CalDAG-GEFI mediates striatal cholinergic modulation of dendritic excitability, synaptic plasticity and psychomotor behaviors.

CalDAG-GEFI (CDGI) is a protein highly enriched in the striatum, particularly in the principal spiny projection neurons (SPNs). CDGI is strongly down-regulated in two hyperkinetic conditions related to striatal dysfunction: Huntington's disease and levodopa-induced dyskinesia in Parkinson's disease. We demonstrate that genetic deletion of CDGI in mice disrupts dendritic, but not somatic, M1 muscarinic receptors (M1Rs) signaling in indirect pathway SPNs. Loss of CDGI reduced temporal integration of excitatory postsynaptic potentials at dendritic glutamatergic synapses and impaired the induction of activity-dependent long-term potentiation. CDGI deletion selectively increased psychostimulant-induced repetitive behaviors, disrupted sequence learning, and eliminated M1R blockade of cocaine self-administration. These findings place CDGI as a major, but previously unrecognized, mediator of cholinergic signaling in the striatum. The effects of CDGI deletion on the self-administration of drugs of abuse and its marked alterations in hyperkinetic extrapyramidal disorders highlight CDGI's therapeutic potential.

Differential Effects of Nicotine and Nicotine Withdrawal on Fear Conditioning in Male Rats.

BACKGROUND: Tobacco use is prevalent in individuals who are routinely exposed to stress. However, little is known about how nicotine affects responses to trauma. We examined in rats how nicotine exposure affects fear conditioning, a procedure often used to study stress-related psychiatric illness. METHODS: We examined 2 methods of nicotine exposure: self-administration, modeling voluntary use, and experimenter-programmed subcutaneous administration, modeling medicinal administration (nicotine patch). For self-administered nicotine, rats trained to self-administer nicotine i.v. were fear conditioned (via light cue preceding foot-shock) either immediately after a 12-hour self-administration session or 12 hours later during a period with somatic signs of nicotine withdrawal. For experimenter-delivered nicotine, rats were conditioned after 1-21 days of nicotine delivered by programmable (12 hours on) subcutaneous mini-pumps. Tests to evaluate acoustic startle responses to the conditioning environment (context-potentiated startle) and in the presence or absence of the light cue (fear-potentiated startle) occurred after a 10-day period. RESULTS: Rats fear conditioned immediately after nicotine self-administration showed reduced responses to the shock-associated context, whereas those trained during nicotine withdrawal showed exaggerated responses. Experimenter-programmed nicotine produced effects qualitatively similar to those seen with self-administered nicotine. CONCLUSIONS: Self-administration or experimenter-programmed delivery of nicotine immediately before exposure to aversive events can reduce conditioned fear responses. In contrast, exposure to aversive events during nicotine withdrawal exacerbates fear responses. These studies raise the possibility of developing safe and effective methods to deliver nicotine or related drugs to mitigate the effects of stress while also highlighting the importance of preventing withdrawal in nicotine-dependent individuals.

Effects of Acute and Chronic Treatments with Dopamine D2 and D3 Receptor Ligands on Cocaine versus Food Choice in Rats.

Dopamine D3 receptor ligands are potential medications for psychostimulant addiction. Medication assessment may benefit from preclinical studies that evaluate chronic medication effects on choice between an abused drug and an alternative, nondrug reinforcer. This study compared acute and chronic effects of dopamine D2- and D3-preferring ligands on choice between intravenous cocaine and palatable food in rats. Under baseline conditions, cocaine maintained dose-dependent increases in cocaine choice and reciprocal decreases in food choice. Acutely, the D2 agonist R-(-)-norpropylapomorphine (NPA) and antagonist L-741,626 [3-[[4-(4-chlorophenyl)-4-hydroxypiperidin-l-yl]methyl-1H-indole] produced leftward and rightward shifts in cocaine dose-effect curves, respectively, whereas the partial agonist terguride had no effect. All three drugs dose-dependently decreased food-maintained responding. Chronically, the effects of R-(-)-norpropylapomorphine and L-741,626 on cocaine self-administration showed marked tolerance, whereas suppression of food-reinforced behavior persisted. Acute effects of the D3 ligands were less systematic and most consistent with nonselective decreases in cocaine- and food-maintained responding. Chronically, the D3 agonist PF-592,379 [5-[(2R,5S)-5-methyl-4-propylmorpholin-2-yl]pyridin-2-amine] increased cocaine choice, whereas an intermediate dose of the D3 antagonist PG01037 [N-[(E)-4-[4-(2,3-dichlorophenyl)piperazin-1-yl]but-2-enyl]-4-pyridin-2-ylbenzamide] produced a therapeutically desirable decrease in cocaine choice early in treatment; however, tolerance to this effect developed, and lower and higher doses were ineffective. D3 ligands failed to significantly modify total cocaine intake but caused persistent decreases in food intake. Thus, D2-and D3-preferring ligands showed distinct profiles, consistent with different pharmacological actions. In addition, these results highlight the role of acute versus chronic treatment as a determinant of test drug effects. With the possible exception of the D3 antagonist PG01037, no ligand was promising in terms of cocaine addiction treatment.

