Sex differences in adolescent methylphenidate sensitization: effects on glial cell-derived neurotrophic factor and brain-derived neurotrophic factor

A neurocomputational account of reward and novelty processing and effects of psychostimulants in attention deficit hyperactivity disorder

Computational models of reinforcement learning have helped dissect discrete components of reward-related function and characterize neurocognitive deficits in psychiatric illnesses. Stimulus novelty biases decision-making, even when unrelated to choice outcome, acting as if possessing intrinsic reward value to guide decisions toward uncertain options. Heightened novelty seeking is characteristic of attention deficit hyperactivity disorder, yet how this influences reward-related decision-making is computationally encoded, or is altered by stimulant medication, is currently uncertain. Here we used an established reinforcement-learning task to model effects of novelty on reward-related behaviour during functional MRI in 30 adults with attention deficit hyperactivity disorder and 30 age-, sex- and IQ-matched control subjects. Each participant was tested on two separate occasions, once ON and once OFF stimulant medication. OFF medication, patients with attention deficit hyperactivity disorder showed significantly impaired task performance (P = 0.027), and greater selection of novel options (P = 0.004). Moreover, persistence in selecting novel options predicted impaired task performance (P = 0.025). These behavioural deficits were accompanied by a significantly lower learning rate (P = 0.011) and heightened novelty signalling within the substantia nigra/ventral tegmental area (family-wise error corrected P < 0.05). Compared to effects in controls, stimulant medication improved attention deficit hyperactivity disorder participants' overall task performance (P = 0.011), increased reward-learning rates (P = 0.046) and enhanced their ability to differentiate optimal from non-optimal novel choices (P = 0.032). It also reduced substantia nigra/ventral tegmental area responses to novelty. Preliminary cross-sectional evidence additionally suggested an association between long-term stimulant treatment and a reduction in the rewarding value of novelty. These data suggest that aberrant substantia nigra/ventral tegmental area novelty processing plays an important role in the suboptimal reward-related decision-making characteristic of attention deficit hyperactivity disorder. Compared to effects in controls, abnormalities in novelty processing and reward-related learning were improved by stimulant medication, suggesting that they may be disorder-specific targets for the pharmacological management of attention deficit hyperactivity disorder symptoms.

Altered Plasma Levels of Glial Cell Line-Derived Neurotrophic Factor in Patients with Internet Gaming Disorder: A Case-Control, Pilot Study

Glial cell line-derived neurotrophic factor (GDNF) has been reported to be involved in negatively regulating the effects of addictive disorders. The objective of this study was to investigate alterations in the levels of GDNF in patients with Internet gaming disorder (IGD) and to assess the relationship between GDNF levels and the severity of IGD indices. Nineteen male patients with IGD and 19 sexmatched control subjects were evaluated for alteration of plasma GDNF levels and for relationship between GDNF levels and clinical characteristics of Internet gaming, including the Young's Internet Addiction Test (Y-IAT). The GDNF levels were found to be significantly low in patients with IGD (103.2±62.0 pg/mL) compared with the levels of controls (245.2±101.6 pg/mL, p<0.001). GDNF levels were negatively correlated with Y-IAT scores (Spearman's rho=-0.645, p=<0.001) and this negative correlation remained even after controlling for multiple variables (r=-0.370, p=0.048). These findings support the assumed role of GDNF in the regulation of IGD.

Repeated Use of the Psychoactive Substance Ethylphenidate Impacts Neurochemistry and Reward Learning in Adolescent Male and Female Mice

Schedule II prescription psychostimulants, such as methylphenidate (MPH), can be misused as nootropic drugs, i.e., drugs that enhance focus and cognition. When users are unable to obtain these prescribed medications, they may seek out novel psychoactive substances (NPSs) that are not yet scheduled. An example of a NPS reportedly being abused is ethylphenidate (EPH), a close analog of MPH but with a higher preference for the dopamine transporter compared with the norepinephrine transporter. Therefore, based upon this pharmacological profile and user self-reports, we hypothesized that repeated EPH exposure in adolescent mice may be rewarding and alter cognition. Here, we report that repeated exposure to 15 mg/kg EPH decreased spatial cognitive performance as assessed by the Barnes maze spatial learning task in adolescent male C57Bl/6 mice; however, male mice did not show alterations in the expression of mature BDNF - a protein associated with increased cognitive function - in key brain regions. Acute EPH exposure induced hyperlocomotion at a high dose (15 mg/kg, i.p.), but not a low dose (5 mg/kg, i.p.). Interestingly, mice exhibited significant conditioned place preference at the low EPH dose, suggesting that even non-stimulating doses of EPH are rewarding. In both males and females, repeated EPH exposure increased expression of deltaFosB - a marker associated with increased risk of drug abuse - in the dorsal striatum, nucleus accumbens, and prefrontal cortex. Overall, our results suggest that repeated EPH use in adolescence is psychostimulatory, rewarding, increases crucial brain markers of reward-related behaviors, and may negatively impact spatial performance.

