Ontogeny of ethanol-induced motor impairment following acute ethanol: assessment via the negative geotaxis reflex in adolescent and adult rats

Abstract

Adolescent rats have been observed to be less sensitive than adults to a number of ethanol effects that may serve as feedback cues to reduce further ethanol intake. Among these findings are a few reports of attenuated sensitivities of adolescents to ethanol-induced motor impairment. The purpose of the present study was to further explore potential age-related differences in ethanol-induced motor impairment in both male and female adolescent (postnatal day [P]28-32), and adult (P68-72) Sprague-Dawley rats using an inclined plane assessment of the negative geotaxis reflex. Adult males displayed significant motor impairment at 1.5 g/kg, whereas adolescent males required higher doses, showing significant motor impairment only at doses of 2.25 g/kg ethanol or greater. Intoxicated practice did not significantly influence level of motor impairment at either age. When female rats of both ages were separately analyzed in terms of their response to ethanol, a dose of 1.5 g/kg ethanol was found to significantly impair adults, whereas adolescent females showed significant motor impairment when challenged with 2.25 g/kg but not 1.5 g/kg ethanol. Yet when the 1.5 g/kg data of females at the two ages were directly compared, no significant age difference was seen at this dose. These data document an attenuated sensitivity of adolescent relative to adult rats to the motor impairing effects of ethanol using a stationary inclined plane test, an effect particularly robust in male animals, and demonstrates the utility of this test for assessment of motor coordination in adolescent and adult rats.

Figure 1

Adult male animals exhibited significant ethanol-induced motor impairment at doses of 1.25 g/kg (at 10 min post-injection; * p≤ 0.05), and 1.5 g/kg (at both the 10 and 30 min tests; † p≤ 0.05) when compared with saline control animals.

Figure 2

Significant ethanol-induced motor impairment was seen in adolescent male animals at the 10 and 30 min tests following 2.25 g/kg (* p≤ 0.05) and at all time points (10, 30, and 60 min) following 2.5 g/kg († p≤ 0.05).

Figure 3

Significant ethanol-induced motor impairment was seen in adult animals at the 10, 30, and 60 min test intervals following 1.5 g/kg ethanol, whereas adolescents did not exhibit impairment at any interval with this same dose. Adults demonstrated significantly longer turn latencies than adolescents at all time intervals following this ethanol challenge dose. Asterisks indicate significant differences from the saline control group at that test interval, as well as significant difference from ethanol-injected adolescent animals at that same time interval (*p ≤ 0.05).

Figure 4

There was no evidence that intoxicated practice facilitated performance on the inclined plane in either adult or adolescent male rats using a test dose at each age (1.5 g/kg and 2.25 g/kg ethanol, respectively) that induced comparable levels of motor impairment. All data are expressed as the mean ± SEM.

Figure 5

(a) Significant ethanol-induced motor impairment was seen in adolescent female animals at all time points (10, 30, and 60 min) following 2.25 g/kg, but not following administration of the lower dose of 1.5 g/kg ethanol. (b) Adult females exhibited significant ethanol-induced motor impairment at the 10 and 30 min tests post-ethanol injection (1.5g/kg) when compared with saline control animals. Asterisks indicate significant differences from the saline control group at that test interval (*p ≤ 0.05).