Insulin replacement restores the behavioral effects of quinpirole and raclopride in streptozotocin-treated rats

Streptozotocin (STZ)-induced diabetes can modulate dopamine (DA) neurotransmission and thereby modify the behavioral effects of drugs acting on DA systems. Insulin replacement, and in some conditions repeated treatment with amphetamine, can partially restore sensitivity of STZ-treated rats to dopaminergic drugs. The present study sought to characterize the role of insulin and amphetamine in modulating the behavioral effects of drugs that selectively act on D2/D3 receptors. In control rats, quinpirole and quinelorane produced yawning, whereas raclopride and γ-hydroxybutyric acid (GHB) produced catalepsy. Raclopride antagonized quinpirole- and quinelorane-induced yawning with similar potency. STZ treatment increased blood glucose concentration, decreased body weight, and markedly reduced sensitivity to quinpirole-induced yawning, quinelorane-induced yawning as well as to raclopride-induced catalepsy, while enhancing sensitivity to GHB-induced catalepsy. Repeated treatment with amphetamine partially restored sensitivity of STZ-treated rats to amphetamine-stimulated locomotion and also produced conditioned place preference, without affecting blood glucose and body weight changes. However, amphetamine treatment did not restore sensitivity to the behavioral effects of quinpirole, raclopride, or GHB, suggesting differential regulation of dopamine transporter activity and sensitivity of D2 receptors in hypoinsulinemic rats. Insulin replacement in STZ-treated rats normalized blood glucose and body weight changes and fully restored sensitivity to quinpirole-induced yawning, as well as to raclopride-induced catalepsy, while reducing sensitivity to GHB-induced catalepsy. Overall, these data indicate that changes in insulin status markedly affect sensitivity to the behavioral effects of dopaminergic drugs. The results underscore the importance of insulin in modulating DA neurotransmission; these effects might be especially relevant to understanding the co-morbidity of eating disorders and substance abuse.