Adult learning deficits after neonatal exposure to D-methamphetamine: Selective effects on spatial navigation and memory

The effects of neonatal D-methamphetamine (MA) treatment on cued and spatia l learning and memory were investigated. MA was administered to neonatal ra ts on postnatal days 11-20. All groups received four subcutaneous injection s per day. Group MA40-4 received 40 mg . kg(-1) . d(-1) of MA in four divid ed doses (10 mg/kg per injection). Group MA40-2 received 40 mg . kg(-1) . d (-1) of MA in two divided (20 mg/kg/injection) and saline for the other two injections per day. Controls received saline for four injections per day. As adults, both MA groups showed no differences in swimming ability in a st raight swimming channel. The MA40-4 group showed no differences in cued lea rning, but was impaired in hidden platform learning in the Morris water maz e on acquisition. They also showed reduced memory performance on probe tria ls. Similar trends were seen on reversal learning and reversal probe trials . Reduced platform-size learning trials caused spatial learning impairments to re-emerge in the MA40-4 group. The MA40-2 group showed no differences i n straight channel swimming, but was slower at finding the visible platform during cued learning. They were also impaired during acquisition and memor y trials in the Morris hidden platform maze. They showed a similar trend on reversal learning and memory trials, but were not different during reduced platform-size learning trials. When the MA40-2 group's performance on hidd en platform learning and memory trials was adjusted for cued trial performa nce, the spatial learning deficits remained. Deficits of spatial learning a nd memory are a selective effect of neonatal methamphetamine treatment irre spective of other learning and performance variables.