Despite the widespread use of stimulant medications for the treatment of attention deficit hyperactivity disorder, few studies have addressed their long-term effects on the developing brain or susceptibility to drug use in adolescence. Here, we determined the effects of chronic methylphenidate (MPH) treatment on brain dopamine (DA) systems, developmental milestones, and later vulnerability to substance abuse in juvenile nonhuman primates. Male rhesus monkeys (approximately 30 months old) were treated daily with either a sustained release formulation of MPH or placebo (N=8 per group). Doses were titrated to achieve initial drug blood serum levels within the therapeutic range in children and adjusted throughout the study to maintain target levels. Growth, including measures of crown-rump length and weight, was assessed before and after 1 year of treatment and after 3-5 months washout. In addition, positron emission tomography scans were performed to quantify binding availability of D2/D3 receptors and dopamine transporters (DATs). Distribution volume ratios were calculated to quantify binding of [¹⁸F]fluoroclebopride (DA D2/D3) and [¹⁸F]-(+)-N-(4-fluorobenzyl)-2β-propanoyl-3β-(4-chlorophenyl)tropane (DAT). Chronic MPH did not differentially alter the course of weight gain or other measures of growth, nor did it influence DAT or D2/D3 receptor availability after 1 year of treatment. However, after washout, the D2/D3 receptor availability of MPH-treated animals did not continue to decline at the same rate as control animals. Acquisition of intravenous cocaine self-administration was examined by first substituting saline for food reinforcement and then cocaine doses (0.001-0.1 mg/kg per injection) in ascending order. Each dose was available for at least five consecutive sessions. The lowest dose of cocaine that maintained response rates significantly higher than saline-contingent rates was operationally defined as acquisition of cocaine reinforcement. There were no differences in rates of acquisition, overall response rates, or cocaine intake as a function of cocaine dose between groups. In an animal model that closely mimics human development; chronic treatment with therapeutic doses of sustained release MPH did not have a significant influence on the regulation of DATs or D2/D3 receptors, or on standard measures of growth. Furthermore, this treatment regimen and subsequent drug washout did not have an impact on vulnerability to cocaine abuse.

The dopamine (DA) D 3 receptor (D3R) has been associated with impulsivity, pathologic gambling, and drug addiction, making it a potential target for pharmacotherapy development. Positron emission tomography studies using the D3R-preferring radioligand [ 11 C]PHNO ([ 11 C](+)-propyl-hexahydro-naphtho-oxazin) have shown higher binding potentials in drug abusers compared with control subjects. Preclinical studies have examined D3R receptor activation using the DA agonist quinpirole and the unconditioned behavior of yawning. However, the relationship between quinpirole-elicited yawning and D3R receptor availability has not been determined. In Experiment 1, eight drug-naive male rhesus monkeys were scanned with [ 11 C]PHNO, and the ability of quinpirole (0.01-0.3 mg/kg i.m.) to elicit yawning was examined. Significant positive (globus pallidus) and negative (caudate nucleus, putamen, ventral pallidum, and hippocampus) relationships between D3R receptor availability and quinpirole-induced yawns were noted. Experiment 2 replicated earlier findings that a history of cocaine self-administration (n = 11) did not affect quinpirole-induced yawning and extended this to examine monkeys (n = 3) with a history of methamphetamine (MA) selfadministration and found that monkeys with experience selfadministering MA showed greater potency and significantly higher quinpirole-elicited yawning compared with controls. Finally, quinpirole-elicited yawning was studied in drug-naive female monkeys (n = 6) and compared with drug-naive male monkeys (n = 8). Sex differences were noted, with quinpirole being more potent and eliciting significantly more yawns in males compared with females. Taken together these findings support the use of quinpirole-elicited yawning as a behavioral tool for examining D3R activation in monkeys and that both drug history and sex may influence individual sensitivity to the behavioral effects of D3R compounds.

Hypofunction of the prefrontal cortex (PFC) contributes to stress-related neuropsychiatric illnesses. Mechanisms leading to prefrontal hypoactivity remain to be determined. Prior evidence suggests that chronic stress leads to an increase in activity of parvalbumin (PV) expressing GABAergic interneurons (INs) in the PFC. The purpose of the study was to determine whether reducing PV IN activity in the Infralimbic (IL) PFC would prevent stress-related phenotypes. We used a chemogenetic approach to inhibit IL PFC PV INs during stress. Mice were first tested in the tail suspension test (TST) to determine the impact of PV IN inhibition on behavioral responses to acute stress. The long-term impact of PV IN inhibition during a modified chronic variable stress (CVS) was tested in the forced swim test (FST). Acute PV IN inhibition reduced active (struggling) and increased passive coping behaviors (immobility) in the TST. In contrast, inhibition of PV INs during CVS increased active and reduced passive coping behaviors in the FST. Moreover, chronic inhibition of PV INs attenuated CVS-induced changes in Fos expression in the prelimbic cortex (PrL), basolateral amygdala (BLA), and ventrolateral periaqueductal gray (vlPAG) and also attenuated adrenal hypertrophy and body weight loss associated with chronic stress. Our results suggest differential roles of PV INs in acute versus chronic stress, indicative of distinct biological mechanisms underlying acute versus chronic stress responses. Our results also indicate a role for PV INs in driving chronic stress adaptation and support literature evidence suggesting cortical GABAergic INs as a therapeutic target in stress-related illnesses.

It has been hypothesized that drugs that serve as substrates for dopamine (DA) and norepinephrine (NE) transporters may be more suitable medications for cocaine dependence than drugs that inhibit DA and NE uptake by binding to transporters. Previous studies have shown that the DA/NE releaser d-amphetamine can decrease cocaine self-administration in preclinical and clinical studies. The present study examined the effects of methylphenidate (MPD), a DA uptake inhibitor, for its ability to decrease cocaine self-administration under conditions designed to reflect clinically relevant regimens of cocaine exposure and pharmacotherapy. Each morning, rhesus monkeys pressed a lever to receive food pellets under a fixed-ratio 50 schedule of reinforcement; cocaine was self-administered under a progressive-ratio schedule of reinforcement in the evening. After cocaine (0.003-0.56 mg/kg per injection, i.v.) dose-response curves were determined, self-administration sessions were suspended and MPD (0.003-0.0056 mg/kg per hour, i.v.; or 1.0-9.0 mg/kg p.o., b.i.d.) was administered for several weeks. A cocaine self-administration session was conducted every 7 days. When a MPD dose was reached that either persistently decreased cocaine self-administration or produced disruptive effects, the cocaine dose-effect curve was re-determined. In most cases, MPD treatment either produced behaviorally disruptive effects or increased cocaine selfadministration; it took several weeks for these effects to dissipate. These data are consistent with the largely negative results of clinical trials with MPD. In contrast to the positive effects with the monoamine releaser d-amphetamine under identical conditions, these results do not support use of monoamine uptake inhibitors like MPD as a medication for cocaine dependence.