Repeated dextromethorphan administration in adolescent rats produces long-lasting behavioral alterations

Abstract

Initiation of non-medical dextromethorphan (DXM) use often occurs in adolescence, yet little is known about the consequences when use begins during this developmental period. The current experiments examined the acute response and the effects of repeated exposure to DXM in adolescence on behavior in adulthood. We examined locomotor activity, locomotor sensitization, and cognitive function, in rats that received repeated administration of DXM. Groups of adolescent (PND 30) and adult (PND 60) male rats were treated with DXM (60 mg/kg) once daily for 10 days. Locomotor activity in response to DXM was assessed following the first injection, on the 10th day of injection (adolescent - PND 39; adult - PND 69), and following 20 days of abstinence (adolescent - PND 59; adult - PND 89). Acute locomotor effects and locomotor sensitization were compared in adolescents and adults; cross-sensitization to ketamine, another dissociative with abuse potential, was also examined. In a separate group of rodents cognitive deficits were assessed following a 20 day abstinence period (adolescent - PND 59; adult - PND 89) in spatial learning and novel object recognition tasks. The locomotor stimulant effect of DXM was much greater in adolescents than adults. Also, only adolescent rats that were repeatedly administered DXM demonstrated locomotor sensitization at the end of 10 days of injection. However, sensitization occurred after the abstinence period in all rats regardless of age. Nonetheless, cross-sensitization to ketamine was only evident in adolescent-treated rats. DXM also led to an increase in perseverative errors in reversal learning only in the adolescent-treated group. We conclude that repeated use of DXM produces long-lasting neuroadaptations that may contribute to addiction. Deficits in cognitive flexibility occur in adolescents, although further work is necessary to confirm these findings. The results extend the understanding of potential long-term consequences of DXM use in adolescents and adults.

Keywords: Addiction; Adolescence; Cognition; Dextromethorphan; Ketamine; Locomotor; Neuroadaptations; Sensitization.

Fig. 1.

Schematic timeline of experimental design. In experiment 1, animals were examined for acute locomotor response, as well as sensitization to dextromethorphan (DXM). Adolescent (PND 30) and adult (PND 60) animals were administered cyclodextrin (CYCLO; vehicle) or DXM (60 mg/kg i.p.) and their locomotor response was measured. They were then given a single injection each day in their home cages for 8 days. On the 10th day, they were injected with DXM and their locomotor response was measured again. After this, they were given an abstinence period of 20 days, and their locomotor response was measured again when adolescents were PND 59 and adults were PND 89. Cross-sensitization to ketamine was measured a week later. In experiment 2, animals received 10 days of injections in their home cages (adolescents PND 30–39; adults PND 60–69). After a 20 day abstinence period, their response in the Novel Object Recognition Task and Barnes Maze were measured in the absence of drug. Following these tasks, their locomotor response to DXM was measured.

Fig. 2.

DXM stimulates acute locomotor response in adolescents but not adults. (A.) Timecourse of horizontal locomotor activity in response to saline or DXM (60 mg/kg) in adults (AD; 60 days of age) and periadolescents (PA; 30 days of age). DXM-induced activity was higher for adolescent (PND 30) animals than adults (PND 60). (B.) Total horizontal locomotor activity for 120 min following injection in each group. Significant differences between DXM AD and DXM PA animals (## = p < 0.01); CYCLO PA and DXM PA animals (** = p < 0.01).

Fig. 3.

DXM produces locomotor sensitization in adolescents but not adults following 10 days of treatment. (A.) Timecourse of horizontal locomotor activity in response to saline or DXM (60 mg/kg) following 10 days of treatment in adults (AD; 69 days of age) and adolescent (PA; 39 days of age). Adolescents treated with DXM exhibited significantly more horizontal activity. (B.) Total horizontal locomotor activity for 120 min following injection in each group. (C.) Total horizontal locomotor activity on Day 1 and Day 10 of treatment. Significant differences between CYCLO PA and DXM PA animals (* = p < 0.05, *** = p < 0.001); DXM AD and DXM PA animals (# = p < 0.05).

Fig. 4.

DXM produces persistent locomotor sensitization in adolescents and adults. (A.) Timecourse of horizontal locomotor activity in response to saline or DXM (60 mg/kg) for adult and formerly adolescent animals. Animals were treated for 10 days and then given a washout period of 20 days. On the day of the test, former adolescents were 59 days of age and adults were 89 days of age. Both DXM-treated groups displayed sensitization relative to cyclodextrin-treated controls and there was no difference in DXM-induced activity between animals treated as adolescents (PA) and adults (AD). (B.) Total horizontal locomotor activity for 120 min following injection in each group. Significant differences between CYCLO AD and DXM AD animals ($ $ = p < 0.01); CYCLO PA and DXM PA animals (** = p < 0.01).

