TERMINAL MORPHOLOGY OF 2 BRANCHES ARISING FROM A SINGLE STEM-AXON OF PRETECTAL (PSM) NEURONS IN THE COMMON CARP

The induction of postsynaptic structures by presynaptic terminals is s uggested in a teleost brain. Neurons in the nucleus pretectalis superf icialis pars magnocellularis (PSm) in the common carp are known to sen d fibers to the corpus mamillare (CM) and the nucleus lateralis valvul ae (NLV). Individual axons of PSm neurons bifurcate (or give off an ax on collateral), both of which reach the target areas in the CM and NLV . The morphology of horseradish peroxidase-labeled terminals in the CM and NLV appears quite different in light microscopy. Terminals in the CM appear as a fine network of beaded (2-4 mu m in diameter) fibers, while those in the NLV are larger (8-12 mu m in transverse diameter) a nd cup-shaped, partially enveloping the soma of individual NLV neurons . In electron microscopy, however, these synapses in the CM and NLV sh are several ultrastructural similarities. Small(0.2 to 0.4-mu m thick, 0.4 to 0.7-mu m long) spine-like protrusions arising from dendrites i n the CM, and from cell bodies in the NLV, invaginate into the axon te rminals, and the synaptic junctions are always formed at the base of t he protrusion in both areas. Development of this unusual morphology is inferred to be directed from the presynaptic side. The morphological similarity of the spine-like protrusions to the ''spinule,'' which is thought to be formed in response to synaptic activation, is discussed. (C) 1997 Wiley-Liss, Inc.