In this scene we shall discuss some of the basic principles of animal endocrinology.
Endocrinology is the study of hormones their chemistry, synthesis, and physiological actions.
What is a hormone? A hormone is a cell to cell signaling chemical found in multicellular
organisms that is synthesized and released by a secretory tissue and transported in the
circulatory system to distant target cells where it exerts a profound physiological response.
There are four types of signaling chemicals that fall into the following categories. Autocrine
signaling is by chemicals that affect their source cell or directly adjacent cells. Paracrine
signaling is the diffusion of a signaling chemical from one cell to nearby non-adjacent cells,
Endocrine signaling is by chemicals called hormones synthesized by specific glandular
cells that is released into the circulatory system to affect distant target ceiis,
Finally, neurosecretory signaling is a fourth category of cell signaling.
Neurons typically secrete signaling chemicals at synaptic junctions and muscle motor end plates
as part of their normal functions in the nervous system. Therefore, it is reasonable
that nervous structures include specific neurosecretory neurons that secrete hormones
which are usually proteins or peptides. How do hormones exert their effects on target tissues?
Target cells contain receptor proteins that bind the hormone and stimulate a target cell response.
There are basically two mechanisms of action depending on the hormones chemical properties.
Oil-soluble hormones such as steroid hormones dissolve in the lipids that comprise cell
membranes and enter into target cells where they bind with receptor proteins in the cytoplasm
and nucleus. The receptor proteins carry the hormones into the nucleus where they interact with
genes to activate gene expression. In summary: the hormone-activated genes transcribe their DNA code
into RNA the RNA then passes from the nucleus to the cytoplasm where the RNA transcript
of the DNA code is translated into the specific proteins required to produce the
hormone-regulated physiological response. In contrast, if the hormone is water-soluble such
as protein and peptide hormones, they act on the cell surface through a g-protein receptor.
G-protein receptors are proteins consisting of seven hydrophobic transmembrane spanning regions.
The receptor is associated with a g-protein that is comprised of alpha, beta and gamma subunits.
The peptide hormone assumes its active configuration, fits onto the receptor
binding site, and conveys its message. The message results in a conformational change in the receptor
that is transferred mechanically to the g-protein. Mechanical activation of the
g-protein results in the alpha subunit replacing guanosine diphosphate with guanosine triphosphate.
The activated alpha subunit separates from the beta and gamma subunits and
migrates within the plasma membrane to an inactive phospholipase-c enzyme.
Energy captured from the hydrolysis of the terminal phosphate of guanosine triphosphate is
transferred through the alpha subunit to activate the phospholipase-c. The process of activating
the phospholipase-c converts the guanosine triphosphate back to guanosine diphosphate,
and the alpha subunit rejoins the beta and gamma subunits to reform the inactive g-protein.
Activated phospholipase-c degrades phospholipids that make up the plasma membrane
to produce two second messengers: inositol-1,4,5-trisphosphate and diacylglycerol.
The inositol trisphosphate is released from the membrane and binds to a receptor on
the endoplasmic reticulum. This intracellular signal results in the release of calcium stored
within the endoplasmic reticulum. The released calcium plus the diacylglycerol from the membrane
activate an inactive protein kinase enzyme in the plasma membrane. The activated protein kinase
facilitates the transfer of a high energy phosphate group from adenosine triphosphate to
an inactive enzyme. Protein kinase phosphorylation activates the enzyme to initiate the physiological
events regulated by the water-soluble peptide hormone. Other water-soluble hormones use a
g-protein activation of adenylyl cyclase to convert ATP to cyclic adenosine monophosphate.
The cyclic AMP then acts as a second messenger to activate a similar cascade of hormone-related
physiological events. By producing multiple intracellular second messengers,
the extracellular primary message by the hormone is magnified within the target cell so that a
single hormone molecule can produce a larger cellular response within an organism. There is an
abundance of ligand-specific g-proteins, and they are a favorite target for pharmaceutical design.
Now that we have discussed the basic concepts of endocrinology, let us turn to insect hormones,
their tissue sources and the endocrine-regulated physiological processes of insects.