RESUMEN
A systematic theoretical study on histamine agonists and their interaction with H1 and H2 receptor models has been carried out utilizing ab initio molecular orbital technique. The effect of substituents on histamine agonists' charge distribution and their agonistic activity has been studied in detail. Drug-receptor interaction models have been studied at the Hartree Fock level of theory with a split valence basis set keeping the cost and efficiency of the calculation in mind. The study indicates that the agonistic activity is controlled either by receptor conformation or by steric hinderances caused by the substituents. The monocationic form of histamine does not appear to be a necessity for a proton relay process which is similar to the one proposed earlier by Weinstein and coworkers. The study also indicates some importance of common cellular ions in neurotransmitter properties of histamine.