Early adolescents show enhanced acute cocaine-induced locomotor activity in comparison to late adolescent and adult rats

Abstract

Initiation of drug use during adolescence is associated with an increased probability to develop a drug addiction. The present study examined dose-response effects of cocaine (0, 5, 10, or 20 mg/kg, i.p.) on locomotor activity in early adolescent (postnatal day (PND) 35), late adolescent (PND 45), and young adults (PND 60) by measuring total distance moved (TDM) and frequency of start-stops. In response to 20 mg/kg cocaine, early adolescents showed the greatest cocaine-induced increase in TDM in comparison to late adolescent and adult rats. At this same dose, early adolescents showed the greatest cocaine-induced attenuation of start-stops relative to older rats. Results suggest that early adolescents engage in more cocaine-induced locomotor activity and less stationary behavior indicating that early adolescents are more sensitive to locomotor activating effects of high dose cocaine than older rats.

FIGURE 1. Total distance moved in a novel environment

Baseline total distance moved (TDM) data are shown collapsed across treatment groups and represent all animals tested at each age: PND 35, n =37; PND 45, n =40; PND 60, n =42. Baseline TDM decreased more rapidly for PND 35 and PND 45 relative to PND 60 rats. *PND 45 less than PND 60; **PND 35 and PND 45 less than PND 60. All data points represent mean and SEM.

FIGURE 2. Age effects in cocaine-induced total distance moved

Each panel represents 1/3 the total number of rats run in the present study. Rats per group were as follows: PND 35 (saline, n =9; 5 mg/kg cocaine, n =10; 10 mg/kg cocaine, n =9; 20 mg/kg cocaine, n =9). PND 45 (saline, n =10; 5 mg/kg cocaine, n =10; 10 mg/kg cocaine, n =10; 20 mg/kg cocaine, n =10). PND 60 (saline, 10; 5 mg/kg cocaine, 11; 10 mg/kg cocaine, 11; 20 mg/kg cocaine, 10). (A) PND 35 rats had greater TDM than PND 45 and PND 60 rats following 5 mg/kg cocaine at the 25 and 30 min time points. ^*PND 35 significantly differs from PND 45 and PND 60. (B) There were no significant age differences in TDM following 10 mg/kg cocaine. ^#Significantly differs from saline (100%). (C) PND 35 rats had greater TDM than PND 45 and PND 60 rats following 20 mg/kg cocaine. *35 significantly differs from PND 45 and 60; **35 significantly differs from PND 60. ^##PND 35, PND 45 and PND 60 significantly differs from saline (100%). All data points represent mean and SEM.

FIGURE 3. Start–stops in a novel environment

Baseline start–stop data are shown collapsed across treatment groups and represent all animals tested at each age: PND 35, n =37; PND 45, n =40; PND 60, n =42. In general, baseline start–stops were greater for early adolescents and late adolescents relative to adults. Each data point represents mean and SEM.

FIGURE 4. Age effects in cocaine-induced start–stops

Each panel represents 1/3 the total number of rats run in the present study. Rats per group were as follows: PND 35 (saline, n =9; 5 mg/kg cocaine, n =10; 10 mg/kg cocaine, n =9; 20 mg/kg cocaine, n =9). PND 45 (saline, n =10; 5 mg/kg cocaine, n =10; 10 mg/kg cocaine, n =10; 20 mg/kg cocaine, n =10). PND 60 (saline, n =10; 5 mg/kg cocaine, n =11; 10 mg/kg cocaine, n =11; 20 mg/kg cocaine, n =10). (A) PND 35 rats had more start–stops than PND 45 and PND 60 rats following 5 mg/kg cocaine at the 20 min time point. ^*35 significantly differs from PND 45 and PND 60. (B) There were no significant age differences in start–stops following 10 mg/kg cocaine. (C) PND 35 rats had less start–stops than PND 45 and PND 60 rats following 20 mg/kg cocaine. ^*35 significantly differs from PND 45 and PND 60. ^#Significantly differs from saline (100%). All data points represent mean and SEM.