Freeze‐fracture study of an invertebrate multiple‐contact synapse: The fly photoreceptor tetrad

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Abstract
Bow‐shaped particle arrays on the P‐faces of the photoreceptor terminals R1–R6 in the lamina ganglionaris of the house fly represent the presynaptic sites of chemically mediated multiple‐contact synapses (Shaw and Stowe, '82, Saint Marie and Carlson, '82). A particle array consists of two polar patches of regularly arranged particles and a central patch of irregularly arranged ones. Corresponding to these P‐face arrays, the receptor E‐faces have lattices of pits opposite the polar patches, and pits and some particles at the center. The presynaptic particle array corresponds in its dimensions to the electron‐dense bar found in thin sections. The center‐to‐center spacing of the regularly arranged particles agrees with the spacing of striations found in the bar overlying the two polar elements of the postsynaptic tetrad. The elements in the two medial postsynaptic positions are hyperpolarizing monopolar cells Ll and L2, which show a strip of P‐face particles within an otherwise bare postsynaptic membrane enclosed by a ridge, and a bare Eface. Comparison with other invertebrate synapses reveals two types of organization of postsynaptic membranes. IMPs fracture with the postsynaptic P‐face in GABAergic and/or inhibitory synapses and with the E‐face in glutaminergic and/or excitatory synapses; the fly photoreceptor synapse thus fits in the former category.