates methylphenidate effect

Methylphenidate (MPD) is a readily prescribed drug for the treatment of attention deficit hyperactivity disorder (ADHD) and moreover is used illicitly by youths for its cognitive-enhancing effects and recreation. MPD exposure in rodents elicits increased locomotor activity. Repetitive MPD exposure leads to further augmentation of their locomotor activity. This behavioral response is referred to as behavioral sensitization. Behavioral sensitization is used as an experimental marker for a drug’s ability to elicit dependence. There is evidence that dopamine (DA) is a key player in the acute and chronic MPD effect; however, the role of DA in the effects elicited by MPD is still debated. The objective of this study was to investigate the role of D1 and/or D2 DA receptors in the acute and chronic effect of MPD on locomotor activity. The study lasted for 12 consecutive days. Seven groups of male Sprague Dawley^® rats were used. A single D1 or D2 antagonist was given before and after acute and chronic MPD administration. Single injection of D1 DA antagonist was able to significantly attenuate the locomotor activity when given prior to the initial MPD exposure and after repetitive MPD exposure, while the D2 DA antagonist partially attenuated the locomotor activity only when given before the second MPD exposure. The results show the role, at least in part, of the D1 DA receptor in the mechanism of behavioral sensitization, whereas the D2 DA receptor only partially modulates the response to acute and chronic MPD.