Antagonism of apomorphine-enhanced startle by alpha 1-adrenergic antagonists

The present study investigated the possible involvement of central noradrenergic neurons in mediating the excitatory effect of the dopamine agonist apomorphine on the acoustic startle response in rats. Experiment 1 assessed the effects of intraperitoneal (i.p.) administration of adrenergic antagonists on apomorphine-enhanced startle. The excitation of startle produced by apomorphine (1.0-3.0 mg/kg i.p.) was blocked by the alpha 1-adrenergic antagonists prazosin (0.03-1.0 mg/kg) and WB-4101 (1.0 mg/kg). Prazosin was very potent in this regard, having an ED50 of 0.03 mg/kg. Blockade of beta-adrenergic receptors with propranolol (20 mg/kg) or blockade of peripheral alpha-adrenergic receptors with phentolamine (10 mg/kg) failed to alter the effect of apomorphine. Prazosin did not block the enhancement of startle produced by other drugs (5-methoxy-N,N-dimethyltryptamine, strychnine), nor did it alter the entry of apomorphine into the brain. The alpha 1-adrenergic antagonists piperoxane (0.03 mg/kg), yohimbine (0.03 mg/kg) or RX781094 (0.07 mg/kg) markedly potentiated apomorphine excitation. These data indicated that specific blockade of central alpha 1-adrenergic receptors prevents apomorphine-enhanced startle. In contrast to the effects of alpha 1-adrenergic antagonists, Experiment 2 found that other drugs that produce an acute (clonidine, 0.040 mg/kg) or chronic (intraventricular 6-hydroxydopamine, 2 X 200 micrograms; DSP4, 50 mg/kg i.p.) disruption of noradrenergic transmission failed to affect apomorphine excitation. Thus, the ability of alpha 1-adrenergic antagonists to block apomorphine's excitation of startle cannot be explained by a simple dopamine-norepinephrine interaction. Alternative hypothesis are discussed.