Long-lasting dopamine receptor up-regulation in amphetamine-treated rats following amphetamine neurotoxicity

Amphetamine (A) (9.2 mg/kg, IP), in combination with iprindole (I) (10.0 mg/kg, IP), caused long-lasting dopamine (DA) depletions in striatum (−49%, 4 weeks) but not in nucleus accumbens following one A/I injection. Striatal DA had recovered by 4 months. DA receptors (DAr) were up-regulated: 1) behavioral responses to a DA receptor agonist (apomorphine) were significantly elevated. These included apomorphine-induced locomotor activity (+ 103% and + 160%, on weeks 3 and 10) and apomorphine-induced stereotypy (day 10). 2) B_max for [³H]spiroperidol binding to striatal D₂ DAr (12 weeks) increased (+53%, week 12.). Injection of the DAr neuromodulator cyclo(leucyl-glycyl) (8 mg/kg/day × 4 days, SC) reversed the B_max increase. Thus toxicity (DA depletion) following high-dose amphetamine appears to induce compensatory changes in DAr. This DAr upregulation may explain the lack of abnormal movements despite enduring DA depletion. Additionally, the A/I paradigm as an animal model of long-lasting DAr up-regulation, could be used to screen neuromodulatory agents, like CLG, that might treat disorders (e.g., tardive dyskinesia and schizophrenia) thought to involve up-regulated DAr.