De Novo Mutation Rate Variation and Its Determinants in Chlamydomonas.

De novo mutations are central for evolution, since they provide the raw material for natural selection by regenerating genetic variation. However, studying de novo mutations is challenging and is generally restricted to model species, so we have a limited understanding of the evolution of the mutation rate and spectrum between closely related species. Here, we present a mutation accumulation (MA) experiment to study de novo mutation in the unicellular green alga Chlamydomonas incerta and perform comparative analyses with its closest known relative, Chlamydomonas reinhardtii. Using whole-genome sequencing data, we estimate that the median single nucleotide mutation (SNM) rate in C. incerta is µ = 7.6 × 10-10, and is highly variable between MA lines, ranging from µ = 0.35 × 10-10 to µ = 131.7 × 10-10. The SNM rate is strongly positively correlated with the mutation rate for insertions and deletions between lines (r > 0.97). We infer that the genomic factors associated with variation in the mutation rate are similar to those in C. reinhardtii, allowing for cross-prediction between species. Among these genomic factors, sequence context and complexity are more important than GC content. With the exception of a remarkably high C→T bias, the SNM spectrum differs markedly between the two Chlamydomonas species. Our results suggest that similar genomic and biological characteristics may result in a similar mutation rate in the two species, whereas the SNM spectrum has more freedom to diverge.

Extensive de novo mutation rate variation between individuals and across the genome of Chlamydomonas reinhardtii.

Describing the process of spontaneous mutation is fundamental for understanding the genetic basis of disease, the threat posed by declining population size in conservation biology, and much of evolutionary biology. Directly studying spontaneous mutation has been difficult, however, because new mutations are rare. Mutation accumulation (MA) experiments overcome this by allowing mutations to build up over many generations in the near absence of natural selection. Here, we sequenced the genomes of 85 MA lines derived from six genetically diverse strains of the green alga Chlamydomonas reinhardtii. We identified 6843 new mutations, more than any other study of spontaneous mutation. We observed sevenfold variation in the mutation rate among strains and that mutator genotypes arose, increasing the mutation rate approximately eightfold in some replicates. We also found evidence for fine-scale heterogeneity in the mutation rate, with certain sequence motifs mutating at much higher rates, and clusters of multiple mutations occurring at closely linked sites. There was little evidence, however, for mutation rate heterogeneity between chromosomes or over large genomic regions of 200 kbp. We generated a predictive model of the mutability of sites based on their genomic properties, including local GC content, gene expression level, and local sequence context. Our model accurately predicted the average mutation rate and natural levels of genetic diversity of sites across the genome. Notably, trinucleotides vary 17-fold in rate between the most and least mutable sites. Our results uncover a rich heterogeneity in the process of spontaneous mutation both among individuals and across the genome.

Higher resources decrease fluctuating selection during host-parasite coevolution.

We still know very little about how the environment influences coevolutionary dynamics. Here, we investigated both theoretically and empirically how nutrient availability affects the relative extent of escalation of resistance and infectivity (arms race dynamic; ARD) and fluctuating selection (fluctuating selection dynamic; FSD) in experimentally coevolving populations of bacteria and viruses. By comparing interactions between clones of bacteria and viruses both within- and between-time points, we show that increasing nutrient availability resulted in coevolution shifting from FSD, with fluctuations in average infectivity and resistance ranges over time, to ARD. Our model shows that range fluctuations with lower nutrient availability can be explained both by elevated costs of resistance (a direct effect of nutrient availability), and reduced benefits of resistance when population sizes of hosts and parasites are lower (an indirect effect). Nutrient availability can therefore predictably and generally affect qualitative coevolutionary dynamics by both direct and indirect (mediated through ecological feedbacks) effects on costs of resistance.

Spontaneous mutation accumulation in multiple strains of the green alga, Chlamydomonas reinhardtii.

