Development and Mechanisms of Cardiorespiratory Responses to Maternal Deprivation in Rat Pups

Abstract

Cardiac and respiratory rates and an index of activity were recorded in rat pups separated from their mothers for 24 hours at 4, 14 and 21 days after birth. The cardiac and respiratory rates of pups separated at 14 days fell by 40%, while the percentage of time they were active was unchanged. In younger pups, the major response was respiratory, while 3-week-old pups showed only marginal trends in these measures. Experiments on 2-week-old pups with autonomic blocking agents and tactile stimulation revealed that the hearts of separated pups were capable of beating at normal rates but failed to do so, probably because of reduced sympathetic tone augmented by some degree of vagal restraint. Separation of 2-week-old pups in a novel environment led to more rapid responses than if pups remained in their home cage nest after removal of the mother. Nonlactating foster mothers failed to prevent the physiologic changes. The importance of the feeding experience to the maintenance of sympathetic tone in young rats is suggested and discussed.Clinical evidence suggests that the development of several classic psychosomatic diseases may depend upon early separation experiences (1), and there are data indicating that illness of many kinds may be precipitated in the setting of object loss in adulthood (2-4). Behavioral effects of maternal deprivation have been studied experimentally in monkeys (5, 6), but the physiologic impact of early maternal separation has received little attention, although a clinical study has documented consistent abnormalities in adrenocortical and pituitary growth hormone function in emotionally deprived children (7). These and other observations suggest that separation, and particularly early maternal deprivation, may elicit biologic responses in a variety of mammalian species. This paper describes such responses occurring in a relatively simple laboratory animal model system where the behavioral, environmental and physiologic factors involved can be analyzed systematically.A previous report described a 40% decrease in cardiac and respiratory rates occurring in 2-week-old rat pups within the first 16 hours of separation from their mothers, despite maintenance of body temperature, adequate nutrition to prevent dehydration-starvation, and irrespective of whether pups were left in their home cage or placed in a novel environment (8). In

Physiological Mechanisms for Cardiac Control by Nutritional Intake after Early Maternal Separation in the Young Rat

“…ECG-EMG, cardiotachometer and impedance pneumograph recordings were made by means of a Grass Model 7 Polygraph from unrestrained infants in their home cages. From 1 to 3 min recording samples, measures were derived for inactive and active heart rate and resting respiratory rate as previously described (4,6).…”

“…Whereas the behavioral effects of separation were caused by absence of behavioral interaction with the mother, the 40% fall in cardiac rate was the result of reduced milk intake. The low heart rates of separated pups were not fixed but rose transiently to normal levels with strong stimulation (4). Starvation and dehydration were not necessary to produce low heart rates (2), but a single intubation feeding with a milk formula made from bovine milk, in sufficient quantity, promptly restored the low heart rates of separated pups to normal levels for 1 to 2 hr (5).…”

The Role of Nutrition in the Physiological and Behavioral Effects of Early Maternal Separation on Infant Rats

“…All litters were culled to 8-10 pups within 72 hours of birth. Rat pups were implanted with fine silver wire (30 gauge) by methods previously described (8) under light ether anesthesia at least 24 hours prior to testing.…”

Presidential Address: Some Comments on the Transduction of Experience by the Brain: Implications for Our Understanding of the Relationship of Mind to Body

“…In Hofer's admirable studies (13) the constellation of events which intervened in the cardiorespiratory and behavioral effects of separation of 14-day-old rats was comprised of the lack of mother's milk, an unfamiliar environment and the physical absence of the mother. The rats studied showed a 40% drop in heart and respiratory rates during the first 12 to 16 hours following separation (14). Future investigations will probably reveal additional physiologic effects of separation.…”