TH protein and mRNA in nigrostriatal dopaminergic neurons are down-regulated by continuous but not intermittent apomorphine.

ABSTRACT

Levels of tyrosine hydroxylase (TH) and TH mRNA were measured after administration of dopamine agonists for a long period of time to elucidate the long-term feedback inhibition of dopamine synthesis in nigrostriatal dopaminergic neurons. Continuous infusion, which desensitized presynaptic dopamine receptors, but not repeated administration, down-regulated TH and TH mRNA levels. This suggests levels of TH protein and mRNA are only feedback inhibited by the continuous stimulation of postsynaptic dopamine receptors.