Ethanol-induced locomotor activity in adolescent rats and the relationship with ethanol-induced conditioned place preference and conditioned taste aversion

Abstract

Adolescent rats exhibit ethanol-induced locomotor activity (LMA), which is considered an index of ethanol's motivational properties likely to predict ethanol self-administration, but few studies have reported or correlated ethanol-induced LMA with conditioned place preference (CPP) by ethanol at this age. The present study assessed age-related differences in ethanol's motor stimulating effects and analyzed the association between ethanol-induced LMA and conventional measures of ethanol-induced reinforcement. Experiment 1 compared ethanol-induced LMA in adolescent and adult rats. Subsequent experiments analyzed ethanol-induced CPP and conditioned taste aversion (CTA) in adolescent rats evaluated for ethanol-induced LMA. Adolescent rats exhibit a robust LMA after high-dose ethanol. Ethanol-induced LMA was fairly similar across adolescents and adults. As expected, adolescents were sensitive to ethanol's aversive reinforcement, but they also exhibited CPP. These measures of ethanol reinforcement, however, were not related to ethanol-induced LMA. Spontaneous LMA in an open field was, however, negatively associated with ethanol-induced CTA.

Figure 1

Assessment of susceptibility to the motor stimulant effect of high-dose ethanol in adolescent (postnatal day 28, PD28) and adult rats (PD74). Left Panel: Horizontal (i.e., forward locomotion) activity (s) in animals given ethanol (2.5 g/kg, i.g.) or vehicle (tap water). Right panel: Wall climbing activity (s) in animals given ethanol (2.5 g/kg, i.g.) or vehicle (tap water). To facilitate data visualization, data have been collapsed across sex. The latter factor did not affect motor behavior nor significantly interacted with the remaining factors. Treatment with 2.5 g/kg ethanol induced a significant increase in horizontal movement, which was similar in adolescent and adult subjects. This significant main effect of ethanol is indicated by the asterisk. The vertical bars indicate SEM.

Figure 2

Ethanol-induced conditioned texture preferences in adolescent rats (postnatal day 32, PD32). Upper panel depicts total time (s) spent on the sandpaper conditional stimulus (CS+) and the smooth surface (CS−, EVA) during the 12-minute test as a function of treatment during conditioning (sandpaper paired or unpaired with ethanol’s effects). During conditioning (PDs 30–31), the Paired group received ethanol-sandpaper parings [US-CS+] whereas the Unpaired group was exposed to sandpaper (CS+) 90 minutes prior to the administration of ethanol (US) [CS+ - US: 90 min delay]. The statistical analysis indicated a significant interaction between texture and conditioning treatment. Paired rats spent significantly more time in sandpaper than in smooth at test and also exhibited greater sandpaper preference than unpaired adolescents. These significant differences are indicated by the asterisk (*) and the pound (#) sign, respectively. Lower panel depicts time spent on sandpaper and the smooth EVA (s) as a function of treatment during conditioning (paired or unpaired) and bin of assessment (1–12 min). Data were collapsed across sex (male or female). The sex factor did not exert a significant main effect or significantly interact with the remaining variables. The vertical bars indicate SEM.

Figure 3

Ethanol-induced conditioned taste aversion in adolescent rats. Saccharin intake (ml/100 g) at test in male and female adolescent rats as a function of ethanol treatment during postnatal day 28 (PD28) and conditioning (PD31). On PD28, the rats were treated with ethanol (3.0 g/kg, i.g.) vehicle (0.0 g/kg) or were left untreated (UT, animals received no intubation). During conditioning (PD31), saccharin intake was paired with ethanol administration (2.5 g/kg, i.g.) or vehicle (tap water, 0.0 g/kg). Two 60-min test sessions were conducted (PDs 33 and 34); the figure depicts average mean saccharin consumption across these tests. The statistical analysis indicated that animals given saccharin-ethanol pairings at training drank significantly less saccharin than vehicle-treated controls, but only if they had not been treated with ethanol on PD 28. These significant differences between ethanol and vehicle-treated subjects are indicated by the asterisk sign. Data were collapsed across the sex factor, which did not exert a significant main effect or significantly interact with the remaining variables. The vertical bars indicate SEM.

Figure 4

Association between spontaneous open-field behaviour and of ethanol-induced conditioned taste aversion. Saccharin intake (ml/100 g of body weight) during extinction test 1 (postnatal day 33, PD 33) as a function of horizontal motor activity in the open field at PD 28, in adolescent rats that remained untreated during the motor activity test (i.e., UT group) and were given parings of saccharin and ethanol (2.5 g/kg, i.g.) on PD31. A Pearson correlation coefficient (r = .64; p < .05) indicated that higher ambulation on the open field predicted higher consumption of the ethanol-paired CS and, therefore, a diminished expression of ethanol-induced conditioned taste aversion.