The 5-Hydroxytryptamine₄ Receptor Agonists Prucalopride and PRX-03140 Increase Acetylcholine and Histamine Levels in the Rat Prefrontal Cortex and the Power of Stimulated Hippocampal θ Oscillations

Abstract

5-Hydroxytryptamine (5-HT) 4 receptor agonists reportedly stimulate brain acetylcholine (ACh) release, a property that might provide a new pharmacological approach for treating cognitive deficits associated with Alzheimer's disease. The purpose of this study was to compare the binding affinities, functional activities, and effects on neuropharmacological responses associated with cognition of two highly selective 5-HT 4 receptor agonists, prucalopride and 6,7-dihydro-4-hydroxy-7-isopropyl-6-oxo-N-[3-(piperidin-1-yl)propyl]thieno[2,3-b]pyridine-5-carboxamide (PRX-03140). In vitro, prucalopride and PRX-03140 bound to native rat brain 5-HT 4 receptors with K i values of 30 nM and 110 nM, respectively, and increased cAMP production in human embryonic kidney-293 cells expressing recombinant rat 5-HT 4 receptors. In vivo receptor occupancy studies established that prucalopride and PRX-03140 were able to penetrate the brain and bound to 5-HT 4 receptors in rat brain, achieving 50% receptor occupancy at free brain exposures of 330 nM and 130 nM, respectively. Rat microdialysis studies revealed that prucalopride maximally increased ACh and histamine levels in the prefrontal cortex at 5 and 10 mg/kg, whereas PRX-03140 significantly increased cortical histamine levels at 50 mg/kg, failing to affect ACh release at doses lower than 150 mg/kg. In combination studies, donepezil-induced increases in cortical ACh levels were potentiated by prucalopride and PRX-03140. Electrophysiological studies in rats demonstrated that both compounds increased the power of brainstem-stimulated hippocampal oscillations at 5.6 mg/kg. These findings show for the first time that the 5-HT 4 receptor agonists prucalopride and PRX-03140 can increase cortical ACh and histamine levels, augment donepezil-induced ACh increases, and increase stimulated-hippocampal power, all neuropharmacological parameters consistent with potential positive effects on cognitive processes.

Identification of Multiple 5-HT4 Partial Agonist Clinical Candidates for the Treatment of Alzheimer’s Disease

“…Intrinsic activity levels for prucalopride reportedly range from 52% to 77% in cell lines expressing the 4l, 4s, and 4e isoforms of the rat 5-HT 4 receptor. 12 Under the test conditions used in this study, prucalopride (10 μM) produced a relaxation that was equivalent to the full agonist 5-HT ( Figure 4A). …”

“…We have previously observed differences between in vitro and in vivo derived K i values for the high intrinsic activity of the 5-HT 4 full agonist prucalopride (11-fold). 12 Since the degree of separation observed between in vitro and in vivo derived K i values appears to generally correlate with the 5-HT 4 agonism, this difference might be due to the degree of interaction occurring with high-and low-affinity binding states, which have been described for 5-HT 4 receptors. 12,21 The free brain % E max values determined using the model showed the same rank order of effect that was observed for E max values determined in vitro for 5-HT 4d receptors (2d, 24%; 3, 35%; 4, 86%).…”

“…From the perspective of treating central nervous systems (CNS) indications, a major limitation of these early generation 5-HT 4 agonists was poor brain penetration likely due to Pglycoprotein (Pgp) mediated efflux. 12 An additional concern is that prolonged exposure of GPCRs to an agonist can result in receptor desensitization, thereby limiting the agent's therapeutic usefulness. Agonist-induced desensitization is often dependent on the degree of intrinsic activity; while weak partial agonists minimize the potential for desensitization, 12 they can in fact act as functional antagonists when low receptor numbers are present.…”

Cholinergic interactions between donepezil and prucalopride in human colon: potential to treat severe intestinal dysmotility

“…Nevertheless, it remains a possibility that in other systems where 5-HT 4 receptors are expressed by cholinergic neurones (e.g. the urinary bladder and the brain; Johnson et al, 2012;Kullmann et al, 2013), similar interactions will occur and adverse events could be noted. Further preclinical and clinical studies are therefore now required to test the efficacy and safety of this combination of drugs.…”

Modulation of hippocampal neuronal network oscillations by α7 nACh receptors

“…1). In addition, hippocampal theta and gamma oscillations are influenced by a number of neurochemically diverse neuronal pathways, including noradrenergic, serotonergic, and histaminergic neurons [11][12][13][14][15][16][17][18]. Therefore, hippocampal network activity is impacted by heterogeneous afferents originating from diverse brain regions.…”