Synaptic vesicle recycling at two classes of release sites in giant nerve terminals of the embryonic chicken ciliary ganglion

Abstract

Rapid synaptic transmission in the embryonic chicken ciliary ganglion occurs through the activation of two distinct classes of nicotinic acetylcholine receptors (AChRs): those containing alpha3 subunits (alpha 3*-AChRs) and those containing alpha7 subunits (alpha 7*-AChRs). alpha3*-AChRs are found on ciliary neurons in clusters at synaptic sites on the cell body, whereas alpha7* -AChRs are found on somatic spines, which historically were thought not to have release sites in the embryo. However, Shoop et al. (Shoop et al. [1999] J. Neurosci. 19:692-704) recently described release sites having pre- and postsynaptic densities on somatic spines. We used transmission electron microscopy to compare the structure of synaptic sites on spines with those on the smooth surfaced part of the cell. We find that the two populations of sites are similar in active zone length, number of vesicles, and distance between vesicles and active zone. To study the functional properties of these sites, we examined their stimulation-dependent uptake and release of the extracellular tracer horseradish peroxidase (HRP). We found that each class of release sites both took up and released HRP in a stimulation- and calcium-dependent manner. The mean fraction of synaptic vesicles labeled with tracer was similar for the two populations, both after loading ( approximately 45%) and after unloading ( approximately 7%). Thus we detect no differences between these two anatomically distinct classes of release sites, other than their incidence: sites on spines occurred only 12% as often as those on the cell body. The release sites on somatic spines presumably underlie synaptic responses attributable to alpha7*-AChRs.

Two Modes of Vesicle Recycling in the Rat Calyx of Held

“…When stimulating with high K ϩ solution for 15 min, 40% of the vesicles in the calyx terminals were labeled (Fig. 2), similar to observations in calyx terminals of the ciliary ganglion after prolonged stimulation (Nguyen and Sargent, 2002). As discussed above, however, the use of high K ϩ solution provides a massive stimulus, and many of the labeled vesicles are expected to have a low release probability during afferent stimulation.…”

Calcium‐induced calcium release contributes to somatic secretion of serotonin in leech Retzius neurons

“…Increasing evidence is showing that somatic exocytosis of transmitters and peptides occurs from neurons of vertebrates and invertebrates (Chen et al, 1995;Huang and Neher, 1996;Matthews, 1997, 1999;Jaffe et al, 1998;Puopolo et al, 2001;Nguyen and Sargent, 2002;Trueta et al, 2003;Soldo et al, 2003). The characteristics of somatic secretion in most of the neuron types studied are different from those of synapses but similar to those of secretion by excitable endocrine cells.…”

“…For example, it is produced from dense-core vesicles in the absence of active zones Matthews, 1997, 1999;Bruns et al, 2000;Puopolo et al, 2001;Trueta et al, 2003); it depends on long depolarizations (Huang and Neher, 1996) or trains of impulses (Soldo et al, 2003;Trueta et al, 2003); its dependence on the intracellular Ca 2ϩ concentration is lower than that of synaptic secretion (Huang and Neher, 1996), and it is coupled to excitation through L-type Ca 2ϩ channels (Puopolo et al, 2001;Trueta et al, 2003). An exception is chick ciliary ganglion neurons that release acetylcholine from clear vesicles at somatic spines (Nguyen and Sargent, 2002).…”

Structure of peripheral synapses: autonomic ganglia

“…These somatic spine "mats" are covered by the presynaptic calyx. In young chickens, about 12% of synaptic contacts are made onto spines in these mats, with the rest being made directly onto the soma (Nguyen and Sargent 2002). The spines express a population of nicotinic receptors (consisting of the alpha7 subunit), which do not normally contribute to fast synaptic transmission, and ganglionic transmission has been thought to be mediated primarily via the axo-somatic contacts (e.g.…”