To breed or not to breed? Empirical evaluation of drug effects in adolescent rats

Abstract

The recent upsurge of research on adolescent rats raises the issue of the extent to which different methods of rodent procurement might affect results. Here, we examined the effects of acute and repeated dosing with two antipsychotics, haloperidol and clozapine, and Delta(9)-tetrahydrocannabinol [Experiments 1 and 2, respectively] in adolescent rats of both sexes that differed in shipping status (i.e., shipped from a commercial breeder at weaning or bred in-house). In each experiment, test drugs produced effects that were characteristic for their respective classes in previous studies with adult rodents. Both haloperidol and clozapine produced catalepsy and haloperidol decreased locomotion in shipped and bred rats of both sexes, with sensitization to haloperidol-induced catalepsy developing with repeated dosing. The most prominent between-status difference in this experiment was greater sensitivity of the shipped rats to haloperidol-induced changes in locomotor activity over a wider dose range, an effect that was especially evident in females. In Experiment 2, vehicle levels of motor activity were decreased in bred rats (which did not occur in Experiment 1), resulting in flattening of the Delta(9)-tetrahydrocannabinol dose-effect curve for this measure in bred rats of each sex. Acutely, Delta(9)-tetrahydrocannabinol produced antinociception, hypothermia and catalepsy in both groups of rats, with tolerance developing after repeated dosing. Status-related differences were sex-dependent. Whereas bred female rats were more sensitive to Delta(9)-tetrahydrocannabinol's antinociceptive effects, shipped male rats were more sensitive to its antinociceptive effects as well as to its hypothermic and cataleptic effects. Together, the results of these descriptive experiments suggest that between-status differences tend to be quantitative rather than qualitative. Further, these results suggest that careful attention to issues related to rodent procurement during adolescence is warranted and may help to account for divergent findings in different labs.

Fig. 1

Effects of haloperidol on catalepsy at 30, 45 or 60 min post-injection (1st, 2nd and 3rd row panels, respectively) in male (left panels) and female (right panels) as a function of age (PN30–PN39) in adolescent rats that were shipped from breeder (filled symbols) or bred and born in-house (open symbols). Catalepsy was measured as the percentage of time during 5-min session spent with forepaws on an elevated bar. Each point represents the mean (±S.E.M.) value for 5–7 adolescent rats. Status × day ANOVAs were performed on the effects of 0.3 mg/kg haloperidol only. Status main effects and status × day interactions were not significant at any assessment time. + Indicates significant main effect of day (vs. PN30). p < 0.05 for all significant differences.

Fig. 2

Effects of haloperidol on locomotor activity (measured as number of photocell beam breaks during a 15-min session) as a function of age (PN30–PN39) in male (top panels) and female (bottom panels) adolescent rats that were shipped from breeder (left panels) or bred and born in-house (right panels). Each point represents the mean (±S.E.M.) value for 5–7 adolescent rats. Three-way (status × dose × day) ANOVA was performed on data for each sex. # Indicates a significant status × dose interaction with post hoc determination of difference between shipped and bred rats in the effects of the specified haloperidol dose on locomotor activity. * Indicates a significant status × dose interaction with post hoc determination of difference in effect of haloperidol dose on activity (vs. saline) for rats of the same status (shipped or bred). $ Indicates significant main effect of haloperidol dose (vs. saline) without regard to shipping status. Circles around a PN day indicate a significant difference in the effects of haloperidol in shipped versus bred rats on that day (status × day interaction). p < 0.05 for all significant differences.

Fig. 3

Effects of clozapine on catalepsy at 30, 45 or 60 min post-injection (1st, 2nd and 3rd row panels, respectively) and on locomotor activity at 65 min post-injection (bottom panels) as a function of age (PN30–PN39) in male adolescent rats that were shipped from breeder (left panels) or bred and born in-house (right panels). Catalepsy was measured as the percentage of time during 5-min session spent with forepaws on an elevated bar. Locomotor activity was measured as number of photocell beam breaks during a 15-min session. Each point represents the mean (±S.E.M.) value for 5–7 adolescent rats. At all three catalepsy assessment times, shipped rats showed less catalepsy than rats bred in-house (three-way ANOVA, main effect of status). $ Indicates significant main effect of clozapine dose (vs. saline) without regard to shipping status. p < 0.05 for all significant differences.

Fig. 4

Effects of clozapine on catalepsy at 30, 45 or 60 min post-injection (1st, 2nd and 3rd row panels, respectively) and on locomotor activity at 65 min post-injection (bottom panels) as a function of age (PN30–PN39) in female adolescent rats that were shipped from breeder (left panels) or bred and born in-house (right panels). Catalepsy was measured as the percentage of time during 5-min session spent with forepaws on an elevated bar. Locomotor activity was measured as number of photocell beam breaks during a 15-min session. Each point represents the mean (±S.E.M.) value for 5–7 adolescent rats. $ Indicates significant, but transient difference, in locomotor activity between dose and saline groups at indicated day (two-way ANOVA, dose × day interaction). p < 0.05 for all significant differences.

Fig. 5

Body weights (in g) of male and female adolescent rats (left and right panels, respectively) that were shipped or bred in-house. Top panels show weights of rats before acute dosing with 0.3 mg/kg haloperidol, 30 mg/kg clozapine or saline (filled bars) and after repeated dosing with indicated treatment (unfilled bars). Bottom panels show weights of rats in the groups that received 100 mg/kg Δ⁹-THC or vehicle before (filled bars) and after repeated dosing Δ⁹-THC (unfilled bars). Each point represents the mean (±S.E.M.) value for 5–7 rats. $ Indicates significant status × drug interaction (at specified PN day) with post hoc difference between weights of in-house bred male rats (vs. shipped rats) that received the same treatment. # Indicates main effect of status for female rats before treatment. * Indicates significant status × drug interaction and post hoc difference between weights in shipped male rats that were to receive vehicle as compared to those that were to receive THC. p < 0.05 for all significant differences.

Fig. 6

Effects of Δ⁹-THC on locomotor activity (top panels), antinociception (2nd row panels), body temperature (3rd row panels), and catalepsy-like behavior (bottom panels) in female and male adolescent rats (left and right panels, respectively) that were shipped from breeder (circles) or bred and born in-house (squares). Unfilled symbols show responses before sub-chronic dosing with Δ⁹-THC and filled symbols show catalepsy after sub-chronic dosing Δ⁹-THC. Each point represents the mean (±S.E.M.) value for 5–7 adolescent rats. A significant main effect of status on locomotor activity was noted for both sexes, with bred rats exhibiting less activity than shipped rats regardless of treatment condition. + Indicates a significant difference between shipped and bred rats in the effects of a specific dose of Δ⁹-THC. # Indicates a significant difference between shipped and bred rats in the overall acute dose–effect function of Δ⁹-THC. * Indicates that the specified dose of Δ⁹-THC produced an effect (compared to respective vehicle) in rats of one status but not the other. p < 0.05 for all significant differences.