Immunohistochemical localization of a synaptic-vesicle antigen in a cholinergic neuron under conditions of stimulation and rest

The Structural Basis of Long-Term Potentiation in Hippocampal Synapses, Revealed by Electron Microscopy Imaging of Lanthanum-Induced Synaptic Vesicle Recycling

Hippocampal neurons in dissociated cell cultures were exposed to the trivalent cation lanthanum for short periods (15–30 min) and prepared for electron microscopy (EM), to evaluate the stimulatory effects of this cation on synaptic ultrastructure. Not only were characteristic ultrastructural changes of exaggerated synaptic vesicle turnover seen within the presynapses of these cultures—including synaptic vesicle depletion and proliferation of vesicle-recycling structures—but the overall architecture of a large proportion of the synapses in the cultures was dramatically altered, due to large postsynaptic “bulges” or herniations into the presynapses. Moreover, in most cases, these postsynaptic herniations or protrusions produced by lanthanum were seen by EM to distort or break or “perforate” the so-called postsynaptic densities (PSDs) that harbor receptors and recognition molecules essential for synaptic function. These dramatic EM observations lead us to postulate that such PSD breakages or “perforations” could very possibly create essential substrates or “tags” for synaptic growth, simply by creating fragmented free edges around the PSDs, into which new receptors and recognition molecules could be recruited more easily, and thus, they could represent the physical substrate for the important synaptic growth process known as “long-term potentiation” (LTP). All of this was created simply in hippocampal dissociated cell cultures, and simply by pushing synaptic vesicle recycling way beyond its normal limits with the trivalent cation lanthanum, but we argued in this report that such fundamental changes in synaptic architecture—given that they can occur at all—could also occur at the extremes of normal neuronal activity, which are presumed to lead to learning and memory.

Changes in the elastic properties of cholinergic synaptic vesicles as measured by atomic force microscopy

Cholinergic synaptic vesicles from Torpedo californica have been probed with the atomic force microscope in aqueous buffers to map and measure their elastic properties. Elastic properties were mapped with a new atomic force microscope technique known as force mapping. Force mapping of vesicles showed that the centers of the vesicles are harder or stiffer than the peripheral areas in the three buffers that were investigated. These were an isoosmotic buffer, a hypoosmotic buffer, and an isoosmotic buffer with 5 mM CaCl2 added. The hardness of the vesicular centers was quantified by calculation of the elastic modulus. Elastic moduli were in the range of 2-13 x 10(5) Pa. Vesicular centers were hardest in calcium-containing buffer and softest in isoosmotic buffer. Hypotheses are presented for the composition and function of the hard centers.

Association of axonally transported heparan sulfate with isolated synaptic plasma membrane

Studies on isolated synaptic plasma membranes (SPM) have detected little if any heparan sulfate or other glycosaminoglycans (GAGs), while more recent studies employing proteoglycan antibodies have localized heparan sulfate proteoglycan in presynaptic plasma membrane of intact tissue. To further address the issue of proteoglycans in synaptic plasma membrane, we have investigated the possible presence of axonally transported GAGs in SPM isolated from the goldfish optic tectum. SPMs isolated from tecta following rapid axonal transport of 35SO4 labeled molecules down the optic nerve, showed specific radioactivity approximately two-fold higher than the starting homogenate. Treatment of the transport labeled SPM with the enzyme heparitinase liberated 21% of the radioactivity, indicating the presence of a significant fraction of transported label in heparan sulfate. In a separate series of experiments a GAG fraction was isolated from transport labeled SPM and was found to consist of heparan sulfate containing 28% of transported radioactivity. Chondroitin (4 or 6) sulfate, which undergoes axonal transport in the goldfish optic system, was not found associated with SPM. Taken together the results support immunological evidence for the presence of heparan sulfate proteoglycans in presynaptic plasma membrane.

Cholinergic vesicle specific proteoglycan: stability in isolated vesicles and in synaptosomes during induced transmitter release

Exposure of synaptosomes isolated from the electric organ of Torpedo marmorata to conditions that promote the release of acetylcholine does not cause the co-release of a vesicle specific proteoglycan. Proteoglycan within synaptosomes is quite stable during various incubation conditions as measured by immune dot blotting. Isolated vesicles from Torpedo also retain their proteoglycan immunoreactivity when exposed to a variety of incubation conditions. Lysis of vesicles in H2O, treatment with pH 11.5 buffer, or exposure to high ionic strength (2 M KCl) results in the loss of acetylcholine or ATP while the proteoglycan is retained by vesicle membranes. Only treatment with Nonidet P-40 releases proteoglycan from vesicles or synaptosomes and free proteoglycan immunoreactivity is then susceptible to degradation by trypsin or heparinase. These results suggest that the proteoglycan is an integral component of vesicle membranes and is at least in the synaptosomal preparation not subject to extensive co-release with acetylcholine or ATP.