Effects of the GLP-1 Agonist Exendin-4 on Intravenous Ethanol Self-Administration in Mice.

BACKGROUND: Glucagon-like peptide 1 (GLP-1) receptor agonists have been shown to decrease ethanol (EtOH) drinking in rodent assays. The GLP-1 system also powerfully modulates food and fluid intake, gastrointestinal functions, and metabolism. To begin to understand the neurobiological mechanisms by which GLP-1 receptor ligands may be able to control EtOH intake, it is important to ascertain whether they can modulate the direct reinforcing effects of EtOH, without the confound of effects on ingestive behaviors generally. METHODS: We trained experimentally naïve, free-fed C57BL/6J mice to self-administer EtOH intravenously. Once stable EtOH intake was acquired, we tested the effect of acute pretreatment with the GLP-1 receptor agonist Exendin-4. Effect of Exendin-4 on operant behavior reinforced by a palatable liquid food was similarly evaluated as a control. RESULTS: Intravenous EtOH functioned as a positive reinforcer in over half the mice tested. In mice that acquired self-administration, EtOH intake was high, indeed, reaching toxic doses; 3.2 µg/kg Exendin-4 decreased intravenous EtOH intake by at least 70%, but had no significant effect on food-maintained operant responding. CONCLUSIONS: This experiment produced 2 main conclusions. First, although technically challenging and yielding only moderate throughput, the intravenous self-administration procedure in mice is feasible, and sensitive to pharmacological manipulations. Second, GLP-1 receptor agonists can powerfully attenuate voluntary EtOH intake by directly modulating the reinforcing effects of EtOH. These findings support the potential usefulness of GLP-1 receptor ligands in the treatment of alcohol use disorder.

Lack of abuse potential in a highly selective dopamine D3 agonist, PF-592,379, in drug self-administration and drug discrimination in rats.

Dopamine D3-preferring agonists are commonly used to treat Parkinson's disease and restless leg syndrome; however, laboratory animal studies suggest that they may possess a moderate abuse potential. These studies aimed to compare the highly selective, full D3 agonist PF-592,379 to the less selective D3 agonist 7-OH-DPAT, and the indirect dopamine agonist cocaine in drug self-administration and discrimination assays. Although rats readily acquired high rates of fixed ratio (FR)1 responding for cocaine, experimentally naive rats failed to acquire responding when 7-OH-DPAT or PF-592,379 was made available during an 18-session acquisition period. Cocaine also maintained dose-dependent levels of responding when available under a FR5 or a progressive ratio (PR) schedule of reinforcement. Although 7-OH-DPAT maintained modest levels of responding when substituted under a FR5, it failed to maintain significant levels of PR responding. PF-592,379 maintained saline-like rates of responding when substituted under FR5 or PR schedules of reinforcement. Similar behavioral profiles were observed in cocaine discrimination assays, with 7-OH-DPAT partially substituting for cocaine, and PF-592,379 producing saline-like effects over a wide range of doses. Together, the results of these studies predict that highly selective D3 agonists, such as PF-592,379, will have low abuse potential in humans.

False positive in the intravenous drug self-administration test in C57BL/6J mice.

