Age-related effects of chronic restraint stress on ethanol drinking, ethanol-induced sedation, and on basal and stress-induced anxiety response

Abstract

Adolescents are sensitive to the anxiolytic effect of ethanol, and evidence suggests that they may be more sensitive to stress than adults. Relatively little is known, however, about age-related differences in stress modulation of ethanol drinking or stress modulation of ethanol-induced sedation and hypnosis. We observed that chronic restraint stress transiently exacerbated free-choice ethanol drinking in adolescent, but not in adult, rats. Restraint stress altered exploration patterns of a light-dark box apparatus in adolescents and adults. Stressed animals spent significantly more time in the white area of the maze and made significantly more transfers between compartments than their non-stressed peers. Behavioral response to acute stress, on the other hand, was modulated by prior restraint stress only in adults. Adolescents, unlike adults, exhibited ethanol-induced motor stimulation in an open field. Stress increased the duration of loss of the righting reflex after a high ethanol dose, yet this effect was similar at both ages. Ethanol-induced sleep time was much higher in adult than in adolescent rats, yet stress diminished ethanol-induced sleep time only in adults. The study indicates age-related differences that may increase the risk for initiation and escalation in alcohol drinking.

Keywords: Adolescent; Adult; Ethanol; Rat; Restraint stress.

Fig. 1. Ethanol-induced behavioral stimulation activity in adolescent and adult rats

Locomotor activity (distance traveled, cm) in adolescent and adult Wistar rats exposed to 5 days of restraint stress (120 min per day), or non-stressed during postnatal days 30–34 or 70–74, for each age, respectively. Two hours following termination of the last stress session on postnatal day 34 or 74, the rats were given ethanol (2.5 g/kg, i.g.) or its vehicle (tap water) before locomotor activity was measured during 5–9 min post-administration time in an open field. The asterisk sign indicates a significant difference between adolescents given ethanol vs. those given vehicle (p < 0.05). The vertical bars indicate SEM.

Fig. 2. Ethanol intake in adolescent and adult rats after restraint stress

Absolute ethanol intake (g/kg) and percent ethanol preference scores (upper and lower panels, respectively) in adolescent and adult Wistar rats as a function of day of assessment (intake test sessions 1, 2, 3, and 4) and stress treatment experienced during postnatal days 30–34 or 70–74 (5 days of restraint stress [120 min per day] or non-stressed). In each daily intake test (duration: 120 min), animals had access to a bottle of water and a bottle of ethanol (ethanol concentration: 3, 4, 5, or 6% v/v, sessions 1 to 4; respectively). Please refer to the text for an account of significant differences across groups. Vertical lines indicate SEM.

Fig. 3

Mean ethanol intake (g/kg ingested and percent preference, left and right panels, respectively), across the 4 intake sessions conducted in Experiment 1, in adolescent and adult Wistar rats as a function of stress treatment experienced during postnatal days 30–34 or 70–74 (5 days of restraint stress [120 min per day] or non-stressed). The asterisk indicates that stress-exposed adolescents drank significantly more ethanol (g/kg) than any of the other groups (p<0.05). The pound sign indicates that stress-exposed adolescents exhibit significantly more ethanol percent preference than either non-stressed adolescents or stressed adults (p<0.05). Vertical lines indicate SEM.

Fig. 4. Baseline and acute-stress induced anxiety response in adolescent and adult rats

Upper panels: response in a 5-min light-dark box (LDB) test (time spent in the white area [sec], latency to escape to dark area [sec], and frequency of transfers between compartments, panel A, B, and C, respectively), in adolescent and adult Wistar rats as a function of stress treatment experienced during postnatal days 30–34 or 70–74 (5 days of restraint stress [120 min per day] or un-manipulated). Lower panels: immediately after the light-dark test, animals were exposed for 5 min to inescapable acute stress (confinement in the white area of the LDB, illuminated with 1200 lux). The panels depict locomotor activity (sec), frequency of wall climbing, and number of fecal boli during acute stress exposure (panels D, E, and F, respectively). The asterisks in panels A and C indicate that the stressed animals, both adolescents and adults, exhibited significantly more time in the bright compartment and moved significantly more often between compartments than control, non-stressed counterparts. The asterisk in panel E indicates that adolescents displayed significantly lower frequency of wall-climbing behaviors than adults. The asterisk in panel D indicates that adults, but not adolescents, with a previous history of stress exhibited lower levels of motor activity than their non-stressed age-matched controls (p<0.025). Vertical lines indicate SEM.

Fig. 5. Baseline and acute-stress induced hormonal response in adolescent and adult rats

Corticosterone response (ng/mL) in 38-day-old adolescent and 78-day-old adult Wistar rats before (i.e., baseline) and immediately after a 5-min exposure to inescapable acute stress (confinement in the white area of a light-dark box, illuminated with 1200 lux). Animals had experienced or not experienced chronic stress treatment during postnatal days 30–34 or 70–74 (5 days of restraint stress [120 min per day] or un-manipulated). The asterisk reflects a main effect of acute stress exposure (i.e., baseline vs. after stress). Vertical lines indicate SEM.

Fig. 6. Sensitivity to the sedative and narcotic effects of ethanol in adolescent and adult rats

Loss of righting reflex (sec) and sleep time (sec) after 4.0 or 4.5 g/kg ethanol (i.p.) in adolescent (panels A and B) and adult (panels C and D) Sprague-Dawley rats as a function of stress treatment experienced during postnatal days 30–34 or 70–74 (5 days of restraint stress [120 min per day] or non-stressed). The asterisk indicates a significant main effect of ethanol dose on sleep time. The pound sign indicates a significant main effect of prior stress exposure on ethanol-induced sleep time in adults. Vertical lines indicate SEM.