Reduced platelet serotonin content in rabbits with dietary hypercholesterolemia

Platelet dysfunction in hypercholesterolemia mice, two Alzheimer's disease mouse models and in human patients with Alzheimer's disease

Alzheimer’s disease (AD) is a severe neurodegenerative disorder characterized mainly by accumulation of amyloid-β plaques and neurofibrillary tangles, synaptic and neuronal loss. Blood platelets contain the neurotransmitter serotonin and amyloid-precursor protein (APP), and may thus be useful as a peripheral biomarker for AD. The aim of the present study was to functionally characterize platelets by FACS, to examine alterations in APP expression and secretion, and to measure serotonin levels in hypercholesterolemia mice with AD-like pathology and in two AD mouse models, the triple transgenic AD model (3xTg) and the APP overexpressing AD model with the Swedish–Dutch–Iowa mutations (APP_SweDI). These data are supplemented with epidermal growth factor (EGF) levels and compared with changes observed in platelets of patients with AD. We observed decreased platelet APP isoforms in 3xTg mice and patients with AD when analysed by means of Western blot. In patients, a significant increase of APP levels was observed when assessed by ELISA. Secreted APPβ proved to be altered amongst all three animal models of AD at different time points and in human patients with AD. Serotonin levels were only reduced in 7 and 14 month old 3xTg mice. Moreover, we found significantly lower EGF levels in human AD patients and could thereby reproduce previous findings. Taken together, our data confirm that platelets are dysfunctional in AD, however, results from AD animal models do not coincide in all aspects, and markedly differ when compared to AD patients. We support previous data that APP, as well as EGF, could become putative biomarkers for diagnosing AD in human platelets.

Hypotensive and antiaggregative effects of eugenosedin-B with serotonin and alpha/beta-adrenoceptor antagonistic activities in rats and human platelets

Eugenosedin-B is able to block serotonin (5-HT) and alpha/beta receptors and to inhibit platelet aggregation. In Wistar rats, intravenous injections of eugenosedin-B (2.4, 7.2, 12 micromoL/kg) caused a dose-dependent decrease in blood pressure and heart rate. In contrast, intracisternal injection of eugenosedin-B (0.3, 0.03 micromoL) and an alpha2-antagonist yohimbine (0.03 micromoL) increased blood pressure and heart rate. Eugenosedin-B and yohimbine prevented hypotension induced by intracisternal injection of an alpha2-agonist clonidine (38 pmol). In in vitro experiments, eugenosedin-B (10, 10, 10 M) competitively antagonized norepinephrine-, clonidine-, and 5-HT (10 to 10 M)-induced vasocontractions in isolated rat aorta. It also competitively antagonized the isoproterenol (10 to 10 M)-induced positive inotropic effects in isolated rat atrium. These findings clearly suggest that eugenosedin-B possesses alpha1, alpha2, beta1, and 5-HT2A receptor blocking activities. In isolated rabbit ear artery sensitized with 16 mM K, eugenosedin-B antagonized 5-nonyloxytryptamine- and 5-HT-induced vasocontractions, indicating it also blocked 5-HT1B and 5-HT2A receptors. In radioligand-binding experiments, eugenosedin-B had significant binding affinities on alpha1, alpha2, beta1, 5-HT1B, and 5-HT2A receptors. In human platelets, eugenosedin-B inhibited epinephrine and 5-HT-induced aggregations. It also had competitive binding effects in human platelet with [H]yohimbine (alpha2), [H]ketanserin (5-HT2A). We conclude that hypotensive and vasorelaxant effects of eugenosedin-B can be attributed to its multiple actions on the blockade of 5-HT1B, 5-HT2A, alpha1/2 and beta1 receptors, and its ability to reduce platelet aggregation attributed to its blockade of alpha2 and 5-HT2A receptors.

AT-1015, a novel serotonin2A receptor antagonist, improves resaturation of exercised ischemic muscle in hypercholesterolemic rabbits

OBJECTIVE

The effect of AT-1015, a serotonin(2A) receptor antagonist, on the resaturation of ischemic muscle in a hypercholesterolemic rabbit model was examined with near-infrared spectroscopy.