The objective of this study was to examine C57BL/6J (B6) mice during extinction conditions, after food training, and for rates and patterns of operant behavior that seems similar to behavior maintained by intravenous cocaine injections. The rationale was to evaluate the potential for false positives in the intravenous self-administration test using protocols common in studies of knockout mice backcrossed to B6. An additional aim was to assess the influence of food-associated and drug-associated cues and mouse strain. Mice were allowed to acquire lever pressing reinforced by sweetened condensed milk under a fixed ratio 1 then fixed ratio 2 schedule of reinforcement accompanied by a flashing light. A catheter base was then implanted for simulation of intravenous self-administration conditions. Mice were allowed to lever press with cues remaining the same as during food training but without further scheduled consequences (i.e. no drug or food reinforcers delivered). All mice sustained lever pressing for several weeks, and over half met commonly used criteria for 'self-administration behavior.' Thus, B6 mice showed perseveration of a previously reinforced behavior that closely resembled rates and patterns of drug self-administration. This effect in B6 mice was greater than with A/J mice, and the lack of extinction was even more robust in the presence of cocaine-associated cues than with food-associated cues. We suggest that a necessary criterion for positive results in the intravenous drug self-administration test include an increase in responding when cocaine is made available after extinction with saline self-administration.

Dramatically decreased cocaine self-administration in dopamine but not serotonin transporter knock-out mice.

There has been much interest in the relative importance of dopamine and serotonin transporters in the abuse-related-effects of cocaine. We tested the hypotheses that mice lacking the dopamine transporter (DAT(-/-)), the serotonin transporter (SERT(-/-)), or both (DAT(-/-)SERT(-/-)) exhibit decreased reinforcing effects of cocaine. We also assessed whether observed effects on self-administration are specific to cocaine or if operant behavior maintained by food or a direct dopamine agonist are similarly affected. We used a broad range of experimental conditions that included acquisition without previous training, behavior established with food training and subsequent testing with food, cocaine or a direct dopamine agonist as reinforcers, fixed ratio and progressive ratio schedules of reinforcement, and a reversal procedure. Wild-type mice readily acquired cocaine self-administration and showed dose-response curves characteristic of the schedule of reinforcement that was used. While some DAT(-/-) mice appeared to acquire cocaine self-administration transiently, almost all DAT(-/-) mice failed to self-administer cocaine reliably. Food-maintained behaviors were not decreased by the DAT mutation, and IV self-administration of a direct dopamine agonist was robust in the DAT(-/-) mice. In contrast to those mice, cocaine's reinforcing effects were not diminished in SERT(-/-) mice under any of the conditions tested, except for impaired initial acquisition of both food- and cocaine-maintained behavior. These findings support the notion that the DAT, but not the SERT, is critical in mediating the reinforcing effects of cocaine.

Attenuation of cocaine's reinforcing and discriminative stimulus effects via muscarinic M1 acetylcholine receptor stimulation.

Muscarinic cholinergic receptors modulate dopaminergic function in brain pathways thought to mediate cocaine's abuse-related effects. Here, we sought to confirm and extend in the mouse species findings that nonselective muscarinic receptor antagonists can enhance cocaine's discriminative stimulus. More importantly, we tested the hypothesis that muscarinic receptor agonists with varied receptor subtype selectivity can blunt cocaine's discriminative stimulus and reinforcing effects; we hypothesized a critical role for the M(1) and/or M(4) receptor subtypes in this modulation. Mice were trained to discriminate cocaine from saline, or to self-administer intravenous cocaine chronically. The nonselective muscarinic antagonists scopolamine and methylscopolamine, the nonselective muscarinic agonists oxotremorine and pilocarpine, the M(1)/M(4)-preferring agonist xanomeline, the putative M(1)-selective agonist (4-hydroxy-2-butynyl)-1-trimethylammonium-3-chlorocarbanilate chloride (McN-A-343), and the novel M(1)-selective agonist 1-(1-2-methylbenzyl)-1,4-bipiperidin-4-yl)-1H benzo[d]imidazol-2(3H)-one (TBPB) were tested as substitution and/or pretreatment to cocaine. Both muscarinic antagonists partially substituted for cocaine and enhanced its discriminative stimulus. Conversely, muscarinic agonists blunted cocaine discrimination and abolished cocaine self-administration with varying effects on food-maintained behavior. Specifically, increasing selectivity for the M(1) subtype (oxotremorine < xanomeline < TBPB) conferred lesser nonspecific rate-suppressing effects, with no rate suppression for TBPB. In mutant mice lacking M(1) and M(4) receptors, xanomeline failed to diminish cocaine discrimination while rate-decreasing effects were intact. Our data suggest that central M(1) receptor activation attenuates cocaine's abuse-related effects, whereas non-M(1)/M(4) receptors probably contribute to undesirable effects of muscarinic stimulation. These data provide the first demonstration of anticocaine effects of systemically applied, M(1) receptor agonists and suggest the possibility of a new approach to pharmacotherapy for cocaine addiction.

