EFFECTS OF INTRAHIPPOCAMPAL INJECTIONS OF SOMATOSTATIN AND CYSTEAMINEON SPATIAL DISCRIMINATION-LEARNING IN MICE

The involvement of hippocampal somatostatin (SS-14) in spatial discrim ination learning in mice was evaluated by bilaterally administering ei ther SS-14 or its depletor, cysteamine, into the hippocampus through c hronically implanted guide cannulae. Two separate experiments were per formed using an eight-arm radial maze. In Experiment 1, mice were trai ned for 4 days on a discrimination task in which only three arms (A, B , and C) were used. In each daily session, subjects were each presente d in 10 trials with the pair AB and the pair BC, with arm B consistent ly baited over trials (e.g., A-B+ and B+C-). Immediately after complet ion of the fourth daily session with these regular trials, they were g iven 10 additional probe trials with free access to all three arms (e. g., A-B+C-). In Experiment 2, animals had free access to all eight arm s. They were trained to discriminate the three arms that were consiste ntly baited over trials (8 trials per day for 7 days). Intrahippocampa l injections were given each day, 30 min before testing. The results o f both experiments showed that, compared with saline, SS-14 administra tion (0.2 mu g/0.2 mu l per hippocampus) resulted in faster acquisitio n rates. In contrast, animals treated with cysteamine (20 mu g) were d ramatically impaired. In Experiment 1, however, the SS-14 treatment gr oup was significantly impaired relative to both the saline and the int act control groups when testing was shifted from regular to probe tria ls. The impaired ability of somatostatin-treated mice to adapt to prob e trials, together with their improved performance on both types of sp atial discrimination task, suggests that this peptide produced a chang e in the strategy the animals used to solve these tasks. Notwithstandi ng, the present findings seem difficult to reconcile with current theo ries about hippocampal function.