We used molecular modeling techniques to examine seven reported alkaloids known as competitive antagonists at the Glycinergic (strychnine, and brucine),  GABAergic ((+)-bicuculline, securinine and d-tubocurarine), Nicotinic-ACh (d-tubocurarine, dihydro-β-erythroidine and erysodine) and Serotoninergic-3 receptors (d-tubocurarine); also, we studied the corresponding neurotransmitters: glycine, GABA, acetylcholine and serotonin, and several representative agonists for each of these receptors. We located a neurotrasmitter-like fragment in each of the above mentioned alkaloids that, when compared to the corresponding neurotransmitter, provides an excellent topological and electronic charge congruence; similar congruence is found by using several characteristic agonists in each of these receptors. Our results are in quite agreement, under a stereoelectronic point of view, with the accepted model in which, within the pentameric configuration of these four complexes, the agonist binding sites are in equivalent regions in the four receptor/channel complexes.