Naloxone attenuation of sham feeding is modified by manipulation of sucrose concentration

The time course of sucrose (5, 10 and 20%; w/v) sham feeding was monitored in one hour tests. Intake levels increased as a function of concentration. Naloxone (1.25 mg/kg, IP) attenuated the sham feeding of 10% sucrose solution in gastric fistulated rats, without affecting initial intake rates. Furthermore, after naloxone the intake pattern of 10% sucrose was identical to that for 5% sucrose in untreated rats. In a second test, substitution of 10% sucrose by a 20% solution after 15 min of sham feeding reversed the effect of naloxone, restoring intake to 10% baseline levels. Thus naloxone's effect appeared to be behaviourally equivalent to that of sucrose dilution and was counteracted by increasing sucrose concentration. Naloxone was apparently more effective against the lower sucrose concentration, suppressing intake at an earlier stage of testing. The data confirm the importance of oropharyngeal stimulation to the suppressive action of naloxone and support opioid mediation of orosensory reward.