Takehiko Saito

Physiological and morphological differences between on- and off-center bipolar cells in the vertebrate retina

The bipolar cells of the vertebrate retinas transfer signals generated by photoreceptors in the distal retina to amacrine and/or to ganglion ceils of the proximal retina. They give rise to the following fundamental signal processes: the segregation of depolarizing (On) and hyperpolarizing (Off) retinal pathways, which are subserved by “On-” and “Off-center bipolar ceils”, the convergence or divergence of rod and cone signals, and the encoding of colour specificity. All vertebrate photoreceptors, both rods and cones, release a chemical transmitter continuously in the dark. Light hyperpolarizes photor~epto~ and then decreases the release of transmitter. A basic question is how diminution of transmitter release leads to depolarization in On-center bipolar cells and hyperpolarization in Off-center bipolar cells. This review summarizes recently acquired knowledge about physiological and morphological differences between Onand Off-center bipolar cells. In view of my own interests, particular emphasis will be placed on three problems related to the above question: (1) whether there is any correlation between the polarity of the light response and photoreceptor-bipolar synaptic contact, (2) whether ionic channels mediating the light responses of Onand Off-center bipolar cells are different or not, and (3) whether pharmacologica! properties of the postsynaptic receptors are identical or not for the two bipolar cell types. Because of space limitations, citation of the literature is mainly selected from the works made by the intracellular microelectrode technique. Limitations of space also have required me to omit many primary references that have formed the framework of important investigations on various aspects of the retinal function. Tbat information and earlier studies have been summarized in other reviews (Cervetto and Fuortes, 1978; Kaneko, 1979; Werblin, 1979; Dowling and Dubin, 1984; Shapley and Enroth-Cugell, 1984). This selective review is supplemented by recent comprehensive reviews (Raneko et al., 1979; Witkovsky, 1980; Fain et al., 1983; Falk, 1985; Slaughter and Miller, 1985a; Miller and Slaughter, 1986).

Effects of Left and Right Vagal Stimulation on Excitation and Conduction of the Carp Heart (Cyprinus carpio)

Summary1.Stimulation to left and right vagi caused an almost equal amount of inhibitory, and occasionally excitatory, effects on pacemaker activity. Both inhibitory and excitatory effects were abolished by atropine. Vagal stimulation hyperpolarized the resting membrane potential of pacemaker fibers in the sino-atrial valve, but did not change their action potential profile.2.The atrial action potential showed a prominent decrease in the action potential amplitude and duration in response to vagal stimulation. The atrial region surrounding the sino-atrial valve was more sensitive to right vagal stimulation.3.The fibers in the atrio-ventricular ring muscle were less sensitive to vagal stimulation than the atrial fibers. Some fibers showed a decrease in the action potential amplitude and duration by vagal stimulation, and other fibers showed a decrease in the amplitude, but a prolongation of the duration as the result of a slowing of the rate of upstroke. The atrial-ventricular conduction delay or block by vagal stimulation may depend on these properties of the action potential of the atrio-ventricular ring muscle.4.The sino-atrial conduction block is explained by the fact that the atrial fibers are more sensitive to vagal stimulation than pacemaker fibers.5.The possible pathways for the sino-ventricular conduction during vagal stimulation are discussed.

Bipolar-amacrine synaptic transmission: Effect of polarization of bipolar cells on amacrine cells in the carp retina*

INTRODUCTION The amacrine cells in the teleost retina are often classified as sustained or transient (1.2). Sustained amacrine cells respond to light with sustained depolarization (ON type) or hyperpolarization (OFF type), while transient amacrine cells respond to both onset and offset of light with a transient depolarization (ON-OFF type). Synaptic transmission from bipolar cells to amacrine cells has been studied by measuring membrane resistance changes accompanying light responses (3), and by examining the effect of chemicals on bipolar and amacrine cell responses (4-7). These studies have suggested that the transmission from bipolar to amacrine cells is excitatory. Furthermore, histological observations on dye-injected bipolar cells and amacrine cells have shown that ON and OFF amacrine cells make synaptic connections with bipolar cells at different sublayers of the inner plexiform layer (8). Thus it is widely believed that ON amacrine cells receive excitatory inputs from depolarizing (ON) bipolar cells, OFF amacrine cells from hyperpolarizing (OFF) bipolar cells, and ON-OFF amacrine cells from both ON and OFF bipolar cells. The above inferences are rather indirect, however, and these observations do not elucidate the kinetics of synaptic transmission between bipolar and amacrine cells. For instance, it remains puzzling how the transient nature of ON-OFF responses is produced by summation of the excitatory inputs from the same ON and OFF bipolar cells that drive the sustained amacrine cells. In this study we approached this problem more directly. We simultaneously recorded responses of a bipolar cell and an amacrine cell with intracellular microelectrodes, while polarizing the bipolar cell with current. A step of depolarizing current injected into ON bipolar cells elicited a steady depolarization in sustained ON amacrine cells, but produced only a transient depolarization at the onset of current in ON-OFF amacrine cells. Steady hyperpolarization of OFF bipolar cells, however, elicited a transient depolarization in ON-OFF amacrine cells at the termination