The 5-HT receptor--G-protein--effector system complex in depression. I. Effect of glucocorticoids

Hormonal modulation of neurotransmission emerged as a concept from the recognition that adrenocortical steroids exert profound effects at the level of receptors, G-proteins and effector units. G-proteins, a family of guanine nucleotide binding regulatory components that couple neurotransmitter receptors to various types of intracellular effector systems, appear to be a key target of glucocorticoid (GC) action in the CNS. It is thought that Gs/Gi mediates stimulation/inhibition of adenylate cyclase (AC system), which forms cyclic AMP as second messenger, while receptors stimulating phospholipase C do so through Go to produce two second messengers, inositol 1,4,5-triphosphate and diacylglycerol (PI system). Recent evidence suggests that GC increase Gs alpha-and decrease Gi alpha-protein subunit expression without affecting Go alpha. Activation of central pre- and postsynaptic 5-HT1A receptors which are linked to the Gi-AC complex, induces hypothermia and ACTH/cortisol release in rodents and humans. Compared with controls, patients with a major depressive disorder exhibit increased basal cortisol secretion associated with decreased hypothermic and ACTH/cortisol responses. The attenuated neuroendocrine and thermoregulatory response to 5-HT1A receptor activation may reflect a GC-dependent feedback inhibition of the hypothalamic-pituitary-adrenal (HPA) system and subsensitivity of the presynaptic 5-HT1A-Gi-AC complex function. Differential regulation of 5-HT1A and 5-HT2 function leading to a relative 5-HT2-Go-PI complex supersensitivity may maintain HPA hyperactivity during the course of depression. These findings corroborate recent reports that GC, via GC-GC receptor (GR) complex activated promotion of gene transcription, modify the expression 5-HT1A-coupled Gi (but not 5-HT2-coupled Go) resulting in altered sensitivity of 5-HT1A-mediated signal transduction and further support the hypothesis of a differential regulation of 5-HT1A and 5-HT2 receptor function and a GC-GR/5-HT1A-G-protein--effector system-related abnormality in depression.