Synthesis and Storage of Neurotransmitters

Fig 1 - Diagram showing the general process of synaptic transmission. Once synthesised, neurotransmitters are stored in vesicles within the synaptic terminal until an action potential arrives, causing their release. Neurotransmitters such as acetylcholine are stored within the small synaptic vesicles, whereas neuropeptides reside within large dense-core vesicles. Importantly, proteases present in the granules cleave the precursors into their mature neuropeptide form during this journey. Synthesised precursors are then packaged into secretory granules and sent to the axonal terminal. These large neurotransmitters are produced within the cell body via transcription in the nucleus and translation in the endoplasmic reticulum. enkephalins) – These are an example of neuropeptides. Enzymes (such as choline acetyltransferase) convert precursors into the neurotransmitter. Its precursors (choline, acetate) are taken into the cell by membrane channels or created as byproducts of other processes. Acetylcholine – This is synthesised within the synaptic terminal of the axon.Some neurotransmitters (eg acetylcholine, ACh) are synthesised in the axon, while others (eg neuropeptides) are made in the cell body. This is the first step of synaptic transmission.

Even simple small molecules such as nitric oxide or reactive oxygen species (ROS) can act as growth factors.Ĭopyright © 2023, StatPearls Publishing LLC.Synthesis and Storage of Neurotransmitters Not all hormones are growth factors, only those affecting cell growth/differentiation. Estrogens, androgens, and progestogens are examples of steroid hormones that are growth factors. Glands produce all hormones and secrete them into the circulatory system. Some growth factors like lipid-soluble steroid hormones, do not have a surface receptor and can directly pass through the cell's plasma membrane, bind to an intracellular protein receptor, or nuclear receptors, and then transmit a growth signal. For example, some growth factors act only on hematopoietic cells, which are cells derived from bone marrow. Growth factor specificity to particular cell types is achievable by the expression of highly specific cell surface receptors. Examples of protein growth factors are vascular endothelial growth factor (VEGF), epidermal growth factor (EGF), and platelet-derived growth factor (PDGF). Granulocyte-macrophage colony-stimulating factor (GM-CSF) is an example of a cytokine growth factor since it promotes the production of white blood cells by stem cells. While all cytokines affect signal transduction pathways, only those cytokines affecting cell growth/differentiation signaling pathways are considered growth factors. Some growth factors are small peptides called cytokines. 'Downstream' here means an event triggered after TGF-beta binding to its receptor. When activated by the binding of TGF-beta cytokines, this receptor can phosphorylate downstream proteins on serine and threonine residues.

An exception is the surface receptors for the transforming growth factor-beta (TGF-beta) family of cytokines. Most cell surface receptors for growth factors show tyrosine kinase activity, meaning they put a phosphate group on a downstream protein tyrosine residue. The receptor-binding site for peptide/protein growth factors is on the outer cell membrane surface, in other words, the extracellular domain. A peptide is defined as having between two and 50 amino acid residues, whereas a protein has more than 50 amino acid residues. Many growth factors are peptides/proteins, and this type of growth factor typically binds with a high affinity to a specific surface receptor, which is a plasma membrane-bound protein. Kinases put a phosphate group on a protein, and phosphatases take off a phosphate group. Enzymes with kinase activity or phosphatase activity are important in this signaling process. Protein phosphorylation cascades play a key role in transmitting growth signals. The general process of transmitting an external molecular signal to a cell to evoke a cellular response is called signal transduction. Growth factors can act on specific cell surface receptors that subsequently transmit their growth signals to other intracellular components and eventually result in altered gene expression. This definition has become expanded to include secreted molecules that promote or inhibit mitosis or affect cellular differentiation.

A growth factor, as initially defined, is a secreted biologically active molecule that can affect the growth of cells.

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