Lack of cocaine self-administration in mice expressing a cocaine-insensitive dopamine transporter.

Cocaine addiction is a worldwide public health problem for which there are no established treatments. The dopamine transporter (DAT) is suspected as the primary target mediating cocaine's abuse-related effects based on numerous pharmacological studies. However, in a previous study, DAT knockout mice were reported to self-administer cocaine, generating much debate regarding the importance of the DAT in cocaine's abuse-related effects. Here, we show that mice expressing a "knockin" of a cocaine-insensitive but functional DAT did not self-administer cocaine intravenously despite normal food-maintained responding and normal intravenous self-administration of amphetamine and a direct dopamine agonist. Our results have three implications. First, they imply a crucial role for high-affinity DAT binding of cocaine in mediating its reinforcing effects, reconciling mouse genetic engineering approaches with data from classic pharmacological studies. Second, they demonstrate the usefulness of knockin strategies that modify specific amino acid sequences within a protein. Third, they show that it is possible to alter the DAT protein sequence in such a way as to selectively target its interaction with cocaine, while sparing other behaviors dependent on DAT function. Thus, molecular engineering technology could advance the development of highly specialized compounds such as a dopamine-sparing "cocaine antagonist."

Sex differences in opioid reinforcement under a fentanyl vs. food choice procedure in rats.

Clinical evidence suggest that men are more sensitive than women to the abuse-related effects of mu-opioid agonists. In contrast, preclinical studies suggest the opposite sex difference. The aim of the present study was to clarify this discrepancy using a fentanyl vs. diluted Ensure® choice procedure to assess sex differences in opioid reinforcement. Sex differences in intravenous (IV) fentanyl self-administration were examined under a fixed-ratio (FR5) schedule, a multi-day progressive-ratio (PR) schedule for behavioral economic analysis, and a concurrent (choice) schedule of fentanyl and diluted Ensure® reinforcement in Sprague-Dawley male and female rats. The fentanyl dose-effect function under the FR5 schedule was significantly shifted upward in females compared to males. Similarly, the reinforcing effectiveness of both fentanyl (3.2 and 10 µg/kg per injection, IV) and diluted Ensure® (18 and 56%) were greater in females than in males as assessed using behavioral economic analysis, irrespective of dose or concentration. However, under a fentanyl vs. foodchoice procedure, males chose 3.2 µg/kg per injection fentanyl injections over 18%, but not 56%, diluted Ensure® at a higher percentage compared to females. Overall, these results suggest that the expression of sex differences in opioid reinforcement depends upon the schedule of reinforcement and that preclinical opioid vs. food choice procedures provide a translationally relevant measure (i.e., behavioral allocation) consistent with the direction of sex differences reported in the clinical literature.

Lack of self-administration of cocaine in dopamine D1 receptor knock-out mice.

