Sari Izenwasser

Dopamine transport function is elevated in cocaine users.

Dopaminergic transmission has been suggested to be a primary mechanism mediating reinforcement, withdrawal and craving associated with psychostimulant addiction. Pyschostimulants attenuate dopamine transporter (DAT) clearance efficiency, resulting in a net increase in synaptic dopamine levels. Re‐uptake rate is determined by the number of functional DAT molecules at the membrane surface. Previous in vivo imaging studies in humans and in vitro studies in post‐mortem human brain have demonstrated that chronic cocaine abuse results in a neuroadaptive increase in DAT‐binding site density in the limbic striatum. Whether this increase in DAT availability represents an increase in the functional activity of the transporter is unknown. Here, we present evidence that DAT function is elevated by chronic cocaine abuse. The effect of increasing post‐mortem interval on the functional viability of synaptosomes was modeled in the baboon brain. Baboon brains sampled under conditions similar to human brain autopsies yielded synaptosomal preparations that were viable up to 24 h post‐mortem. Dopamine (DA) uptake was elevated twofold in the ventral striatum from cocaine users as compared to age‐matched drug‐free control subjects. The levels of [3H]DA uptake were not elevated in victims of excited cocaine delirium, who experienced paranoia and marked agitation prior to death. In keeping with the increase in DAT function, [3H]WIN 35,428 binding was increased in the cocaine users, but not in the victims of excited delirium. These results demonstrate that DA uptake function assayed in cryopreserved human brain synaptosomes is a suitable approach for testing hypotheses of the mechanisms underlying human brain disorders and for studying the actions of addictive drugs in man.

Chronic nicotine differentially alters cocaine-induced locomotor activity in adolescent vs. adult male and female rats.

Tobacco use is prevalent in the adolescent population. It is a major concern because tobacco is highly addictive and has also been linked to illicit drug use. There is not much research, however, on the interaction between nicotine and other stimulant drugs in animal models of early adolescence. This study examined the effects of chronic nicotine alone and on cocaine-stimulated activity in male and female periadolescent rats compared to male and female adult rats. During the seven-day nicotine pretreatment period, nicotine increased locomotor activity in all groups compared to vehicle controls. Male and female adult rats and female periadolescent rats developed sensitization to the locomotor-activating effects of nicotine over the 7-day treatment period, while male periadolescent rats did not. All groups treated with nicotine, however, exhibited sensitization to nicotine-induced repetitive motion over the 7-day nicotine treatment period. On day 8, male periadolescent rats pretreated with nicotine were more markedly sensitized to the locomotor-activating effects of cocaine than male adult rats, while female rats pretreated with nicotine were not sensitized to cocaine. In contrast, male and female periadolescent rats, but not adult rats, had increased amounts of repetitive beam breaks induced by cocaine after nicotine pretreatment. Overall, it appears that cross-sensitization to cocaine is greater in periadolescent than in adult rats, and that males are more sensitized than females. Thus, it may be that nicotine use during adolescence carries a greater risk than during adulthood and that male adolescents may be particularly vulnerable to the risk of cocaine abuse after nicotine use. This information should be taken into account so as to help us better understand the development of drug addiction in adolescents compared to adults.

Comparison of the effects of cocaine and other inhibitors of dopamine uptake in rat striatum nucleus accumbens olfactory tubercle and medial prefrontal cortex

It is thought that inhibition of dopamine reuptake into neurons may play a major role in the mechanisms by which cocaine produces its reinforcing effects. The striatum, while rich in dopamine terminals, is not implicated in drug reinforcement, whereas the mesolimbic dopamine pathway appears to play a primary role. It is therefore possible that the properties and drug sensitivities of the dopamine uptake systems in the nigrostriatal, mesolimbic, and mesocortical tracts differ. The effects of cocaine, GBR 12909, amfonelic acid, and methylphenidate on dopamine uptake in the striatum, nucleus accumbens, olfactory tubercle, and medial prefrontal cortex were examined. Over 80% of the dopamine uptake in each of the 4 regions was sodium-dependent and exhibited Km values of approximately 100 nM. Cocaine, GBR 12909, amfonelic acid, and methylphenidate each biphasically inhibited uptake in the striatum, nucleus accumbens and olfactory tubercle with GBR 12909 and amfonelic acid being approximately 50-fold more potent than cocaine or methylphenidate. In the medial prefrontal cortex, cocaine and GBR 12909 could inhibit only about 40% of the [3H]dopamine uptake. There are similarities in the properties and drug sensitivities of the dopamine uptake systems in brain areas which are implicated in drug reinforcement and those which are not.