Premature responding is associated with approach to a food cue in male and female heterogeneous stock rats

RATIONALE

Disorders of behavioral regulation, including attention deficit hyperactivity disorder (ADHD) and drug addiction, are in part due to poor inhibitory control, attentional deficits, and hyper-responsivity to reward-associated cues.

OBJECTIVES

To determine whether these traits are related, we tested genetically variable male and female heterogeneous stock rats in the choice reaction time (CRT) task and Pavlovian conditioned approach (PavCA). Sex differences in the response to methylphenidate during the CRT were also assessed.

METHODS

In the CRT task, rats were required to withhold responding until one of two lights indicated whether responses into a left or right port would be reinforced with water. Reaction time on correct trials and premature responses were the operational definitions of attention and response inhibition, respectively. Rats were also pretreated with oral methylphenidate (0, 2, 4 mg/kg) during the CRT task to determine whether this drug would improve performance. Subsequently, during PavCA, presentation of an illuminated lever predicted the delivery of a food pellet into a food-cup. Lever-directed approach (sign-tracking) and food-cup approach (goal-tracking) were the primary measures, and rats were categorized as "sign-trackers" and "goal-trackers" using an index based on these measures.

RESULTS

Sign-trackers made more premature responses than goal-trackers but showed no differences in reaction time. There were sex differences in both tasks, with females having higher sign-tracking, completing more CRT trials, and making more premature responses after methylphenidate administration.

CONCLUSIONS

These results indicate that response inhibition is related to reward-cue responsivity, suggesting that these traits are influenced by common genetic factors.

Sex differences in subjective and objective responses to a stimulant medication (methylphenidate): Comparisons between overweight/obese adults with and without binge-eating disorder

OBJECTIVE

The purpose of this study was to examine sex differences in response to a single dose of a psychomotor-stimulant medication (methylphenidate: MP) and to assess whether expected differences were moderated by binge-eating disorder (BED) status. It is anticipated that findings will shed light on factors that contribute to response variation in the use of stimulant pharmacotherapy to treat BED.

METHOD

The study employed a double-blind, drug-placebo, cross-over design in overweight/obese adults with BED (n = 90) and without BED (n = 108). Emotional/mood ratings were assessed every 15 minutes after oral administration of the drug/placebo, and appetite, cravings, and consumption were assessed during a laboratory-based snack-food challenge.

RESULTS

Women reported earlier and more sustained "overall" effects of the drug-including "feeling high"-than the men. There was also a significantly greater suppression in appetite ratings, food cravings, and food consumption from the placebo to the drug condition among the women. Indeed, among men there were no significant differences between the two conditions on any of the food-related variables. BED status also did not moderate any of the drug-placebo differences.

DISCUSSION

These findings are relevant to the use of stimulant pharmacotherapy for BED, and raise the possibility that overweight/obese men may be relatively less responsive to this form of treatment. © 2015 Wiley Periodicals, Inc. (Int J Eat Disord 2016; 49:473-481).

Prescription stimulant use is associated with earlier onset of psychosis

A childhood history of attention deficit hyperactivity disorder (ADHD) is common in psychotic disorders, yet prescription stimulants may interact adversely with the physiology of these disorders. Specifically, exposure to stimulants leads to long-term increases in dopamine release. We therefore hypothesized that individuals with psychotic disorders previously exposed to prescription stimulants will have an earlier onset of psychosis. Age of onset of psychosis (AOP) was compared in individuals with and without prior exposure to prescription stimulants while controlling for potential confounding factors. In a sample of 205 patients recruited from an inpatient psychiatric unit, 40% (n = 82) reported use of stimulants prior to the onset of psychosis. Most participants were prescribed stimulants during childhood or adolescence for a diagnosis of ADHD. AOP was significantly earlier in those exposed to stimulants (20.5 vs. 24.6 years stimulants vs. no stimulants, p < 0.001). After controlling for gender, IQ, educational attainment, lifetime history of a cannabis use disorder or other drugs of abuse, and family history of a first-degree relative with psychosis, the association between stimulant exposure and earlier AOP remained significant. There was a significant gender × stimulant interaction with a greater reduction in AOP for females, whereas the smaller effect of stimulant use on AOP in males did not reach statistical significance. In conclusion, individuals with psychotic disorders exposed to prescription stimulants had an earlier onset of psychosis, and this relationship did not appear to be mediated by IQ or cannabis.