Endocrine-Disrupting Pesticides - Pesticides: Evaluation of Environmental Pollution - page 612

Several hormones are produced by insects. A few of them, namely the brain hormone,
juvenile hormone, and ecdysone produced by endocrine organs, are described in the fol-
lowing lines.
Brain hormone : The NSC of the brain synthesizes it, so it is called a neurohormone. It is
believed to be peptidergic (proteinaceous) in nature, but its chemical structure is still to be
determined. It is also known as activation hormone (AH) due to the fact that it possesses
two adenotropic (gland-activating) factors—the allatotropic hormone or allatotropin and
the prothoracicotropic hormone or prothoracicotropin—which activate the CA and PTG,
respectively, to produce their own hormones.
Juvenile hormone : The CA produces a hormone that is known as the juvenile hormone
(JH), which was initially known as neotinin, due to its ability to keep the larva in perma-
nent juvenile condition. Chemically, the JH is an isoprenoid.
Ecdysone : It is produced by the PTG. It is a steroid, and it brings about molting in insects,
so it is also known as the molting hormone (MH). The MH contains two hormones:
a-ecdysone and b-ecdysone. The a-ecdysone is a prohormone produced by the PTG that
is converted into b-ecdysone in the peripheral tissues of the body. The latter is also called
20-hydroxyecdysone, ecdysterone, and crustecdysone. In fact, it has been found to be
responsible for bringing about molting in insects and is the true MH.
For further information, see Nijhout (1994).
20.5 Chemical Communication Between Cells
The regulation of growth and metabolism of complex multicellular organisms depends
heavily upon chemical messages sent between cells. This includes secretion of hormones
into the circulatory system, chemical transfer of information through communicating cell
junctions, and passage of signals between neurons in the brain.
20.5.1 Chemical Regulators
Hormones belong to the group of chemical regulators. In addition to hormones, there are
two categories of regulators: neurocrines and cytocrines. Neurocrine regulators are chem-
icals secreted by the nervous system. Neurocrines are divided in two groups. The first
group, consisting of neurohormones, is secreted into the blood. The second group consists
of neurotransmitters and neuromodulators. These are secreted in the extracellular fluid of
the synapses between neurons and their target cells such as other neurons, muscle cells,
and gland cells. A neurotransmitter triggers or inhibits an action in the target cells of
the synapse. A neuromodulator increases or decreases the response of target cells to the
neurotransmitter.
Cytocrines are local chemical regulators. They are issued in the extracellular tissue fluid
and can diffuse a short distance to their target cells. Cytocrines are, in turn, divided into
autocrine and paracrine regulators. Paracrine regulators affect other cell types, and auto-
crine regulators affect the secretory cell type.
A chemical regulator goes through several steps: synthesis, release, transport (free or
bound to a protein), binding to a specific receptor, feedback, degradation, and excretion
(by binding to other substances).
A neurotransmitter is a molecule that is used for signal transmission between nerve cells
(neurons) in the nervous system. The synapse is the place where this signal is transmitted.