Estimates of mutational parameters, such as the average fitness effect of a new mutation and the rate at which new genetic variation for fitness is created by mutation, are important for the understanding of many biological processes. However, the causes of interspecific variation in mutational parameters and the extent to which they vary within species remain largely unknown. We maintained multiple strains of the unicellular eukaryote Chlamydomonas reinhardtii, for approximately 1000 generations under relaxed selection by transferring a single cell every ~10 generations. Mean fitness of the lines tended to decline with generations of mutation accumulation whereas mutational variance increased. We did not find any evidence for differences among strains in any of the mutational parameters estimated. The overall change in mean fitness per cell division and rate of input of mutational variance per cell division were more similar to values observed in multicellular organisms than to those in other single-celled microbes. However, after taking into account differences in genome size among species, estimates from multicellular organisms and microbes, including our new estimates from C. reinhardtii, become substantially more similar. Thus, we suggest that variation in genome size is an important determinant of interspecific variation in mutational parameters.

Estimate of the spontaneous mutation rate in Chlamydomonas reinhardtii.

The nature of spontaneous mutations, including their rate, distribution across the genome, and fitness consequences, is of central importance to biology. However, the low rate of mutation has made it difficult to study spontaneous mutagenesis, and few studies have directly addressed these questions. Here, we present a direct estimate of the mutation rate and a description of the properties of new spontaneous mutations in the unicellular green alga Chlamydomonas reinhardtii. We conducted a mutation accumulation experiment for ∼350 generations followed by whole-genome resequencing of two replicate lines. Our analysis identified a total of 14 mutations, including 5 short indels and 9 single base mutations, and no evidence of larger structural mutations. From this, we estimate a total mutation rate of 3.23 × 10(-10)/site/generation (95% C.I. 1.82 × 10(-10) to 5.23 × 10(-10)) and a single base mutation rate of 2.08 × 10(-10)/site/generation (95% C.I., 1.09 × 10(-10) to 3.74 × 10(-10)). We observed no mutations from A/T → G/C, suggesting a strong mutational bias toward A/T, although paradoxically, the GC content of the C. reinhardtii genome is very high. Our estimate is only the second direct estimate of the mutation rate from plants and among the lowest spontaneous base-substitution rates known in eukaryotes.

Selection on non-social traits limits the invasion of social cheats.

While the conditions that favour the maintenance of cooperation have been extensively investigated, the significance of non-social selection pressures on social behaviours has received little attention. In the absence of non-social selection pressures, patches of cooperators are vulnerable to invasion by cheats. However, we show both theoretically, and experimentally with the bacterium Pseudomonas fluorescens, that cheats may be unable to invade patches of cooperators under strong non-social selection (both a novel abiotic environment and to a lesser extent, the presence of a virulent parasite). This is because beneficial mutations are most likely to arise in the numerically dominant cooperator population. Given the ubiquity of novel selection pressures on microbes, these results may help to explain why cooperation is the norm in natural populations of microbes.

Host-parasite coevolutionary arms races give way to fluctuating selection.

Host-parasite coevolution is a key driver of biological diversity and parasite virulence, but its effects depend on the nature of coevolutionary dynamics over time. We used phenotypic data from coevolving populations of the bacterium Pseudomonas fluorescens SBW25 and parasitic phage SBW25Φ2, and genetic data from the phage tail fibre gene (implicated in infectivity evolution) to show that arms race dynamics, typical of short-term studies, decelerate over time. We attribute this effect to increasing costs of generalism for phages and bacteria with increasing infectivity and resistance. By contrast, fluctuating selection on individual host and parasite genotypes was maintained over time, becoming increasingly important for the phenotypic properties of parasite and host populations. Given that costs of generalism are reported for many other systems, arms races may generally give way to fluctuating selection in antagonistically coevolving populations.

Comparative analysis of myxococcus predation on soil bacteria.

