Role of tissue plasminogen activator in the rewarding effect of morphine

Tissue plasminogen activator (tPA) is a serine protease that catalyzes the conversion of plasminogen (plg) to plasmin. The tPA-plasmin system plays a role in synaptic plasticity and remodeling. In this review, we focused on the role of tPA-plasmin system in the rewarding effect of morphine. A single morphine treatment induced tPA mRNA and protein expression in a naloxone-sensitive manner, which was associated with an increase in the enzyme activity in the nucleus accumbens (NAc). The acute effect of morphine in inducing tPA expression was diminished after repeated administration. No differences were observed in the morphine-induced antinociceptive effect between wild-type and tPA knockout (tPA-/-) mice. Morphine-induced conditioned place preference and hyperlocomotion were significantly reduced in tPA-/- and pLg-/- mice, being accompanied by a loss of morphine-induced dopamine release in the NAc. Microinjection of either exogenous tPA or plasmin into the NAc significantly potentiated morphine-induced dopamine release in the NAc of ICR mice. In contrast, plasminogen activator inhibitor-1 (PAI-1) dose-dependently reduced morphine-induced dopamine release. Furthermore, the defect of morphine-induced dopamine release and hyperlocomotion in tPA-/- mice was reversed by microinjections of either exogenous tPA or plasmin into the NAc. Our findings demonstrate a novel function of the tPA-plasmin system in regulating dopamine release in the NAc, which is involved in the morphine reward.