Histamine H 3 ‐receptor‐mediated [ 35 S]GTPγ[S] binding: evidence for constitutive activity of the recombinant and native rat and human H 3 receptors

Constitutive activity of the recombinant and native rat and human H 3 receptors (H 3 Rs) was studied using H 3 R‐mediated [ 35 S]GTPγ[S] binding and [ 3 H]‐arachidonic acid release. Ciproxifan, an inverse agonist at the rat H 3 R (rH 3 R), decreased [ 3 H]arachidonic acid release from CHO cells expressing moderate densities (∼200–300 fmol mg −1 protein) of the human H 3 R (hH 3 R). This effect occurred with the same magnitude than at the rH 3 R. The expression of the hH 3 R was associated with an increase in [ 35 S]GTPγ[S] binding to membranes of CHO cells. Ciproxifan decreased [ 35 S]GTPγ[S] binding to membranes of CHO (hH 3 R) cells. Both effects were correlated to receptor density and revealed that constitutive activity of the hH 3 R, although lower than that of the rH 3 R in this assay, was again observed at physiological densities (<500 fmol mg −1 protein). Ciproxifan was less potent at the human than the rat receptor, not only as an antagonist ( K i =45 n M ), but also as an inverse agonist (EC 50 =15 n M ). Constitutive activity of the hH 3 R was also evidenced using inhibition of [ 35 S]GTPγ[S] binding by unlabelled GTPγS. The expression of the hH 3 R generated a high affinity binding for GTPγS which was increased by imetit, but partially decreased by ciproxifan, therefore acting as a partial inverse agonist. [ 35 S]GTPγ[S] binding to rat brain membranes was decreased in several regions by thioperamide, ciproxifan and FUB 465, three inverse agonists at the H 3 R, whose effects were blocked by proxyfan, a neutral antagonist. [ 35 S]GTPγ[S] binding was also decreased by an A 1 ‐adenosine receptor inverse agonist, but remained unchanged in the presence of inverse agonists at D 2 /D 3 dopamine, H 1 and H 2 histamine, α 2 ‐adrenergic and δ opioid receptors. In conclusion, the present study shows that the recombinant rat and human H 3 receptors expressed at physiological densities display constitutive activity and suggests that constitutive activity of native H 3 Rs is one of the highest among G‐protein‐coupled receptors present in rat brain.British Journal of Pharmacology (2002) 135 , 383–392; doi: 10.1038/sj.bjp.0704490