BLOCKADE OF NICOTINIC RESPONSES BY PHYSOSTIGMINE, TACRINE AND OTHER CHOLINESTERASE-INHIBITORS IN RAT STRIATUM

1 The acetylcholinesterase inhibitors physostigmine, neostigmine, tetr ahydroaminoacridine (tacrine; THA) and diisopropylfluorophosphate (DFP ) were tested for possible direct nicotinic actions in rat striatal sy naptosomes preloaded with [H-3]-dopamine. In this preparation, nicotin ic cholinoceptor activation evoked [H-3]-dopamine release. 2 Antagonis t activity was examined by giving a brief nicotine (1 mu M) challenge after 30 min superfusion with an acetylcholinesterase (AChE) inhibitor (0.3-300 mu M). Physostigmine, neostigmine and tacrine produced a con centration-dependent blockade. Physostigmine and tacrine were particul arly potent (IC(50)s approx. 10 mu M and 1 mu M, respectively). DFP re duced nicotinic responses only at the highest concentration tested (30 0 mu M). 3 Nicotinic blockade produced by superfusion with physostigmi ne (30 mu M) was insurmountable when tested against nicotine (0.1-100 mu M). 4 Physostigmine (30 mu M) also reduced responses to the nicotin ic agonists 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP) and cytisi ne, but did not alter responses to high K+ or (+)-amphetamine. A highe r concentration of physostigmine (300 mu M) completely blocked respons es to nicotine, somewhat reduced responses to amphetamine, and did not alter responses to high K+. Tacrine (3 mu M) reduced responses to nic otine and to high K+ but did not affect responses to amphetamine. 5 Ph ysostigmine (0.3-300 mu M), given as a brief pulse, did not produce a nicotinic agonist-like effect. 6 Physostigmine, neostigmine, tacrine a nd DFP (all at 30 mu M) each produced near-total (>96%) inhibition of AChE activity. However, DFP at a concentration (60 mu M) that produced a degree of AChE inhibition equal to that of physostigmine 30 mu M, d id not significantly reduce nicotine-induced dopamine release. 7 It th us appears that physostigmine blocks CNS nicotinic receptors in an ins urmountable and pharmacologically selective manner, independent of its ability to inhibit acetylcholinesterase. Tacrine reduced nicotinic re sponses, quite possibly by an indirect mechanism. The possibility of d irect or indirect blockade of nicotinic receptor-mediated actions may complicate the interpretation of preclinical studies that have employe d physostigmine and tacrine.