Fig. 5.

Pretreatment with DXM produces persistent cross-sensitization to ketamine in adolescents. (A.) Timecourse of horizontal locomotor activity in response to KET challenge for adult (AD; 96 days of age) and formerly adolescent (PA; 66 days of age) animals. (B.) Total horizontal locomotor activity for 15 min following injection in each group. Both DXM-treated groups appeared to show an increase in ketamine-induced activity relative to cyclodextrin-treated controls, however the increase was statistically significant only animals treated with DXM as adolescents. There were no significant differences between the DXM-treated groups. Significant differences between CYCLO PA and DXM PA animals (* = p < 0.05).

Fig. 6.

DXM produces persistent deficit in novel object recognition (NOR) in Adults but not Adolescents. Animals were treated with DXM (60 mg/kg) or cyclodextrin, once daily for 10 days as periadolescents (PA; 30 days of age) or adults (AD; 60 days of age), and then given a 20-day washout period to allow for clearance of the drug. They were tested in the absence of drug following the washout. (A.) Total exploration time (in seconds) for Trial 1 (Sample Trial). (B.) Total exploration time (in seconds) for Trial 2 (Test Trial). There were no differences in total exploration time in either trial. (C.) Discrimination Index (D=N/N + F) of time spent with novel object (N) versus time spent with familiar object (F). There were no differences among groups. However, only animals treated with DXM as adults failed to show a significant difference from chance (one-sample t-test), suggesting that this group performed worse in novel object recognition. Significant difference in one-sample t-test (compared to 0.5) (A, p < 0.05).

Fig. 7.

No persistent effects of DXM on training on the Barnes Maze. Animals were treated with DXM (60 mg/kg) or cyclodextrin, once daily for 10 days as periadolescents (PA; 30 days of age) or adults (AD; 60 days of age), and then given a 20-day washout period to allow for clearance of the drug. They were tested in the absence of drug following the washout. (A.) Number of errors made across trials during the training session. (B). Mean number of errors in the training session. (C.) Time to goal box across trials during the training session. (D.) Total time to the goal box in the training session. The CYCLO PA group performed significantly more errors during Trial 1 of the training session than the CYCLO AD group (@ = p < 0.05). There were no other significant differences between groups during training.

Fig. 8.

No persistent effects of DXM on testing in the Barnes Maze. Animals were treated with DXM (60 mg/kg) or cyclodextrin, once daily for 10 days as periadolescents (PA; 30 days of age) or adults (AD; 60 days of age), and then given a 20-day washout period to allow for clearance of the drug. They were tested in the absence of drug following the washout. (A.) Number of errors made across trials during the test session. (B.) The mean number of errors in the test session. (C.) Time to goal box across trials during the test session. (D.) Total time to the goal box in the test session. There were no significant differences among groups in errors, but the CYCLO PA group did take more time to reach the goal box than the CYCLO AD group in the first trial of the test session (@ = p < 0.05).

Fig. 9.

Persistent effect of DXM on reversal in the Barnes Maze in adolescents but not adults. (A.) The number of errors made across trials per group during the reversal session. An ANOVA with repeated measures on trial demonstrated that animals efficiently learned the new set of rules required to successfully navigate the maze. (B.) The mean number of perseverative errors in the reversal session. (C.) Latency (time) to goal box across trials during the reversal session. (D.) The total time to the goal box in the reversal session. Animals treated with DXM as adolescents demonstrated a greater number of total perseverative errors and a trend toward increased latency to the goal box than age-matched controls. Significant differences between CYCLO PA and DXM PA animals (** = p < 0.01).

Fig. 10.

DXM produces persistent locomotor sensitization in adults. (A.) Timecourse of horizontal locomotor activity in response to saline or DXM (60 mg/kg) for adult (AD; 96 days of age) and formerly adolescent (PA; 66 days of age) animals. Animals were treated for 10 days and then given a washout period of 20 days. There was no difference in DXM-induced activity between animals treated as adolescents (PA) and adults (AD). (B.) Total horizontal locomotor activity for 120 min following injection in each group. These results were similar to the results in Experiment 1 (Fig. 4) in that DXM-treated animals showed increased activity compared to age-matched controls, however results were statistically significant only in adult-treated animals. Significant differences between CYCLO AD and DXM AD animals ($ = p < 0.01). There were no significant differences between DXM AD and DXM PA groups.