A History of Low-Dose Ethanol Shifts the Role of Ventral Hippocampus during Reward Seeking in Male Mice

Abstract

Although casual drinkers are a majority of the alcohol drinking population, understanding of the long-term effects of chronic exposure to lower levels of alcohol is limited. Chronic exposure to lower doses of ethanol may facilitate the development of alcohol use disorders, potentially because of ethanol effects on reward learning and motivation. Indeed, our previously published findings showed that chronic low-dose ethanol exposure enhanced motivation for sucrose in male, but not female, mice. As the ventral hippocampus (vHPC) is sensitive to disruption by higher doses of chronic ethanol and tracks reward-related information, we hypothesized that this region is impacted by low-dose ethanol and, further, that manipulating vHPC activity would alter reward motivation. In vivo electrophysiological recordings of vHPC population neural activity during progressive ratio testing revealed that vHPC activity was suppressed in the period immediately after reward seeking (lever press) in ethanol-naive controls, whereas suppression of vHPC activity anticipated reward seeking in ethanol-exposed mice. In both ethanol-naive and exposed mice, vHPC activity was suppressed before a reward magazine entry. Temporally selective inhibition of vHPC using optogenetics increased motivation for sucrose in ethanol-naive controls, but not in ethanol-exposed mice. Further, regardless of exposure history, vHPC inhibition promoted checking of the reward magazine, indicating a role for vHPC in reward tracking. There was no effect of chemogenetic inhibition of the vHPC either during training or testing on sucrose reward motivation. These results reveal novel ethanol-induced alterations in vHPC neural activity that shift how vHPC activity is able to regulate reward seeking.

Keywords: electrophysiology; ethanol; hippocampus; motivation; operant; optogenetics.

Figure 1.

Blood ethanol concentrations (BECs) and behavioral effects of low-dose ethanol exposure. A, Blood ethanol concentrations were elevated at 30 min following injection of low-dose ethanol (0.5 g/kg, i.p.). At 60 min, BECs were not different from baseline. B, Daily exposure to low-dose ethanol daily following training did not impact the number of days it takes to acquire stable responding on an FR1 schedule for a sucrose reward. C, Ethanol-exposed male mice exhibited sustained, accelerated responding during the PR test as compared with saline-exposed mice (**p < 0.01; for BEC data, n = 4 per time point; saline n = 12; ethanol n = 10; error bars represent SEM).

Figure 2.

vHPC firing rates during progressive ratio responding. A, A timeline of the behavioral experiment and MEA placements in the vHPC. There was no effect of ethanol on days to acquire stable responding (B), basal reward seeking (C), or break points during the PR test (D). E, vHPC mean firing rates were higher immediately before reaching the break point than the start of the session, independent of ethanol exposure condition. F, vHPC mean firing rates did not correlate with response rates (**p < 0.01; training: saline n = 6, ethanol n = 6; testing: saline n = 4, ethanol n = 6; error bars represent SEM).

Figure 3.

vHPC population activity surrounding discrete behavioral events. A, Normalized vHPC firing rate around lever pressing in ethanol and saline mice. B, vHPC activity was significantly reduced after a lever press in saline controls. In mice with a history of low-dose ethanol exposure, vHPC activity was suppressed before a lever press. C, Normalized vHPC firing rate around magazine entries in ethanol and saline mice. D, vHPC activity was significantly reduced before a magazine entry independent of exposure condition (**p < 0.01, *p < 0.05; saline n = 4, ethanol n = 6).

Figure 4.

Effects of vHPC optogenetic manipulation in saline-exposed mice. A, Schematic of optogenetic inhibition timing for paired and unpaired groups and optic fiber placements for saline mice. B, ArchT-expressing mice acquired stable responding in fewer days than GFP-expressing mice. C, There was no effect of ArchT expression on basal reward seeking. There was no association between days it took to acquire stable responding and break points during either the unpaired (D) or paired (E) condition. vHPC inhibition increased break points (F) and response rates (G) regardless of light pairing, showing that this inhibition is able to increase motivated behavior. Inhibition pairing condition did not affect the percent change in either break points (H) or response rates (I), again supporting that this is not impacted by whether vHPC inhibition occurs temporally close to a lever press. J, vHPC inhibition significantly reduced latencies to a magazine entry during the unpaired condition, showing that this inhibition is able to drive magazine checking behavior. K, There was no effect of vHPC inhibition on latencies to a lever press, indicating that this is specific to magazine entries (****p < 0.0001, *p < 0.05; GFP n = 6, ArchT n = 6; error bars represent SEM).

Figure 5.

Effects of vHPC optogenetic manipulation in ethanol-exposed mice. A, Experimental timeline and optic fiber placements for ethanol mice. B, There was no effect of ArchT expression on days it took to acquire stable responding. C, ArchT-expressing mice had higher basal reward seeking than GFP-expressing mice. There was no association between days it took to acquire stable responding and break points during either the unpaired (D) or paired (E) condition. vHPC inhibition had no effect on break points (F) and response rates (G) regardless of light pairing. There was no effect of inhibition pairing on differences in break points (H), but paired vHPC inhibition did increase response rates during the PR test (I). J, vHPC inhibition significantly reduced latencies to a magazine entry regardless of pairing condition. K, There was no effect of vHPC inhibition on latencies to a lever press (ns = not significant; ***p < 0.001; *p < 0.05; GFP n = 10, ArchT n = 16; error bars represent SEM).

Figure 6.

Effects of vHPC chemogenetic manipulation on reward seeking. A, Experimental timeline and representative placements of minimal and maximal virus expression. Chemogenetic vHPC inhibition during task acquisition did not affect the days it took to acquire stable responding (B) or basal reward seeking (C). Neither a history of vHPC inhibition during acquisition (D) nor expression of motivated behavior (E) affected break points on the PR test (GFP + CNO n = 8, DREADD + Sal n = 8, DREADD + CNO n = 7; error bars represent SEM).