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A variety of visually evoked responses are recorded in the fish optic tectum using single-cell recording technique. Based on indirect criteria (frequency power spectrum of spikes, spike waveform, receptive field size), they may be divided into two groups: those presumably recorded from axon terminals of retinal ganglion cells projecting to the tectum (precynaptic recording), and those recorded from tectal neurons (postsynaptic recording). In the present study, we used cobalt, a reversible blocker of synaptic transmission, as a more crucial criterion to identify the source of these responses. After cobalt application, some units (such as ON- and OFF-types of direction-selective units, orientation-selective and spontaneously active units) were visually responsive, while others (including ON-OFF direction-selective units with large receptive fields) ceased firing. Discrimination of the units by the use of cobalt has been found to coincide with that by the indirect physiological criteria. Thus, the differences in frequency power spectrum of spikes, spike waveform, and receptive field size may be used for efficient and reliable discrimination between pre- and post-synaptic recordings in the fish tectum.

The review deals with the morphology, physiology, topography, and central projections of direction-selective cells of the accessory optic system in vertebrates.

Direction selective cells have been found in the retina, the first level of the visual system, in mammals and recently also in the archer fish. These cells are involved in a variety of fast neural computation processes, from the control of eye movements to the detection of prey by the archer fish. The standard model for this mechanism in mammalian retina is well understood and is based on the asymmetry of inhibitory and excitatory inputs to the retinal ganglion cells. However, it remains unclear whether the mechanism that underlies direction selectivity is similar across animal classes. This study reports a pharmacological investigation designed to elucidate the mechanism that underlies motion detection in the archer fish retina. Direction selectivity in the retina was characterized under the influence of specific channel blockers that are known to be present in the different types of neurons of the retina. The results show that the direction-selective mechanism in the archer fish retina is modified only when the inhibitory channels of GABA and Glycine are manipulated. This suggests that the mechanism of direction selectivity in the archer fish retina is fundamentally different from the mechanism of direction selectivity in the mammalian retina.

Alpha-type retinal ganglion cells (alpha cells) of the same class in mammalian retina are connected by gap junctions. Electrical synapses between alpha cells were examined using combined techniques of dual patch-clamp recordings, intracellular labeling and electron microscopy in the albino rat retina. In simultaneous dual whole-cell recordings from pairs of neighboring alpha cells, bidirectional electrical synapses with symmetrical junction conductance were observed in pairs with cells of the same morphological type. Regulatory domains of gap junction protein subunit connexins in electrical synapses between alpha cells by extracellular and intracellular ligands investigated by dual whole-patch clamp recordings. I examined how passage currents through electrical synapses between alpha cells are modulated by specific antibodies against connexin36 proteins, and extracellular or intracellular application of ligands. Control conditions led us to observe large passage currents between connected cells and adequate transjunctional conductance (Gj) (1.35$\pm$0.51nS). Experimental results show that high level of intracellular cyclic AMP within examined cells suppress electrical synapses between the neighboring cells. Gj between examined cells reduced to 0.15$\pm$0.04nS. Under application of dopamine (1.25$\pm$0.06nS) or intracellular cyclic GMP (0.98$\pm$0.23nS), however, Gj also remains as in the control level. Intracellular application of an antibody against the cytoplasmic loop of connexin36 reduced G_j (0.98$\pm$0.23nS). Cocktail of the antibody against cytoplasmic connexin36 and intracellular cyclic AMP leaves Gj as in the level by single involvement of the cytoplasmic antibody. The elimination of Gj by the cytoplasmic antibody was in a dose-dependent manner. These results suggest that binding domains against cyclic AMP may be present in the cytoplasmic sites of connexin proteins to regulate channel opening of gap junctions between mammalian retinal alpha ganglion cells.