Milk intake and nipple attachment behaviors were studied in the natural suckling situation after gastrointestinal preloading. Rat pups, deprived of their dam for 9 hr at 1, 10, or 20 days of age, were preloaded by gavage with volumes ranging from 2 to 16% of their body weight and returned to suckle. Preloads of artificial bitch's milk decreased the intake of mother's milk in a volume-related manner at all ages. At 1 and 10 days 4% preloads decreased milk intake without affecting attachment behaviors; larger preloads of 8 and 16% decreased intake and reduced the incidence of attachment. At 20 days of age small and large preloads decreased both incidence of attachment and milk intake. Preloads up to 8% of body weight had no effect on latency to attach at any age. Complete subdiaphragmatic bilateral vagotomy increased milk intake of 7-9 day-old pups fed automatically through an anterior mouth cannula in a nonsuckling situation. Vagotomy combined with spinal cordotomy (T2-T3) resulted in a synergistic hyperphagia and massive distension of the upper GI tract. The results indicate that suckling rats can control their intake of mother's milk while remaining attached to a nipple as early as 1 day of age. The suppression of ingestion in response to GI filling appears to be mediated by visceral afferent activity. Conversely, attachment behaviors are less affected by GI fill. This suggests that ingestive behaviors and attachment behaviors have different controls during the 1st 10 days of postnatal development.

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Three neurochemical agents known to arouse ingestion in adult rats were studied in milk-sated rat neonates ingesting fluids away from their dams. All were given intracerebroventricular (ICV) injection. Drinking of both milk and water by 2-day-olds injected with angiotensin II (Ang II) was confirmed. The characteristic adult dipsogenic response, intake of water greater than that of milk, appeared at 8 days. ICV carbachol (Carb), on the other hand, did not elicit drinking before 4 days, despite the fact that these dipsogens (Carb and Ang II) may both affect receptors in the subfornical organ. ICV norepinephrine (NE) increased milk intake beginning at 9 to 10 days, which coincides with the age at which central adrenergic receptors believed to mediate NE's orexigenic effect are appearing in the developing forebrain. NE had no effect on water intake at any age. The effects of all three agents on ingestive behaviors were dose-dependent. These data suggest that the neural systems utilized by Ang II, Carb, and NE are competent for mediation of specific ingestive behaviors before they reach full anatomical and biochemical maturity. Last, NE did not increase milk intake of pups suckling from their dam at any age. The noradrenergic system that is developing in the rat brain during the suckling period appears to be a nascent control of subsequent adult feeding rather than a functioning control of ongoing suckling.