Evidence suggests a critical role for dopamine in the reinforcing effects of cocaine in rats and primates. However, self-administration has been less often studied in the mouse species, and, to date, "knock-out" of individual dopamine-related genes in mice has not been reported to reduce the reinforcing effects of cocaine. We studied the dopamine D1 receptor and cocaine self-administration in mice using a combination of gene-targeted mutation and pharmacological tools. Two cohorts with varied breeding and experimental histories were tested, and, in both cohorts, there was a significant decrease in the number of D1 receptor knock-out mice that met criteria for acquisition of cocaine self-administration (2 of 23) relative to wild-type mice (27 of 32). After extinction of responding with saline self-administration, dose-response studies showed that cocaine reliably and dose dependently maintained responding greater than saline in all wild-type mice but in none of the D1 receptor knock-out mice. The D1-like agonist SKF 82958 (2,3,4,5,-tetrahydro-6-chloro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine hydrobromide) and the D2-like agonist quinelorane both functioned as positive reinforcers in wild-type mice but not in D1 receptor mutant mice, whereas food and intravenous injections of the opioid agonist remifentanil functioned as positive reinforcers in both genotypes. Finally, pretreatment with the D1-like antagonist SCH 23390 [R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepine-7-01] produced surmountable antagonism of the reinforcing effects of cocaine in the commonly used strain C57BL/6J. We conclude that D1 receptor knock-out mice do not reliably self-administer cocaine and that the D1 receptor is critical for the reinforcing effects of cocaine and other dopamine agonists, but not food or opioids, in mice.

Effects of acute and chronic aripiprazole treatment on choice between cocaine self-administration and food under a concurrent schedule of reinforcement in rats.

RATIONALE: Dopamine D2-like partial agonists such as aripiprazole have received some attention as potential pharmacotherapies for the treatment of psychostimulant addiction. However, the preclinical evaluations so far have focused on acute effects of aripiprazole. OBJECTIVES: We tested the hypothesis that aripiprazole, both as acute and as chronic treatment, would preferentially decrease cocaine self-administration while sparing behavior maintained by a natural reinforcer, resulting in a shift in the allocation of behavior from cocaine-taking towards the alternative reinforcer. MATERIALS AND METHODS: Rats were trained to self-administer intravenous cocaine in a concurrent choice procedure, with a palatable food as the competing reinforcer, under a fixed ratio (FR) 1 FR 5 chain schedule. Aripiprazole was then administered as continuous infusion by osmotic minipumps for 5 days, during which performance in the choice procedure was assessed daily. RESULTS: An intermediate dose of aripiprazole decreased cocaine self-administration and shifted the cocaine choice curve to the right as an acute treatment. However, as a chronic treatment, aripiprazole failed to decrease cocaine self-administration or cocaine choice, despite a dose-dependent decrease in overall response rates and food-maintained behavior. CONCLUSIONS: Our results confirm and extend earlier findings and indicate that acute administration of aripiprazole can decrease cocaine self-administration. However, based on the present data, chronic treatment with aripiprazole does not show much promise as a potential pharmacotherapy for cocaine addiction. Both acute and chronic treatment data are in agreement with published clinical findings, suggesting that the concurrent choice procedure in rats has predictive validity of efficacy in humans.

Effects of selective dopamine D1-like and D2-like agonists on prepulse inhibition of startle in inbred C3H/HeJ, SPRET/EiJ, and CAST/EiJ mice.

RATIONALE: Prepulse inhibition (PPI) and locomotor activity have been used to investigate the effects of antipsychotic and stimulant drugs and their underlying dopaminergic mechanisms. Whereas D2-like agonists consistently decreased PPI and increased locomotion in rats in previous studies, we recently reported that these hallmark behavioral effects were not observed in several mouse strains. Nevertheless, we recently identified three mouse strains (C3H/HeJ, SPRET/EiJ, and CAST/EiJ) that exhibited locomotor hyperactivity after administration of a selective D2-like agonist. OBJECTIVES: We hypothesized that, similar to rats, C3H/HeJ, SPRET/EiJ, and CAST/EiJ mice would exhibit decreased PPI after administration of a D2-like agonist. RESULTS: Administration of the D2-like agonist quinelorane dose-dependently decreased PPI in C3H/HeJ and SPRET/EiJ mice. In agreement with previous reports in rats and other strains of mice, the D1-like agonist R-6-Br-APB also decreased PPI in C3H/HeJ and SPRET/EiJ mice. In contrast, CAST/EiJ mice had low levels of baseline PPI in our standard test session and quinelorane and R-6-Br-APB had no effect on PPI under those conditions. Through the optimization of session parameters, we obtained higher baseline PPI in CAST/EiJ mice and found that quinelorane but not R-6-Br-APB decreased PPI. In summary, similar to rats and unlike previous published reports on several strains of mice, we have now identified three strains of mice in which a D2-like agonist decreased PPI. CONCLUSIONS: The C3H/HeJ, SPRET/EiJ, and CAST/EiJ mice may more closely mirror the Sprague Dawley rat than most other mouse strains and may confer advantages in cross-species behavioral pharmacology studies related to D2 receptor function.

