Effects of infraorbital nerve section on survival, growth, and suckling behaviors of developing rats

Previous studies on sensory control of nursing in the rat have emphasized local olfactory cues. This study examined the role of tactile sensory processes by surgical section of the infraorbital nerve in rats at 17, 12, 7, and less than 1 day of postnatal age. All sectioned rats showed anesthesia of snout, upper lip, and vibrissal pads, and severe impairment of nipple attachment for at least the first day after denervation. Seven-day-old pups failed to recover, and all died after 3-6 days of relentless weight loss. Seventeen-day-old pups showed no weight loss after denervation if already weaned to solid food, but they lost weight for 3 days, and 25% failed to recover. Behavioral observations showed that denervated pups were activated by the mother's ventrum and probed for the nipple in the appropriate area but failed to focus head sweeping or licking and mouthing on the nipple itself and did not attach even when the nipple was inserted into the intact sensory areas of the mouth. Recovery after nerve section did not appear to be due to nerve regeneration. The data show that tactile cues are not necessary for normal nursing in the rat and suggest that important tactile sensorimotor adaptations may occur during the early and late phases of nursing.