Cellular and behavioral effects of 5-HT1A receptor agonist 8-OH-DPAT in a rat model of levodopa-induced motor complications.

5-HT1A autoreceptor stimulation can act to attenuate supraphysiological swings in extracellular dopamine levels following long-term levodopa treatment and may be useful in the treatment and prevention of the motor complications. The purpose of this study was to investigate cellular and behavioral effects of 5-HT1A receptor agonist 8-OH-DPAT in a rat model of levodopa-induced motor complications. Two sets of experiments were performed. First, animals were treated with levodopa (50 mg/kg with benserazide 12.5 mg/kg, twice daily), intraperitoneally (i.p.) for 22 days. On day 23, animals received either 8-OH-DPAT (1 mg/kg, i.p.) or 8-OH-DPAT plus WAY-100635 (0.1 mg/kg, i.p) or vehicle with each levodopa dose. In the second set, animals were treated either with levodopa (50 mg/kg, i.p.) plus 8-OH-DPAT (1 mg/kg, i.p.) or levodopa (50 mg/kg, i.p.) plus vehicle, administered twice daily for 22 consecutive days. Our study showed that 8-OH-DPAT plus levodopa both prolonged the duration of the motor response and reduced peak turning. 8-OH-DPAT plus levodopa also decreased the frequency of failures to levodopa. Co-administration of WAY-100635, a 5-HT1A receptor antagonist, with 8-OH-DPAT eliminated the effect of 8-OH-DPAT on motor complications indicating that the observed 8-OH-DPAT responses were probably mediated at the 5-HT1A autoreceptor. Moreover, 8-OH-DPAT plus levodopa significantly reduced hyperphosphorylation of GluR1 at serine 845, which was closely associated with levodopa-induced motor complications.