Agonist-induced homologous and heterologous sensitization to D-1- and D-2-dependent contraversive turning.

A low dose of the D-1 receptor agonist SKF 38393 (2.0 mg/kg s.c.) 14 days post-lesion failed to induce contraversive turning in male Sprague-Dawley rats lesioned unilaterally in the medial forebrain bundle with 6-hydroxydopamine (6OHDA). Priming with a single administration of the specific D-2 agonist LY 171555 (0.2 mg/kg s.c. 3 days before), which itself elicited contraversive turning, made SKF 38393 (2.0 mg/kg s.c.) very active to produce contraversive turning. Priming with LY 171555 (0.2 mg/kg s.c. 3 days before) potentiated the contraversive turning in response to a low dose of LY 171555 (0.05 mg/kg s.c.). In naive 6OHDA-lesioned rats a rather high dose of SKF 38393 (10 mg/kg s.c.) was needed to induce a low intensity contraversive turning. Priming with a single administration of SKF 38393 (10 mg/kg s.c. 3 days before) made an otherwise ineffective dose of SKF 38393 (2.0 mg/kg s.c.) very effective to produce contraversive turning. The results indicate that the sensitization to the behavioural expression of supersensitivity for a given DA-receptor type follows not only priming with agonists for the same DA-receptor type (homologous sensitization) but also priming with agonists for a different DA-receptor type (heterologous sensitization).