Daily cocaine treatment produces a persistent reduction of [3H]dopamine uptake in vitro in rat nucleus accumbens but not in striatum

Daily administration of cocaine (15 mg/kg i.p. x 3 days) led to a decrease in the total amount of 15 nM [3H]dopamine uptake in rat nucleus accumbens with no change in uptake in the striatum when tested 24 h after the last injection. There was an increase in the Km for dopamine uptake in the nucleus accumbens of cocaine-treated rats, with no change in the Vmax. Furthermore, cocaine was a more potent inhibitor of [3H]dopamine uptake in vitro in the nucleus accumbens of rats treated with cocaine than in those which had received saline.

Differential effects of methamphetamine and cocaine on conditioned place preference and locomotor activity in adult and adolescent male rats

Human and animal laboratory studies show that adolescents and adults respond differently to drugs and that drug administration during adolescence leads to different behavioral effects than during adulthood. Although there are a number of studies on the effects of cocaine, little is known about the effects of methamphetamine in adolescent vs adult rats. In the present study, sensitivity to the conditioned reward of multiple doses of methamphetamine or cocaine was evaluated in male adolescent (PND 34) and adult (PND 66) rats using a conditioned place preference (CPP) paradigm. In addition, the locomotor-activating effects of methamphetamine were determined across a 5-day period of administration. After 3 days of training with cocaine, both adolescent and adult male rats developed CPP to cocaine, however, the dose-effect curve for cocaine CPP was shifted to the left in adolescent compared to adult rats. In contrast to the development of CPP to cocaine in both groups after 3 days of conditioning, methamphetamine CPP occurred only in adolescent, and not in adult rats. After 5 days of training, however, both adolescent and adult rats exhibited identical responses to multiple doses of methamphetamine and a significant CPP was observed in both groups. Daily administration of methamphetamine increased locomotor activity in both adolescent and adult rats, with a greater effect seen in the adults. In neither group, was there evidence of a significant sensitization to the locomotor-activating effects of methamphetamine. These data show that adolescents are more sensitive to psychostimulant reward and thus to the conditioned rewarding properties of cocaine or methamphetamine than adults. A better understanding of this difference may lead to age-specific preventions and treatments for psychostimulant abuse.

Sensitivity to cocaine conditioned reward depends on sex and age

Human and animal laboratory studies show that females and males respond differently to drugs and that drug administration during adolescence leads to different behavioral effects than during adulthood. Adult female rats are more sensitive to the behavioral effects of cocaine than adult males, but it is not known if the same effect of sex exists during adolescence. In the present study, sensitivity to the conditioned reward of cocaine was evaluated using a conditioned place preference (CPP) paradigm where adolescent (PND 34) and adult (PND 66) male and female rats were trained and tested for the development of CPP to multiple doses of cocaine. Female rats developed CPP at lower doses than males, regardless of age. In addition, adolescent male and female rats established a CPP at lower doses of cocaine than adult male and female rats, respectively. Thus, both age and sex altered cocaine conditioned reward with the order of sensitivity being adolescent females > adult females > adolescent males > adult males. These data show that adolescents are more sensitive to the conditioned rewarding properties of cocaine than adults and that females respond to lower doses of cocaine compared to males regardless of age.

Cocaine differentially alters behavior and neurochemistry in periadolescent versus adult rats.

