In vivo characterisation of novel efficacious muscarinic receptor agonists

An assay of drug-induced emesis in the squirrel monkey (Saimiri sciureus)

BACKGROUND

Emesis has significant evolutionary value as a defense mechanism against ingested toxins; however, it is also one of the most common adverse symptoms associated with both disease and medical treatments of disease. The development of improved antiemetic pharmacotherapies has been impeded by a shortage of animal models.

METHODS

The present studies characterized the responses of the squirrel monkey to pharmacologically diverse emetic drugs. Subjects were administered nicotine (0.032-0.56 mg/kg), lithium chloride (150-250 mg/kg), arecoline (0.01-0.32 mg/kg), or apomorphine (0.032-0.32 mg/kg) and observed for emesis and prodromal hypersalivation.

RESULTS

Nicotine rapidly produced emesis and hypersalivation. Lithium chloride produced emesis with a longer time course without dose-dependent hypersalivation. Arecoline produced hypersalivation but not emesis. Apomorphine failed to produce emesis or hypersalivation.

CONCLUSIONS

The squirrel monkey is sensitive to drug-induced emesis by a variety of pharmacological mechanisms and is well-positioned to examine antiemetic efficacy and clinically important side effects of candidate antiemetic pharmacotherapies.

Arecoline, a major alkaloid of the areca nut, causes neurotoxicity through enhancement of oxidative stress and suppression of the antioxidant protective system

Arecoline, an areca nut alkaloid, has been noted for its potential cognition-enhancing effects in patients with Alzheimer dementia. However, it has been confirmed that areca nut use is associated with oral and pharyngeal cancers. In addition, arecoline is genotoxic and cytotoxic both in vitro and in vivo through oxidative stress-dependent mechanisms. The aim of this study was to investigate whether arecoline would interfere with the antioxidant defense system and induce cytotoxicity in rat primary cortical neurons. Results indicate that arecoline (50-200 μM) induces neuronal cell death, and catalase, NADPH oxidase inhibitors (diphenyleneiodonium chloride and apocynin), and a caspase inhibitor (z-VAD-fmk) can prevent arecoline-induced cell death. Furthermore, arecoline increased reactive oxygen species production and upregulated protein expression and mRNA levels of NADPH oxidase 2, which could be attenuated by catalase and NADPH oxidase inhibitors. Arecoline also attenuated neuronal antioxidant defense by decreasing glutathione (GSH) level and superoxide dismutase activity. In addition, arecoline enhanced the expression of proapoptotic proteins (cytochrome c, Bax, caspase-9, and caspase-3) and attenuated the expression of the antiapoptotic protein Bcl-2. Moreover, NADPH oxidase inhibitors could attenuate the arecoline-induced GSH depletion and reverse arecoline-induced changes in proapoptotic and antiapoptotic proteins. In conclusion, the results indicate that arecoline could induce neuronal apoptotic death by attenuating antioxidant defense and enhancing oxidative stress.

The muscarinic receptor agonist L-658,903 modulates the in vivo accumulation of inositol monophosphates in mouse brain

In the present study we examined the effects of lithium chloride and the muscarinic receptor agonists pilocarpine hydrochloride and L-658,903 (3-(3-methyl-1,2,4-oxadiazol-5-yl) quinuclidine hydrochloride) upon the accumulation of inositol monophosphates in mouse brain using a radiometric technique. Lithium was able to stimulate dose dependently the accumulation of inositol monophosphates with a minimal effective dose (MED) of 3 mEq/kg s.c. and maximal effect seen at 20 mEq/kg. This corresponded to an increase in the radioactivity in the inositol monophosphate fraction from 1.4 +/- 0.06% to 4.6 +/- 0.60%. The response was time-dependent, with a peak effect observed at 4 h post administration and returning to basal levels by 48 h. The muscarinic receptor agonist pilocarpine (MED 10 mg/kg i.p.) was able to enhance dose dependently the response to 10 mEq/kg lithium, with a maximum response seen at 30 mg/kg (9.3% of the total brain radioactivity present in the inositol monophosphate fraction). The efficacious oxadiazole muscarinic receptor agonist L-658,903 also enhanced the response to lithium, producing a maximal effect of 10.4% of the total brain radioactivity present in the inositol monophosphate fraction at 1 mg/kg i.p. This stimulation was blocked by 1 mg/kg scopolamine i.p. but not by 1 mg/kg N-methylscopolamine. These results demonstrate the linkage of muscarinic receptors to the accumulation of inositol monophosphates in vivo, and confirm that following peripheral administration L-658,903 is a potent efficacious at muscarinic receptors within the central nervous system.

