The topographic order of the retinocollicular projection in the rat was examined from birth until maturity. Small, localized deposits of rhodamine-filled latex microspheres were placed into the superior colliculus at different locations. To minimize labeling fibers of passage deposit sites were typically, although not exclusively, placed into the caudal-lateral pole of the colliculus. Examination of the area and density of labeled cells in the retinae of these animals led to the following conclusions: (1) At each age examined, the location of the majority of labeled cells was observed to be in appropriate topographic register with the deposit site in the superior colliculus. (2) Confirming the work of previous investigators, errors in topographic projection were observed. These were present in both the contralateral and ipsilateral retinae and decreased with increasing postnatal age. The mature pattern was present by P10. (3) Quantitatively, the number of retinal ganglion cells terminating nontopographically within the colliculus constituted a relatively minor proportion of the total number of labeled cells in both retinae. It is concluded that the majority of the retinal ganglion cells make topographically appropriate terminations within the superior colliculus during development.

Isolated retinal rods of the frog consisting of the outer segment and the ellipsoid were patch-clamped and recorded in the whole-cell mode. The recording pipettes were filled with solutions of different composition in order to alter the cytoplasmic content of sodium, phosphate, and calcium ions, and guanine nucleotides. When a simple medium with potassium as the principal cation was used, the dark voltage slowly approached more negative values. This tendency of spontaneous hyperpolarization was reduced significantly when cGMP or GTP were present in the pipette medium. Sodium ions, on the other hand, clearly increased the speed of hyperpolarization. In the presence of sodium (20 mM), the stabilizing effect of GTP did not occur and that of cGMP was clearly diminished. Phosphate (20 mM) neutralized the sodium effect. High calcium levels (100 μM) did not measurably influence the time course of hyperpolarization. We conclude that the normal cytoplasmic sodium level in rods does not exceed 10 mM and that higher internal sodium concentrations interfere with the sodium–calcium exchange mechanism.

A perfusion system was used to monitor the release of [3H]-GABA from isolated retinas of Xenopus laevis. Measurable release was stimulated by glycine at concentrations as low as 200 μM. Glycine-stimulated release was blocked by strychnine, and was not reduced in “calcium-free” Ringer's solution (0 Ca2+/20 mM Mg2+). Glutamate also stimulated calcium-independent release, using concentrations as low as 100 μM. In contrast, release stimulated by 25 mM potassium was reduced by 80% in calcium-free medium.

In most experiments, agonists were applied in six consecutive 4-mm pulses separated by 10-mm washes with Ringer's solution. Under these conditions, the release stimulated by 0.5 mM glutamate or 25 mM potassium decreased by at least 50% from the first to the second pulse, and then gradually decreased with successive applications. In contrast, the response to 0.5 mM glycine at first increased and then only gradually decreased with successive pulses. These patterns of response to different agonists were similar in calcium-free medium.

Somatostatin (—14 or —28) also stimulated release, and this effect was inhibited by AOAA, an inhibitor of GABA degradation. In the presence of AOAA, somatostatin had little effect, except at high concentrations of somatostatin (5 μM), which increased both basal and glycine-stimulated release. In contrast to somatostatin, glycine-stimulated release was much larger in the presence of AOAA.

Autoradiography was used to investigate which cell types released [3H]-GABA under our conditions. Autoradiograms showed that horizontal cells and a population of apparent “off” bipolar cells were well-labeled by [3H]-GABA high-affinity uptake. In addition, light labeling was seen over numerous amacrine cells. After application of glycine, glutamate, or potassium, there was a decrease in label density over horizontal cells.

This paper describes experiments on GABA-activated whole-cell membrane currents in bipolar cells freshly isolated from the adult rat retina. The main goal was to determine whether bipolar cell responses to GABA could be resolved in terms of mediation by the GABAA receptor, the GABAB receptor, or both. Bipolar cells were isolated by gentle enzymatic dissociation and identified by their distinct morphology. GABA agonists and antagonists were applied focally by pressure and the resultant currents were recorded under whole-cell voltage clamp. In all bipolar cells tested, GABA (0.1–100 μM) induced a monophasic response associated with a conductance increase (IGABA). The shift in reversal potential for IGABA as a function of pipet [CI] paralleled that predicted based on the Nernst equation for Cl−. IGABA was mimicked by muscimol (5–20 μM) and antagonized by bicuculline (20–100 μM). Baclofen (0.1–1.0 mM) produced no apparent conductance change. “Hot spots” of sensitivity to GABA which might be associated with regions of synaptic contact were not found; both the soma and processes of all bipolar cells were responsive to focally applied GABA. Furthermore, all bipolar cells tested responded to glycine.