Decreased prepulse inhibition and increased sensitivity to muscarinic, but not dopaminergic drugs in M5 muscarinic acetylcholine receptor knockout mice.

RATIONALE: Schizophrenic patients show decreased measures of sensorimotor gating, such as prepulse inhibition of startle (PPI). In preclinical models, these measures may be used to predict antipsychotic activity. While current antipsychotic drugs act largely at dopamine receptors, the muscarinic acetylcholine receptors offer promising novel pharmacotherapy targets. Of these, the M(5) receptor gene was recently implicated in susceptibility to schizophrenia. Due to the lack of selective ligands, muscarinic receptor knockout mice have been generated to elucidate the roles of the five receptor subtypes (M(1)-M(5)). OBJECTIVES: Here, we used M(5) receptor knockout (M(5)-/-) mice to investigate the involvement of M(5) receptors in behavioral measures pertinent to schizophrenia. We tested the hypothesis that disruption of M(5) receptors affected PPI or the effects of muscarinic or dopaminergic agents in PPI or psychomotor stimulation. MATERIALS AND METHODS: We measured PPI in M(5)-/-, heterozygous and wild-type mice without drugs, and with clozapine (0.56-3.2 mg/kg) or haloperidol (0.32-3.2 mg/kg) alone, and as pretreatment to D: -amphetamine. In addition, we evaluated locomotor stimulation by the muscarinic antagonist trihexyphenidyl (0.56-56 mg/kg) and by cocaine (3.2-56 mg/kg). RESULTS: The M(5)-/- mice showed decreased PPI relative to wild-type mice, and clozapine appeared to reduce this difference, while haloperidol increased PPI regardless of genotype. The M(5)-/- mice also showed more locomotor stimulation by trihexyphenidyl than wild-type mice, while cocaine had similar effects between genotypes. CONCLUSIONS: These data suggest that disruption of the M(5) receptor gene affected sensorimotor gating mechanisms, increased sensitivity to clozapine and to the psychostimulant effects of muscarinic antagonists without modifying the effect of dopaminergic drugs.

Reduced cocaine self-administration in muscarinic M5 acetylcholine receptor-deficient mice.

The reinforcing effects of cocaine have been related to increased extracellular concentrations of dopamine in the ventral striatum. Several studies suggest that M5 muscarinic receptors facilitate striatal dopamine release. We tested the hypothesis that the reinforcing effects of cocaine are decreased in M5 receptor-deficient mice using chronic intravenous cocaine self-administration in extensively backcrossed mice. We also assessed whether operant performance generally, rather than cocaine self-administration specifically, was altered in the mutant mice. To this end, we evaluated both food-maintained operant behavior and cocaine self-administration under a fixed ratio 1 and a progressive ratio (PR) schedule of reinforcement. We also evaluated acquisition of self-administration in experimentally naive mice using several doses of cocaine. M5 receptor deletion decreased self-administration of low to moderate doses of cocaine under a PR schedule of reinforcement and diminished acquisition of self-administration of a low dose in experimentally naive mice. We found no differences between genotypes in food-maintained behavior. The present study extends our previous findings using backcrossed mice and covering various experimental conditions. Our results indicate that M5 receptor deletion diminished the reinforcing effects of low doses of cocaine and identified specific conditions under which this may be observed.

Cocaine self-administration under fixed and progressive ratio schedules of reinforcement: comparison of C57BL/6J, 129X1/SvJ, and 129S6/SvEvTac inbred mice.

