Journal of Neuroscience, Vol 1, 812-826, Copyright © 1981 by Society for Neuroscience
An immunohistochemical study of the neuropeptidergic bag cells of Aplysia
AY Chiu and F Strumwasser
The bag cell neurosecretory system of Aplysia californica synthesizes and
releases a neuropeptide, the egg-laying hormone (ELH), which can induce
egg-laying behavior. While ELH acts as a neurohormone on distant neuronal
and non-neuronal targets, it also can moderate the electrical activity of
neurons within the abdominal ganglion. Using antibodies generated against
pure ELH, our immunohistochemical study provides direct morphological
support for this dual function of ELH by defining the distribution of this
neuropeptide within the abdominal ganglion of A. californica. We also find
ELH cross-reactivity in the bag cell systems of three other species of
Aplysia. In sections of abdominal ganglia stained for ELH by the
peroxidase-antiperoxidase method, bag cell clusters were found to be
homogeneous populations of ELH- immunoreactive neurons, extending processes
in a complex array within the connective tissue sheath overlying the
ganglion and the proximal regions of neighboring nerves. This association
of neurites with the sheath is the neurohemal organ which facilitates
release of ELH into circulating hemolymph. Stained processes also invade
the interior of the ganglion in association with septal sheets of
connective tissue and as scattered fibers in the neuropil. Local modulation
by ELH probably occurs through release from these intraganglionic sites. A
few ELH- positive ectopic cells were seen outside of the confines of the
bag cell clusters as far displaced as the base of the branchial nerve; they
appear to be bag cells which have mis-migrated during development.
Specificity of the antiserum was confirmed by the total lack of staining of
all other neurons within the abdominal ganglion as well as the absence of
staining of bag cells with pre-immune serum. Although th ELHs of Aplysia
vaccaria, Aplysia braziliana, and Aplysia dactylomela have not been
characterized biochemically, it is known that abdominal ganglion extracts
from A. californica can induce egg laying in all three species. We find
that antiserum against A. californica ELH stains the bag cell systems of A.
vaccaria, A. braziliana, and A. dactylomela and not other parts of their
abdominal ganglia. It is likely that this staining represents
cross-immunoreactivity of the egg-laying neurohormones from these species
which also share cross-bioactivity.
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