Protean agonism at histamine H ₃ receptors in vitro and in vivo

Abstract

G protein-coupled receptors (GPCRs) are allosteric proteins that adopt inactive (R) and active (R*) conformations in equilibrium. R* is promoted by agonists or occurs spontaneously, leading to constitutive activity of the receptor. Conversely, inverse agonists promote R and decrease constitutive activity. The existence of another pharmacological entity, referred to as ''protean'' agonists (after Proteus, the Greek god who could change shape), was assumed on theoretical grounds. It was predicted from the existence of constitutive activity that a same ligand of this class could act either as an agonist or an inverse agonist at the same GPCR. Here, we show that proxyfan, a high-affinity histamine H 3-receptor ligand, acts as a protean agonist at recombinant H 3 receptors expressed in the same Chinese hamster ovary cells. In support of the physiological relevance of the process, we show that proxyfan also behaves as a protean agonist at native H 3 receptors known to display constitutive activity. On neurochemical and behavioral responses in rodents and cats, proxyfan displays a spectrum of activity ranging from full agonism to full inverse agonism. Thus, protean agonism demonstrates the existence of ligand-directed active states LR* different from, and competing with, constitutively active states R* of GPCRs, and defines a pharmacological entity with important therapeutic implications.

The H3 receptor protean agonist proxyfan enhances the expression of fear memory in the rat

“…This is in agreement with the hypothesis that competition between the active and inactive forms of the receptor may be responsible for the heterogeneity of pharmacological responses in different brain regions (Gbahou et al, 2003). But the pharmacology of H 3 receptors is very complex, and when evaluating behavioural responses, other factors such as polymorphism (both in humans and rodents) with a differential distribution of splice variants in the CNS, and potential coupling to different g-protein signaling pathways (see Passani and Blandina, 2004 for a review) should be kept in mind.…”

“…t-MeHA, in the cortex, hence behaving as an inverse agonist, which presumes a high efficacy of the constitutively active H 3 receptor (Gbahou et al, 2003). This is in agreement with the hypothesis that competition between the active and inactive forms of the receptor may be responsible for the heterogeneity of pharmacological responses in different brain regions (Gbahou et al, 2003).…”

“…Accordingly, rats that received systemic administrations of proxyfan showed a stronger response to the footshock-context association, as they spent more time freezing in the retention test. Full agonism of proxyfan was observed also in the cat, where it significantly increased deep slow-wave sleep, without affecting wakefulness, whereas in the mouse proxyfan behaved as a full inverse agonist (Gbahou et al, 2003), mimicking the arousal effect of the antagonist/inverse agonist ciproxifan (Parmentier et al, 2002), and as a partial agonist in the dipsogenia model (Fox et al, 2002b). Presumably, the agonist effect of proxyfan in our model implies that in the rat amygdala H 3 receptors are predominantly in a quiescent state and constitutive activity has a low efficacy.…”

“…Several of the classical H 3 antagonists, such as thioperamide and ciproxifan, behave as potent inverse agonists, as they block the intracellular pathways associated with active H 3 receptors in transfected cells Rouleau et al, 2002). The high-affinity H 3 -receptor ligand proxyfan Morisset et al, 2001), on the other hand, is a ''protean'' agonist, which displays the full spectrum of pharmacological activities from full agonism to full inverse agonism (Gbahou et al, 2003). Little is known of the behavioural effects of proxyfan.…”

“…In a dipsogenia test in mice proxyfan behaves as an antagonist/partial agonist (Fox et al, 2002b). Furthermore, proxyfan modulates the sleep-wake cycle in a different manner depending on the animal species (Gbahou et al, 2003), hence depending on the conformation of the receptors and their constitutive activity. The H 3 receptor is apparently one of the most highly constitutively active receptors so far detected, hence we used proxyfan to study its functional significance in a memory test that requires the activation of the amygdala, one of the brain regions with the highest density of H 3 receptor mRNA and binding sites in the rat (Pillot et al, 2002).…”