Intrahippocampal injections of somatostatin dissociate acquisition from the flexible use of place responses.

Previous studies showed that injections of somatostatin (SS-14) into the hippocampus facilitate the acquisition of spatial tasks in mice. The present study was aimed at better understanding the learning and memory processes that could be affected by hippocampal SS-14 stimulation. Balb/c mice were submitted to a two-stage learning paradigm. In stage 1, they were trained for acquisition of a spatial discrimination task in a radial maze and, in stage 2, were submitted to a probe test aiming at evaluating their ability to use flexibly their previously acquired knowledge. Injections of vehicle or SS-14 were given during the acquisition phase and/or before the probe test using a 2 x 2 factorial design. Mice receiving SS-14 during acquisition failed to succeed in the probe test despite showing a trend to faster acquisition of the initial spatial discrimination task. By contrast, when given only prior to probe trials, SS-14 did not yield any behavioural effects. Thus, SS-14 interfered with the establishment of a flexible form of memory, not with its expression per se, and therefore dissociated the acquisition of place responses from their flexible use. The theoretical issues raised by the present findings are discussed.