Findings are summarized about basic intracellular signalling pathways influencing neurotransmission and involved in neurodegenerative or neuropsychiatric disorders. Psychotropic drugs used in the therapy of a series of mental disorders, mood disorders especially, show neurotrophic or neuroprotective effects after long-term treatment. Thus, beyond adenylate cyclase, guanylate cyclase and calcium system, attention has been paid to the tyrosine kinase pathway and Wnt pathway. New neurochemical hypotheses of mood disorders are disclosed; they were formulated on the basis of known effects of antidepressants or mood stabilizers on intracellular signal transduction, i.e. on the function, plasticity and survival of neurons. These hypotheses focus on the constituents of intracellular signalling pathways that could be studied as biological markers of mood disorders: transcription factor CREB, neurotrophin BDNF and its trkB receptor, anti-apoptotic factor Bcl2, pro-apoptotic enzyme GSK3, caspases, calcium, and a number of mitochondrial functions related to brain energy metabolism.