amines, once released and through various receptors in
target territories will selectively in¯uence the properties
of many central neurons, and probably also peripheral
targets like exoskeletal muscles, the heart, and sensory
neurons (Battelle and Kravitz 1978; Florey and Rath-
mayer 1978; Dixon and Atwood 1989; Pasztor and Bush
1989). These actions will alter the ways in which the
target neurons function in circuits, generating new out-
puts from existing circuits or perhaps even molding new
circuits. These, in turn, will help shape the behavioral
patterns that have been initiated by sensory signals
announcing the arrival of an adversary.
Are the serotonergic neurosecretory neurons
important in ®ghting behavior?
One serious concern is that the A1- and T5-5HT ne-
urosecretory cells, which have been the focus of our
studies and those of other investigators, appear to be
mainly involved with the postural components of ®ght-
ing behavior. While posturing is an important part of
®ghting behavior in essentially all species of animals, its
control probably lies with decision-making neurons in
higher centers of the CNS. In lobsters, these neurons
most likely will be found in the supraesophageal
ganglion, the brain of decapod crustaceans. While our
studies also suggest that 5HT plays an important role in
decision making, since 5HT injections reverse the un-
willingness of losing animals to ®ght (Huber et al.
1997a), much less is known about how brain ser-
otonergic neurons function (see Sandeman and Sande-
man 1987, 1994; Sandeman et al. 1995). In addition,
there are short- and long-term consequences of winning
and losing ®ghts, and these too must be a part of any
discussion of the role of 5HT in aggression. Defeated
animals will not ®ght with winners for many days after
an initial encounter and show reduced levels of ®ghting
for up to a week, when animals are separated after their
®rst ®ghts (Rutishauser et al. 1999; for studies with
adults see Karavanich and Atema 1998a). If animals are
housed together after the ®rst ®ght, then no changes are
seen in the status of the paired animals, unless some
traumatic event occurs, like the winning animal under-
going a molt (shedding of its old cuticle).
Why do ®ghts end?
Fights appear to be decided when one animal ``gives
up''; it refuses to ®ght any longer. While this is decided
on rare occasions by damage to the losing animal, gen-
erally we see no precipitous event that causes one animal
to ``give up''. If fairly closely matched in size, animals
readily engage each other in combat and easily move to
higher levels of intensity throughout the ®ght. Suddenly,
at some point, the loser backs o and avoids further
encounters. We have no clear idea of why this happens.
Perhaps some balance of humoral factors within the
brain is involved in decision-making of this sort, like for
example, changing 5HT/OCT ratios in key brain areas.
Perhaps animals recognize chemical cues released in the
urine of their opponents during the ®ghts (Breithaupt
et al. 1999). Animals unable to release urine, or unable
to ``smell'' the released urine of opponents, ®ght for
longer periods of time than control animals (Karavanich
and Atema 1998b). In this regard, key metabolites of
amines, like the expensive-to-synthesize sulfate conju-
gates (Kennedy 1978; Huber et al. 1997b), are released
in the urine of animals. If lobsters can detect these me-
tabolites, and can distinguish between the 5HT and
OCT forms, then each animal would be capable of
evaluating the patterns of usage of the two amines in the
nervous system of the other. Whatever the reason, it is
unlikely that changes in gene expression account for
``giving-up''. The event happens too suddenly and after
too short a time for much in the way of changes in levels
of expression of genes to reach synaptic regions of
neurons. The decision once made is not reversed. It is
very rare to see a ``loser'' advance on a ``winner'' after a
®ght has been decided, unless 5HT is injected into the
``loser''. Then we see a transient ``willingness'' of the
loser to ®ght again, an event invariably reversed after
some tens of minutes. From preliminary studies, it ap-
pears that uptake of 5HT, and presumably subsequent
release, plays an important role in this reversal (see
above). If animals are housed together after a hierarchy
is established, however, there are longer-term conse-
quences of being winners and losers, which may involve
changes in gene expression (see below). We do not yet
know if 5HT injection will cause a behavioral reversal in
animals sharing a tank for extended periods of time.
Postulated roles of the A1- and T5-5HT
neurosecretory neurons during ®ghts
How might the 5HT-neurosecretory neurons function
during ®ghts? In dissected preparations, A1-5HT neu-
rons usually are spontaneously active, ®ring at 0.5±3 Hz.
If the cells ®re at similar rates in intact animals, 5HT
should be released tonically into the general circulation
and into neuropil regions in the central nervous system
via the two sets of endings of the cells. The dierent rates
of spontaneous ®ring of cells presumably were set at
some earlier time, possibly relating to earlier experiences
of the animals. Although small, these dierences may
contribute in important ways to the functioning of the
neurons. For example, neurons ®ring more rapidly
should release more 5HT and may also release higher
levels of the proctolin that co-localizes with 5HT in the
cells. Moreover the duration of the autoinhibition seen
after high frequency ®ring of the cells (see above) is
inversely related to the initial ®ring rate, suggesting that
small dierences in ®ring rates could play large roles in
how cells are used (Heinrich et al. 1999). We anticipate
that there will be a phasic component to the actions of
these neurons as well: that is that the rate of ®ring will
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