Physiology to functionality: the brain and neurotransmitter activity.

The monoamine hypothesis of depression proposes that the biological basis for depression is a deficiency in the neurotransmitters serotonin or noradrenaline, or both. Over the past 30 years this hypothesis has been refined as more experimental and clinical evidence has emerged. The selective serotonin reuptake inhibitors in particular have made a significant contribution to our understanding of the role of serotonin in depression. Our knowledge of the noradrenergic system is less complete, mainly because, until recently, no equivalent agents were available for this system. With the advent of reboxetine, the first selective noradrenaline reuptake inhibitor, attention is again focusing on the role of noradrenaline in depression. To an extent, the action of the selective inhibitors can be predicted through knowledge of the neuroanatomy of the central and peripheral nervous systems. With regard to depression, the most important pathways are those of the serotonergic and noradrenergic neurones projecting to the prefrontal cortex, from the raphe nucleus and locus coeruleus, respectively. However, increasing the levels of the monoamines in the central nervous system affects many other pathways and a wide range of serotonin and adrenergic receptors, leading to a multiplicity of potential beneficial and adverse events. In addition, the complex intracellular responses are beginning to be examined, again with the aid of the selective antidepressants.