Developmental differences in the effects of alcohol and stress on heart rate variability

• Effects of stress and ethanol on heart rate variability during ontogeny were assessed.
• A possible ethanol-induced sympathetic deactivation in adolescents was noted.
• Ethanol, but not stressor, effects on sympathovagal balance were seen in adolescents.
• Ethanol-induced autonomic effects were more pronounced in adults following stress.
• Effects of prior stress on hormone levels were less in adolescents than adults.

Adolescent rats differ in their responses to stress and ethanol from their adult counterparts, although not much is known about the contribution of the autonomic nervous system (ANS) to these differences. This study assessed the impact of stress, ethanol, and their combination on parameters of heart rate variability (HRV) in adolescent and adult male Sprague–Dawley rats. Animals were habituated to the testing box and neck sensors (MouseOX, STARR Life Sciences Corp.) used for recording heart rate (HR). After 8–10 min of baseline recording, animals were restrained for 90 min or returned home, followed by intraperitoneal injection of 0, 0.5, 1.0, or 1.5 g/kg ethanol. The 8–10 min test recording occurred 30 min post-injection. Ethanol-related decreases in LF (an index of sympathetic activity) were evident under non-stressed conditions in adolescents but only after stress in adults, perhaps in part due to apparent ethanol-induced sympathetic deactivation in adolescents. Parasympathetic tone, indexed by HF, was unaffected by both ethanol and stress in adolescents, while again both the 1.0 and 1.5 g/kg ethanol doses decreased HF in adults following stress. Ethanol also decreased low frequency/high frequency tone (LF/HF), an index of sympathovagal balance, only in adolescents, with no decrease evident in adults. Further, stressed adults, and not adolescents, had significantly lower CORT and PROG values than their non-stressed counterparts. Taken together, these results demonstrate notable age differences in the ANS response to ethanol under stressful vs. non-stressful circumstances, reflected by ethanol-mediated autonomic effects that were more pronounced following stressor exposure in adults but under non-stressed conditions in adolescents.