Chemical synapses, particle arrays, pseudo‐gap junctions and gap junctions of neurons and glia in the buccal ganglion of Helisoma

Abstract

The nervous system of the snail, Helisoma trivolvis, has been utilized for a wide range of studies of neuronal plasticity; however, the ultrastructural features of this tissue were previously unknown. The present study examined the nature of synaptic interactions of neurons and glia and considered several plasma membrane specializations of these cells. The symmetrical pair of buccal ganglia consisted of a ring of unipolar neurons surrounding a central neuropil. The neurons were separated by two morphologically distinct types of glia: type I were most numerous and possessed an electron-dense homogeneous cytoplasm, whereas type II glia were of lower electron density, possessed a heterogeneous cytoplasm, and appeared to be phagocytic. Gap junctions were abundant between glia and were occasionally found between neuronal processes, including those of neurons 19 injected with horseradish peroxidase (HRP). Comparison of neuron and glial gap junction widths (16.4 and 17.6 nm, respectively) in thin sections and their intramembrane particle diameters (13.1 and 13.7 nm, respectively) by freeze fracture, did not elucidate significant differences. A heterogeneous population of putative chemical synapses, similar to those reported in other molluscs, was also observed between axonal collaterals in the neuropil. Additionally, examination of freeze-fractured neuropil revealed rhombic arrays of particles localized on neuronal membranes; these arrays do not appear to form intercellular junctions but may represent postsynaptic receptor sites. Freeze fracture also revealed small, square arrays consisting of 7-9 nm diameter particles on glial membranes which may correspond to pentalaminar membrane contacts (pseudo-gap junctions) seen in thin sections between glia situated around dilated extracellular spaces (lacunae).

Connexin36 vs. connexin32, “miniature” neuronal gap junctions, and limited electrotonic coupling in rodent suprachiasmatic nucleus

“…Gap junctions were identified based on well-established criteria [ (Brightman and Reese, 1969;Sloper, 1972;Berdan et al, 1987;Rash et al, 1997); reviewed in Rash et al, (1998a)], including: a) uniform 8-9 nm diameter intramembrane particles in their P-faces and uniform 8-9 nm diameter pits in their E-faces; b) close packing of IMPs/pits, often in regular hexagonal array; c) maintained alignment of the rows of IMPs and pits across the step from E-to P-face within the gap junction; and d) narrowing of the extracellular space from 10-20 nm to 2-3 nm within the border of those gap junctions in which both P-and E-faces were exposed. For additional criteria, see Kamasawa et al, (2006).…”

“…Regardless, each SCN neuron appears to be detectably coupled to one or two other neurons, but may be more weakly coupled to additional neurons by a small but unknown number of "mini" gap junctions. Conventional thin-section TEM is not capable of identifying "mini" gap junctions because they are smaller than the 60-90 nm thickness of typical TEM sections (Sotelo and Korn, 1978;Berdan et al, 1987;Rash et al, 1998a). For example, in thin-section studies of adult rat spinal cord, only large gap junctions had been detected, and then, only in the caudal extent of the lumbosacral enlargement in the nucleus of the bulbocavernosus (Matsumoto et al, 1988(Matsumoto et al, , 1989.…”

Identification of Cells Expressing Cx43, Cx30, Cx26, Cx32 and Cx36 in Gap Junctions of Rat Brain and Spinal Cord

“…Thus, in LM when not all cell types are separately visualized, fluorescence images cannot provide unambiguous localization of connexins to any of the three cell types likely to be present near the surface of a neuron labeled with a fluorescent marker. Moreover, TEM localization of Cx32 and Cx26 to putative neuronal gap junctions (Alvarez-Maubecin et al 2000) is uncertain because labeled membranes did not conform to established criteria for identifying gap junctions (Brightman and Reese 1969;Sloper 1972;Berdan et al 1987;reviewed in Rash et al 1998, Nagy et al 1999, and the level of apparent labeling was not demonstrated to be above nonspecific "background".…”

Cell-Specific Expression of Connexins and Evidence of Restricted Gap Junctional Coupling between Glial Cells and between Neurons

“…Moreover, the limited resolution of light microscopy in tissue slices (ϳ0.3-0.5 m) precludes its use for assigning connexins to plasma membranes of either of two apposed cells or to intervening unresolved cell processes. Likewise, with thin-section electron microscopy (TEM), identification of close plasma membrane appositions as gap junctions is particularly difficult in the CNS unless strict criteria for identifying gap junctions are rigorously applied (Brightman and Reese, 1969;Sloper, 1972;Berdan et al, 1987;Rash et al, 1998a). Although unambiguous labeling of identified gap junctions in identified glial cells has been demonstrated Nagy et al, 1999), TEM sampling methods limit quantitative analysis of rare coupling combinations.…”