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

Previous studies showed that injections of somatostatin (SS-14) into the hi ppocampus facilitate the acquisition of spatial tasks in mice. The present study was aimed at better understanding the learning and memory processes t hat could be affected by hippocampal SS-14 stimulation. Balb/c mice were su bmitted 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 stag e 2, were submitted to a probe test aiming at evaluating their ability to u se flexibly their previously acquired knowledge. Injections of vehicle or S S-14 were given during the acquisition phase and/or before the probe test u sing a 2 x 2 factorial design. Mice receiving SS-14 during acquisition fail ed to succeed in the probe test despite showing a trend to faster acquisiti on 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 w ith its expression per se, and therefore dissociated the acquisition of pla ce responses from their flexible use. The theoretical issues raised by the present findings are discussed.