Serial feature positive and feature negative discrimination learning in a taste avoidance preparation: implications...

Psychoactive drugs produce interoceptive stimuli that can guide appropriate behaviors by initiating or inhibiting responding.

The current study investigated whether an interoceptive morphine state produces similar patterns of serial feature positive (FP) and feature negative (FN) discrimination learning under comparable conditions in a taste avoidance design.

Male Sprague-Dawley rats were trained under 10 cycles of FP or FN discrimination. In the FP task, morphine (10 mg/kg, IP) signaled that a saccharin solution was followed by LiCl (1.2 mEq, IP), while the vehicle (saline) signaled that the LiCl was withheld. In the FN task, the contingency was reversed.

The FP-trained rats acquired the discrimination after three training cycles, consuming significantly less saccharin on morphine, than on vehicle, sessions ( P  < 0.05). The FN-trained rats acquired the discrimination after six training cycles, consuming more on morphine than on vehicle sessions ( P < 0.05). However, FN-trained rats never recovered saccharin consumption to baseline levels and 40% of the rats continued to avoid saccharin (consuming 0 ml) on morphine sessions. Control rats that never received LiCl consumed high levels of saccharin on morphine and vehicle sessions, indicating that morphine did not produce unconditioned suppression of saccharin consumption.

The difficulty to acquire FN discrimination might reflect the limitations of learning about safety contingencies in the taste avoidance design. The rapidity of FP learning when a drug state signals an aversive contingency may have implications for the general role of interoceptive stimuli in the control of behavior.