Novelty modulates the stimulating motor effects of ethanol in preweanling rats

Abstract

During early ontogeny heterogeneous rats are sensitive to ethanol's stimulating effects. In adulthood locomotor activity in a novel environment is a valuable predictor of acute sensitivity to the activating effects of various drugs, including ethanol. Environmental novelty modulates response to ethanol and other drugs in adult rats. The present study analyzed the role of novelty in the acute locomotor response induced by ethanol earlier in development, during the preweanling period, a stage characterized by enhanced sensitivity to ethanol's reinforcing effects. In Experiment 1 we evaluated the predictive value of baseline locomotor activity upon ethanol-induced locomotor effects in 12-day-old rats. In Experiment 2 we tested whether repeated familiarization with the testing environment would reduce the stimulating effects induced by ethanol on postnatal day 12. Individual differences in response to an inescapable novel environment significantly predicted the locomotor activating effects of ethanol, but not other acute effects of the drug, such as hypothermia, motor impairment or sedation. Behavioral activation induced by ethanol during the preweanling period was attenuated after familiarization with the testing environment, suggesting that environmental novelty is critical for activating effects of ethanol.

Figure 1

Locomotor activity scores on PD11 (baseline) and PD12 as a function of ethanol dose (0.0, 1.25 or 2.5 g/kg) in pups tested 5–10 (Figure 1a) or 25–30 (Figure 1b) minutes after ethanol administration. Vertical lines illustrate standard errors of the means.

Figure 2

Latency (seconds) to complete the righting reflex as a function of post-administration time (Figure 2a: 5–10 or Figure 2b: 25–30 min) and ethanol treatment (0.0, 1.25 or 2.5 g/kg ethanol). Vertical lines illustrate standard errors of the means.

Figure 3

Rectal temperature in preweanling rats as a function ethanol treatment (0.0, 1.25 or 2.5 g/kg ethanol) and post-administration time (Figure 3a: 5–10 or Figure 3b: 15–20 min). Vertical lines illustrate standard errors of the means.

Figure 4

Correlations and regression lines comprising baseline activity and locomotor activity on P12 when preweanling rats received the ethanol treatment (0, 1.25 or 2.5 g/kg) 5–10 (Figure 4a) or 25–30 (Figure 4b) after drug treatment. “R²” represents the determination coefficients. Baseline activity correlated positively and significantly with locomotor activity at testing only in the case of pups given 2.5 g/kg ethanol and tested 5–10 minutes postadministration time.

Figure 5

Figure 5a depicts locomotor activity during the pre-exposure phase (PDs 9, 10 and 11) as a function of the preexposure treatment (ig or no-ig). Figure 5b represents locomotor activity at testing as a function of the prior experience with the testing environment (context or no-context) and ethanol treatment (0 or 2.5 g/kg). Vertical lines illustrate standard errors of the means.