Journal of Neuroscience, Vol 14, 576-589, Copyright © 1994 by Society for Neuroscience
Colocalization of somatostatin with GABA or glutamate in distinct afferent terminals presynaptic to the Mauthner cell
C Sur, H Korn and A Triller
Laboratoire de Neurobiologie Cellulaire, INSERM U261, Departement des Biotechnologies, Institut Pasteur, Paris, France.
The presence of somatostatin in afferent fibers impinging on the goldfish
Mauthner (M-) cell was determined using immunohistochemical methods,
combined with confocal and electron microscopy, and the relationship of
this peptide with inhibitory and excitatory terminals was studied.
Somatostatin-reactive boutons were present only on the distal part of the
M-cell's lateral dendrite. Somatostatin immunoreactivity was observed in
typical large myelinated club endings (LMCEs) corresponding to mixed
(electrical and chemical) eighth nerve primary afferent fibers. The
axoplasm of these fibers contained dense- core vesicles (DCVs) dispersed
among round vesicles. We have made a novel finding that the excitatory
transmitter glutamate is present in LMCEs. Colocalization of this amino
acid with somatostatin was detected in 75% of these endings using
postembedding staining with gold particles of various sizes. The other
structures labeled by somatostatin antibody were found to be small vesicle
boutons (SVBs), which establish symmetrical synapses and contain a
population of pleiomorphic vesicles with DCVs scattered among them. Double
labeling with antibodies against glutamic acid decarboxylase and GABA
allowed the definition of three types of biochemically characterized
terminals: [somatostatin-GABA], [GABA], and [somatostatin]. However, the
occurrence of DCVs in SVBs stained for GABA alone suggests that
neuropeptides other than somatostatin may also coexist with GABA in this
class of boutons. The coexistence of somatostatin with both inhibitory and
excitatory neurotransmitters acting on the same region of a postsynaptic
cell is discussed in relation to the role postulated for this peptide in
synaptic plasticity.