PT  - JOURNAL ARTICLE
AU  - R Leurs
AU  - M Kathmann
AU  - R C Vollinga
AU  - W M Menge
AU  - E Schlicker
AU  - H Timmerman
TI  - Histamine homologues discriminating between two functional H3 receptor assays. Evidence for H3 receptor heterogeneity?.
DP  - 1996 Mar 01
TA  - Journal of Pharmacology and Experimental Therapeutics
PG  - 1009--1015
VI  - 276
IP  - 3
4099  - http://jpet.aspetjournals.org/content/276/3/1009.short
4100  - http://jpet.aspetjournals.org/content/276/3/1009.full
SO  - J Pharmacol Exp Ther1996 Mar 01; 276
AB  - We studied several histamine homologues as potential ligands for the histamine H3 receptor in two binding assays ([125l]iodophenpropit and N alpha-[3H]methylhistamine binding to rat brain cortex membranes) and two functional H3 receptor models (inhibition of the neurogenic contraction in the guinea pig jejunum and of [3H]noradrenaline release in mouse brain cortex slices). The histamine homologues acted all as competitive H3 antagonists at the guinea pig jejunum. The potency in this model and/or the affinity for N alpha-[3H]methylhistamine binding was higher for the butylene (pA2 = 7.7; pKi = 9.4) and pentylene homologue (impentamine, pA2 = 8.4; pKi = 9.1) than for the propylene, hexylene and octylene homologues (pA2 = 5.9-7.8; pKi = 6.1-7.6). In the mouse brain cortex the propylene, butylene and pentylene homologues acted as partial agonists (alpha = 0.3-0.6) and the hexylene and octylene homologues acted as antagonists. [125I]Iodophenpropit binding was displaced monophasically by the propylene, hexylene and octylene homologues and biphasically by the butylene and pentylene homologues. Biphasic displacement curves were converted to monophasic ones by 10 microM guanosine-5'-O-(3-thiotriphosphate. In conclusion, the homologue of histamine with five methylene groups is a more potent H3 receptor antagonist in the guinea pig jejunum than the other homologues tested. Furthermore, the propylene, butylene and pentylene homologues can discriminate between the two functional H3 receptor models in the guinea pig jejunum and mouse brain. These data are discussed in relation to the efficiency of receptor coupling and receptor heterogeneity.