Early maternal separation affects ethanol-induced conditioning in a nor-BNI insensitive manner, but does not alter ethanol-induced locomotor activity

Abstract

Early environmental stress significantly affects the development of offspring. This stress has been modeled in rats through the maternal separation (MS) paradigm, which alters the functioning of the HPA axis and can enhance ethanol intake at adulthood. Infant rats are sensitive to ethanol's reinforcing effects, which modulate ethanol seeking and intake. Little is known about the impact of MS on sensitivity to ethanol's appetitive and aversive effects during infancy. The present study assessed ethanol-induced conditioned place preference established through second-order conditioning (SOC), spontaneous or ethanol-induced locomotor activity and ethanol intake in preweanling rats that experienced normal animal facility rearing (AFR) or daily episodes of maternal separation (MS) during postnatal days 1-13 (PDs 1-13). Low-ethanol dose (0.5 g/kg) induced appetitive conditioned place preference (via SOC) in control rats given conventional rearing but not in rats given maternal separation in early infancy, whereas 2.0 g/kg ethanol induced aversive conditioned place preference in the former but not the latter. The administration of a kappa antagonist at PD 1 or immediately before testing did not alter ethanol-induced reinforcement. High (i.e., 2.5 and 2.0 g/kg) but not low (i.e., 0.5 g/kg) ethanol dose induced reliable motor stimulation, which was independent of early maternal separation. Ethanol intake and blood alcohol levels during conditioning were unaffected by rearing conditions. Pups given early maternal separation had lower body weights than controls and showed an altered pattern of exploration when placed in an open field. These results indicate that, when assessed in infant rats, earlier maternal separation alters the balance between the appetitive and aversive motivational effects of ethanol but has no effect on the motor activating effects of the drug.

Figure 1

Methods for analyzing the effects of early maternal separation on ethanol-mediated second-order conditioning. Phase 1, first-order conditioning, postnatal day (PD) 14: Pups were removed from the maternal cage, cannulated and then briefly (10 min) habituated to the experimental context. They were then given ethanol (0.0, 0.5 or 2.0 g/kg, Experiment 1; 0.0 or 2.0 g/kg, Experiment 2) and stimulated with a conditioned stimulus (CS1) consisting of intraoral pulses of water. CS1 delivery occurred 5-20 min after ethanol. Phase 2, second-order conditioning, postnatal day 15: Animals were stimulated with water pulses while placed in a sandpaper-lined compartment (CS2). Phase 3, tactile preference test, postnatal day 15: Time spent on sandpaper was recorded during a 5 min preference test. The figure and legend were adapted with permission from Pautassi et al., 2011a.

Figure 2

Ethanol-induced second-order conditioning in infant rats that experienced either normal animal facility rearing (AFR, upper panel) or daily episodes of maternal separation (MS, lower panel) during postnatal days (PD) 1 to 13, with percent time spent in the rough CS2 texture (sandpaper) during the test as a function of ethanol treatment. On PD1 animals were administered nor-BNI (0.0, 10 or 30 mg/kg, ip). During conditioning on PD14 animals were given ethanol administration (0.5, 2.0 or 0.0 g/kg, i.g.), and were then stimulated with intraoral pulses of water (CS1). The following day, animals experienced CS1-CS2 pairings and were then tested for CS2 preference. To facilitate data visualization, data have been collapsed nor-BNI treatment. The latter factor did not affect tactile preference scores nor significantly interacted with the remaining factors. Asterisks (*) indicate significant differences between an ethanol-treated group and its corresponding vehicle-treated control (p < 0.05).Vertical bars indicate the SEM.

Figure 3

Ethanol-induced second-order conditioning in infant rats that experienced daily episodes of maternal separation during postnatal days (PD) 1 to 13, with percent time spent in the rough CS2 texture (sandpaper) during the test as a function of nor-BNI treatment (0.0 or 10 mg/kg, ip) and ethanol treatment (0.0 or 2.0 g/kg) On PD13, after termination of the last episode of maternal deprivation, animals were administered with nor-BNI. During conditioning on PD14 animals were given ethanol administration and were then stimulated with intraoral pulses of water (CS1). The following day animals experienced CS1-CS2 pairings and were then tested for CS2 preference. Ethanol (2.0 g/kg) readily induced aversive second-order conditioning and nor-BNI treatment did not alter this result. The ANOVA indicated a main effect of ethanol treatment (F_1,37 = 8.13, p < 0.01), that did not interact with sex or nor-BNI treatment. Vertical bars indicate the SEM.

Figure 4

Ethanol-induced locomotor activity (distance traveled, cm) 5-9, 10-14, and 15-19 min after ethanol administration (bins 1, 2, and 3, respectively) in infant rats that experienced either normal animal facility rearing (AFR, left panel) or daily episodes of maternal separation (MS, right panel) during postnatal days (PD) 1 to 13. Animals experienced 60 min of maternal deprivation before the onset of testing, which was conducted at PD 14. Vertical bars indicate the SEM.

Figure 5

Ethanol-induced locomotor activity (distance traveled, cm) 5-9, 10-14, and 15-19 min after ethanol administration (bins 1, 2, and 3, respectively) in infant rats that experienced either normal animal facility rearing (AFR, left panel) or daily episodes of maternal separation (MS, right panel) during postnatal days (PD) 1 to 13. Animals experienced 240 min of maternal deprivation before the onset of testing, which was conducted PD 14. Vertical bars indicate the SEM.