More complex forms of signal transduction of membrane-bound receptors involve a coupling of ligand and receptor followed by the activation of different intracellular signal transduction pathways.
After binding of the ligand, the signal is transferred into the cell and leads to the activation of "second messengers", like cyclic nucleotides (cAMP, cGMP), calcium (Ca2+), inositol-trisphosphate (IP3), diacylglycerol (DAG), as well as to the phos-phorylation of proteins. Phosphorylation involves in many cases the activity of cAMP-dependent protein kinase (PKA) and DAG-activated protein kinases.
The downstream response modifies intracellular processes, for example the release of neuroactive substances, an altered activity of ion channels or changes in enzymatic activity, particularly kinase cascades. The modifications are induced slowly and the resulting effects can be long-lasting.
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