RATIONALE: Combining strains to generate mutant mice may obscure conclusions regarding the targeted gene. Specifically, cocaine may have reduced reinforcing effects in 129 substrains compared to the C57BL/6 strain, commonly used for ES cells and breeding, respectively. OBJECTIVES: We tested the hypothesis that reinforcing effects of cocaine differ between the C57BL/6J strain and two substrains of 129, 129X1/SvJ and 129S6/SvEvTac. METHODS: To assess and reduce performance differences, operant responding was established with liquid food as a reinforcer and evaluated under fixed and progressive ratio schedules. Dose-effect functions for intravenous cocaine self-administration were then determined under both schedules. Finally, reinforced and nonreinforced manipulanda were reversed to assess acquisition of self-administration using a previously nonreinforced response. RESULTS: Relative to C57BL/6J mice, 129X1/SvJ mice showed decreased reinforcing effects of low-magnitude food and cocaine reinforcers. Dose-effect functions for cocaine self-administration were comparable between C57BL/6J and 129S6/SvEvTac mice, despite delayed acquisition of operant behaviors and rightward shifts in the food concentration-effect functions in 129S6/SvEvTac mice. A high cocaine dose clearly served as a positive reinforcer in all three strains in a reversal procedure. CONCLUSIONS: Relative to C57BL/6J mice, the reinforcing effects of cocaine were diminished in 129X1/SvJ mice, but only for low cocaine doses, and a similar profile was observed with food reinforcement. 129S6/SvEvTac mice required more extensive operant training than C57BL/6J mice did, but after acquisition, reinforcing effects of cocaine were similar in the two strains. We suggest that comparable phenotypes observed in gene-targeting studies may result from genetic background, whereas more profound or qualitatively different phenotypes may be more confidently attributed to targeted mutations.

Role of dopamine D2-like receptors in cocaine self-administration: studies with D2 receptor mutant mice and novel D2 receptor antagonists.

Dopamine receptor subtypes have been classified generally as D1-like (e.g., D1, D5) or D2-like (D2, D3, D4), and converging evidence suggests that D2-like receptors may be especially important in mediating the abuse-related effects of cocaine. However, it has been difficult to differentiate the roles of the D2-like receptor subtypes in the behavioral effects of cocaine because of the relatively low selectivity of drugs for D2, D3, and D4 receptors in vivo. The goal of the present series of studies was to investigate the contributions of D2-like receptor subtypes in the reinforcing effects of cocaine using new genetic and pharmacological tools. First, we evaluated cocaine self-administration behavior, and related effects of nonselective D2-like drugs, in mutant mice that lack the D2 receptor but express D3 and D4 receptors. When high doses of cocaine on the descending limb of the cocaine dose-effect function were available, D2 mutant mice self-administered at higher rates than their heterozygous or wild-type littermates, but the ascending limb of the cocaine dose-effect function did not differ between genotypes. Elevated rates of drug intake were not attributable to nonspecific increases in response rate, because response rates maintained by presentation of a range of food concentrations were significantly lower in D2 mutant mice than in wild-type mice. In wild-type mice, pretreatment with the D2-like antagonist eticlopride increased rates of self-administration of high doses of cocaine, and the D2-like agonist quinelorane served as a positive reinforcer when substituted for cocaine. However, these effects of eticlopride and quinelorane were not observed in mice that lacked the D2 receptor. Next, we compared the effects of novel antagonists selective for different D2 receptor subtypes on cocaine self-administration behavior in outbred rats. In rats, a D2 selective antagonist increased rates of self-administration of high doses of cocaine and also combinations of cocaine and the D2-like agonist quinelorane, whereas D3/D4 antagonists were ineffective. Collectively, these findings suggest that the D2 receptor is not necessary for cocaine self-administration, but this receptor subtype is involved in mechanisms that limit rates of high-dose cocaine self-administration. Our results also suggest that D3 and D4 receptors do not play major roles in the modulation of cocaine self-administration by D2-like drugs.

Effect of gonadectomy and gonadal hormone replacement on cocaine self-administration in female and male rats.

Both sex and gonadal steroid hormones may influence the abuse-related behavioral effects of cocaine under some conditions, but there is considerable inconsistency in the literature. In the present study, rats were trained under a fixed ratio (FR) 5 schedule of food presentation and were then allowed to self-administer cocaine (1.0 mg/kg/injection) until behavior stabilized. Subsequently, complete dose-effect functions for cocaine self-administration (0.032-3.2 mg/kg/injection) were determined in female and male rats before and after gonadectomy, and in gonadectomized female and male rats before and during chronic treatment with estradiol or testosterone, respectively. Sex, gonadectomy, and gonadal hormones did not alter the shape or position of dose-effect functions for cocaine self-administration. These results suggest that sex, estrogen, and testosterone levels are not critical determinants of cocaine's reinforcing effects in rats under these conditions. This study differed from earlier studies in that complete dose-effect functions for cocaine were determined. These findings suggest that the behavioral training history, the unit dose of cocaine, and the schedule of reinforcement are important variables in studies of sex and gonadal hormone effects on cocaine self-administration.

