Effects of selective breeding for infant rat ultrasonic vocalization on cardiac responses to isolation

Heart rates (HR) and autonomic control of heart period (HP) during maternal separation and isolation were measured at postnatal days (P) 10 and 18 in rat pups selectively bred for high, low, and random rates of infant ultrasonic vocalization. P10 pups displayed no significant line differences in HRs, strong tonic basal sympathetic control of HP and little sympathetic reactivity to isolation, and no evidence of tonic baseline or phasic parasympathetic responses to isolation. At P18, during isolation and recovery, Low HRs were significantly higher compared with Random and High pups. High HRs were significantly higher compared with the Random line in the last third of isolation and through recovery. The higher HRs of Low pups during stress were due to greater parasympathetic withdrawal, but greater sympathetic activation could not be ruled out. Higher HRs in High pups were due to increased sympathetic activation. Thus, increases in HR responses in the selected lines are due to different underlying autonomic processes.