Vulnerability of synaptic plasticity in the striatum of methamphetamine-sensitized rats

Treatment of the psychostimulant-sensitized animal model of schizophrenia

Behavioral sensitization to psychostimulants in rodents is associated with the alteration of dopaminergic neurotransmission, and has been proposed as a useful model of schizophrenia due to its progressively intensifying, easily relapsing, and long-lasting features. Pharmacological treatments that reverse the established sensitization may have potential therapeutic values for schizophrenia. The present aim is to review pharmacological treatments that induce the reversal of established sensitization to psychostimulants. In addition, we discuss possible mechanisms for the reversal of sensitization. Reversal of sensitization is induced by chronic dopamine D1 receptor agonism, D2 or D1/D2 receptor agonism combined with mild N-methyl-D-aspartate (NMDA) receptor antagonism or serotonin (5-HT(2A) or 5-HT(3) ) receptor antagonism, 5-HT(1A) receptor agonism, and 5-HT(2A) or 5-HT(3) receptor antagonism. Chronic treatments with these drugs likely adjust altered dopaminergic neurotransmission in sensitized animals. Especially, chronic dopamine D1 receptor agonism, which may adjust mesolimbic hyperdopaminergic and mesocortical hypodopaminergic functions in sensitized animals, is an attractive therapeutic approach for schizophrenia.

Reversal of methamphetamine-induced behavioral sensitization by repeated administration of a dopamine D1 receptor agonist

Repeated intermittent administration of methamphetamine (MAP) produces an enduring hypersensitivity to the motor stimulant effect of MAP, termed behavioral sensitization. Dopamine plays a critical role in the development and expression of behavioral sensitization. Here, we investigated whether a dopamine D1 receptor agonist could reverse behavioral sensitization to MAP. Administration of MAP (1.0 mg/kg, i.p.) to rats once every 3 days for a total of 5 times (days 1-13) induced the enhancement of locomotor activity after MAP challenge (0.5 mg/kg, i.p.) on day 20, verifying the development of behavioral sensitization. The MAP-sensitized rats then received a dopamine D1 agonist, R-(+)-SKF38393 (3.0 mg/kg, i.p.), once a day for 7 consecutive days (days 21-27). Behavioral analysis on days 30 and 41 revealed that the enhanced locomotor activity was reversed by repeated R-(+)-SKF38393 administration. Moreover, repeated R-(+)-SKF38393 administration reversed the increased dopamine release in the striatum after MAP challenge on day 41. Thus, repeated administration of the dopamine D1 receptor agonist induces the reversal of established behavioral sensitization to MAP and of increased dopamine release in the striatum, lasting for at least 2 weeks. Dopamine D1 receptor agonists may be useful therapeutic agents for the treatment of psychostimulant addiction.

Effect of methamphetamine and imipramine on cerebral ischemia-induced hyperactivity in Mongolian gerbils

Ischemia-induced hyperactivity is recognized several hours after both common carotid arteries' occlusion for 5 min in Mongolian gerbils, and it continues for at least 7 days. The aim of this study is to investigate the possible mechanisms of this abnormal behavior. Methamphetamine (MAP) (1 and 3 mg /kg) was administered for 7 days and imipramine (IMP) (5 and 10 mg/kg) was administered for 7 or 14 days. Bilateral carotid artery was occluded for 5 min 24 h after the last administrations of these drugs. MAP, which had been administered every day for 1 week, showed marked inhibition in the ischemia-induced hyperactivity. However, IMP did not have any effect even though it had been injected every day for 2 weeks. Hippocampal CA1 neuronal changes also appeared in the MAP- and IMP-administered groups. As the dopaminergic neurotransmission is facilitated by the repeated administration of MAP, the ischemia-induced hyperactivity may be related to abnormalities in dopaminergic function. The participation of the other neurotransmitters is also discussed.

Repeated adenosine pre-treatment potentiates the acute effect of methamphetamine in rats

Adenosine was intraperitoneally (i.p.) injected to Wistar rats every 3 days with a total of 5 administrations. After a 7-day withdrawal, the animals were challenged with methamphetamine (0.5 mg/kg, i.p.). The effect of methamphetamine on locomotor activity was significantly potentiated by repeated adenosine pretreatment. Moreover, methamphetamine-induced dopamine release was also increased in the striatum. Methamphetamine-induced hyperactivity and dopamine release were significantly potentiated by repeated pretreatment of an adenosine A1 agonist, N6-cyclohexyladenosine (0.5 mg/kg, i.p.). These results suggest that the acute effect of methamphetamine is potentiated by repeated pre-treatment of adenosine via adenosine A1 receptors.