A FREEZE-FRACTURE AND THIN SECTION STUDY OF SYNAPSES ON THE DISTAL HALF OF THE LATERAL DENDRITE OF THE GOLDFISH MAUTHNER CELL

REBECCA TUTTLE, Purdue University

Abstract

Thin sectioning and double replica freeze-fracture techniques were employed to study synapses on the distal half of the Mauthner cell lateral dendrite. Five morphologically distinct types of synapses were found: the large myelinated club ending (LMCE) synapse, three small-vesicle bouton (SVB) synapses, types A, B, and C, and the small myelinated club ending synapse. The LMCEs of primary eighth nerve afferents form mixed synapses. Electrical synaptic junctions (gap junctions) were occupied 10-20% of the synaptic contact. Specializations characteristic of excitatory chemical synaptic junctions were most prominent at the periphery of the synaptic contact. These specializations consisted of postsynaptic E face particle aggregates which were subjacent to presynaptic active zones. Quantitative analysis of gap junction particle (connexon) number at five LMCEs revealed 24,000-106,000 connexons per LMCE. Comparison with data from electrophysiological studies of single LMCEs indicates that only a small fraction of the connexon channels are open at any given time during electrotonic transmission at an LMCE synapse. One or more of the three types of SVB synapses mediates a remote dendritic inhibition. The SVB A synapse is characterized by mostly oval and round synaptic vesicles, a discrete presynaptic active zone with a highly variable shape, and a postsynaptic active zone with no apparent particle aggregate in either the E or P face. At the SVB B synapse, most of the synaptic vesicles are quite flat. A very high particle density is present throughout the presynaptic P face, and vesicle attachment sites are dispersed over much of the presynaptic membrane. The postsynaptic active zone is characterized by P face particle aggregates which are often large and anastomotic. The SVB C synapse is characterized by pleiomorphic synaptic vesicle profiles, and a cytoplasm which is lacks the flocculent material that characterized SVBs A and B. The presynaptic active zones at the SVB C synapse are discrete, and macular or oblong. No particle aggregates are apparent in the postsynaptic active zone. Small, macular particle aggregates were found in the postsynaptic E face of all three types of SVBs. Small subsurface cisterns were also observed underlying the M cell membrane at all three types of SVB synapses. Both of these structures were often seen adjacent to the active zones. Short, straight rows of particles and short cylinders were often seen in both pre- and postsynaptic surrounding zone regions of SVB A and C synapses. These structures are thought to represent tight junctions.

Degree

Ph.D.

Subject Area

Neurology

COinS