Induction of tolerance of dopaminergic responses in man
Schizophrenia may reflect a sensitization of dopaminergic (DA) function. Apomorphine (Apo), a DA receptor agonist, induces both sensitization and tolerance of DA function in rodents depending on dose intervals. We investigated sensitization and tolerance to Apo in healthy male volunteers. After a period of acclimatization to the experimental setting (Day 1) subjects were assigned randomly to two groups: Group A subjects received seven injections of placebo (physiological saline) (PLA) and Group B subjects received seven injections of Apo HCl (7 μg/kg sc) under double-blind conditions at 2 h intervals commencing at 0930 hours (Day 2) after an overnight fast. Twelve hours after the seventh injection, i.e. on Day 3, after an overnight fast all subjects received an injection of Apo. Serial samples of blood commencing at 0900 hours were drawn after the first and last injection in both groups for assay of growth hormone (GH), prolactin (PRL) and cortisol by radioimmunoassay; sleepiness was measured using the Analog Sleepiness Rating Scale and yawning recorded by video recorder. The GH response in Group B (N = 8) was (a) decreased after the eighth injection of Apo compared with the first injection of Apo (P = 0.03) and (b) decreased after the eighth injection of Apo compared with the first injection of Apo in Group A (N = 10) (P = 0.001). The number of yawns in Group B was significantly decreased after the eighth injection of Apo compared with the first injection of Apo (P = 0.042). PRL, cortisol and sleepiness were not significantly different between the first and eighth injection of Apo. Sensitization was not observed in any of the measures studied. These results are compatible with induction of acute tolerance of DA-mediated GH and yawning responses. The method used provides a safe pharmacological paradigm to examine plasticity of DA mechanisms in man. Results are discussed in the context of possible therapeutic implications for schizophrenia.