Oropharyngeal control of ingestion in rats: acquisition of sham-drinking patterns

Rats with esophageal fistulas, offered a concentrated glucose solution, sham drank relatively small amounts on the first session. This was true whether or not such a solution was familiar. Intake rose to very high levels over ensuing sham-drinking sessions, a result showing that the initial small volumes were not attributable to fixed properties of the solution itself (e.g., aversiveness, viscosity, or input to "satiety" mechanisms). The gradual acquisition of copious sham drinking was not affected by previous "practice" at sham drinking other commodities; it was not simply an alteration in motor habits. Finally, the gradual acquisition of the sham-drinking pattern did not occur with more dilute solutions; in response to these, sham drinking was copious and continuous from the outset. The following conclusions are made: (a) Rats must learn to respond, by continuous drinking, to the absence of the postingestive inhibition normally produced by concentrated solutions. (It is a separate and open question whether the response to the presence of such inhibitory factors must also be learned.) (b) No such learning is required in the case of more dilute solutions. This is further evidence that controlling factors with differing properties operate at different levels even of the single commodity, glucose in solution.