Comparison of donepezil-, tacrine-, rivastigmine- and metrifonate-induced central and peripheral cholinergically mediated responses in the rat

There are now several acetylcholinesterase inhibitors in clinical use for the treatment of Alzheimer's disease, however, no systematic comparative studies of their central and peripheral cholinergic mediated effects in rats appear to have been reported. The present study investigated the dose-response characteristics of donepezil, tacrine, rivastigmine and metrifonate in inducing tremor, lacrimation, salivation and hypothermia and the duration of action of these compounds in Lister hooded rats. Data obtained were compared with the clinical observations on these drugs. Three doses of each compound were given orally to establish a dose-response curve for each behaviour, Tremor and lacrimation were scored, salivation was measured by weighing swabs applied to the mouth area and hypothermia was measured with a rectal probe. ED50 values were calculated for tremor. Using a just sub-maximal tremorigenic dose, the duration of response was examined. All four compounds produced dose-dependent increases in tremor and hypothermia. Only tacrine also produced marked salivation and lacrimation. The order of potency (ED50 value in mu mol/kg) was rivastigmine (3.7), donepezil (18.0), tacrine (37.5), metrifonate (470). Tremor following tacrine (150 mu mol/kg) and donepezil (20 mu mol/kg) was prolonged (> 6 h) with a similar hypothermic response. The duration of these responses following metrifonate (777 mu mol/kg) and rivastigmine (12.5 mu mol/kg) did not exceed 3 h. Tacrine had poor selectivity for central (tremor) versus peripheral (salivation/lacrimation) effects compared to the other compounds. Donepezil also had a sustained duration of action. The data are consistent with clinical results and indicate that simple in-vivo models may assist in the selection of acetylcholinesterase inhibitors with a suitable response profile for use in the symptomatic treatment of Alzheimer's disease.