Predator-prey relationships among prokaryotes have received little attention but are likely to be important determinants of the composition, structure, and dynamics of microbial communities. Many species of the soil-dwelling myxobacteria are predators of other microbes, but their predation range is poorly characterized. To better understand the predatory capabilities of myxobacteria in nature, we analyzed the predation performance of numerous Myxococcus isolates across 12 diverse species of bacteria. All predator isolates could utilize most potential prey species to effectively fuel colony expansion, although one species hindered predator swarming relative to a control treatment with no growth substrate. Predator strains varied significantly in their relative performance across prey types, but most variation in predatory performance was determined by prey type, with Gram-negative prey species supporting more Myxococcus growth than Gram-positive species. There was evidence for specialized predator performance in some predator-prey combinations. Such specialization may reduce resource competition among sympatric strains in natural habitats. The broad prey range of the Myxococcus genus coupled with its ubiquity in the soil suggests that myxobacteria are likely to have very important ecological and evolutionary effects on many species of soil prokaryotes.

Impact of bacterial mutation rate on coevolutionary dynamics between bacteria and phages.

Mutator bacteria are frequently found in natural populations of bacteria and although coevolution with parasitic viruses (phages) is thought to be one reason for their persistence, it remains unclear how the presence of mutators affects coevolutionary dynamics. We hypothesized that phages must themselves adapt more rapidly or go extinct, in the face of rapidly evolving mutator bacteria. We compared the coevolutionary dynamics of wild-type Pseudomonas fluorescens SBW25 with a lytic phage to the dynamics of an isogenic mutator of P. fluorescens SBW25 together with the same phage. At the beginning of the experiment both wild-type bacteria and mutator bacteria coevolved with phages. However, mutators rapidly evolved higher levels of sympatric resistance to phages. The phages were unable to "keep-up" with the mutator bacteria, and these rates of coevolution declined to less than the rates of coevolution between the phages and wild-type bacteria. By the end of the experiment, the sympatric resistance of the mutator bacteria was not significantly different to the sympatric resistance of the wild-type bacteria. This suggests that the importance of mutators in the coevolutionary interactions with a particular phage population is likely to be short-lived. More generally, the results demonstrate that coevolving enemies may escape from Red-Queen dynamics.

Selective breeding for differential saccharin intake as an animal model of drug abuse.

A positive relationship between the consumption of sweetened dietary substances (e.g. saccharin and sucrose) and drug abuse has been reported in both the human and other animal literature. The proposed genetic contribution to this relationship has been based on evidence from behavioral, neurobiological, and linkage studies in heterogeneous and homogeneous animal populations. Initial work in several laboratories indicated that rodents that are selected for high alcohol consumption also display an increased preference for sweets compared with low alcohol-consuming animals. More recently, Sprague-Dawley rats have been selectively bred based on high saccharin (HiS) or low saccharin (LoS) consumption, and these lines represent an ideal opportunity to determine whether a reciprocal genetic relationship exists between the consumption of sweetened substances and self-administration of drugs of abuse. The purpose of this review is to examine a series of studies on the HiS and LoS rats for drug-seeking and drug-taking behavior using laboratory animal models that represent critical phases of drug abuse in humans. The data support the hypothesis that sweet consumption and drug self-administration are closely related and genetically influenced. Other characteristics of HiS and LoS rats are discussed as possible mediators of the genetic differences such as activity, impulsivity, novelty reactivity, stress, and emotionality. The interaction of sweet preference with biological variables related to drug abuse, such as age, sex, and hormonal influences, was considered, as they may be additive vulnerability factors with consumption of sweet substances. In the studies that are discussed, the HiS and LoS lines emerge as ideal addiction-prone and addiction-resistant models, respectively, with vulnerability or resilience factors that will inform prevention and treatment strategies for drug abuse.

Higher locomotor response to cocaine in female (vs. male) rats selectively bred for high (HiS) and low (LoS) saccharin intake.

