09.01 Introduction

Every step, in the physiological system for a neurotransmitter, is a possible step at which drugs can modify the effects of the neurotransmitter. Thus, most neurotransmitters are synthesised, stored, released, and have receptor mediated effects involving cell signaling, before the neurotransmitter is inactivated by being broken down (metabolism) or be transport back into the nerve. Thus, synthesis, release, receptors, cell signaling, and inactivation are all sites at which drugs can modify the actions of neurotransmitters. In theory, drugs can increase or decrease any of the steps. In this chapter, one or two important examples of drugs acting at each of these steps is considered.