Lecture
#16: Chemical Messengers in Animals
I. INTRODUCTORY
REMARKS (hormone = Greek for excite)
1. Two major
systems of internal communication are nervous and endocrine systems:
a. the nervous
system conveys high speed signals via neuronal axons (synapses).
b. the endocrine
system conveys chemical substances called hormones slowly via fluids.
c. the two
systems oftentimes work in a coordinated fashion.
2. The classical
definition of a hormone:
"A
specific molecule synthesized and secreted by a group of specialized cells
usually referred to as an endocrine gland; it is released by these cells into
the circulatory system; and it travels to another area of the body, where it
elicits biological responses from selected target cells."
3. There are
three general classes of hormones:
a. steroid
hormones (cholesterol derivatives) are fat soluble with nuclear receptors.
b. peptide
hormones (TRH, insulin, etc.) are water soluble and membrane receptors.
c. amino acid
derivatives (usually tyrosine) like epinephrine react with membrane.
4. Overview of
cell signaling (Fig.
11.5)
5. Target
cells have receptors either on their surface or inside in the nucleus. (Fig. 45.3)
a. surface
receptors are usually “serpentine”—spanning membrane 7 times. (Fig. 11.6)
(1) membrane
(surface) receptors are usually G-protein-linked receptors (Fig. 11.7)
(2) cyclic-AMP
as a second messenger (Fig. 11.13)
b. cytoplasmic
receptors respond to steroid hormones (Fig. 11.10)
(1) steroid is
lipid soluble and moves freely in and out of cells.
(2) in the
cytoplasm or the nucleus, steroid reacts with a protein receptor.
(3)
steroid-receptor complex binds with acceptor on DNA.
(4) this promotes
transcription of new mRNA.
(5) mRNA sets up
shop on ribosomes and translates message into new protein.
(No amount of testosterone will produce masculine
features if no testosterone receptors.)
5. Modern
concepts of hormone substances: (beyond classical definition of hormones)
a. Methods of
signaling: paracrine, synaptic, classical. (Fig. 11.3)
b. pheromones
communicate between animals for mate attractants, territorial markers, or alarm
substances (skunks).
c. growth
factors (paracrine hormones) like NGF, EGF, stimulate proliferation.
d. nitric
oxide is highly reactive (neurotransmitter) and potentially toxic (used by
WBCs).
d. prostaglandins
(arachidonate cascade) like PGE & PGF have a variety of effects on blood
vessels, smooth muscles, reproductive organs, etc.
6. One chemical
signal (i.e., one hormone) can act on different target cells (Fig. 45.4)
II. HORMONAL
REGULATION OF INSECT DEVELOPMENT (Fig. 45.2)
1. Example of how
hormones operate with one another and with nervous system.
2. Brain
hormone from corpora cardiaca stimulates ecdysone from prothoracic
gland.
3. Ecdysone
triggers larval molts (loss of exoskeleton).
4. Juvenile
hormone from corpus allatum promotes retention of larval traits
(i.e., it prevents pupa formation).