POSTNATAL-DEVELOPMENT OF THE PUTATIVE NEUROPEPTIDE-Y-MEDIATED SYMPATHETIC-PARASYMPATHETIC AUTONOMIC INTERACTION

Objective: intense stellate ganglion stimulation causes a long-lasting inhibition of cardiac vagal responses in adult dogs. This inhibition is thought to result from the release of neuropeptide Y from sympathet ic nerve terminals, which, in turn, blocks the release of acetylcholin e from parasympathetic neurons, The purpose of this study was to chara cterize the developmental expression of this autonomic interaction in the dog, Methods: We studied and compared the effects of 5-min trains of right stellate ganglion stimulation on cardiac chronotropic respons es to supramaximal vagal stimulation trains in 10 neonatal dogs, 8 one -month-old puppies, 8 two-month-old puppies and 8 adult dogs. Results: In the adult group, after 5 min of stellate stimulation, inhibition o f the vagal chronotropic response was observed in 7 of 8 (87.5%), Inhi bition was observed in 100% of the one-month-olds and in 87.5% of the two-month-olds. In contrast, in the neonates, inhibition was observed in only 4 of 10 (40%) (P < 0.05). The maximum percent inhibition of th e cardiac vagal response was significantly less in the neonates than i n the older puppies (P < 0.001) and adults (P < 0.01), and the summate d inhibition also tended to be less in the neonates (P < 0.05 compared to one- and two-month-old puppies), Finally, in 60% of the neonates a nd 37.5% of all other animals vagal responses after stellate stimulati on were facilitated, i.e. at least 20% greater than the pre-stellate s timulation values. Conclusion: The putative neuropeptide-Y-mediated, s ympathetic-parasympathetic interaction is not fully expressed in the c anine neonate. It appears to develop quite rapidly postnatally, being fully expressed by 1 month of age, We hypothesize that this developmen tal change is likely the result of maturation of sympathetic nervous s ystem function after birth. The facilitation of the vagal chronotropic response, observed in some animals after stellate stimulation, is a n ew finding, and may represent yet another type of autonomic interactio n.