NALOXONE DECREASES INTAKE OF 10-PERCENT SUCROSE IN PREWEANLING RATS

To investigate the role of opioids in the mediation of sucrose intake in the preweanling rat pup, we measured the effect of naloxone on inta ke of pups licking 10% sucrose from the floor of a beaker (independent ingestion test) and of pups ingesting 10% sucrose that was continuous ly infused through an anterior, sublingual oral catheter (oral cathete r test). Pups were tested only once to eliminate the possible effect o f test experience. Pups were tested in the second postnatal week (PN7, 9, 10, 11, and 14 days) with naloxone (1 mg/kg) or vehicle controls. Fourteen-day-old pups were also tested with 0.1 and 0.5 mg/kg. Naloxon e began to be efficacious for inhibiting intake on PN10 in the oral ca theter test and on PN11 in the independent ingestion test. On PN14, th e inhibition of intake was dose related and naloxone was more potent f or inhibiting intake in independent ingestion tests than in oral cathe ter tests. Naloxone not only decreased intake, it also decreased the i ncidence of licking, increased mouthing and resting, and had no signif icant effect on locomotion. The site of the inhibitory effect of nalox one on intake was in the central nervous system, presumably in the bra in, because naloxonemethiodide, an analogue of naloxone that does not cross the blood-brain barrier, did not inhibit sucrose in either test. These results demonstrate that the intake of 10% sucrose depends on e ndogenous opioids as early as PN10 and that this opioid mechanism oper ates when pups have not had prior test experience and in a test (oral catheter test) where intake is not dependent on appetitive behaviors.