COLOCALIZATION OF SOMATOSTATIN WITH GABA OR GLUTAMATE IN DISTINCT AFFERENT TERMINALS PRESYNAPTIC TO THE MAUTHNER CELL

The presence of somatostatin in efferent fibers impinging on the goldf ish Mauthner (M-) cell was determined using immunohistochemical method s, combined with confocal and electron microscopy, and the relationshi p of this peptide with inhibitory and excitatory terminals was studied . Somatostatin-reactive boutons were present only on the distal part o f the M-cell's lateral dendrite. Somatostatin immunoreactivity was obs erved in typical large myelinated club endings (LMCEs) corresponding t o mixed (electrical and chemical) eighth nerve primary afferent fibers . The axoplasm of these fibers contained dense-core vesicles (DCVs) di spersed among round vesicles. We have made a novel finding that the ex citatory 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. Th e other structures labeled by somatostatin antibody were found to be s mall vesicle boutons (SVBs), which establish symmetrical synapses and contain a population of pleiomorphic vesicles with DCVs scattered amon g them. Double labeling with antibodies against glutamic acid decarbox ylase and GABA allowed the definition of three types of biochemically characterized terminals: [somatostatin-GABA], [GABA], and [somatostati n]. However, the occurrence of DCVs in SVBs stained for GABA alone sug gests that neuropeptides other than somatostatin may also coexist with GABA in this class of boutons. The coexistence of somatostatin with b oth inhibitory and excitatory neurotransmitters acting on the same reg ion of a postsynaptic cell is discussed in relation to the role postul ated for this peptide in synaptic plasticity.