The ultrastructure of rat rod synaptic terminals: Effects of dark‐adaptation

The ultrastructure of synaptic terminals of rat rod photoreceptors was studied in light‐adapted and dark‐adapted states. In the light‐adapted state, horizontal cell processes embedded in the photoreceptor terminal show smooth surface contours and are ovoid in cross section. In the dark‐adapted state, fingerlike protrusions of photoreceptor cytoplasm extend into the horizontal cell processes on either side of the synaptic ribbon. An electron‐dense structure lies under the horizontal cell membrane that surrounds these fingerlike protrusions. This submembrane specialization is generally associated with areas of concavity of the horizontal cell membrane; it occurs in goldfish rods as well as rat rods. The protrusions and the horizontal cell specialization may be involved in membrane dynamics during synaptic activity.