Influence of dopamine agonists on plasma and brain levels of homovanillic acid

The response of the plasma dopamine (DA) metabolite, homovanillic acid (HVA), to two DA agonists was investigated in the rat. Apomorphine administered i.p. (2 mg/kg) produced, within one hour, a significant decrease in plasma HVA. The response of plasma HVA to apomorphine was also investigated after pretreatment with debrisoquin, a drug which selectively blocks peripheral HVA production by inhibition of MAO. Pretreatment with debrisoquin did not significantly alter the decrement in plasma HVA produced by apomorphine indicating that a substantial portion of the plasma HVA response to apomorphine is due to the drug's action on brain. Bromocriptine (2 mg/kg) was also found to produce a significant decrease in plasma HVA. Since the response of brain HVA to DA agonists reflects the sensitivity of the DA receptor, the plasma HVA response to DA agonists might be a practical method of assessing brain DA receptor sensitivity in humans.