Developmental differences in hypothermic and behavioral responses to ethanol treatment in Alcohol Preferring and Non-Preferring Rats

Abstract

Differences in voluntary consumption of ethanol have been negatively correlated with differences in initial sensitivity and tolerance to ethanol’s pharmacological effects. From this perspective, both adolescent and adult alcohol-nonpreferring (NP) rats would be expected to be initially more sensitive to the sedative and hypothermic effects of ethanol and fail to acquire tolerance to those effects than preferring (P) rats. The first objective of this experiment was to assess alcohol-induced hypothermia and locomotor sedation in adolescent and adult P and NP rats over five consecutive daily administrations (saline, 1.5 g/kg, or 3.0 g/kg ethanol 17%v/v), testing the hypothesis that the P rats would acquire tolerance to the hypothermic response whereas the NP rats would not show changes across days. In addition, it was hypothesized that there would be age-related differences in initial sensitivity to ethanol, evident by adolescent rats displaying less ethanol-induced hypothermia and locomotor sedation than adult rats on Day 1. The second objective was to determine if conditioning was occurring between the administration environment and the hypothermic response and locomotor sedation elicited by ethanol exposure, via a sixth injection of saline. Female rats were surgically implanted with intraperitoneal Mini Mitter telemetry probes on postnatal day 25 or 85 and experimental manipulations began five days later. Data were collected every minute; temperature data were then converted to change from baseline scores and locomotor data were totaled for each session. On Day 1, maximum temperature reduction elicited by the 3.0 g/kg dose was greater in the NP rats than the P rats, regardless of age. That dose also produced greater levels of locomotor sedation in the adult rats compared to the adolescent rats, regardless of line. The 1.5 g/kg dose of ethanol produced a greater hypothermic response in adult rats compared to adolescent rats, locomotor activity was reduced equally across the groups. With repeated administrations, NP adult rats displayed sensitization to the hypothermic response elicited from the 3.0 g/kg dose; in contrast, tolerance to the hypothermic response was found within the 1.5 g/kg dose for the adolescent P, adult P, and the adult NP rats. Repeated saline administrations also resulted in tolerance to the hypothermic response associated with administration in the adult NP and adolescent P rats. On the Day 6 saline administrations, adult rats which had previously been exposed to the 3.0 g/kg dose, maintained their baseline body temperatures better than both of the other exposure groups. Adolescent rats failed to show any signs of conditioning when administered saline on Day 6. Contrary to prediction the P rats failed to acquire tolerance to the 3.0 g/kg dose for either measure; and the line difference in ethanol-induce hypothermia was due to sensitization of the hypothermic response in adult NP rats. These results also provide further support that adolescent rats are less sensitive to the initial aversive effects of ethanol at the 1.5 g/kg dose for ethanol-induced hypothermia and the 3.0 g/kg dose for locomotor activity. The current experiment provides evidence that initial sensitivity as well as the acquisition of tolerance to ethanol-induced hypothermia may be behavioral phenotypes correlated with selection for high and low alcohol drinking preference.