Rats selectively bred for high saccharin consumption (HiS) self-administer more oral ethanol and i.v. cocaine than those selectively bred for low saccharin consumption (LoS). Male and female drug-seeking-prone (HiS) and -resistant (LoS) rats were used in the present experiment to test the prediction that cocaine-induced locomotor activity and sensitization varied with sex and their selective breeding status (HiS and LoS). All rats were intermittently exposed over 2 weeks to pairs of sequential saline and cocaine injections, separated by 45 min. The first 5 pairs of injections, each separated by 2-3 days (10-12 days total), were given to examine the development of cocaine-induced locomotor activity and the development of locomotor sensitization, which was determined by comparing the effects of cocaine injection 1 with injection 6 (given 2 weeks after the 5 pairs of intermittent injections). Results indicated that after the first injection pair (saline, cocaine) the HiS and LoS groups did not differ (saline vs. cocaine) in locomotor activity; however, after cocaine injection pairs 1, 5, and 6, HiS females were more active than HiS males and LoS females. There were also significant phenotype differences (HiS>LoS) in locomotor activity after cocaine injections 5 and 6. There was a weak sensitization effect in cocaine-induced locomotor activity in HiS females after cocaine injection 5 (compared to 1); however it was not present after injection 6 or in other groups. The lack of a strong sensitization effect under these temporal and dose conditions was inconsistent with previous reports. However, the results showing HiS>LoS and females>males on cocaine-induced activity measures are consistent with several measures of cocaine-seeking behavior (acquisition, maintenance, escalation, extinction, and reinstatement), and they suggest that cocaine-induced locomotor activity and sensitization are behavioral markers of drug-seeking phenotypes.

Impulsivity (delay discounting) for food and cocaine in male and female rats selectively bred for high and low saccharin intake.

Previous research in rats indicates that delay discounting for food, a model of impulsivity, predicted the rate of acquisition of cocaine self-administration. In other studies, rats bred for high saccharin intake (HiS) acquired cocaine self-administration at higher rates than those with low saccharin intake (LoS), and female (F) rats acquired cocaine self-administration more rapidly than males (M). The purpose of this study was to examine a possible connection between impulsivity, saccharin intake, and sex by comparing M and F rats from the HiS and LoS selectively bred lines on measures of impulsivity; i.e., their rate of delay discounting for food or i.v. cocaine infusions. The adjusting delay procedure allowed rats access to 2 response levers, and a pellet dispenser or an i.v. drug infusion pump. In 4 groups (HiS M, HiS F, LoS M, LoS F) responses under a fixed-ratio (FR) 1 schedule on one lever resulted in one 45 mg pellet immediately, and responses on the other lever resulted in 3 or 6 pellets after a delay. Four additional groups received either a small cocaine (0.2, 0.4, or 0.8 mg/kg) infusion immediately or a delayed larger infusion (3x the amount of the small infusions). The delay to the larger reinforcer began at 6 s and increased or decreased by 1 s following responses on the delay or immediate levers, respectively. A mean adjusted delay (MAD) was calculated over 30 choice trials during each daily 3-hour session, and it was used as a quantitative measure of impulsivity. In groups maintained by food, HiS rats were more impulsive (lower MADs) than LoS rats, and LoS females were more impulsive than LoS males. There were no phenotype or sex differences in delay discounting for cocaine. Understanding the relationship between impulsivity and other predictors of drug abuse (e.g., sex, saccharin intake) is important in developing prevention and treatment strategies.

Experimental coevolution with bacteria and phage. The Pseudomonas fluorescens--Phi2 model system.

Parasites are ubiquitous in biological systems and antagonistic coevolution between hosts and parasites is thought be a major ecological and evolutionary force. Recent experiments using laboratory populations of bacteria and their parasitic viruses, phage, have provided the first direct empirical evidence of antagonistic coevolution in action. In this article we describe this model system and synthesise recent findings that address the causes and consequences of antagonistic coevolution.

Differential impact of simultaneous migration on coevolving hosts and parasites.

