title: Cone synapses in macaque fovea: II. Dendrites of OFF midget bipolar cells exhibit Inner Densities similar to their Outer synaptic Densities in basal contacts with cone terminals.
abstract: As described in the companion paper, the synaptic terminal of a cone photoreceptor in macaque monkey makes an average of 35 or 46 basal contacts with the tips of the dendrites of its OFF midget bipolar cell. Each basal contact has one or more symmetrically thickened dense regions. These "Outer Densities," averaging 48 or 67 in number, harbor clusters of ionotropic glutamate receptors and are ~0.8 μm (and ~1-ms diffusion time) from active zones associated with synaptic ribbons. Here, we show similarly appearing "Inner Densities," averaging 53 or 74 in number, located more proximally on the dendrites of these OFF midget bipolar cells, ~0.4 μm inward from the tips of the dendrites and out of contact with the basal surface of the cone terminal. Compared to desmosome-like junctions, Inner Densities are closer to the terminal and are less dense and less thick. Each Inner Density is shared with another cell, the partners including diffuse bipolar cells, ON midget bipolar cells, and horizontal cells. Given the diversity of the partners, the OFF midget bipolar cells are unlikely to be in a synaptic relationship with the partners. Instead, Inner Densities are near enough to the active zones associated with synaptic ribbons to receive pulses of glutamate at concentrations effective for glutamate receptors. The role of Inner Densities is not known, but they might represent additional clusters of glutamate receptors.
authors.affiliation: Department of Psychology, Franz Hall, University of California, Los Angeles, Los Angeles, California, USA.
authors.forename: Steve
authors.initials: S
authors.lastname: Herr
authors.forename: Ivy Tran
authors.initials: IT
authors.lastname: Ngo
authors.forename: Teresa M
authors.initials: TM
authors.lastname: Huang
authors.forename: Karl
authors.initials: K
authors.lastname: Klug
authors.forename: Peter
authors.initials: P
authors.lastname: Sterling
authors.forename: Stan
authors.initials: S
authors.lastname: Schein
mesh_terms: D000818:Animals; D003712:Dendrites / Q000502:physiology* / Q000648:ultrastructure; D005584:Fovea Centralis / Q000166:cytology / Q000502:physiology* / Q000648:ultrastructure; D018698:Glutamic Acid / Q000378:metabolism; D007091:Image Processing, Computer-Assisted; D008252:Macaca fascicularis; D008297:Male; D008854:Microscopy, Electron; D008959:Models, Neurological; D017729:Presynaptic Terminals / Q000502:physiology / Q000648:ultrastructure; D011956:Receptors, Cell Surface / Q000502:physiology; D058468:Receptors, Ionotropic Glutamate / Q000502:physiology; D051245:Retinal Bipolar Cells / Q000502:physiology* / Q000648:ultrastructure; D017949:Retinal Cone Photoreceptor Cells / Q000502:physiology* / Q000648:ultrastructure; D013569:Synapses / Q000502:physiology* / Q000648:ultrastructure
chemical_list: D011956:Receptors, Cell Surface; D058468:Receptors, Ionotropic Glutamate; D018698:Glutamic Acid