SR 46559A: a novel and potent muscarinic compound with no cholinergic syndrome

The cholinergic activities of SR 46559A, 3-[N-(2 diethyl-amino-2-methylpropyl)-6-phenyl-5-propyl] pyridazinamine sesquifumarate, have been investigated in vitro and in vivo, in rodents. Using rat brain cortical membranes, SR 46559A was a competitive ligand (Ki = 112 nM) at muscarinic M1 receptors, its affinity for muscarinic M2 (cardiac) and M3 (glandular) receptors being 6-7 times lower. SR 46559A did not interact with brain nicotinic receptors and high affinity choline uptake sites nor did it inhibit brain acetylcholinesterase activity. In contrast to reference muscarinic agonists, SR 46559A (1 mM) did not inhibit the forskolin-induced activation of cAMP synthesis nor did it stimulate phosphoinositides breakdown in various brain preparations. However, this compound enhanced (+67% at 1 mM) diacylglycerol formation in rat striatal miniprisms, an effect fully reversed by atropine. As shown with reference agonists, SR 46559A inhibited (IC50 = 10 microM) the K(+)-evoked release of [3H]GABA from rat striatal slices and reduced at 0.5 and 1 microM, the population spike amplitude of the CA1 pyramidal cells induced by stimulation of the Schaffer's collateral commissural pathway in rat hippocampal slices. In mice, SR 46559A at a near lethal dose (200 mg/kg PO) did not induce the typical cholinergic syndrome nor did it modify at 30 mg/kg PO the oxotremorine-induced hypothermia. Like muscarinic agonists, SR 46559A (1 mg/kg PO) potentiated haloperidol-induced catalepsy in rats and inhibited (ED50 = 0.12 mg/kg PO) rotations induced in mice by intrastriatal injection of pirenzepine.(ABSTRACT TRUNCATED AT 250 WORDS)

The design of novel muscarinic partial agonists that have functional selectivity in pharmacological preparations in vitro and reduced side-effect profile in vivo

Antagonist/agonist binding ratios (NMS/Oxo-M ratio) were used as an index of the efficacy of novel compounds acting at muscarinic receptors. These binding ratios have been used with a range of functional pharmacological assays to investigate the effects of varying the efficacy of muscarinic agonists. This strategy has been used as a means of obtaining functional receptor selectivity by exploiting differences in effective receptor reserves. The oxadiazole and pyrazine muscarinic agonists L-670,548 (NMS/Oxo-M ratio 1100) and L-680,648 (NMS/Oxo-M ratio 690) are amongst some of the most potent and efficacious agonists known. Decreasing the efficacy of compounds from these series, resulted in compounds with functional selectivity. The chloropyrazine L-689,660 (NMS/Oxo-M ratio 28) was an agonist on the rat superior cervical ganglion (M1), a partial agonist on the guinea-pig ileum (M3), but was an antagonist in the guinea-pig atria (M2). Synthesis of compounds with even lower predicted efficacy, such as the cyclopropyloxadiazole L-687,306 (NMS/Oxo-M ratio 15), maintained agonist activity in the ganglion, but showed antagonist activity in the M3 ileal, as well as the M2 atrial preparations. When tested in vivo these compounds did not produce many of the side effects associated with more efficacious agonists, particularly those associated with the cardiovascular system. However, they were active in reversing scopolamine-induced deficits in a variety of behavioural paradigms. This approach shows how functional selectivity for muscarinic receptor subtypes can be achieved in vitro, that in vivo reduces the dose-limiting side effects normally associated with muscarinic agonists.