What is reaction between alkaloid?

Well alkaloids, as you know, are nitrogen containing compounds commonly seen in pharmacology as narcotics. Now your question is horribly incomplete because you could mean what are the reactions between two alkaloids or between an alkaloid and something else; morphane family compounds use their substitution about the nitrogen (and the closeby Quaternary carbon) to position electron pairs in the right orientations of the neuro-receptor to induce the narcotic effect (in a nutshell) - on another note, the less substituted Nitrogens on some other alkaloids can be protected with a number of methods in synthesis or even used as drugs for receptors different from those of the morphane family. Don't ask questions that are... well... not good questions - elaboration is your best friend.