Adolescent but not adult rats exhibit ethanol-mediated appetitive second-order conditioning

Abstract

Background: Adolescent rats are less sensitive to the sedative effects of ethanol than older animals. They also seem to perceive the reinforcing properties of ethanol. However, unlike neonates or infants, ethanol-mediated appetitive behavior is yet to be clearly shown in adolescents. Appetitive ethanol reinforcement was assessed in adolescent (postnatal day 33, P33) and adult rats (P71) through second-order conditioning (SOC).

Methods: On P32 or P70, animals were intragastrically administered ethanol (0.5 or 2.0 g/kg) paired with intra-oral pulses of sucrose (CS(1), first-order conditioning phase). CS(1) delivery took place either 5-20 (early pairing) or 30-45 minutes (late pairing) following ethanol administration. The time interval between CS(1) exposure and ethanol administration was 240 minutes in unpaired controls. On P33 or P71, animals were presented the CS(1) (second-order conditioning phase) in a distinctive chamber (CS(2), second-order conditioning). Then they were tested for CS(2) preference.

Results: Early and late paired adolescents, but not adults, had greater preference for the CS(2) than controls, a result indicative of ontogenetic variation in ethanol-mediated reinforcement. During the CS(1)-CS(2) associative phase, paired adolescents given 2.0 g/kg ethanol wall-climbed more than controls. Blood and brain ethanol levels associated with the 0.5 and 2.0 g/kg doses at the onset of each conditioning phase did not differ substantially across age, with mean blood ethanol concentration of 38 and 112 mg%.

Conclusions: These data indicate age-related differences between adolescent and adult rats in terms of sensitivity to ethanol's motivational effects. Adolescents exhibited high sensitivity for ethanol's appetitive effects. These animals also showed ethanol-mediated behavioral activation during the SOC phase. The SOC preparation provides a valuable conditioning model for assessing ethanol's motivational effects across ontogeny.

Figure 1

Methods for the analysis of motivational properties of ethanol in adolescent and adult rats. Phase 1, first-order conditioning, postnatal day 32 or 70, P32 or 70: animals were administered ethanol (0.5 or 2.0 g/kg, intragastric) and then given a conditioned stimulus (CS₁) consisting of intraoral pulses of sucrose. CS₁ delivery took place either 5-20 or 30-45 min after EtOH (groups Early pairing and Late pairing, respectively). In Unpaired controls, CS₁ exposure and EtOH administration were separated by 240 min. Unpaired adolescents were assigned to two conditions (early or late), as a function of the interval between habituation and CS₁ exposure (5 or 30 min, respectively). These conditions did not differed in terms of either CS₁ responsiveness or CS₂ preference and, for the purpose of the statistical analyses, have been collapased in a single condition. At adulthood, only early unpaired groups were employed. All groups underwent an initial, non-reinforced habituation phase (duration: 10 min). Phase 2, second-order conditioning, P33 or P71: animals were briefly stimulated with 10% sucrose (trial duration: 4 min) while placed in a visually and tactile distinctive chamber (CS₂). Duration of wall climbing, frequency of head-shakings and general locomotion were registered. Phase 3, locational preference test, P33 or P71: time spent on the CS₂ chamber was recorded in a 12 min, place preference test.

Figure 2

Texture preferences at test in adolescent subjects (postnatal day 33).A. Total time (s) spent on the sandpaper lined chamber (top panel) and its corresponding percentage preference for the chamber (bottom panel) during the 12-minute test session as a function of treatment during conditioning (Unpaired, Early pairing and Late Pairing) and Ethanol Dose (0.5 and 2.0 g/kg, intragastric). The statistical analysis for either dependent variable indicated significant main effects of treatment during conditioning. B. Total time (s) spent on the sandpaper lined chamber (top panel) and its corresponding percentage preference for the chamber (bottom panel) during the 12-minute test session as a function of treatment during conditioning. In Fig. 2B, asterisks indicate significant differences from the Unpaired group (p < 0.05). Vertical bars indicate the standard error of the mean.

Figure 3

Texture preferences at test in adult subjects (postnatal day 71). Total time (s) spent on the sandpaper-lined floor (top panel) and its corresponding percentage preference for the chamber (bottom panel) during the 12-minute test session as a function of treatment during conditioning (Unpaired, Early pairing and Late Pairing) and Ethanol Dose (0.5 and 2.0 g/kg, intragastric). The statistical analysis revealed that absolute and percent preference for sandpaper were not affected by ethanol dose or treatment during conditioning. Vertical bars indicate the standard error of the mean.

Figure 4

Blood and brain ethanol levels (mg %) in adolescent and adult rats (P32 and P 70, respectively) given 0.5 or 2.0 g/kg ethanol (intragastric). Blood and brain samples were collected either at 7.5 or 32.5 min postadministration (early and late postadministration time, PAT; respectively). A detailed account of the statistical findings and signficant differences across groups can be found in the Results section. Vertical bars indicate the standard error of the mean.