Altered Autonomic Nervous System Function in Chickens Divergently Selected for Body Weight

Autonomic nervous system activity is related to body weight regulation. Based on the MONA LISA hypothesis it has been suggested that most obese subjects and animals have low sympathetic nervous system activity. The aim of this study was to investigate whether there are differences in autonomic nervous system activity between lines of chickens selected for either high (HWS) or low body weight (LWS). In Exp. 1, various pharmacological agents were injected intravenously, and the changes in blood pressure (BP) and heart rate (HR) of both HWS and LWS chickens were compared. The results showed that the HWS birds had a greater increase in BP and HR than the LWS following injection of atropine, a muscarinic receptor blocker, and LWS birds had a greater decrease in BP and HR to propranolol, a beta- adrenergic receptor blocker than the HWS birds. These results suggested that HWS chickens have higher parasympathetic tone, whereas LWS chickens have a higher sympathetic nervous system tone regulating the cardiovascular system. HWS and LWS chickens displayed a similar response in BP and HR following injection of the ganglion blocker tetraethylammonium chloride. These results suggest that there is no significant difference in the central autonomic nervous system in the cardiovascular regulation between HWS and LWS together. Since there does not appear to be any differences in the activity of the autonomic nervous system activity at the level of the central nervous system, these findings imply that the difference in response to atropine and propranolol could be caused by differences in adrenal activity. The ratio of heart rate and blood pressure after the injection of phenylephrine showed significant difference between these two lines of birds, but not when phenylephrine was injected following atropine. This result indicated that HWS are more dependent on the parasympathetic nervous system to regulate the baroreceptor reflex. The percentage of adrenal and sympathetic impact on the regulation of heart rate showed that LWS females required greater adrenal activity than the other birds. In Exp. 2, the body weight and food intake responses of HWS and LWS chickens to ip injections of reserpine were compared. Reserpine caused a transitory decrease in food intake and body weight in both lines of birds. However HWS chickens recovered more slowly from the depression caused by reserpine than the LWS chickens. This could be due to lower sympathetic nervous system activity. In conclusion, it appears that HWS may have lower sympathetic activity than LWS. Combining the results of both experiments, it appears that the HWS birds have lower sympathetic and higher parasympathetic activity. Furthermore central nervous system autonomic activity in BP and HR regulation is not different between HWS and LWS, but the activity of the adrenal gland may be different between these two lines of birds.