Review of the histamine system and the clinical effects of H₁ antagonists: Basis for a new model for understanding the effects of insomnia medications

The pharmacologic management of insomnia has long been dominated by agents that facilitate gamma amino butyric acid inhibition. These agents have served as the clinical model for understanding the pharmacodynamic effects of insomnia agents according to which sleep effects parallel plasma drug levels (pharmacokinetic effects). Agents with other mechanisms also exist for treating insomnia; however, their effects are less well understood. Many of these diminish the activity in one or more of the key wake-promoting systems. This review focuses on one such mechanism, blockade of the wake promoting effects of histamine via H₁ receptor antagonism. Although drugs with H₁ antagonist effects have long been available, this review was prompted by new studies of a selective H₁ antagonist, which provide the first indication of the effects that are specifically associated with H₁ antagonism. The findings do not conform to our long-standing model of insomnia agents in that factors other than drug blood level are needed to explain the clinical effects. We suggest a model for understanding these unique effects based on a review of the basic neurobiology of the histamine system. In addition to drug blood level, clinical effects reflect circadian variation in activity in the histamine system and other parallel wake promoting systems as well as factors such as pain and stress. We hypothesize that significant sleep enhancing effects are likely when the histamine system is relatively active and the activity in other parallel wake promoting systems is relatively minimal. Although the focus of this review is on the novel properties of H₁ antagonism, the principles that emerge from this analysis are most likely relevant to all agents that selectively block wake promoting systems, and as such, this review provides a new paradigm for understanding the effects of insomnia medications.