This study examined whether there are differences in the behavioral and neurochemical effects of cocaine in periadolescent rats compared to adult rats. Periadolescent (postnatal days 28-35) and adult rats were injected with cocaine or vehicle for 7 days. Ten days later (day 17), rats either were challenged with cocaine, or dopamine transporter and receptor and serotonin transporter binding were examined. Adult rats became sensitized to the locomotor-activating effects of cocaine and there were increases in dopamine transporter density in the caudate putamen compared to vehicle-treated adult rats. In addition, serotonin transporter densities were increased in the ventromedial caudate putamen, nucleus accumbens shell, and the olfactory tubercle in cocaine-treated adult rats compared to vehicle-treated adult rats. In contrast, periadolescent rats did not show sensitization to cocaine and there was no effect of cocaine on either dopamine or serotonin transporter densities. These findings suggest that there are different neurochemical and behavioral adaptations to repeated cocaine administration in periadolescent versus adult rats.

Inhibition of dopamine uptake by cocaine and nicotine : tolerance to chronic treatments

Chronic administration of cocaine (50 mg/kg/day for 7 days, s.c. via an osmotic minipump) produced tolerance to inhibition of [3H]dopamine uptake by cocaine in rat striatum but did not produce cross-tolerance to inhibition of [3H]dopamine uptake by nicotine. Chronic nicotinic treatment (6 mg/kg/day for 7 days, s.c. via an osmotic minipump), however, produced tolerance to the inhibition of [3H]dopamine uptake by nicotine and cross-tolerance to inhibition of uptake by cocaine in rat striatum. Nicotine did not inhibit uptake of [3H]dopamine in the nucleus accumbens of saline-treated animals. In this tissue, both cocaine and nicotine treatments produced tolerance to cocaine, as in striatum. Unlike the effects on [3H]dopamine uptake, chronic cocaine infusion did not have any effect on the ability of cocaine to inhibit [3H]serotonin uptake in either brain region.

Chronic methylphenidate alters locomotor activity and dopamine transporters differently from cocaine

Continuous infusion of cocaine produces partial behavioral tolerance to its locomotor activating effects, while daily injections produce sensitization. Methylphenidate binds with a similar affinity to cocaine at the dopamine transporter, but has a much lower affinity for the serotonin transporter than does cocaine. This study was done to compare the effects of chronic methylphenidate with chronic cocaine. The pattern of locomotor activity over a 7 day treatment period was significantly different from cocaine. Methylphenidate elevated activity on each day, compared to saline, yet neither tolerance to a continuous infusion of the drug, nor sensitization to repeated daily injections was produced. We have previously shown that neither of these treatments with cocaine produces significant alterations in dopamine transporter density 1 day after the end of treatment. In contrast, methylphenidate injections significantly decreased dopamine transporters in rostral caudate putamen, with no change in nucleus accumbens. Continuous infusion of methylphenidate had no effect on dopamine transporters in either brain region. These findings provide further evidence that different classes of dopamine uptake inhibitors may interact with the dopamine transporter in qualitatively different manners. Furthermore, it is possible that the inhibition of serotonin uptake by cocaine may contribute to the adaptations in behavioral activity that are seen during chronic treatment.

Social and physical environment alter cocaine conditioned place preference and dopaminergic markers in adolescent male rats

This study was done to determine whether social and environmental factors alter cocaine reward and proteins implicated in mediating drug reward in rats during early adolescence. On postnatal day (PND) 23, rats were housed under conditions where both social (number of rats per cage) and environmental (availability of toys) factors were manipulated. Socially isolated rats were housed alone impoverished with no toys (II) or enriched with toys (IE). Social rats were housed two rats/cage with no toys (SI2) or with toys (SE2), or three/cage with (SE3) or without (SI3) toys. On PND 43, cocaine conditioned place preference (CPP) sessions began with the post-test done on PND 47. Cocaine CPP was established in response to 5 or 10 mg/kg cocaine in II rats, and CPP was decreased with the addition of cage mates or toys. No CPP was seen to any dose in SI3 or SE3 rats. Enriched housing (SE3) increased dopamine transporter (DAT) protein in the nucleus accumbens compared to II. There also were differential effects of cocaine on tyrosine hydroxylase and DAT depending on housing, with both increased by cocaine in II but not SE3 rats. DARPP-32 was unchanged by housing or cocaine, while phospho-Thr(34)-DARPP-32 was increased by cocaine treatment across conditions. Thus, both social and environmental enrichment decrease cocaine CPP during adolescence and different housing alters proteins that regulate dopaminergic neurotransmission in a manner that may account for the observed differences in cocaine-induced reward.