Protocol for Devaluation in an Instrumental Task

General Overview
We have developed a protocol for assessment of devaluation in an instrumental task that is well-adapted for testing mice. Typically mice that are trained to respond on two levers, each associated with a distinct reward, will learn to associate each response with its predicted rewarding outcome. Subsequent prefeeding with one of the rewards (i.e., sensory-specific satiety) will result in a reduction that reward’s motivational and incentive value. This devaluation is manifest in a test, by suppression of the response associated with the ‘devalued’ reward, but not the other response associated with the non-devalued reward. Note that this change in behavior can occur spontaneously when mice are tested without any reward in the devaluation test. Mice that are unable to adapt to changes in outcome value, or to guide behavior based on the current value of predicted outcomes (such that they act in a habitual manner) will fail to show this devaluation effect. A direct test of reward consumption is also included in the protocol (rewarded choice test) to assess the motivational effect of satiation and the ability of mice to suppress responses to a devalued reward itself, rather than to the events that predict reward.

Apparatus
All mice are trained in identical individual chambers with aluminum front and back walls, clear polycarbonate sides, and a floor made of stainless steel rods (Med Associates, St Albans, VT). Each chamber has been modified to include a liquid dispenser into which 0.1ml of liquid can be delivered, in addition to a vacuum attached to the bottom of the food cup (to suction off residual solution at the end of a trial). An infrared photocell placed inside the food cup monitors time spent and number of entries into the food cup. Retractable ultrasensitive mouse levers (Med Associates, St Albans, VT) are available on the left and right sides of the food cup. Ambient illumination for the chamber is provided by a 28V, 100 mA house light mounted on the inside wall of the sound attenuating chamber. An IBM-compatible computer equipped with Med-PC software (Med Associates, St Albans, VT) controls and records all stimuli and responses.

Procedure

Food Cup Training
Initially mice are given food cup training, which consists of two 45-min sessions per day (one in the A.M. and one in the P.M) for a total of two days. In the A.M. session mice are given random presentations (Random Time-30 second schedule) of 0.1-ml orange-flavored sucrose solution (“orange”) and in the other, P.M. session, a 0.1-ml grape-flavored sucrose solution (“grape”) is delivered. The order of the two sessions is fully counterbalanced. The solutions are 0.1% Koolaid brand flavorings in 10% sucrose (w/v).

Lever Press Training
Mice then receive two instrumental training sessions per day (i.e., A.M. and P.M.), one with only the left lever present and one with only the right lever present, with the order of the two sessions alternating daily. For half the mice in each group, left lever responses result in delivery of grape and responses on the right lever produce delivery of orange. The remaining mice are assigned the opposite response-outcome contingencies. For the first two days mice are given 60-min sessions in which each response is reinforced. For the remaining 8 days the session duration is reduced to 20-min and reward delivered on a random ratio (RR) 5 schedule (i.e., on average every 5 responses results in reward delivery) on days 3 and 4, and RR10 on days 5-8, and an RR15 on days 9 and 10. Thus, mice are given a total of 10 sessions of instrumental training on each lever.

Devaluation Treatment (i.e., sensory-specific satiety)
The next day, mice receive a sensory-specific devaluation treatment by prefeeding each mouse with one of the two outcomes for a 2-hour period. Each mouse is placed in a separate holding cage, with a cube filled with 2-ml of either grape or orange, fully counterbalanced across the prior response-outcome contingencies. The experimenter monitors and records reward consumption during this phase. As the solution is consumed, it is replaced in 1-ml increments to ensure continuous reward availability for each mouse.

Devaluation Test
Immediately following the devaluation treatment, the mice are given a 30-min extinction test session in the experimental chamber during which responses are not reinforced with reward delivery. Unlike in training, both levers are available in this test session. To the extent that responding is controlled by the current value of the reward anticipated after each of the two responses (left and right lever presses), mice should preferentially perform the response associated with the reward that was not prefed (i.e., the non-devalued response).

Rewarded Choice Test
Finally, the effectiveness of the prefeeding devaluation treatment in altering the mice’s preference for the rewards themselves is assessed. On a separate day mice are given prefeeding identical to that used in the previous devaluation treatment. On completion, the mice are given access to two cubes, one containing 2-ml of the prefed reward and other containing 2-ml of the other reward. Mice are then given 30-min to consume each reward, with the expectation that consumption will be greater for the non-devalued reward.