BACKGROUND: The dynamics of antagonistic host-parasite coevolution are believed to be crucially dependent on the rate of migration between populations. We addressed how the rate of simultaneous migration of host and parasite affected resistance and infectivity evolution of coevolving meta-populations of the bacterium Pseudomonas fluorescens and a viral parasite (bacteriophage). The increase in genetic variation resulting from small amounts of migration is expected to increase rates of adaptation of both host and parasite. However, previous studies suggest phages should benefit more from migration than bacteria; because in the absence of migration, phages are more genetically limited and have a lower evolutionary potential compared to the bacteria. RESULTS: The results supported the hypothesis: migration increased the resistance of bacteria to their local (sympatric) hosts. Moreover, migration benefited phages more than hosts with respect to 'global' (measured with respect to the whole range of migration regimes) patterns of resistance and infectivity, because of the differential evolutionary responses of bacteria and phage to different migration regimes. Specifically, we found bacterial global resistance peaked at intermediate rates of migration, whereas phage global infectivity plateaued when migration rates were greater than zero. CONCLUSION: These results suggest that simultaneous migration of hosts and parasites can dramatically affect the interaction of host and parasite. More specifically, the organism with the lower evolutionary potential may gain the greater evolutionary advantage from migration.

Regulation of intravenous cocaine self-administration in rats selectively bred for high (HiS) and low (LoS) saccharin intake.

RATIONALE: Rats selectively bred for high saccharin (HiS) intake consume more alcohol and acquire intravenous (i.v.) cocaine self-administration more rapidly than their low saccharin (LoS)-consuming counterparts. The present experiment was designed to determine whether HiS and LoS rats differed in other aspects of drug abuse. OBJECTIVE: The purpose of the present experiment was to use a two-lever dose self-selection procedure to investigate the regulation/dysregulation of i.v. cocaine self-administration in female HiS and LoS rats. MATERIALS AND METHODS: HiS and LoS rats were trained to self-administer eight different doses of cocaine during daily 5-h sessions, with the cocaine doses ranging from 0.2 to 1.6 mg/kg in steps of 0.2 mg/kg. The dose size increased after a response on one lever (infusion duration lengthened by 3 s) and decreased after a response on the other lever (infusion duration shortened by 3 s), with a lower limit of 0 s and with an upper limit of 24 s (a corresponding range of 0 to 1.6 mg/kg); the animals increased or decreased their self-administered dose in nine discrete steps. RESULTS: The HiS rats showed less precise regulation of their postinfusion interval (PII) than LoS rats based on the size of the previously self-administered cocaine dose. Correlations between these variables were lower for HiS than for LoS rats during the acquisition and maintenance phases, and HiS rats had lower PIIs than LoS rats after many of the cocaine doses. The HiS rats also self-administered significantly more cocaine infusions during the maintenance phase than the LoS rats, especially at the highest dose. CONCLUSIONS: These data indicate that HiS rats are more likely to self-administer more cocaine infusions, at higher doses, and with less precise dose-time regulation than LoS rats. Thus, rats selectively bred for HiS showed less inhibitory control over their cocaine intake than LoS rats, suggesting that a genetic predisposition for saccharin preference is related to the rewarding effects of i.v. cocaine.

Escalation of i.v. cocaine self-administration and reinstatement of cocaine-seeking behavior in rats bred for high and low saccharin intake.

