Prototype nicotinic agonist; naturally occuring isomer Nicotine is a plant alkaloid, found in the tobacco plant, and addictive central nervous system (CNS) stimulant that causes either ganglionic stimulation in low doses or ganglionic blockage in high doses. Nicotine acts as an agonist at the nicotinic cholinergic receptors in the autonomic ganglia, at neuromuscular junctions, and in the adrenal medulla and the brain. Nicotine's CNS-stimulating activities may be mediated through the release of several neurotransmitters, including acetylcholine, beta-endorphin, dopamine, norepinephrine, serotonin, and ACTH. As a result, peripheral vasoconstriction, tachycardia, and elevated blood pressure may be observed with nicotine intake. This agent may also stimulate the chemoreceptor trigger zone, thereby inducing nausea and vomiting.
Title: Structure-activity relationship of reversible cholinesterase inhibitors including paraquat Seto Y, Shinohara T Ref: Archives of Toxicology, 62:37, 1988 : PubMed
The inhibitory effect of paraquat on cholinesterase activity was investigated in comparison with four paraquat derivatives, six monoquaternary ammoniums and six anticholinergic drugs. Inhibitor concentrations to cause 50% inhibition (I50) and Hill coefficients for three enzymes, human erythrocyte acetylcholinesterase (AChE), Electrophorus electricus AChE and human plasma butyrylcholinesterase (BCHE) were measured. The results obtained were as follows. The I50 for erythrocyte AChE was similar to the I50 for eel AChE. Secondary to edrophonium, diethylparaquat, paraquat, morfamquat and monoquat showed lower I50 for AChE, and possessed higher inhibition selectivity (IS), expressed as the ratio of I50 for BCHE to I50 for erythrocyte AChE. However, diquat showed higher I50 for AChE and lower IS, similar to the other monoquaternary ammoniums. A negative correlation was observed between log [I50 for erythrocyte AChE] and log [IS], among paraquat and its derivatives, monoquaternary ammoniums and anticholinergic drugs, respectively. With respect to Hill coefficients, these inhibitors could be classified into four groups, [1] competitive inhibitors: diquat, edrophonium, choline, tetramethylammonium and trimethylphenylammonium, [2] inhibitors showing negative cooperativity: paraquat, diethylparaquat, morfamquat, d-tubocurarine, atropine, gallamine and nicotine, [3] moderate type inhibitors: monoquat, hexamethonium and decamethonium. [4] the other type inhibitors showing positive cooperativity for erythrocyte AChE: tetraethylammonium and ethyltrimethylammonium.
