The multiple signalling systems and activation cascades described in this chapter may seem to be far removed from the problems normally encountered in psychiatry. However, it should be remembered that any drug which interacts with a receptor will of necessity either activate or inhibit at least one if not more of these second-messenger systems, and its pharmacological and therapeutic effects will thus be a direct consequence of the changes in the intracellular effector systems induced. For antidepressant drugs, the systems activated by serotonin and noradrenaline receptors may be particularly important, while for antipsychotics the intracellular effectors activated by binding to dopamine receptors are presumably involved in their actions. It is possible that drugs may attenuate some signal transduction cascades while activating others, depending on the cell type and precise combination of G proteins expressed in particular neurones. Changes in transcription of discrete sets of genes may follow, with the genes being turned on or off to yield a favourable clinical response. Furthermore, disease states such as depression or schizophrenia may themselves result from alterations in activity of distinct signal transduction cascades. Appreciation of these processes will therefore contribute in the long run to increased understanding of both the pathology of mental illness and psychopharmacology.
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