RATIONALE: Rats selectively bred for high saccharin (HiS) intake consume more alcohol, acquire intravenous (i.v.) cocaine self-administration more rapidly, and show more dysregulated patterns of cocaine self-administration than their low saccharin-consuming (LoS) counterparts. OBJECTIVES: The purpose of the present study was to determine whether HiS and LoS rats also differ in the escalation, maintenance, extinction, and reinstatement of i.v. cocaine self-administration. MATERIALS AND METHODS: Two experiments were conducted in separate groups of rats. In the first experiment, HiS and LoS female rats were allowed to self-administer cocaine [0.4 mg/kg; fixed ratio (FR) 1] under short (ShA, 2 h per day) or long (LgA, 12 h per day) access conditions for 21 days. Session lengths were subsequently equated (2 h), and FR1-maintained cocaine self-administration was examined. In the second experiment, additional groups of HiS and LoS female rats were given access to cocaine (0.4 mg/kg; FR 1) self-administration during 2-h sessions for 10 days. Subsequently, saline was substituted for cocaine, and responding was extinguished. After a 14-day extinction period, saline- and cocaine-[5, 10, and 15 mg/kg, intraperitoneal (i.p.)] induced reinstatement of drug-seeking behavior was measured. RESULTS: HiS LgA rats escalated their cocaine intake more rapidly than LoS rats, and during the 2 h sessions after escalation cocaine self-administration was significantly higher in HiS LgA rats, compared to LoS LgA rats. HiS rats responded on the cocaine-paired lever more than LoS rats during maintenance, extinction, and cocaine-(15 mg/kg) induced reinstatement. CONCLUSIONS: These results suggest that HiS and LoS rats have distinct drug-seeking and drug-taking profiles. The HiS and LoS rats differ along a wide range of behavioral dimensions and represent an important model to study the interactions of excessive intake of dietary substances and vulnerability to drug abuse.

Sex differences in physical dependence on orally self-administered phencyclidine (PCP) in rhesus monkeys (Macaca mulatta).

Withdrawal from orally self-administered phencyclidine (PCP) has been shown to alter operant baselines of food-maintained responding. The goal of the present study was to determine whether there are sex differences in these alterations. Seven female and 7 male rhesus monkeys (Macaca mulatta) were given concurrent access to PCP and water under fixed ratio (FR) 8 schedules during 2 daily sessions that alternated with 2 sessions during which pellet deliveries were contingent on lever presses under an FR 64 schedule. After operant responding stabilized, PCP was replaced by water for 10 days, and food access remained under the same schedule. Subsequently, concurrent PCP and water access was reintroduced for 10 days. This procedure was repeated with 3 PCP concentrations (0.125, 0.25, and 0.50 mg/ml) and 3 FR requirements for food-reinforced responding (64, 128, and 256). Disruptions in operant responding for food served as a quantitative measure of withdrawal severity. During PCP withdrawal, males showed a greater suppression of food-maintained behavior than females at the 2 highest PCP concentrations and the lowest FR requirement tested. Males responded more than females for PCP; however, when weight was taken into consideration, PCP intake (milligrams per kilogram) in males and females was equal. The data suggest that males may experience more severe withdrawal effects than females, and the duration of the adverse effects of withdrawal lasts longer in males than in females. This study is the 1st to use nonhuman primates to document sex differences in withdrawal severity as measured by a quantifiable baseline.

The effect of migration on local adaptation in a coevolving host-parasite system.

Antagonistic coevolution between hosts and parasites in spatially structured populations can result in local adaptation of parasites; that is, the greater infectivity of local parasites than foreign parasites on local hosts. Such parasite specialization on local hosts has implications for human health and agriculture. By contrast with classic single-species population-genetic models, theory indicates that parasite migration between subpopulations might increase parasite local adaptation, as long as migration does not completely homogenize populations. To test this hypothesis we developed a system-specific mathematical model and then coevolved replicate populations of the bacterium Pseudomonas fluorescens and a parasitic bacteriophage with parasite only, with host only or with no migration. Here we show that patterns of local adaptation have considerable temporal and spatial variation and that, in the absence of migration, parasites tend to be locally maladapted. However, in accord with our model, parasite migration results in parasite local adaptation, but host migration alone has no significant effect.

Sex differences in the escalation of oral phencyclidine (PCP) self-administration under FR and PR schedules in rhesus monkeys.

