The arousal of ingestive behaviors by chemical injection into the brain of the suckling rat

Abstract

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.