The attribution of incentive salience to Pavlovian alcohol cues: a shift from goal-tracking to sign-tracking

Abstract

Environmental stimuli that are reliably paired with alcohol may acquire incentive salience, a property that can operate in the use and abuse of alcohol. Here we investigated the incentive salience of Pavlovian alcohol cues using a preclinical animal model. Male, Long-Evans rats (Harlan) with unrestricted access to food and water were acclimated to drinking 15% ethanol (v/v) in their home-cages. Rats then received Pavlovian autoshaping training in which the 10 s presentation of a retractable lever served as the conditioned stimulus (CS) and 15% ethanol served as the unconditioned stimulus (US) (0.2 ml/CS; 12 CS presentations/session; 27 sessions). Next, in an operant test of conditioned reinforcement, nose pokes into an active aperture delivered presentations of the lever-CS, whereas nose pokes into an inactive aperture had no consequences. Across initial autoshaping sessions, goal-tracking behavior, as measured by entries into the fluid port where ethanol was delivered, developed rapidly. However, with extended training goal-tracking diminished, and sign-tracking responses, as measured by lever-CS activations, emerged. Control rats that received explicitly unpaired CS and US presentations did not show goal-tracking or sign-tracking responses. In the test for conditioned reinforcement, rats with CS-US pairings during autoshaping training made more active relative to inactive nose pokes, whereas rats in the unpaired control group did not. Moreover, active nose pokes were positively correlated with sign-tracking behavior during autoshaping. Extended training may produce a shift in the learned properties of Pavlovian alcohol cues, such that after initially predicting alcohol availability they acquire robust incentive salience.

Keywords: autoshaping; conditioned reinforcement; conditioned stimulus; ethanol; motivation; rat.

Figure 1

The timing of experimental events during Pavlovian autoshaping sessions for paired and unpaired groups. The conditioned stimulus (CS) occurred synchronously for both groups and consisted of the insertion of a lever into the conditioning chamber for 10 s. For the paired group, retraction of the lever-CS was immediately followed by the delivery of 0.2 ml of 15% ethanol unconditioned stimulus (US) across 6 s into a fluid port for oral consumption. For the unpaired group, US delivery occurred halfway between two lever-CS presentations. For both groups, the variable interval between offset of one lever-CS and onset of the next lever-CS was 260 s on average, excluding the 6 s over which ethanol was delivered.

Figure 2

Alcohol intake and preference increased across 12 sessions in which access to 15% ethanol was provided in the home-cage for 24 h. In this and subsequent graphs, black symbols represent the paired group (n = 11) and white symbols represent the unpaired group (n = 12). Data are expressed as mean ± SEM for each session. (A) Alcohol intake in grams of ethanol consumed as a function of rat weight (g/kg/24 h). (B) Alcohol preference calculated as grams of ethanol solution consumed divided by grams of total fluid consumed in the same session and expressed as a percentage (%).

Figure 3

Conditioned responding elicited by the lever-CS shifted from initial goal-tracking responses to robust sign-tracking behavior with extended Pavlovian autoshaping training. Data are expressed as mean ± SEM for each training session. (A) Photograph depicting goal-tracking behavior, defined as entries into the fluid port during the lever-CS. (B) Normalized port entries during the lever-CS across session. To calculate a normalized measure that accounted for differences in baseline levels of behavior, port entries during a 10 s pre-CS interval were subtracted from port entries during the corresponding lever-CS. (C) Latency to enter the fluid port after presentation of the lever-CS. (D) Photograph depicting sign-tracking behavior, defined as activation of the lever-CS. (E) Number of lever-CS activations across session. (F) Latency to activate the lever-CS.

Figure 4

Response bias shifted from primarily goal-tracking to predominantly sign-tracking with extended Pavlovian autoshaping training. Response bias was calculated for each subject in the paired group using the formula: (number of lever-CS activations minus number of port entries)/(sum of lever-CS activations and port entries). The x-axis represents the identification number of individual rats represented in (A) session 8 and (B) session 27. A response bias score between −1 and 0 indicates a preference toward goal-tracking and a response bias score between 0 and 1 indicates a preference toward sign-tracking. Individual rats that demonstrated a shift in preference from goal-tracking in session 8 to sign-tracking in session 27 are depicted with black bars.

Figure 5

Higher levels of sign-tracking behavior were associated with lower levels of goal-tracking responses at the end of Pavlovian autoshaping training. Data represent mean ± SEM normalized port entries and lever-CS activations averaged across sessions 19–27. Each symbol represents data from an individual rat in (A) the paired group or (B) the unpaired group. Pearson's r-values are indicated in each graph. ^*p < 0.05.

Figure 6

Both paired and unpaired groups learned to approach the fluid port when alcohol was delivered during Pavlovian autoshaping training. Data represent mean ± SEM number of port entries made during the 6 s when alcohol was delivered in to the fluid port.

Figure 7

A lever-CS that was previously paired with alcohol functioned as a conditioned reinforcer. Black bars represent the paired group and white bars represent the unpaired group. Data are expressed as mean ± SEM for tests 1–4. (A–D) Number of nose pokes into the active and inactive apertures. (E–H) Number of lever-CS presentations earned. (I–L) Number of times the lever-CS was activated when it was presented as the result of active nose pokes. ^∧p < 0.01, paired active vs. inactive. ^*p < 0.05 and ^**p < 0.01, paired vs. unpaired.

Figure 8

Correlations between sign-tracking behavior at the end of Pavlovian autoshaping training and response measures obtained in each of the 4 tests for conditioned reinforcement in all rats. In each graph, lever-CS activations averaged across sessions 26 and 27 of Pavlovian autoshaping training are plotted on the x-axis for each rat in the paired (black circles) and unpaired (white circles) groups. (A–D) Correlation between active nose poke responses and sign-tracking behavior. (E–H) Correlation between inactive nose poke responses and sign-tracking behavior. (I–L) Correlation between lever-CS presentations earned and sign-tracking behavior. (M–P) Correlation between lever-CS activations and sign-tracking behavior. ^*p < 0.05 and ^**p < 0.01.