Previous exposure to cocaine in vivo reduces the incidence of STD LTP in VTA neurons. A–E , Time course of mean EPSPs before and after the STD LTP induction protocol indicated by the arrow in naive animals ( A ; n = 9), animals injected 3–5 h earlier with saline ( B ; n = 8) or cocaine (15 mg/kg, i.p.; C ; n = 12), and animals injected 24 h earlier with one injection of cocaine (15 mg/kg, i.p.; D ; n = 6) or the last injection of five daily cocaine injections (15 mg/kg, i.p.; E ; n = 6). A–C , Representative EPSP traces before and 30 min after the induction protocol are shown above graphs. F , Summary of the magnitude of LTP under various conditions showing a significant reduction of LTP in cocaine-treated animals compared with time-matched saline-treated animals. Coc, Cocaine. **p < 0.01. Error bars represent SEM.