The histamine H3 receptor is a G protein-coupled receptor (GPCR) drug target 
that is highly expressed in the CNS, where it acts as both an auto- and 
hetero-receptor to regulate neurotransmission. As such, it has been considered 
as a relevant target in disorders as varied as Alzheimer's disease, 
schizophrenia, neuropathic pain and attention deficit hyperactivity disorder. A 
range of competitive antagonists/inverse agonists have progressed into clinical 
development, with pitolisant approved for the treatment of narcolepsy. Given the 
breadth of compounds developed and potential therapeutic indications, we 
assessed the comparative pharmacology of six investigational histamine H3 
agents, including pitolisant, using native tissue and recombinant cells. Whilst 
all of the compounds tested displayed robust histamine H3 receptor inverse 
agonism and did not differentiate between the main H3 receptor splice variants, 
they displayed a wide range of affinities and kinetic properties, and included 
rapidly dissociating (pitolisant, S 38093-2, ABT-239) and slowly dissociating 
(GSK189254, JNJ-5207852, PF-3654746) agents. S 38093-2 had the lowest histamine 
H3 receptor affinity (pKB values 5.7-6.2), seemingly at odds with previously 
reported, potent in vivo activity in models of cognition. We show here that at 
pro-cognitive and anti-hyperalgesic/anti-allodynic doses, S 38093-2 
preferentially occupies the mouse sigma-1 receptor in vivo, only engaging the 
histamine H3 receptor at doses associated with wakefulness promotion and 
neurotransmitter (histamine, ACh) release. Furthermore, pitolisant, ABT-239 and 
PF-3654746 also displayed appreciable sigma-1 receptor affinity, suggesting that 
this property differentiates clinically evaluated histamine H3 receptor 
antagonists and may play a role in their efficacy.