Taste reactivity in suckling rat pups was assessed. Five-day-old rats in Experiment I were equipped with tongue cannulae seated 2 mm rostral, 0-3 mm caudal, or 4-6 mm caudal to the intermolar eminence. Either water, 1.4 x lo-' QHCI, SOM NH,Cl, or .43M NaCl was delivered in one of three fashions during suckling: (1) continuous: fluid was continuously available from a reservoir connected to the cannula, and could be obtained by exerting suction; (2) discrete: same procedure as above, save that fluid was available for only 30 sec every 3 min of the 30-min test; (3) pulsatile: here, .05 ml fluid was delivered by infusion pump 8-10 sec every 3 min, provided the pups were attached t o a nipple. Based on intake, time and behavioral measures, we conclude behavioral reactivity exists especially to NH,C1 and NaC1. As to quinine, Day 5 pups were responsive only when QHCl was delivered in the continuous mode. In Experiment 11, rats 10,15, and 20 days of age received the above solutions by infusions with the cannulae located in one of the three positions. Quinine reactivity developed sequentially in an anterior to posterior direction. NH4C1 disrupted behavior in 5-, lo-, and especially 20-day-old rats. Remarkably, Day 15 rats were not behaviorally reactive to NH,C1. Sodium chloride, especially in the anterior and middle positions, was treated as an aversive solution during suckling throughout development. Based on existing electrophysiological data, the last finding was unexpected.
The bases of nipple attachment and suckling behaviors have been extensively investigated during the past decade. Nipple attachment in rats 10 days of age and younger appears to be an end in and of itself. It is sustained for at least 12 hours in the absence of milk letdown (Cramer, Hall, & Blass, 1980). Rats, until day 17, do not prefer to suckle lactating rather than nonlactating nipples (Kenny, Stoloff, Bruno, & Blass, 1979). Rats younger than 2 weeks, when suckling, do not seem to react to either the immediate or delayed consequences of gastrointestinal malaise accompanying gastric distention (Cramer & Blass, 1983a,b; H a l l & Rosenblatt, 1977) or lithium toxicity (Martin & Alberts, 1979). This apparent insensitivity may be restricted to suckling situations because very young rats, eating a liquid diet away from the nipple, respond to both immediate (Hall, 1979) and long-term ingestive consequences (Gemberling & Domjan, 1982;Gemberling, Domjan, & Amsel, 1980). Finally, 3-day-old rats reject certain concentrated solutions injected into their mouths when away from the nipple (Hall & Bryan, 1981).