Effects of dopamine indirect agonists and selective D1-like and D2-like agonists and antagonists on cocaine self-administration and food maintained responding in rats.

A procedure is described for comprehensive evaluation of the effects of acute drug pretreatments on the reinforcing effects of cocaine using the rat self-administration assay in combination with a novel control assay of liquid-food maintained responding. In sessions comprised of five 20-min components, either complete dose-effect functions for cocaine self-administration or complete concentration-effect functions for liquid-food maintained responding were evaluated. The schedule of reinforcement (FR 5 TO 20-s), drug pretreatment doses and time intervals (0-30 min), and duration of sessions (108 min) were identical for cocaine- and food-reinforced test sessions. Whereas acute pretreatment with indirect dopamine agonists (D-amphetamine, GBR 12909) and D2-like agonists (7-OH-DPAT, quinelorane) produced dose-dependent leftward shifts in dose-effect functions for cocaine self-administration, D1-like agonists (SKF 82958, R-6-Br-APB) and dopamine antagonists (D1-like, SCH 39166; D2-like, eticlopride) shifted dose-effect functions for cocaine downward and rightward, respectively. Interestingly, with the indirect dopamine agonists but not the D2-like agonists, increased responding maintained by low cocaine doses was paralleled by increased responding maintained by low food concentrations. Moreover, three of the four direct agonists were moderately selective (< or =5-fold more potent) in decreasing cocaine self-administration relative to food maintained responding. When data were analyzed according to alterations in total cocaine intake, all of the agonists uniformly decreased total cocaine intake, whereas both antagonists increased total cocaine intake. Overall, this procedure was sensitive to leftward, downward and rightward shifts in cocaine dose-effect functions and should be useful for evaluating the nature of pharmacological interactions between novel compounds and self-administered cocaine, as well as the potential for altering cocaine self-administration selectively with candidate treatments for cocaine abuse and dependence.

Acute and chronic effects of the M1/M4-preferring muscarinic agonist xanomeline on cocaine vs. food choice in rats.

RATIONALE: We previously showed that the M1/M4-preferring muscarinic agonist xanomeline can acutely attenuate or eliminate cocaine self-administration in mice. OBJECTIVE: Medications used to treat addictions will arguably be administered in (sub)chronic or repeated regimens. Tests of acute effects often fail to predict chronic effects, highlighting the need for chronic testing of candidate medications. METHODS: Rats were trained to lever press under a concurrent FR5 FR5 schedule of intravenous cocaine and food reinforcement. Once baseline behavior stabilized, the effects of 7 days once-daily injections of xanomeline were evaluated. RESULTS: Xanomeline pretreatment dose-dependently (1.8-10 mg/kg/day) shifted the dose-effect curve for cocaine rightward (up to 5.6-fold increase in A 50), with reallocation of behavior to the food-reinforced lever. There was no indication of tolerance, rather effects grew over days. The suppression of cocaine choice appeared surmountable at high cocaine doses, and xanomeline treatment did not significantly decrease total-session cocaine or food intake. CONCLUSIONS: In terms of xanomeline's potential for promoting abstinence from cocaine in humans, the findings were mixed. Xanomeline did produce reallocation of behavior from cocaine to food with a robust increase in food reinforcers earned at some cocaine/xanomeline dose combinations. However, effects appeared surmountable, and food-maintained behavior was also decreased at some xanomeline/cocaine dose combinations, suggesting clinical usefulness may be limited. These data nevertheless support the notion that chronic muscarinic receptor stimulation can reduce cocaine self-administration. Future studies should show whether ligands with higher selectivity for M1 or M1/M4 subtypes would be less limited by undesired effects and can achieve higher efficacy.