FOS expression induced by an ethanol-paired conditioned stimulus

Abstract

To identify brain areas involved in ethanol-induced Pavlovian conditioning, brains of male DBA/2J mice were immunohistochemically analyzed for FOS expression after exposure to a conditioned stimulus (CS) previously paired with ethanol (2 g/kg) in two experiments. Mice were trained with a procedure that normally produces place preference (Before: ethanol before the CS) or one that normally produces place aversion (After: ethanol after the CS). Control groups received unpaired ethanol injections in the home cage (Delay) or saline only (Naïve). On the test day, mice were exposed to the 5-min CS 90 min before sacrifice. Before groups showed a conditioned increase in activity, whereas the After group showed a conditioned decrease in activity. FOS expression after a drug-free CS exposure was significantly higher in Before-group mice than in control mice in the bed nucleus of the stria terminalis (Experiment 1) and anterior ventral tegmental area (Experiments 1-2). Conditioned FOS responses were also seen in areas of the extended amygdala and hippocampus (Experiment 2). However, no conditioned FOS changes were seen in any brain area examined in After-group mice. Overall, these data suggest an important role for the mesolimbic dopamine pathway, extended amygdala and hippocampus in ethanol-induced conditioning.

Figure 1

Mean activity counts per minute (±SEM) during each 5-min ethanol (CS+) and saline (CS-) conditioning trial in Experiment 1. Each group contained 12 mice.

Figure 2

Mean activity counts per minute (±SEM) during the final 5-min test session in Experiment 1. Each group contained 12 mice.

Figure 3

Mean activity counts per minute (±SEM) during each 5-min ethanol (CS+) and saline (CS-) conditioning trial in Experiment 2. Data are collapsed across test drug within the Before and Delay conditions because this factor was irrelevant during the conditioning phase and because there were no differences between mice assigned to the saline or ethanol test groups. Each group contained 23-24 mice.

Figure 4

Mean activity counts per minute (±SEM) during the final 5-min test session in Experiment 2. Each group contained 11-12 mice.

Figure 5

Photomicrographs showing FOS immunostaining in the BST (A) and VTAant (B) from representative animals in each group in Experiment 1. In the BST, Before-S (CPP-trained) mice had higher FOS counts than all other groups. In the VTAant, Before-S mice had higher FOS counts than Delay-S and Naïve-S control animals. Abbreviations for neuroanatomical landmarks: ac = anterior commissure; f = fornix; fr = fasciculus retroflexus; v = blood vessel. Scale bar corresponds to 500 μm.