title: Receptor changes during chronic dopaminergic stimulation.
abstract: The underlying cause of the long term complications of L-DOPA or dopamine agonist therapy in Parkinson's disease remains unknown. Previous studies of repeated administration of L-DOPA or bromocriptine to rodents have shown increases, decreases or no change in brain dopaminergic activity. For this reason we have re-examined the effects of chronic L-DOPA or dopamine agonist administration on brain dopamine receptor function in rats. Repeated intraperitoneal administration of L-DOPA to rats for 21 days followed by 3 days drug withdrawal caused an enhancement of apomorphine-induced stereotypy but no apparent alteration in striatal dopamine receptor numbers or affinity (as judged by 3H-spiperone; 3H-NPA and 3H-piflutixol binding). Chronic oral administration of L-DOPA plus carbidopa to rats for one year was without effect on apomorphine-induced stereotypy or striatal D-2 dopamine receptors. Similarly, no effects were observed on striatal dopamine function following one year's administration of bromocriptine. Pergolide produced an enhancement of apomorphine-induced stereotypy but a decrease in D-2 receptor density as judged by 3H-spiperone binding. In rats with a unilateral 6-OHDA lesion of the medial forebrain bundle the oral administration of L-DOPA plus carbidopa for 4 weeks, followed by 4 days withdrawal, enhanced the rate of apomorphine-induced contraversive rotation. It appears difficult, at least in rats, to manipulate striatal dopamine receptors with L-DOPA or dopamine agonist drugs. An enhancement of motor behaviour can occur in the presence of no change or a decrease in dopamine receptor numbers identified by in vitro ligand binding to tissue homogenates.
mesh_terms: D015632:1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine; D000818:Animals; D001522:Behavior, Animal / Q000187:drug effects; D001927:Brain Diseases / Q000139:chemically induced / Q000188:drug therapy / Q000523:psychology; D001971:Bromocriptine / Q000494:pharmacology; D004298:Dopamine / Q000502:physiology*; D006801:Humans; D006892:Hydroxydopamines; D007274:Injections, Intraperitoneal; D007980:Levodopa / Q000494:pharmacology; D018377:Neurotransmitter Agents / Q000502:physiology; D016627:Oxidopamine; D010300:Parkinson Disease / Q000378:metabolism; D010479:Pergolide / Q000494:pharmacology; D011323:Primates; D011725:Pyridines / Q000494:pharmacology; D011954:Receptors, Dopamine / Q000187:drug effects / Q000502:physiology*; D013375:Substance Withdrawal Syndrome / Q000503:physiopathology
chemical_list: D006892:Hydroxydopamines; D018377:Neurotransmitter Agents; D011725:Pyridines; D011954:Receptors, Dopamine; D010479:Pergolide; D001971:Bromocriptine; D007980:Levodopa; D016627:Oxidopamine; D015632:1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine; D004298:Dopamine