REVERSIBLE INACTIVATION OF DORSAL HIPPOCAMPUS BY TETRODOTOXIN IMPAIRSBLOCKING OF TASTE-AVERSION SELECTIVELY DURING THE ACQUISITION BUT NOTTHE RETRIEVAL IN RATS

The role of the dorsal hippocampus in the different stages of blocking was examined in a taste aversion learning task. Blocking is a learnin g effect in which one previously conditioned element of a compound mak es the conditioning of the added element difficult, An effective block ing procedure with one trial in each stage was tested in unoperated ra ts, In the first stage, rats drank saccharin and later received lithiu m chloride by i.p. injection. In the second stage, they were presented with a serial compound saccharin-cider vinegar before lithium injecti on. In a one-bottle, test a reduced aversion to cider vinegar was obse rved in this group compared to control groups with no previous sacchar in aversion. Bilateral tetrodotoxin (TTX) injection (10 ng/mu l) in th e dorsal hippocampus attenuated blocking if this was applied before dr inking the compound stimulus during the second stage, but it produced no effect applied either before drinking saccharin in the first stage or before testing. Non-specific retrieval deficit produced by ?TX inje ction applied before testing was ruled out in a control group subjecte d to a conventional cider vinegar aversion learning which showed compl ete retrieval of the aversion under TTX. It is concluded that the hipp ocampal function relevant for blocking takes place during the compound phase.