Conditioned Place Preference Protocol

General Overview
Conditioned place preference (environmental place conditioning) is a commonly used technique to evaluate preferences for environmental stimuli that have been associated with a positive or negative reward. In general, this procedure involves several trials where the animal is presented with the positive stimulus (e.g., food or the effects of a drug of abuse) paired with placement in a distinct environment containing various cues (e.g., tactile, visual, and/or olfactory). When later tested in the normal state, approaches and the amount of time spent in the compartments previously associated with the positive stimulus serves as an indicator of preference and a measure of reward learning.

Apparatus
Each place conditioning apparatus consists of an open field enclosed in separate light- and sound-attenuating chambers. General activity and location in the open field is monitored by video recording. The floor of the open field consists of interchangeable halves made of one of two textures. The combination of floor textures was selected on the basis of previous studies showing that mice spend an average of about 50% time on each floor type during preference tests (Cunningham et al. 1992, 1997; Cunningham 1995). Thus, the apparatus is "unbiased". Specifically, the floors are either a "grid" made from stainless-steel rods or a "hole" floor made from perforated stainless-steel. Prior to each session the open field and floors are cleaned using a 70% ETOH solution.

Procedure
Place conditioning consists of the following four phases.

(1) Habituation phase (2 days):
On two 30-min sessions (1 session/day) mice are able to freely explore the entire open field apparatus using a non-discriminative flat plastic floor.

(2) Pre-conditioning bias test (1 day):
To determine whether a pre-existing bias exists, a single 30 min test session is conducted during which mice are allowed to freely explore the open field with half of the arena containing the "grid" floor and half of the arena containing the "hole" floor.

(3) Conditioning (max 8 days):
Mice are randomly assigned to one of two groups: For one group, placement into the apparatus with one floor-type is paired with a positive stimulus and for a second group the apparatus floor is paired with a placebo procedure. All mice receive four 30-60 min conditioning sessions of this type, one session every second day. On the intervening days, mice are exposed to the alternate floor type and given a placebo treatment. On all sessions, mice have access to the entire apparatus with the same floor texture on both sides.

(4) Post-conditioning bias test (3 days):
The 30-min floor preference tests are conducted 24h, 7d and 30d after the last conditioning session. The conditions of these tests are identical to those during the pre-conditioning bias test. The number of seconds spent on the target (previously paired) floor is measured and the results are compared with those from the pre-conditioning bias test.

All place conditioning experiments follow this procedure but can differ in the reinforcing stimulus used during the conditioning phase. Examples of experiments using either a food-related positive reinforcing stimulus or a drug-related positive reinforcing stimulus are described below:

1. Food place preference conditioning
Mice are food-restricted as described. During the conditioning phase, mice are randomly assigned to a food-paired or control group (n=8-10 each). The paired mouse receives access to 20 mg food or sucrose pellets (Noyes Precision Pellets, Research Diets Inc, New Brunswick, N.J., USA) presented in a small dish on one floor type whilst the control mouse has access to an empty dish, for a total of 40 min. On alternate days all mice receive access to an empty dish on the alternate floor type. To assess food consumption the food dish is weighed prior to and following each session.

2. Drug place preference conditioning
During the conditioning phase, mice are randomly assigned to a drug-paired or a control group. Drug-paired mice receive an injection of drug and their respective control mice receive an injection of vehicle, prior to placement on one of the floor types. All mice will undergo four 30-45 min conditioning session of this type, one session every other day. On the intervening days, all mice receive an injection of vehicle prior to placement on the alternate floor type.