MORPHOLOGICAL SUBCLASSES OF LATERAL OLIVOCOCHLEAR TERMINALS - ULTRASTRUCTURAL ANALYSIS OF INNER SPIRAL BUNDLE IN CAT AND GUINEA-PIG

The lateral olivocochlear efferent pathway terminates in vesicle-fille d swellings in the inner spiral bundles under inner hair cells (IHCs) and has been suggested to include at least two chemically distinct sub classes (see, e.g., Vetter et al. [1991] Synapse 7:21-43). In the pres ent study, the ultrastructure and peripheral targets of vesicle-filled swellings in the IHC area of the cat and guinea pig cochleas were qua ntitatively analyzed to determine 1) whether morphological subclasses could be defined based on swelling size or on the density, size, or sh ape of clear and dense-cored vesicles and 2) whether swellings with di fferent postsynaptic targets differed morphologically. In both cat and guinea pig, all swellings contained large, round, clear vesicles and a variable number of dense-core vesicles. Although evidence of clear-c ut subclasses was not compelling, the smallest swellings tended to be rich in dense-core and poor in clear vesicles and rarely formed synapt ic contacts. Most of the larger swellings, which tended to contain few dense-core vesicles and a rich complement of clear round vesicles, fo rmed synapses with radial afferent fibers. However, there were no morp hological differences between swellings contacting afferents originati ng on the modiolar vs. pillar sides of the IHC (the source of afferent s with low and high spontaneous discharge rates, respectively). We con clude that 1) if distinct gamma-amino butyric acid (GABA)ergic and cho linergic subclasses of lateral olivocochlear (LOG) fibers exist, then the vesicle morphology of their terminals does not differ as it does i n the central nervous system and that 2) if peptide neurotransmitters, such as calcitonin gene-related peptide and enkephalins, are packaged in dense-core vesicles, then the LOC terminals synapsing with IHC aff erent fibers are not particularly rich in these peptides. (C) 1996 Wil ey-Liss, Inc.