Biology Reference
In-Depth Information
The electrical signal is transmitted between cells at special sites called
syn-
apses
, which occur between two nerve cells as well as between nerve cells and
effectors such as muscle cells. The signal is relayed by a chemical neurotrans-
mitter that is packaged in membrane-bound vesicles. When an action poten-
tial reaches the pre-synaptic terminal, the depolarization activates Ca channels
in the presynaptic membrane and the subsequent influx of Ca ions leads to the
release of neurotransmitter. The neurotransmitter diffuses to the post-synaptic
cell and interacts with specific receptors on that cell surface. Receptors are
activated in response to binding of the specific neurotransmitter molecules.
Generally the size and duration of a synaptic potential reflects the amount of
transmitter released by the pre-synaptic terminal. By depolarizing the post-
synaptic cell above the threshold, the synaptic potential triggers the generation
of an action potential, which continues the signaling one step further along the
neural pathway (
Ganetzky and Wu 1989
).
11.5.6.2 Neurotransmitters
Acetylcholine (ACh) is the major neurotransmitter in the central nervous system
of
Drosophila
and other insects. Choline acetyltransferase (ChAT) is the biosyn-
thetic enzyme, and acetylcholinesterase (AChE) is the degradative enzyme. AChE
terminates synaptic transmission by rapidly hydrolyzing acetylcholine (
Fournier
et al. 1989, Ganetzky and Wu 1989
).
The acetylcholinesterase gene (
Ace
) from
Drosophila
is 34kb in size and is
split into 10 exons, with the splicing sites of the two last exons precisely con-
served among
Drosophila
and vertebrate cholinesterases (
Fournier et al. 1989
).
The deduced mature
Ace
transcript is 4.2kb long. A gene for an acetylcholine
receptor subunit has been identified, and the amino-acid sequence of this AChR
shares similarity with vertebrate sequences.
11.5.6.3 Ion Channels
Two types of ion channels, permeable to sodium (Na) or potassium (K) ions, are
responsible for membrane electrical phenomena. Multigene families encode the
Na and K channels. Mutated genes that affect Na channels include
n
o-
a
ction-
p
otential,
t
emperature-
s
ensitive, or
nap
ts
, and paralytic, or
para
(
Salkoff et al.
1987, Kernan et al. 1991
). The mutation
para
ts
is a temperature-sensitive muta-
tion that causes instantaneous paralysis of adults at 29°C and of larvae at 37°C
(
Loughney et al. 1989
). Mutations of several different genes (
Shaker
,
Shal
,
Shab
,
and
Shaw
) alter K currents (
Covarrubias et al. 1991
). One of the best-studied
mutations is
Shaker
and
D. melanogaster
carrying the
Shaker
allele exhibit
aberrant behavior, shaking their legs when anesthetized with ether (
Papazian
et al. 1987
).