In conclusion, we have established the presence of GABAA receptors on rat retinal bipolar cells. Our data suggest further that these cells lack GABAB receptors. Finally, our observation that bipolar cells in the rat retina are relatively homogeneous in terms of their sensitivity to GABA and glycine lead us to postulate that the functional significance of the presence of receptors and their distribution on a neuron may be dictated more by the topography of the presynaptic inputs than by its inherent chemosensitivity.

The effects of cobalt ions on 502-nm rod- and 575-nm cone-driven components of the b-wave of the electroretinogram were studied in the isolated frog retina. Addition of 100–150 μM cobalt initially caused a suppression of rod-driven responses and an enhancement of cone-driven responses. In the continued presence of cobalt, however, the rod-driven responses gradually recovered and the cone-driven responses became suppressed. These concentrations of cobalt had no effect on the rod- and cone-driven mass receptor potentials which were isolated in the presence of 4 mM glutamate. At higher concentrations of cobalt (1 mM or greater), both rod- and cone-driven b-wave responses were eliminated and there was no recovery in the continued presence of cobalt. The results suggest that cobalt has markedly different, time-dependent effects on signal transmission from rods and cones to second-order cells.

The anatomy and physiology of the retinotectal pathway of the perch was investigated using physiological and histological techniques. Massed responses of the optic nerve to single shocks exhibited five distinct peaks. Single-unit responses to shocks indicate two groups of fast fibers correlating well with peaks I and II of the massed response. The flash-evoked response in nerve and tectum has three major phases (PSPI-III), with a marked low-threshold fast component. Patterns of flash-evoked response from single fibers vary, but the responses of fast transient fibers coincide with the timing of PSPI, and longer latency groups with PSPII-III. Units reflexly activated by efferents were also seen, and 12% of units were photically inexcitable.

Surprisingly, few fibers responded well to a scanned spot light, unlike tectal cells, and receptive fields were often large (>70 deg). ON/OFF responses, evoked either by whole field or local illumination, were much commoner than pure ON or OFF responses.

Effects of electrical stimulation or cautery of the tectum on the flash-evoked response of fiber bundles, via the efferents were marginal, but repetitive stimulation or section of the optic nerve produced clear-cut deficits in the slow components of the flash-evoked response of the nerve. Stimulation of the eighth nerve produced a complex long-latency, large-amplitude response in the optic nerve.

The fiber spectrum of the optic nerve taken from electron micrographs revealed the presence of a relatively small group (less than 1%) of thick fibers with diameters between 3 μm and 10 μm that could be correlated with fast responses recorded from the optic nerve, and the remainder with axon diameters down to 0.2 μm providing the slow responses. The distribution of cell-body diameters from sectioned and wholemount material indicated a marked distinction between small and large ganglion cells. The total number of fibers in the nerve was estimated 868,840.

The ability of freely-flying honeybees to track moving targets was examined by training to collect a reward on a target, and then videotaping their approach to the target while it was in motion. Training experiments were carried out with several groups of bees, using various colors for the target and the background. Computer-aided frame-by-frame analysis of video recordings was used to plot the instantaneous positions of the target, as well as the position and orientation of the approaching bee in three dimensions. The results show that bees are perfectly capable of tracking moving targets and landing on them. When the distance of the target is greater than 15 cm, approaching bees correct for angular deviations of the target from the midline, both in the horizontal and in the vertical plane. In either plane, the input vaariables that are important to the tracking system seem to be (1) the angular bearing of the target with respect and (2) the angular velocity of the target with respect to the eye. The tracking control system tends to orient the bee such that the target is located frontally, at an angle of Ca. 35 deg below the bee's long axis. The chromatic properties of tracking behavior were investigated by employing combinations of colors for the target and background such that the boundary between the target and the background presented a contrast that was visible either only to the green-sensitive receptors of the bee's eye, or only to theblue-sensitive receptors. The results of these experiments suggest that, in controlling tracking, the measurement of the angular velocity of the target is derived almost exclusively from signals from the green-sensitive receptors, as is the case with previously studied movement-sensitive behavior. However, the measurement of the angular bearing of the target is derived from the blue-sensitive receptors as well as the green-sensitive noes. When the target is closer than Ca. 15 cm, approaching bees use translational maneuvers, in addition to rotational ones, to track the moving target. Translational target tracking appears to be driven primarily by signals from the green-sensitive receptors.