Motivational effects of intraorally-infused ethanol in rat pups in an operant self-administration task

Abstract

Motivational effects of self-administered ethanol have rarely been studied in preweanling rats due primarily to the lack of age-appropriate operant tasks. The present experiments assessed the hedonic effects of intraoral ethanol in infant rats self-administered by activating a touch sensor. On postnatal day (PD) 13 pups were pre-exposed to the drug's pharmacological and/or sensory effects. Operant sessions were conducted during PDs 14-16 (Experiments 1 and 2). Paired animals were placed in chambers equipped with a touch-sensitive disk and received an intraoral infusion of ethanol (3 or 5% v/v, 5 microl) after each sensor contact. Yoked controls were equated for number and distribution of ethanol infusions but had no control over the contingency between operant behavior and intraoral infusion. In Experiment 2, training trials were preceded by a non-reinforced phase. Paired pups performed fewer operant responses than controls and decreased their operant responses across sessions. These results suggest that intraoral self-administered ethanol has an aversive hedonic value in two-week old rats. Operant behavior seems to have been associated with aversive orosensory effects derived from intraoral ethanol infusion.

Figure 1

1a. Methods for the analysis of reinforcing properties of intraorally delivered ethanol in infant rats (Experiment 1). Stage I (preexposure, postnatal day 13, PD 13), pups were given an intragastric (i.g.) administration of ethanol (0.5 g/kg) or water (0.0 g/kg). A third group was intraorally stimulated with oral ethanol (7% v/v, 30 infusions of 5 ul each) in addition to the i.g delivery of 0.5 g/kg ethanol. In the fourth pre-exposure group pups were given only the same intraoral ethanol infusion. Stage II (operant training, PDs 14-16), pups underwent daily sessions (20 min) in which each contact with a dime-sized disk yielded an intraoral infusion of ethanol (3 or 5% v/v, 5 ul). Yoked controls equated for ethanol exposure (but not in regards to contingency on behavior) were employed. Stage III (extinction, PD 17), pups underwent a 20-min session in which the reinforcer was withheld following the target response. 1b. Mean number of target operant responses (sensor contacts) in Experiment 1 during training (postnatal days 14-16, PDs 14-16) and extinction (PD 17) sessions as a function of the contingency between behavior and delivery of the reinforcer [target behavior either paired or unpaired (yoked control) with reinforcer availability]. Data has been collapsed across ethanol concentration and pre-exposure manipulations. These factors did not affect operant behavior scores. The vertical dashed line in the y-axis indicates that PD 16 was the last day that ethanol was delivered. Vertical lines represent standard errors of the mean.

Figure 2

Ethanol consumption scores (g/kg) in Experiments 1 and 2 (Figs. 2a and 2b, respectively) during daily 20 min operant sessions (postnatal days 14, 15 and 16, PDs 14-16) as a function of learning condition (paired or yoked) and ethanol concentration (3 or 5% v/v). Vertical lines represent standard errors of the mean.

Figure 3

Location scores (time spent in the section of the cage where the sensor was located relative to time spent in the opposite quadrant) in Experiment 1 during training (postnatal days 14-16, PDs 14-16) and extinction (PD 17) sessions as a function of learning condition (Paired or Yoked) and ethanol concentration (3 or 5%, v/v). The vertical dashed line in the y-axis indicates that PD 16 was the last day that ethanol was delivered. Vertical lines represent standard errors of the mean.

Figure 4

Mean number of target operant responses (sensor contacts) in Experiment 2 during training (postnatal days 14-16, PDs 14-16) and extinction (PD 17) sessions as a function of the contingency between behavior and delivery of the reinforcer [target behavior either paired or unpaired (yoked control) with reinforcer availability]. The vertical dashed line in the y-axis indicates that PD 16 was the last day that ethanol was delivered. Vertical lines represent standard errors of the mean.

Figure 5

Location preference scores (time spent in the section of the cage where the sensor was located relative to time spent in the opposite quadrant) in Experiment 2 during training (postnatal days 14-16, PDs 14-16) and extinction (PD 17) sessions as a function of learning condition (Paired ot Yoked) and nature of the preexposure treatment (Ethanol or Water). The vertical dashed line in the y-axis indicates that PD 16 was the last day that ethanol was delivered. Vertical lines represent standard errors of the mean