Fos activation patterns related to acute ethanol and conditioned taste aversion in adolescent and adult rats

Abstract

Studies in rats have revealed marked age differences in sensitivity to the aversive properties of ethanol, with a developmental insensitivity to ethanol aversion that is most pronounced during pre- and early adolescence, declining thereafter to reach the enhanced aversive sensitivity of adults. The adolescent brain undergoes significant transitions throughout adolescence, including in regions linked with drug reward and aversion; however, it is unknown how ontogenetic changes within this reward/aversion circuitry contribute to developmental differences in aversive sensitivity. The current study examined early adolescent (postnatal day [P]28-30) and adult (P72-74) Sprague-Dawley male rats for conditioned taste aversion (CTA) after doses of 0, 1.0, or 2.5 g/kg ethanol, and patterns of neuronal activation in response to ethanol using Fos-like immunohistochemistry (Fos+) to uncover regions where age differences in activation are associated with ethanol aversion. An adolescent-specific ethanol-induced increase in Fos+ staining was seen within the nucleus accumbens shell and core. An age difference was also noted within the Edinger-Westphal nucleus (EW) following administration of the lower dose of ethanol, with 1 g/kg ethanol producing CTA in adults but not in adolescents and inducing a greater EW Fos response in adults than adolescents. Regression analysis revealed that greater numbers of Fos+ neurons within the EW and insula (Ins) were related to lower consumption of the conditioned stimulus (CS) on test day (reflecting greater CTA). Some regionally specific age differences in Fos+ were noted under baseline conditions, with adolescents displaying fewer Fos+ neurons than adults within the prelimbic (PrL) cortex, but more than adults in the bed nucleus of the stria terminalis (BNST). In the BNST (but not PrL), ethanol-induced increases in Fos-immunoreactivity (IR) were evident at both ages. Increased ethanol-induced activity within critical appetitive brain regions (NAc core and shell) supports a role for greater reward-related activation during adolescence, possibly along with attenuated responsiveness to ethanol in EW and Ins in the age-typical resistance of adolescents to the aversive properties of ethanol.

Keywords: Adolescent; Conditioned taste aversion; Ethanol; Fos; Nucleus accumbens.

Figure 1.

Adolescents displayed attenuated intake of the CS only at the highest ethanol dose (2.5 g/kg), while adults showed aversion at both ethanol doses (1.0 and 2.5 g/kg). *denotes significant difference from age-matched saline control

Figure 2.

In the nucleus accumbens core, there was a significant interaction between Age and Dose, with adolescents displaying an increase in Fos positive neurons following administration of ethanol, while there was no change in adults (A). Adolescents had significantly less Fos+ neurons following saline administration compared to adults. Photomicrographs showing representative Fos IR in the NAc core at 10X magnification in adolescents (B, D, F) and adults (C, E, G) after treatment with saline (B, C), 1.0 (D, E), and 2.5 (F, G) g/kg ethanol. *denotes significant difference from age-matched saline control #denotes significant difference from dose-matched adult

Figure 3.

In the nucleus accumbens shell, adolescents displayed an increase in Fos positive neurons following administration of ethanol, while a decrease was seen in adults following the 1.0 g/kg dose (A). Adolescents also had significantly less Fos+ neurons compared to adults, an effect driven by the saline control group. Photomicrographs showing representative Fos IR in adolescents (B, D, F) and adolescents (C, E, G) in the Nac shell following administration of saline (B, C), 1.0 (D, E) and 2.5 (F, G) g/kg ethanol. *denotes significant difference from age-matched saline control #denotes significant difference from dose-matched adult

Figure 4.

In the Edinger Westphal nucleus (EW), administration of ethanol resulted in significant increases in Fos neurons, with adolescents displaying significantly less Fos positive neurons than adults following the 1.0 g/kg dose (A). Adults also had fewer Fos positive neurons compared to adolescents. Inserts showing representative Fos IR in the EW at 10X magnification in adolescents (C, E, G) and adults (C, E, G) after treatment with saline (B, C), 1.0 (D, E), and 2.5 (F, G) g/kg ethanol. $ denotes significant difference between dose-matched adolescents and adults

Figure 5.

A. In the Edinger Westphal nucleus (EW), the number of Fos immunoreactive cells was significantly correlated with milliliters (mls) consumed on test day. B. In the insula, the number of Fos+ neurons were significantly correlated with milliliters (mls) consumed on test day.