Characterization of Highper, an ENU-induced mouse mutant with abnormal psychostimulant and stress responses

Abstract

Rationale: Chemical mutagenesis in the mouse is a forward genetics approach that introduces random mutations into the genome, thereby providing an opportunity to annotate gene function and characterize phenotypes that have not been previously linked to a given gene.

Objectives: We report on the behavioral characterization of Highper, an N-ethyl-N-nitrosourea (ENU)-induced mutant mouse line.

Methods: Highper and B6 control mice were assessed for locomotor activity in the open field and home cage environments. Basal and acute restraint stress-induced corticosterone levels were measured. Mice were tested for locomotor response to cocaine (5, 20, 30, and 45 mg/kg), methylphenidate (30 mg/kg), and ethanol (0.75, 1.25, and 1.75 g/kg). The rewarding and reinforcing effects of cocaine were assessed using conditioned place preference and self-administration paradigms.

Results: Highper mice are hyperactive during behavioral tests but show normal home cage locomotor behavior. Highper mice also exhibit a twofold increase in locomotor response to cocaine, methylphenidate, and ethanol and prolonged activation of the hypothalamic-pituitary-adrenal axis in response to acute stress. Highper mice are more sensitive to the rewarding and reinforcing effects of cocaine, although place preference in Highper mice appears to be significantly influenced by the environment in which the drug is administered.

Conclusions: Altogether, our findings indicate that Highper mice may provide important insights into the genetic, molecular, and biological mechanisms underlying stress and the drug reward pathway.

Fig. 1

Highper mice are hyperactive in the open field. a Two G3 outliers (circled) from the same pedigree were identified as affected mutants. b Comparison of B6 wild-type and Highper F1 mice generated by crossing an affected F2 back to B6 to determine mode of inheritance. c Open-field data from presumed homozygous Highper mice at the F2–F8 generations and B6 controls show that the Highper mutation is likely a recessive mutation with approximately 50 % penetrance. Solid and broken lines represent the mean and two SD from the mean, respectively. d Highper mice do not exhibit hyperactivity in the home cage environment in either the dark or the light part of the circadian cycle. Error bars are SEM

Fig. 2

Habituation alters open-field locomotor response to an acute dose of 20 mg/kg cocaine in Highper mice. a Cocaine-induced psychomotor response is significantly greater in Highper mice when the drug is administered following habituation to the open-field arena. b Dose response to four doses of cocaine in Highper and B6 mice. Cocaine-induced locomotion was normalized to saline-induced locomotion to correct for baseline activity differences. Error bars are SEM. *p < 0.05

Fig. 3

Highper mice exhibit increased context-specific place preference and acute locomotor response to 20 mg/kg cocaine. a Acute locomotor response was determined by comparing activity in the conditioned place preference apparatus on day 3 (first cocaine treatment) and day 2 (saline). b Cocaine was administered either in the black or white CPP chamber. Place preference was significantly higher in Highper mice when cocaine was paired with the white chamber but not the black chamber. c Locomotor activity was higher in the black chamber of the place preference apparatus during habituation (day 1) and conditioning (days 2–9). Chamber-specific activity differences were present in both saline- and cocaine-treated animals. Error bars are SEM. *p < 0.05, **p < 0.01, and ***p < 0.001

Fig. 4

Highper mice are sensitive to the rewarding effects of cocaine. a Highper mice self-administered more cocaine at the lower doses (0.125, 0.25, and 0.5 mg/kg/inf). b, c Female, but not male, Highper mice worked harder for cocaine in a progressive ratio schedule of self-administration. d All mice show extinction of cocaine-seeking behavior across ten sessions, whereas Highper males had higher levels of responding relative to B6 males. Error bars are SEM

Fig. 5

Highper mice have prolonged HPA activation in response to restraint-induced stress. Corticosterone levels (nanograms per milliliter) were assessed at baseline and 30- and 120-min post-restraint for female (a) and male mice (b). Error bars are SEM. *p < 0.05