2.4. Behavioral tests

Two sets of rats, one for sucrose preference, and the other for social interaction and forced-swim test were tested as follows. The time points (Fig. 2A) for social interaction and forced swim tests were chosen based on the results of the sucrose preference test. The rat groups were coded to mask the investigators to drug treatment. The experimental designs and resultant data were verified by at least another trained researcher.

Rats were acclimatized to single housing with access to unlimited rat chow and two bottles of water. The effect of systematic administration of JZL195 on sucrose preference was assessed using a two-bottle choice paradigm. Rats were offered access to pre-weighed bottles containing tap water and 1% sucrose and consumption was monitored overnight for 4 consecutive days after the final injection. The WKY rats were pre-exposed to sucrose and water bottles before the drug treatment and were divided into two groups of equal sucrose consumption. Two bottles containing water and 1% sucrose were placed on the cage without rat to control spillage and evaporation. The positions of water and sucrose bottles were alternated every day to avoid place preference. The amount of sucrose intake is expressed as gram of sucrose solution consumed/kg body weight and sucrose preference as percentage of sucrose consumption over total liquid (sucrose and water) consumption.

The social interaction test was conducted 24 hr after the final injection in a solid acrylic box (30 cm × 120 cm × 30 cm) consisting of 3 chambers defined as social, neutral, and anti-social. Each rat was habituated to the arena for 5 min prior to the test. A novel rat (Sprague Dawley) of the same sex was placed in the social chamber before introducing the subject rat. The social behavior of rat (time spent in each chamber and interaction [sniffing and touching]) was monitored for 5 min.

The effect of JZL195 was assessed in the forced swim test as it is a sensitive and reliable method to assess the antidepressant property of the drugs with high predictive validity (Gobbi et al., 2005; Rittenhouse et al., 2002). After 48 hr following the last injection, rats were tested in the forced swim test. The main behaviors: immobility (no or minimum movement), swimming and climbing were videotaped and assessed during the last 5 min of the 6 min testing session. Rats were considered immobile when floating in water without struggling and making only those movements necessary to keep their heads above the water surface.