It was hypothesized that if modafinil acts primarily via
It was hypothesized that if modafinil acts primarily via a noradrenergic mechanism, Dbh −/− mice should be non-responsive since they completely lack NE. In contrast, if modafinil acts mainly through DA systems, these mice should be hypersensitive. Modafinil was tested in Dbh −/− mice using both locomotor and sleep latency paradigms as behavioral readouts, and NE–DA interactions were further explored by examining the effect of NE and DA receptor antagonist pretreatments.
Discussion Although the effects of modafinil appear to involve both NE and DA, the exact contribution of these two monoamines to the mechanism of modafinil action remains unclear. Dbh −/− mice completely lack NE but have hypersensitive DA signaling. Thus, it was hypothesized that if modafinil acts primarily via NE, then the behavioral effects of modafinil would be attenuated in these mice, while if modafinil acts primarily via DA, Dbh −/− mice would be hypersensitive. As Dbh −/− mice were hypersensitive to both the wake-promoting and locomotor-activating effects of modafinil, it is tempting to conclude that modafinil acts primarily via the dopaminergic system. However, this finding requires reconciliation with the reported effects of α1AR antagonists in attenuating the effects of modafinil. Lesioning of the locus coeruleus (LC), the major brainstem noradrenergic nucleus, using the selective noradrenergic neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4), had no effect on modafinil-induced wake behavior (Wisor and Eriksson, 2005). Because α1AR blockade attenuated the effects of modafinil in both intact and LC-lesioned animals, these authors proposed that modafinil acts by blocking DAT and increasing extracellular DA, which then directly stimulates α1ARs to promote wake (Wisor and Eriksson, 2005). However, there are a number of caveats to this model. Firstly, the potency of DA at cloned α1ARs is approximately 100-fold lower than that of NE (Zhang et al., 2004). Secondly, DSP-4 does not completely eliminate LC neurons, and in fact Fidaxomicin ventral brainstem adrenergic and noradrenergic nuclei (e.g. A1, A2, C1, C2) intact (Fritschy and Grzanna, 1991). This is an important point, as projections from these nuclei provide the majority of the noradrenergic innervation to dopaminergic areas (i.e., ventral tegmental area, nucleus accumbens, periaquaductal grey (PAG)) and supplies NE and EPI to the hypothalamus (Jones et al., 1977, Woulfe et al., 1990, Delfs et al., 1998), which is a likely site of the wake-promoting effects of modafinil (Delfs et al., 1998, Engber et al., 1998a, Jones et al., 1977, Lin et al., 1996, Scammell et al., 2000, Woulfe et al., 1990). This hypothesis was tested in the present study by examining the effects of the α1AR antagonist, prazosin, in Dbh −/− mice. If modafinil acts by facilitating the ability of DA to directly stimulate α1ARs, then blocking α1ARs should attenuate the behavioral effects of modafinil whether or not NE is present. However, while prazosin attenuated modafinil-induced locomotor activity in control mice, it failed to do so in Dbh −/− mice. In contrast, the DA receptor antagonist, flupenthixol, attenuated the effects of modafinil in both control and Dbh −/− mice. Thus, an alternate mechanism for modafinil-induced arousal may be proposed that partially depends on NE–DA interactions (Fig. 7, “right” pathway). The noradrenergic system provides excitatory drive onto DA neurons via α1ARs, which are critical for DA release and responses to dopaminergic drugs like psychostimulants (Weinshenker and Schroeder, 2007). This is consistent with the hypothesis that modafinil produces its behavioral effects via weak blockade of both DAT and NET (Gallopin et al., 2004, Madras et al., 2006). NET blockade increases extracellular NE, which in turn activates α1ARs and promotes the firing of DA neurons and DA release. DAT blockade prevents the reuptake of the released DA, which then promotes the behavioral effects of modafinil by activating DA receptors. NET blockade also increases NE in other brain regions involved in sleep–wake regulation, such as the hypothalamus (Fig. 7, “left” pathway). Although Dbh −/− mice lack NE, they can bypass the requirement for α1AR stimulation because of hypersensitive DA receptors.