Large dense-core vesicle exocytosis and membrane recycling in the mossy fibre synapses of the rabbit hippocampus during epileptiform seizures

Abstract

The ultrastructure of the hippocampal mossy fibre layer was studied in ultrathin sections and freeze-fracture preparations of rabbits under deep Nembutal anaesthesia, after recovery from ether anaesthesia, and 40 min after a single injection of methoxypyridoxine, that is, during the second generalized seizure discharge. The giant mossy fibre boutons contain two types of vesicles: evenly distributed, small round clear vesicles (50 nm) and a few scattered large dense-core vesicles (100 nm). In rare instances fusion of dense-core vesicles with the presynaptic membrane was observed. No differences in the morphology of the mossy fibre synapses were found between anaesthetized and unanaesthetized animals. During epileptiform seizures, however, the size and shape of clear and dense-core vesicles varied greatly. The active synaptic zones were covered with large, core-containing omega profiles or bumps and indentations. Only dense-core vesicles seem to undergo exocytosis. A fusion of clear vesicles with presynaptic membrane was not observed. Various explanations for the fact that only dense-core vesicles seem to undergo exocytosis are discussed. The hypothesis is put forward that in the mossy fibre bouton two morphologically and functionally distinct populations of synaptic vesicles exist and that only one of them undergoes visible irreversible exocytosis, whereas the majority, that is, the small vesicles discharge their transmitter by reversible fusion. After MP injection features of membrane retrieval were also prominent. Frequently, at the borders of the active synaptic zones coated membrane convolutes of both pre- and postsynaptic membranes had invaded the terminals as well as the postsynaptic spine. Thus, in contrast to electrical stimulation, the self-sustained seizures allows energy-expensive processes such as extensive membrane internalization to take place during the interictal pauses.

Morphometry of a peptidergic transmitter system: Dynorphin B-like immunoreactivity in the rat hippocampal mossy fiber pathway before and after seizures

“…Within 3 to 12 hr of the initiation of KA-induced seizures, dyn A-LI levels in both studies had dropped to roughly 50%, suggesting a seizure-associated depletion of peptide reserves, most likely through release. In support of this hypothesis, ultrastructural analysis of mossy fiber terminals during or immediately after seizures have shown large numbers of DCVs in the process of exocytosis (Nitsch and Rinne, 1981;Hochi et al, 1987). During this time period there is a substantial (to 2x: Lason et al, 1992; or 15x: Douglass et al, 1991) transient rise in prodynorphin mRNA levels, proposed to represent a response to the depletion of dynorphin and/or early gene activation (Helton and McGinty, 1993).…”

Exocytotic release of neurotransmitter substances from nerve endings in the taste buds of rat circumvallate papillae.

“…In the authentic neurons, using a conventional method, NITSCH and RINNE (1981) found exocytotic figures of large cored vesicles at only the synaptic sites in the hippocampus mossy fibers of the rabbits being symptomatic of epileptif orm seizures by the administration of methoxy pyridoxine. On the other hand, using the TART method, BUMA and ROUBOS (1986) demonstrated the non-synaptic release of large cored vesicles in the central nervous system of snails, insects and rats.…”

Structure, Distribution, and Function of Neuronal/Synaptic Spinules and Related Invaginating Projections

“…Purkinje cell spines that form synapses with climbing fibers also sometimes may invaginate almost completely into the climbing fiber terminal (Palay and Chan-Palay 1974). In the giant mossy fiber terminals of the hippocampus (see “Small Postsynaptic Invaginating Projections: Spinules” section), the postsynaptic thorny excrescences, although not technically spines, can be nearly completely invaginated into the mossy terminals (Nitsch and Rinne 1981; Chicurel and Harris 1992; Petralia and Wenthold 1992; Petralia et al 2011). Inner hair cells of the mouse cochlea (see subsequent sections below) can be involved in a three-way circuit among a hair cell body, and afferent and efferent terminals, and all three processes may include some invaginating postsynaptic spine processes (Sobkowicz et al 2003).…”