Reversible inactivation of dorsal hippocampus by tetrodotoxin impairs blocking of taste aversion selectively during the acquisition but not the 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 learning effect in which one previously conditioned element of a compound makes the conditioning of the added element difficult. An effective blocking procedure with one trial in each stage was tested in unoperated rats. In the first stage, rats drank saccharin and later received lithium chloride by i.p. injection. In the second stage, they were presented with a serial compound saccharin-cider vinegar before lithium injection. In a one-bottle, test a reduced aversion to cider vinegar was observed in this group compared to control groups with no previous saccharin aversion. Bilateral tetrodotoxin (TTX) injection (10 ng/microliters) in the dorsal hippocampus attenuated blocking if this was applied before drinking 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 TTX injection applied before testing was ruled out in a control group subjected to a conventional cider vinegar aversion learning which showed complete retrieval of the aversion under TTX. It is concluded that the hippocampal function relevant for blocking takes place during the compound phase.