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Comorbidity between major depressive disorder (MDD), anxiety (generalized anxiety, panic disorder, social anxiety disorder) and pain is a major complicating factor in the diagnosis and treatment of psychiatric and neurological disorders. Although numerous neurotransmitters and/or neuromodulators may be involved, abnormalities in the GABAergic and glutamatergic systems seem to be a common factor in all these disorders. Neuroactive steroids (NASs) have been the object of considerable interest in this area in recent years since they appear to act predominantly on GABA-A and glutamate NMDA receptors. An overview of the possible involvement of NASs in MDD, anxiety and pain is provided in this chapter.

Although it is well known that there is a high degree of comorbidity between chronic pain and mood and anxiety disorders, the mechanisms involved in these co-occurrences are not clear. It appears that numerous neurotransmitters and neuromodulators are involved, and this chapter focuses on the monoamine neurotransmitters noradrenaline, 5-hydroxytryptamine (5-HT, serotonin), and dopamine and the amino acid neurotransmitters GABA (γ-aminobutyric acid) and glutamate in chronic pain and depression. Numerous preclinical and clinical neurochemical, neuroanatomical, pharmacological and molecular biological studies as well as clinical pharmacological treatment investigations implicate noradrenaline, 5-HT and, to a lesser extent, dopamine in the etiology of pain and depression. Similarly, preclinical and clinical studies on GABAergic and glutamatergic mechanisms as well as reports on the actions of neuroactive steroids suggest that GABA and glutamate play an important role in the etiology of pain and depression and may contribute to comorbidity.