Muscarinic antagonist enhances one-trial food-aversion learning in the mollusc Pleurobranchaea

One hour before training, in two replicate studies on the sea slug Pleurobranchaea californica, all animals (N = 114) received body-cavity injections of scopolamine, oxotremorine, or the equivalent volume of the saline/seawater vehicle that was used to inject the drugs. The low drug doses (2 mumol/kg) were near the threshold for generating observable neurophysiological responses, but did not affect feeding thresholds arising to a stimulus derived from beer (Sbr) and to one derived from squid (Ssq). Before training, the animals did not discriminate between Sbr and Ssq, as indicated by similar thresholds to both stimuli. During training, experimental animals in each injection group received Sbr alone for 10 sec and then paired with electric shocks for 50 sec; control animals received shocks 1 hr after Sbr. Postconditioning tests began 12 hr after training and were repeated once daily thereafter. After training, all experimental groups exhibited the expected aversive behavior to Sbr, as indicated by 100- to 1000-fold increases in feeding thresholds, and retained low thresholds to Ssq, but the scopolamine animals were better able to discriminate between Sbr and Ssq than either of the other experimental groups. The aversive responses to Sbr increased over a 3-day period, but there appeared to be no difference between injection groups on such a long-duration "consolidation" phase. Of the control groups, only the scopolamine animals exhibited low feeding thresholds to both Sbr and Ssq. The other control groups exhibited similar behavior as the experimental animals, indicating that associative factors relating to the 1-hr separation between Sbr and shocks may have produced the behavior in these control animals. Thus, by comparison to the other injections, scopolamine 1) increased the ability of the experimental animals to make the discrimination between Sbr and Ssq, and 2) prevented learning to avoid Sbr in the control animals. An accompanying paper provides a detailed characterization of muscarinic receptor pharmacology in Pleurobranchaea.