Physiology & Behavior, Vol. 17, pp. 131-136. Pergamon Press and Brain Research Publ., 1976. Printed in the U.S.A. Evidence that Maternal Ventral Skin Substances Promote Suckling in Infant Rats MYRON A. HOFER, HARRY SHAIR AND PAULINE SINGH Department of Psychiatry, Albert Einstein College of Medicine, Montefiore Hospital and Medical Center 111 East 210th Street, Bronx, NY 10467 (Received 13 February 1976) HOFER, M. A., H. SHAIR AND P. SINGH. EvMence that maternal ventral skin substances promote suckling in infant rats. PHYSIOL. BEHAV. 17(1) 131-136; 1976.- Washing a lactating rat's ventral skin with acetone, alcohol and water markedly reduces the effectiveness with which her 2-week-old infants can initiate suckling on the anesthetized mother. Control experiments with detailed thermal and behavioral measures give evidence tending to rule out lowered skin temperature, altered thermal gradients, changed tactile characteristics of abdominal fur or nipples and aversion to remaining traces of the organic solvents as responsible for the infants' failure to find and attach to nipples. The data suggest that some substance(s) on the skin of the mother's ventrum act as olfactory and/or gustatory cues for the infants' orientation and attachment to nipples. Olfaction Development Nursing Mother-infant interaction Pheromones YOUNG rats have been shown to locate their mother from a distance by responding to a pheromonal cue released by lactating females after the 14th postnatal day [1,2]. In younger infant rats, we found that olfactory denervation disrupts orientation to the mother's ventrum and nipple attachment as early as 7 days of age, resulting in severe weight loss, starvation and death [5,6]. These latter studies led us to hypothesize that infant rats depend upon olfactory cues on the mothers' ventrum for initiation and maintenance of suckling. Olfactory denervation procedures may have direct CNS or toxic side effects, however, which complicate interpretation of the observed disruption of nursing behavior. Therefore, we designed the present studies in order to explore more directly the properties of the mother's ventrum which might be necessary for infant rats to orient, attach and suckle. We utilized the strategy of attempting to remove all substances possibly acting as olfactory or gustatory cues from the mother's ventrum by a procedure of extraction with organic solvents [4] and elution with water. Finding that this effectively prevented nipple attachment, we carried out a number of control experiments to find out whether the washing procedure exerted its effects by altering thermal and tactile cues or by making the mother aversive to the infants. GENERAL METHOD Animals and Housing Rats of the Wistar strain (Carworth CFN), born and raised in our laboratory, were housed in 6 gal. terraria, under conditions of regulated temperature and humidity. All litters were culled to 8-10 pups within 72 hr of birth and housed under a 12 : 12 hr reversed day-night cycle The litters were left undisturbed until 14 days postnatal age. Behavioral Testing Infants from each litter were tested twice for nipple attachment on their anesthetized mother, both before and after an extensive washing of her ventrum as described below. On the day of the experiment, half the litter was removed from the mother and placed in a plastic holding cage with thermoregulated heat supplied under its floor, but without food and water. The remaining infants were left in the home cage with their mother. At the end of a 4 hr period, the mother was anesthetized with 30 mg/kg of pentobarbital, placed on her back along the edge of a tray of clean pine shavings and skin temperatures were recorded as described below. For the first test, each of the infants which had not been allowed to nurse for 4 hr were picked up and placed singly across the mid-ventrum of the anesthetized mother, facing the edge of the tray. A 2-minute observation period was allowed for each infant. The latency to suckling and the position of the nipple on the mother's ventrum were recorded. At the conclusion of the 2 min, the pups were returned to the holding cage, and remained there while one of a number of different washing or control procedures was carried out on the mother. The behavioral testing procedure was then repeated with the same group of pups (2nd test). l This research supported by a Research Scientist Development Award MH K-1 38,632 (M.A.H.), NIMH Project Grant 16929 and an NIMH Interdepartmental Fellowship (P.S.). ~We thank Elizabeth Margoshes and Shidan Tavana for their help in the conduct of these experiments. 131