Fig. 4.
Restoration of the reduced LTP by re-exposure of the chronic drug-treated animals to opiates. Twelve hours after the termination of chronic opiate treatment, rats were reinjected with opiates or NS (subcutaneously, as indicated by thetriangle), respectively, 30 min before LTP induction.A, B, In freely moving animals, reinjection of 10 mg/kg morphine (Mor;A) or 1 mg/kg heroin (Her;B) 30 min before high-frequency stimulation correspondingly restored the reduced LTP to the normal level compared with the NS group. C,D, The reduced LTP after chronic opiate treatment as above was also restored in anesthetized rats by subcutaneous administration of 10 mg/kg morphine (Mor;C) or 1 mg/kg heroin (Her;D) 30 min before LTP induction, respectively, compared with the NS control (n = 6–8 in each group). NMDA receptor antagonist MK-801 (3 mg/kg, i.p.), could block the restoration of LTP by morphine (C). Arrowsindicate high-frequency stimulation; triangles indicate drug injection.