Increased consumption but not operant self-administration of ethanol in mice lacking the RIIbeta subunit of protein kinase A

Abstract

Background: Accumulating evidence indicates that adenosine monophosphate (cAMP)-dependent protein kinase A (PKA) is involved in the neurobiological responses to ethanol. Previous reports indicate that mice lacking the RIIbeta subunit of PKA (RIIbeta(-/-)) voluntarily consume more ethanol than wild-type controls (RIIbeta(+/+)) using 2-bottle testing procedures. Although such procedures primarily measure consummatory behavior, operant self-administration procedures allow analysis of consummatory as well as appetitive or "ethanol-seeking" behavior (i.e., lever pressing is required to gain access to the ethanol solution). Therefore, we determined whether the high ethanol consumption characteristic of RIIbeta(-/-) mice would be complemented by increased appetitive ethanol-seeking behavior in an operant paradigm.

Methods: RIIbeta(-/-) (n=8) and RIIbeta(+/+) (n=8) mice were initially sucrose-faded until they were lever responding for nonsweetened ethanol (10, 14, and 18%). Following the self-administration testing, RIIbeta(+/+) and RIIbeta(-/-) mice were given access to 2 bottles, one containing water and the other ethanol to replicate the voluntary ethanol drinking data previously from our laboratory. Finally, immediately after voluntary consumption all mice were again tested for self-administration of 10% ethanol. Alterations in the reinforcement schedule were also explored as RIIbeta(+/+) and RIIbeta(-/-) mice were tested for self-administration of 10% ethanol at FR-3 and FR-5 schedules.

Results: The RIIbeta(-/-) mice displayed lower operant responding for ethanol and food reinforcement compared with RIIbeta(+/+) controls. However, this effect was driven by a significant increase in lever responses made by female RIIbeta(+/+) mice. When the excessive lever responses of the female RIIbeta(+/+) mice are accounted for, the RIIbeta(-/-) mice show ethanol lever responses comparable to controls. Following operant self-administration testing, RIIbeta(-/-) mice of both sexes consumed more ethanol solution compared with RIIbeta(+/+) mice during 2-bottle testing.

Conclusions: Increased ingestion of ethanol by RIIbeta(-/-) mice is likely the result of altered PKA activity within neuronal pathways that control ethanol-consummatory behaviors. Conversely, the RIIbeta subunit of PKA appears not to play a critical role in neuronal pathways that regulate appetitive behaviors directed at obtaining ethanol. Finally, increased operant self-administration of food and ethanol by female wild-type mice was absent in female RIIbeta(-/-) mice, suggesting that normal PKA signaling may be part of a general, and sex-dependent, mechanism involved with reinforcement-seeking behavior.

Figure 1

Operant self-administration (FR-1) of 10% ethanol (v/v), food, and water in RIIβ ^+/+ and RIIβ^−/− mice (a–c), for all figures male mice appear as (▪) and female mice (□). Operant self-administration of 14% ethanol (v/v), food, and water in RIIβ ^+/+ and RIIβ^−/− mice (D–F). Operant self-administration of 18% ethanol (v/v), food, and water in RIIβ ^+/+ and RIIβ^−/− mice (g–i). All data are mean ± SEM. * p<0.05, female RIIβ ^+/+ mice relative to all other groups.

Figure 2

Homecage two-bottle voluntary ethanol consumption (a), ethanol preference (b), and water consumption (c) in male and female RIIβ ^+/+ and RIIβ^−/− mice. Male RIIβ ^+/+ mice appear as (•) and female RIIβ ^+/+ mice (▪). Male RIIβ^−/− mice appear as (○) and female RIIβ^−/− mice (□). All data are mean ± SEM. * p<0.05, overall significant genotype effect (RIIβ^−/− mice drank more ethanol compared to RIIβ ^+/+ mice).

Figure 3

Operant self-administration of 10% ethanol (v/v), on FR-1, FR-3 and FR-5 reinforcement schedules in RIIβ ^+/+ and RIIβ−/− mice following homecage ethanol consumption (a–c), for all figures male mice appear as (▪) and female mice (□). All data are mean ± SEM. * p<0.05, female RIIβ ^+/+ mice relative to all other groups.