Psychotomimetic-induction of tissue plasminogen activator mRNA in corticostriatal neurons in rat brain

We have studied in the rat the effects of acute subcutaneous injection of psychotomimetics including methamphetamine (MAP), cocaine and phencyclidine (PCP) on the expression of a brain plasticity-related molecule, tissue plasminogen activator (tPA) mRNA, using non-radioactive in situ hybridization histochemistry. In addition to the constitutive expression of tPA mRNA in cerebellar Purkinje cells, ventricular ependymal cells and meningeal blood vessel-associated cells, MAP (1-4 mg/kg), cocaine (30 mg/kg) and PCP (1.25-5 mg/kg) caused a transient and dose-dependent induction of the transcript with its peak at 3 h postinjection in a group of neurons of the medial and insular prefrontal cortices, and the piriform cortex. Another indirect dopamine agonist nomifensine (20-40 mg/kg) mimicked the tPA mRNA induction in the prefrontal cortical areas. Moreover, MAP induction of tPA mRNA was markedly inhibited by pretreatment with a D1 (R(+)-SCH23390: R(+)-7-chloro-8-hydroxy-9-methyl-1-phenyl-2,3,4,5-tetra-hydro-1H-3-benzazepine hydrochloride) or a D2 (haloperidol) dopamine receptor-preferring antagonist. Intramedial striatum, but not intrathalamic, application of a fluorescent tracer, fluorogold, retrogradely labelled the cortical cells expressing tPA mRNA. The present results suggest that acute injections of the above psychotomimetic drugs may induce tPA mRNA in a group of the prefrontal cortical neurons that project to the medial striatum. This tPA mRNA expression may be due to the activation of the dopamine neurotransmission. Because it is well documented that single or repeated administration of methamphetamine, cocaine and PCP produces enduring changes in responses to these drugs in humans and experimental animals (e.g. behavioural sensitization), the psychotomimetic-induction of tPA mRNA could be implicated in an initial step in the plastic rearrangements in the neuronal circuits underlying long-lasting changes in behavioural expression.