Effects of haloperidol and SCH 23390 on acoustic startle in animals depleted of dopamine as neonates: implications for neuropsychiatric syndromes.

Animals depleted of dopamine (DA) in the neonatal period and tested in adulthood exhibit some similarities to patients with schizophrenia, including increased sensitivity to DA agonists, altered sensitivity to DA receptor antagonists, and abnormalities of the acoustic startle response (ASR). In this study, we examined the contributions of D1-like and D2-like DA receptors to ASR measures in animals depleted of DA as neonates. Male rat pups received intracerebroventricular injections of 6-hydroxydopamine (DA depleted) or its vehicle (controls) at 3 days of age. Animals underwent startle testing as adults (60-75 days of age) after administration of DA antagonists (haloperidol: 0.1 or 0.3 mg/kg, SCH 23390:0.01 or 0.05 mg/kg) with and without DA agonist administration (apomorphine 0.5 mg/kg). ASR amplitude and prepulse inhibition (PPI: percentage decrease in startle amplitude due to a low intensity prepulse) were measured. DA depleted animals showed increased ASR amplitude and reduced PPI compared to controls. Administration of D1-like or D2-like DA antagonists significantly reduced overall ASR and increased PPI in both control and DA depleted animals, with DA depleted animals showing a relatively greater sensitivity to the D1-like antagonist SCH 23390. Findings are discussed in terms of the role of residual DA in mediating ASR phenomena in depleted animals, differences between D1/D2 DA receptor mediation of ASR compared to other behaviors in DA depleted animals, and potential implications for neuropsychiatric syndromes such as schizophrenia.