RATIONALE: Studies with male rats indicate that long access (LgA) vs short access (ShA) to i.v. cocaine and heroin self-administration leads to an escalation of drug intake and a subsequent upward shift of the dose-response function. OBJECTIVE: The purpose of this experiment was to extend these results to male and female rhesus monkeys and oral phencyclidine (PCP) self-administration under fixed-ratio (FR) and progressive-ratio (PR) schedules. METHODS: Adult rhesus monkeys (seven females and nine males) orally self-administered PCP (0.25 mg/ml) and water under concurrent FR 16 FR 16 schedules during daily ShA 3-h sessions. Since females weighed less than males, each liquid delivery (0.6 ml) represented a higher unit dose mg/kg for females than males, but drug concentration mg/ml remained constant. Concurrent PR PR schedules were then used to obtain a concentration-response function (0.125, 0.25, 0.5, and 1.0 mg/ml). Next, PCP and water were available during LgA 6-h sessions under concurrent FR 16 FR 16 schedules for 21 days. The monkeys were then retested under the concurrent FR 16 FR 16 and PR PR conditions during ShA sessions. RESULTS: Under the initial ShA concurrent FR 16 FR 16 schedules, females and males did not differ on PCP deliveries or intake (mg/kg); however, during LgA, males and females had more PCP deliveries compared with ShA. During LgA, males exceeded females in PCP deliveries, but females were higher than males in mg/kg PCP intake. Also, PCP (but not water) deliveries and mg/kg PCP intake significantly increased from the first 3 days to the last 3 days of the 21-day LgA period in both males and females. The subsequent ShA FR 16 FR 16 performance did not differ by sex, but it was significantly elevated above the first ShA period in both sexes. The concentration-response function for PCP break point under the PR PR schedules and PCP intake (mg/kg) were significantly shifted upward during the second (vs first) ShA period, and females' mg/kg intake significantly exceeded males'. CONCLUSIONS: Male and female rhesus monkeys both showed escalation of PCP self-administration during LgA to PCP and during ShA that occurred after (vs before) LgA. Both showed vertical upward shifts in the concentration x intake (mg/kg) function under the PR schedule, and females exceeded males on this measure. These findings with PCP and monkeys are consistent with vertical upward shifts of cocaine dose-response functions in previous escalation studies in male rats and reports of sex differences (F>M) during several other phases of drug abuse.

Escalation of intravenous cocaine self-administration, progressive-ratio performance, and reinstatement in rats selectively bred for high (HiS) and low (LoS) saccharin intake.

RATIONALE: Rats selectively bred for high saccharin (HiS) intake consume more alcohol and acquire intravenous (IV) cocaine self-administration more rapidly than their low saccharin (LoS) consuming counterparts. OBJECTIVES: The purpose of the present study was to determine whether HiS and LoS rats also differ in the escalation, maintenance, and reinstatement of IV cocaine self-administration. METHODS: LoS and HiS female rats were allowed to self-administer cocaine [0.4 mg/kg; fixed ratio (FR) 1] under short (ShA, 2 h per day) or long (LgA, 12 h per day) access conditions for 21 days. Session lengths were subsequently equated (2 h) and (1) FR1-maintained cocaine (0.4 mg/kg) self-administration, (2) progressive ratio (PR)-maintained cocaine (0.2-1.6 mg/kg) self-administration, and (3) saline-induced and cocaine (10 mg/kg, IP)-induced reinstatement of drug-seeking behavior were examined. RESULTS: HiS LgA rats escalated their cocaine intake more rapidly and self-administered more cocaine (mg/kg) than LoS LgA rats; however, there was no LoS versus HiS phenotype difference in the number of infusions self-administered by Day 21. Post-escalation cocaine self-administration under an FR1 schedule did not differ as a function of phenotype (LoS versus HiS) or access condition (ShA versus LgA); however, LoS rats responded more for cocaine under the PR schedule than HiS rats, and they showed a greater reinstatement of cocaine-seeking behavior than HiS rats. In contrast, ShA versus LgA did not affect PR or reinstatement performance in the LoS and HiS groups. CONCLUSIONS: These results suggest that LoS and HiS rats have distinct drug-seeking and drug-taking profiles that differ as a function of the experimental phase and access condition. The LoS and HiS rats differ along a wide range of behavioral dimensions and represent an important model to study the interactions of feeding, emotionality, and other factors related to vulnerability to drug abuse.