Effects of l-dihydroxyphenylalanine (l-Dopa) and d,l,5-hydroxytryptophan (d,l,5-HTP) on reserpine-induced amnesia

In a series of experiments the effects of reserpine, l-Dopa, and d,l,5-hydroxytryptophan (d,l,5-HTP) on retention of a passive avoidance training in mice were investigated. Reserpine (2.5 mg/kg) produced amnesia when given at 120 min before but not at 30 min before or at 0, 10, 30, or 90 min following training. This time-dependent reserpine effect did not appear to be due to either an alteration in footshock sensitivity during training or to the drug producing state-dependent learning. The amnesic effect of reserpine could be blocked when both l-Dopa and d,l,5-HTP were also administered up to 10 min but not at 30 or 90 min following training. The drugs, l-Dopa or d,l,5-HTP, given alone or in higher doses, could not at any time counteract the reserpine effect. The retrograde effects of the combined administration of these biogenic amine precursors on the reserpine-induced amnesia are discussed in terms of the possible role of biogenic amines in memory formation.