Role of hydrophobic substituents on the terminal nitrogen of histamine in receptor binding and agonist activity: development of an orally active histamine type 3 receptor agonist and evaluation of its antistress activity in mice.

The terminal nitrogen atom of histamine was modified with lipophilic substituents to investigate the structure-activity relationship of histamine type 3 receptor (H3R) agonists. The introduction of an alkylated benzene rings maintained or increased the H3R binding affinity. The most potent compound, 4-(2-(4-tert-butylphenylthio)ethyl)-1H-imidazole, possessed in vivo agonistic activity, decreasing brain N(tau)-methylhistamine levels in mice after oral administration. It also exhibited antistress activity in the mouse resident-intruder test.

Anxiolytic-like profiles of histamine H3 receptor agonists in animal models of anxiety: a comparative study with antidepressants and benzodiazepine anxiolytic.

RATIONALE: Histamine H3 receptor functions as a presynaptic auto- and hetero-receptor on histaminergic and non-histaminergic neurons in the brain regulating the synaptic release of numerous neurotransmitters. Therefore, the ligands for this receptor have been proposed to be of therapeutic interest for the treatment of various neuropsychiatric disorders. At present, however, the psychopharmacological profiles of H3 ligands, particularly H3 agonists, have not been extensively studied. OBJECTIVE: The present study investigated the anxiolytic-like profiles of H3-selective agonists in a variety of classical (benzodiazepine-sensitive) and atypical (antidepressant-effective) animal models of anxiety. Comparator drugs used were diazepam and both fluvoxamine and desipramine in the former and latter models, respectively. RESULTS: H3 agonist R-alpha-methylhistamine and immepip were inactive in rat elevated plus maze test and Vogel type conflict test where diazepam (5 mg/kg) produced significant anxiolytic-like effects. Meanwhile, these H3 agonists (10-30 mg/kg) significantly reduced isolation-induced vocalizations in guinea pig pups and isolation-induced aggressive behavior in mouse resident-intruder test. Moreover, in rat conditioned fear stress test, R-alpha-methylhistamine (30 mg/kg) and immepip (10 mg/kg) significantly decreased freezing time, which were completely reversed by concomitant treatment with H3 antagonist, thioperamide (10 mg/kg). In contrast to the limited efficacy obtained with desipramine (30 mg/kg), fluvoxamine (20-60 mg/kg) exhibited anxiolytic-like effects in all the latter three atypical models. CONCLUSIONS: These data suggest that the H3 agonists may have anxiolytic-like effects similar to those of selective serotonin reuptake inhibitors but not benzodiazepine anxiolytics and represent a novel strategy for the treatment of some anxiety disorders in which selective serotonin reuptake inhibitors are prescribed.