Effects of serotonin-norepinephrine reuptake inhibitors on locomotion and prefrontal monoamine release in spontaneously hypertensive rats

Catecholamine neurotransmission in the prefrontal cortex plays a key role in the therapeutic actions of drugs for attention-deficit/hyperactivity disorder (ADHD). Recent clinical studies show that several serotonin-norepinephrine reuptake inhibitors have potential for treating ADHD. In this study, we examined the effects of acute treatment with serotonin-norepinephrine reuptake inhibitors on locomotion and the extracellular levels of monoamines in the prefrontal cortex in spontaneously hypertensive rats (SHR), an animal model of ADHD. Adolescent male SHR exhibited greater horizontal locomotion in an open-field test than male WKY control rats. Psychostimulant methylphenidate (0.3 and 1 mg/kg), the selective norepinephrine reuptake inhibitor atomoxetine (1 and 3 mg/kg), and serotonin-norepinephrine reuptake inhibitors duloxetine (10 mg/kg), venlafaxine (10 and 30 mg/kg) and milnacipran (30 mg/kg) reduced the horizontal activity in SHR, but did not affect in WKY rats. The selective norepinephrine reuptake inhibitor reboxetine (10 mg/kg) and the tricyclic antidepressant desipramine (10 and 30 mg/kg) also reduced the horizontal activity in SHR, whereas the selective serotonin reuptake inhibitor citalopram (30 mg/kg) did not. Microdialysis studies showed that atomoxetine, methylphenidate, duloxetine, venlafaxine, milnacipran, and reboxetine increased the extracellular levels of norepinephrine and dopamine in the prefrontal cortex in SHR. Citalopram did not affect norepinephrine and dopamine levels in the prefrontal cortex, although it increased the serotonin levels. Neither duloxetine nor venlafaxine increased the dopamine levels in the striatum. These findings suggest that serotonin-norepinephrine reuptake inhibitors, similar to methylphenidate and atomoxetine, have potential for ameliorating motor abnormality in the SHR model.