Selective breeding for intravenous drug self-administration in rats: a pilot study

Decreased anxiety-like behavior in a selectively bred high nicotine-preferring rat line

Genetic vulnerability contributes significantly to the individual variability observed in nicotine dependence. Selective breeding for sensitivity to a particular effect of abused drugs has produced rodent lines useful for studying genetic vulnerability to drug addiction. Previous research showed that anxiety-related personality traits are associated with nicotine dependence. Therefore, we examined the differences in anxiety-like behavior between a high nicotine-preferring rat line and their controls. At the beginning of the study, all rats, naïve to any drug, were exposed sequentially to open field arena, marble-burying and elevated plus-maze paradigms. In the second step, all rats received nicotine in drinking water for 7 weeks. Behavioral tests were rerun on the final 2 weeks of chronic nicotine treatment. Elevated plus-maze testings under basal condition and during chronic nicotine treatment showed that the time spent on the open arms, preference for being in the open arms, and the latency to enter the closed arms were higher, whereas open arm avoidance index was lower in nicotine-preferring rats compared to the controls. In the open field test, nicotine-preferring rats spent longer time in the central zone and excreted less fecal pellets; they buried less marbles in the marble-burying test. These findings indicate a lower level of anxiety-like behavior in nicotine-preferring rat line under basal conditions and during chronic nicotine treatment. We conclude that lower anxiety level in nicotine-preferring rat line is consistent with novelty-seeking personality type and may increase vulnerability to nicotine dependence in this rat line.

Changes Depression- and Anxiety- like Behaviors following Selective Breeding for Cocaine Reinforcement

The LS and HS are rat lines selectively bred for altered cocaine self-administration. Given the importance of mental health in substance use, these lines were evaluated for putative depression- and anxiety- like behaviors through forced swimming and exploration of a plus maze. We found increases of struggling in LS males, climbing in LS females, and swimming in HS males; with biphasic effects on immobility in the HS strain. HS rats had fewer entries into and less time spent in open arms of the plus maze, consistent with greater anxiety-like behavior, which may contribute to enhanced drug taking.

Multigenerational and transgenerational effects of paternal exposure to drugs of abuse on behavioral and neural function

Addictions are highly heritable disorders, with heritability estimates ranging from 39% to 72%. Multiple studies suggest a link between paternal drug abuse and addiction in their children. However, patterns of inheritance cannot be explained purely by Mendelian genetic mechanisms. Exposure to drugs of abuse results in epigenetic changes that may be passed on through the germline. This mechanism of epigenetic transgenerational inheritance may provide a link between paternal drug exposure and addiction susceptibility in the offspring. Recent studies have begun to investigate the effect of paternal drug exposure on behavioral and neurobiological phenotypes in offspring of drug-exposed fathers in rodent models. This review aims to discuss behavioral and neural effects of paternal exposure to alcohol, cocaine, opioids, and nicotine. Although a special focus will be on addiction-relevant behaviors, additional behavioral effects including cognition, anxiety, and depressive-like behaviors will be discussed.

Extracellular dopamine, acetylcholine, and activation of dopamine D1 and D2 receptors after selective breeding for cocaine self-administration in rats

RATIONALE

The low self-administration (LS)/Kgras (LS) and high self-administration (HS)/Kgras (HS) rat lines were generated by selective breeding for low- and high-intravenous cocaine self-administration, respectively, from a common outbred Wistar stock (Crl:WI). This trait has remained stable after 13 generations of breeding.

OBJECTIVE

The objective of the present study is to compare cocaine preference, neurotransmitter release, and dopamine receptor activation in LS and HS rats.

METHODS

Levels of dopamine, acetylcholine, and cocaine were measured in the nucleus accumbens (NA) shell of HS and LS rats by tandem mass spectrometry of microdialysates. Cocaine-induced locomotor activity and conditioned-place preference were compared between LS and HS rats.

RESULTS

HS rats displayed greater conditioned-place preference scores compared to LS and reduced basal extracellular concentrations of dopamine and acetylcholine. However, patterns of neurotransmitter release did not differ between strains. Low-dose cocaine increased locomotor activity in LS rats, but not in HS animals, while high-dose cocaine augmented activity only in HS rats. Either dose of cocaine increased immunoreactivity for c-Fos in the NA shell of both strains, with greater elevations observed in HS rats. Activation identified by cells expressing both c-Fos and dopamine receptors was generally greater in the HS strain, with a similar pattern for both D1 and D2 dopamine receptors.

CONCLUSIONS

Diminished levels of dopamine and acetylcholine in the NA shell, with enhanced cocaine-induced expression of D1 and D2 receptors, are associated with greater rewarding effects of cocaine in HS rats and an altered dose-effect relationship for cocaine-induced locomotor activity.

Long-lasting decreases in cocaine-reinforced behavior following treatment with the cholinesterase inhibitor tacrine in rats selectively bred for drug self-administration

Tacrine is a centrally acting, reversible cholinesterase inhibitor that increases synaptic levels of acetylcholine (ACh) and can potentiate the actions of dopamine (DA). The present study was conducted to evaluate effects of tacrine on cocaine-reinforced responding in a rat line selectively bred for high levels of drug self-administration (the HS line). HS rats self-administered different doses of cocaine under a fixed-ratio-5 (FR-5) schedule. Over a four-day period, vehicle or tacrine (1.0, 3.2, or 10 mg/kg-day) was infused when animals were maintained in home cages (21 h per day). Tacrine dose-dependently decreased cocaine self-administration. Actions of tacrine differed for self-administration that was initiated within 20 min of pretreatment (described as early sessions), and for self-administration that occurred between one and three days after administration of tacrine was discontinued (late sessions). Tacrine's potency for attenuating self-administration during late sessions was greater for cocaine- relative to food-reinforcement in HS rats, and for HS relative to outbred rats. In a subset of tacrine-treated HS rats, cocaine self-administration was persistently attenuated by more than 80% from pretreatment baseline levels over a one-week period during which no further tacrine was administered. In summary, pretreatment with tacrine can produce a long-lasting attenuation of cocaine-reinforced responding.