The upregulation of D 1 receptors associated with the downregulation of opioid receptors in the limbic areas may play an important role in the behavioral syndrome produced by sleep deprivation. Indeed, enhanced dopaminergic transmission may result from the upregulation of D1 receptors in limbic areas combined with the desensitization of those opioid receptors localized on DA nerve terminals, which tonically inhibit DA release (Spanagel et al., 1992). Desensitization of these receptors should result in disinhibition of DA nerve terminals from opioid inhibitory control. However, if this hypothesis is correct, why then is naloxone so effective in suppressing the behavioral syndrome? One might postulate that opioid receptors, whose activation produces enhanced dopaminergic firing, do not desensitize, unlike those receptors localized presynaptically on DA neurons. This possibility is supported by the finding that the number of opioid receptors in the mesencephalon remains unaltered after sleep deprivation (Fadda et al., 1991, 1992).