Attenuation of SCH 23390-Induced Alteration of Striatal Dopamine D₁ Receptor Ontogeny by Prolyl-Leucyl-Glycinamide in the Rat

Long-term postnatal treatment of rats with SCH 23390 is associated with a reduction in the development of dopamine D1 receptors in the striatum. Because the tripeptide, l-prolyl-l-leucylglycinamide (PLG) attenuates the neuroleptic-induced increase in D2 receptors in the striatum in adult rats, this study was undertaken with the objective of determining whether PLG could modulate a developmental alteration in the D1 subtype of receptor. Rats were treated with the dopamine D1 receptor antagonist, SCH 23390 (R[+]-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1-H-3benzazepine) (0.30 mg/kg/d i.p.) for 32 successive days from birth, while D1 receptors in the striatum were assessed at 5 and 8 weeks from birth. Postnatal treatment with SCH 23390 reduced in vitro binding of [3H]SCH 23390 to homogenates in the striatum by 70% at 8 weeks. Scatchard analysis at 5 weeks determined that the Bmax for the binding of [3H]SCH 23390 was reduced by 78%, while the Kd was unaltered. When PLG (1.0 mg/kg/d i.p.) was administered together with SCH 23390 for the initial 32 days from birth, the binding of [3H]SCH 23390 to homogenates of the striatum was unchanged from that of the control group at 8 weeks. Also, at 5 weeks the Bmax and Kd were unaltered from control in the group that was treated with both SCH 23390 and PLG. The binding of [3H]SCH 23390 was not altered from control in the group treated with PLG alone. Also, PLG given in vitro did not alter the binding of [3H]SCH 23390 to control homogenates of the striatum. These findings indicate that PLG is able to attenuate neuroleptic-induced alterations in dopamine d1 receptors in the striatum.