After oral administration, the nonimidazole histamine H 3 receptor antagonist, 6-[(3-cyclobutyl-2,3,4,5-tetrahydro-1H-3-benzazepin-7-yl)oxy]-N-methyl-3-pyridinecarboxamide hydrochloride (GSK189254), increased histamine release from the tuberomammillary nucleus, where all histaminergic somata are localized, and from where their axons project to the entire brain. To further understand functional histaminergic circuitry in the brain, dual-probe microdialysis was used to pharmacologically block H 3 receptors in the tuberomammillary nucleus, and monitor histamine release in projection areas. Perfusion of the tuberomammillary nucleus with GSK189254 increased histamine release from the tuberomammillary nucleus, nucleus basalis magnocellularis, and cortex, but not from the striatum or nucleus accumbens. Cortical acetylcholine (ACh) release was also increased, but striatal dopamine release was not affected. When administered locally, GSK189254 increased histamine release from the nucleus basalis magnocellularis, but not from the striatum. Thus, defined by their sensitivity to GSK189254, histaminergic neurons establish distinct pathways according to their terminal projections, and can differentially modulate neurotransmitter release in a brain region-specific manner. Consistent with its effects on cortical ACh release, systemic administration of GSK189254 antagonized the amnesic effects of scopolamine in the rat object recognition test, a cognition paradigm with important cortical components. The discovery of the histamine H 3 receptor (H 3 R) back in 1983 was a major scientific breakthrough that provided key new perspectives in histamine research (Arrang et al., 1983). Originally detected on varicosities of histaminergic axons as an autoreceptor that restricts histamine synthesis and release (Arrang et al., 1987), the H 3 R is also located on histaminergic somata where it provides a tonic inhibition. Indeed, application of the H 3 R antagonist thioperamide enhanced the firing of histaminergic neurons (Haas and Panula, 2003). These neurons arise solely from the tuberomammillary nucleus (TMN) of the posterior hypothalamus, from where they send diffuse projections throughout the entire central nervous system (Inagaki et al., 1988;Panula et al., 1989). Consistent with this wideranging output, histamine is directly and indirectly involved in a variety of basic homeostatic and higher brain functions, such as the control of the sleep-wake cycle, appetite, nociception, cognition, and emotion (Haas and Panula, 2003;Passani et al., 2004), and H 3 R antagonists have been shown to increase wakefulness, improve cognitive performance, and reduce body weight in animal mod-