METHODS

New Zealand White male rabbits were fed normal chow or cholesterol-rich chow. Ischemia was induced in the right hindlimb by ligation of the femoral artery, accompanied by balloon injury of the iliac artery. At 3 days after induction of ischemia, the bilateral gastrocnemius muscles were subjected to passive contraction for 2 minutes. The oxygen resaturation time of the gastrocnemius muscle after exercise was measured by near-infrared spectroscopy. AT-1015 was orally administered for 3 days after induction of ischemia. Assay of serotonin level in platelet-poor plasma and histologic examination of muscle and artery were performed in another set of rabbits.

RESULTS

Oxygen resaturation time of the ischemic gastrocnemius was significantly prolonged in hypercholesterolemic rabbits compared with in normal rabbits without AT-1015, whereas there was no difference between both groups of rabbits that were administered AT-1015. Plasma level of serotonin in hypercholesterolemic rabbits was significantly increased compared with that in normal rabbits. No histologic differences were found in both muscle and artery among all groups.

CONCLUSIONS

A serotonin(2A) receptor antagonist improved the oxygen resaturation of ischemic calf muscle after exercise in hypercholesterolemia. The interaction between plasma free serotonin and the serotonin(2A) receptor may play an important role in muscle oxygenation in ischemic limbs.

Pharmacological profiles of R-96544, the active form of a novel 5-HT2A receptor antagonist R-102444

We examined the pharmacology of (2R,4R)-4-hydroxy-2-[2-[2-[2-(3-methoxy)phenyl]ethyl]phenoxy]ethyl-1-methylpyrrolidine hydrochloride (R-96544), the active form of a novel 5-HT(2A) receptor antagonist, (2R,4R)-4-lauroyloxy-2-[2-[2-[2-(3-methoxy)phenyl]ethyl]phenoxy]ethyl-1-methylpyrrolidine hydrochloride (R-102444). R-96544 produced a concentration-dependent inhibition of platelet aggregation induced by serotonin (5-hydroxytryptamine, 5-HT) alone or in combination with ADP in platelets from humans, monkeys, cats, rabbits, rats and mice. An intravenous administration of R-96544 to rabbits significantly inhibited ex vivo platelet aggregation induced by 5-hydroxytryptamine (5-HT) combined with epinephrine. An oral administration of R-102444 to rats also resulted in significant inhibition of ex vivo platelet aggregation, whereas R-102444 was ineffective in an in vitro platelet aggregation assay. These antiplatelet effects of R-96544 and R-102444 were more potent than those of two other 5-HT(2A) receptor antagonists, sarpogrelate and its active metabolite (+/-)-1-[2-[2-(3-methoxyphenyl)ethyl]phenoxy]-3-(dimethylamino)-2-propanol hydrochloride (M-1). A binding study using cat platelet membranes showed that R-96544 has high affinity for 5-HT(2A) receptors but no effect on non-serotonergic [3H]ketanserin-binding sites. R-96544 caused a parallel shift to the right of concentration-response curves for 5-HT in rat caudal artery contraction mediated by 5-HT(2A) receptors. Schild plot analysis gave a pA(2) value of 10.4 with a slope near unity (1.04). R-96544 also inhibited 5-HT(2A) receptor-mediated contraction of guinea pig trachea but not 5-HT(3) receptor-mediated contraction of guinea pig ileum and 5-HT(2B) receptor-mediated contraction of rat fundus preparation. R-96544 (i.v.) attenuated the pressor responses evoked by 5-HT (15 microg/kg, i.v.) but not by phenylephrine (5 microg/kg, i.v.) and angiotensin II (0.1 microg/kg, i.v.), after ganglionic blockade in anesthetized spontaneously hypertensive rats. These results show that R-96544, the active form of R-102444, is a novel 5-HT receptor antagonist with potent, competitive, and 5-HT(2A)-selective activity.