Seventeen adult cats were chronically implanted with electrodes for polygraphic recordings in order to assess the role of catecholamines in the arousal effects of oral administrations of modafinil, a presumed noradrenergic agonist, and amphetamine, a well-known catecholamine-releasing agent. Whereas both modafinil (1, 2.5 and 5 mg/kg) and amphetamine (0.25, 0.5 and 1 mg/kg) caused a significant and dose-dependent increase in wakefulness and brain temperature, amphetamine, but not modafinil, elicited marked signs of behavioral excitation.
Pretreatments with alpha-methyl-DL-p-tyrosine methyl ester (50 mg/kg, i.p.), an inhibitor of catecholamine synthesis, almost completely prevented the effects of amphetamine (0.25 and 1 mg/kg), but only slightly reduced the duration of the waking effect of modafinil (2.5 and 5 mg/kg). Pretreatments with phentolamine (10 mg/kg, i.p.), prazosin (1.5 mg/kg, per os) and propranolol (5 mg/kg, i.p.), an alpha-, alpha 1- and beta-receptor antagonist, respectively, attenuated significantly the arousal effect of modafinil (1 mg/kg, the same as below) but not of amphetamine (0.25 mg/kg, the same as below). Intraperitoneal injections of haloperidol (0.5 mg/kg), a dopamine-receptor antagonist, blocked significantly the arousal of amphetamine but not of modafinil.
The effects of both modafinil and amphetamine were enhanced by a pretreatment with yohimbine (1 mg/kg, i.p.), an alpha 2-receptor antagonist. These results suggest that the arousal effect of modafinil does not depend on the availability of the endogenous catecholamines but results from an enhancement of alpha 1- and beta-receptor activity and that the waking and behavioral effects of amphetamine may be mainly due to an increase in dopamine release.