Parametric studies on phencyclidine enhancement of 3H quinuclidinyl benzilate accumulation in vivo

Acetylcholinesterase inhibition increases in vivo N-(2-[18F]fluoroethyl)-4-piperidyl benzilate binding to muscarinic acetylcholine receptors

Although the inhibition of acetylcholinesterase remains the primary treatment of Alzheimer's disease, little is known of the results of increased acetylcholine levels on muscarinic receptor occupancy or function. Using N-(2-[18F]fluoroethyl)-4-piperidyl benzilate ([18F]FEPB), a moderate affinity (Ki = 1.7 nmol/L) nonsubtype-selective muscarinic receptor antagonist, the authors examined the sensitivity of equilibrium in vivo radioligand binding in rat brain with changes in endogenous acetylcholine levels produced by treatments with acetylcholinesterase inhibitors. Phenserine administration 30 minutes before resulted in a dose-dependent into muscarinic cholinergic receptors, reaching a maximum increase of 90% in the striatum at a dose of 5 mg/kg intraperitoneally. Constant infusion of physostigmine at a dosage of 250 microg/kg/min produced an identical increase in radioligand binding. This agonist-induced increase of in vivo mAChR radioligand binding offers a new method for monitoring of the efficacy of acetylcholinesterase inhibitors or other drugs to enhance acetylcholine actions at the muscarinic receptors.

Effect of central injection of phencyclidine on QNB accumulation and motor activity

The purpose of the present research was to determine if central administration of phencyclidine (PCP) would increase the in vivo accumulation of the muscarinic ligand, [3H]quinuclidinyl benzilate (QNB) in mouse brain as has been reported after peripheral administration of PCP. Secondly, in order to provide a behavioral reference against which to compare any biochemical changes, the effects of peripheral vs. central PCP administration on motor activity were compared. When given into the lateral ventricle (i.c.v.) 20 min before QNB, PCP increased the accumulation of QNB in brain striatum and cortex, but not cerebellum. This finding replicates results of previous studies in which the drug was given i.p. The dose required to enhance QNB binding after central injection was, on a mg/kg body weight basis, approximately 10-fold less than that previously found necessary after i.p. injection. In contrast, PCP given centrally was markedly more potent in the motor activity tests than when given i.p. In addition, the behaviorally effective dose (0.4 microgram) was substantially less than that needed for the QNB change (25.6 micrograms). Therefore, though PCP was more active in both measures after central than peripheral administration, the changes in QNB accumulation cannot, at this time, be related to changes in motor activity.