Adolescent oxytocin exposure causes persistent reductions in anxiety and alcohol consumption and enhances sociability in rats

Abstract

Previous studies have suggested that administration of oxytocin (OT) can have modulatory effects on social and anxiety-like behavior in mammals that may endure beyond the time of acute OT administration. The current study examined whether repeated administration of OT to male Wistar rats (n = 48) during a key developmental epoch (early adolescence) altered their physiology and behavior in later-life. Group housed rats were given intraperitoneal injections of either 1 mg/kg OT or vehicle during early adolescence (post natal-days [PND] 33-42). OT treatment caused a transient inhibition of body weight gain that recovered quickly after the cessation of treatment. At PND 50, the rats pre-treated with OT displayed less anxiety-like behavior on the emergence test, while at PND 55 they showed greater levels of social interaction. A subgroup of OT pre-treated rats examined at PND 63 showed a strong trend towards increased plasma OT levels, and also displayed significantly increased OT receptor mRNA in the hypothalamus. Rats pre-treated with OT and their controls showed similar induction of beer intake in daily 70 min test sessions (PND 63 onwards) in which the alcohol concentration of beer was gradually increased across days from 0.44% to 4.44%. However, when given ad libitum access to beer in their home cages from PND 72 onwards (early adulthood), consumption of beer but not water was significantly less in the OT pre-treated rats. A "booster" shot of OT (1 mg/kg) given after 25 days of ad libitum access to beer had a strong acute inhibitory effect on beer intake without affecting water intake. Overall these results suggest that exogenous OT administered during adolescence can have subtle yet enduring effects on anxiety, sociability and the motivation to consume alcohol. Such effects may reflect the inherent neuroplasticity of brain OT systems and a feed-forward effect whereby exogenous OT upregulates endogenous OT systems.

Figure 1. Experimental procedures.

There were 24 VEH and 24 OT subjects for inital drug administration, washout and residual testing. Eight VEH and Eight OT subjects from the initial 48 subjects then underwent alcohol induction on PND 63–72 and were used in the subsequent experiments conducted between PND 72–97. Another five VEH and five OT subjects taken from the initial 48 subjects were culled on PND 63 and their brain and blood was later analysed. Abbreviations: VEH = vehicle; OT = oxytocin; PND = post natal day.

Figure 2. Emergence test.

Results from the 5 min emergence test conducted 8 days after the cessation of 10 consecutive days of once per day treatments with either 1 mg/kg OT or VEH. A. Mean (+SEM) seconds spent in the open field and in the third of the open field farthest from the hide box (far zone). B. Mean (+SEM) metres travelled during the test. N = 24 per condition. * Significantly different to VEH, p<0.05 ** Significantly different to VEH, p<0.01.

Figure 3. Social interaction test.

Results from a 5 min social interaction test conducted 13 days after the cessation of 10 consecutive days of once per day treatments with either 1 mg/kg OT or VEH. A. Mean (+SEM) seconds the two subjects spent in close proximity (within one body length). B. Mean (+SEM) number of active social contacts between the subjects. C. Mean (+SEM) meters travelled by each pair of subjects (on average) during the social interaction test. N = 24 per condition. * Significantly different to VEH, p<0.05.

Figure 4. “Near-beer” and beer consumption during the 9 day alcohol induction phase in the lickometer apparatus.

Mean (± SEM) consumption (in mls) of increasing concentrations of beer (from .44% “near-beer” up to 4.44% EtOH beer). The 9 day induction period began 21 days after the cessation of 10 consecutive days of once per day treatments with either 1 mg/kg OT or equivalent VEH saline. Subjects were placed in the lickometer for one 70 min session per day. N = 8 per condition.

Figure 5. Alcohol and water consumption under continuous access.

Mean (± SEM) 4.44% EtOH beer and water consumption (in mls) for each of the 25 days of continuous access to beer and water is illustrated. Continuous access began 30 days after the cessation of 10 consecutive days of once per day treatments with either 1 mg/kg OT or equivalent VEH. Subjects were housed individually and had 24 h access to both beer and water over the entire 25 days. N = 8 per condition. * indicates sig. difference in total beer consumption between OT and VEH over the final seven days of continuous access, p<0.05.

Figure 6. Alcohol and water consumption following the OT “booster” shot.

Mean (+SEM) consumption (mls) of 4.44% EtOH beer and water over the 2.5 h following injection of a “booster” of either 1 mg/kg OT or equivalent VEH saline. The “booster” shot was administered after the final measurement of intakes on the final day of continuous access to beer and water. N = 8 per condition. ** Significantly different to VEH, p<0.01.

Figure 7. RT-PCR and ELISA.

Results from the RT-PCR and ELISA conducted on the 10 rats (5 OT, 5 VEH) randomly selected 21 days after the cessation of 10 consecutive days of once per day treatments with either 1 mg/kg OT or equivalent VEH saline. A. Fold change (±SE) in OT and OTR mRNA expression in the OT pre-treated subjects compared to VEH. B. Mean (+SEM) blood plasma concentration of OT (pg/ml) in the OT and VEH pre-treated subjects. *Significant fold change, p<0.05. + Trend towards significant difference compared to VEH, p = 0.057.