Nobuyoshi Matsumoto

Principal neuronal organization in the frog optic tectum revealed by a current source density analysis.

In the frog optic tectum, the spatiotemporal pattern of neuronal activity evoked by electrical stimulation of the optic tract was examined by means of a current source density (CSD) analysis. The CSD depth profile was highly reproducible in different experiments. In all seven CSD profiles, three current sinks A, B, and D were observed in the retinorecipient layers. Four out of the seven profiles show additional two sinks C and E below the retinorecipient layers. Very small and short lasting sinks related to afferent fiber activities precede sinks A and B by about 1 ms, which could be accounted for by monosynaptic delay, in the corresponding depth region. The earliest prominent sink A at the bottom of the retinorecipient layers reflects only excitatory monosynaptic activities derived from R3 and/or R4 retinal ganglion cells. The second prominent sink B in the superficial retinorecipient layer is composed partly of excitatory monosynaptic activity from medium-sized myelinated optic fibers. It may involve excitatory monosynaptic activity from unmyelinated optic fibers and further polysynaptic activity. The fourth prominent sink D in the intermediate retinorecipient layer partially reflects excitatory monosynaptic activity derived from unmyelinated optic fibers. It may also involve further polysynaptic activity. In contrast with these three sinks, the third prominent sink C and fifth sink E exclusively reflect intratectal polysynaptic activity that has not been reported in any previous CSD studies in the frog optic tectum. These sinks almost overlap spatially in the tectal layer. We also measured the intratectal resistance changes and computed inhomogeneous CSD depth profiles to show that the results from homogeneous CSD computation assuming constant conductivity are valid for our present study. Finally, we compared the present results with previously reported CSD studies on the frog optic tectum and discuss consistencies and discrepancies among these experiments.

Spreading depression in vivo potentiates electrically-driven responses in frog optic tectum

This is the first description of an in vivo potentiation phenomenon associated to spreading depression (SD) in the frog optic tectum. Field potential responses electrically-elicited from the optic tract and recorded in the optic tectum disappeared during KCl-elicited SD and recovered 10-20 min thereafter. Post-SD responses reached amplitudes 10-30% higher than their pre-SD values (P<0.05), indicating a potentiation effect. Current source density analysis of the tectal depth profiles of field-potential responses, as well as the calculation of the post-SD intratectal conductance changes, also supported the potentiation phenomenon. This in vivo potentiation lasted for 40-90 min, suggesting a post-SD enhancement of synaptic transmission, which may be important in understanding mechanisms of brain disfunctions like epilepsy.

Intracellular and current source density analyses of somatosensory input to the optic tectum of the frog

This is the first report of current source density (CSD) and intracellular analyses of non-optic processing in the frog optic tectum. Sciatic nerve stimulation was used to test for somatosensory input to the optic tectum. To demonstrate the distribution of somatosensory input, field potentials were recorded from the whole surface of both tecta. Two components were observed. An early component was found in the whole area, but a late component was detected only in medial and caudal regions of the contralateral tectum. The effect of different stimulus intensity suggested that the optic tectum receives mainly the tactile sensation with fast conducting, low threshold level afferents from the sciatic nerve. The result of CSD analysis suggests that somatosensory afferents terminate on the tectal neurons with vertically expanding dendrites at the medial site of the contralateral optic tectum where the late component was found. Intracellular recordings demonstrated postsynaptic potentials in the middle and deeper layers, which is consistent with results from mammalian superior colliculus in earlier studies. Additional stimulation of the optic tract demonstrated that some somatosensory neurons had bimodal responses. The responses of those in the middle layers appeared to participate in avoidance behavior, based upon previous CSD analysis of the tectum using optic tract stimulation. All somatosensory responses elicited in these neurons were IPSPs. The findings imply that the somatosensory input to the optic tectum gives a suppressive effect on avoidance behavior. A somatosensory effect on prey-catching behavior could not be found in the present small number of intracellular data.

Synaptic connection patterns between frog retinal ganglion cells and tectal neurons revealed by whole-cell recordings in vivo.

The synaptic connections between frog retinal ganglion cells and tectal neurons were examined by whole-cell recordings in vivo combined with nystatin perforated patch-clamp technique and pulse-triggered averaging analysis. In five tectal neurons, monosynaptic EPSPs from seven R3 retinal ganglion cells were observed. One tectal neuron was found to receive EPSPs from both R1/2 and R3 retinal ganglion cell simultaneously. Thus we could provide the first evidence for convergence of multiple types of retinal axons upon a single tectal neuron.

Postsynaptic potentials of tectal neurons evoked by electrical stimulation of the pretectal nuclei in bullfrogs (Rana catesbeiana).

Postsynaptic responses of the tectal cells to electrical stimulation of pretectal (Lpd/P) nuclei were intracellularly recorded in the bullfrog (Rana catesbeiana). The pretectal stimulation elicited mainly two types of responses in the ipsilateral tectum: an EPSP followed by an IPSP and a pure IPSP. The latter predominates in the tectal cells responding to ipsilateral pretectal stimulation. In a few cells, biphasic hyperpolarization appeared under stronger stimulus intensities. Only one type of response was found in the contralateral tectum, a pure IPSP. The antidromically invaded tecto-pretectal projecting cells were recorded in both tecta, which revealed reciprocal connections between the tectum and particular pretectal nuclei. This paper demonstrates the synaptic nature underlying pretectotectal information transfer. EPSPs with short latencies were concluded to be monosynaptic. Most IPSPs were generated through polysynaptic paths, but monosynaptic IPSPs were also recorded in both optic tecta. Nearly 98% of impaled tectal cells (except for intra-axonally recorded and antidromically invaded cells) showed inhibitory responses to pretectal stimulation. The results provide strong evidence that pretectal cells broadly inhibit tectal neurons as suggested by behavioral and extracellular recording studies.

Current source density analysis of contra- and ipsilateral isthmotectal connections of the frog

The nucleus isthmi (NI) of the frog receives input from the ipsilateral optic tectum and projects back to both optic tecta. After ablation of NI, frogs display no visually elicited prey-catching or threat avoidance behavior. Neural mechanisms that underlie the loss of such important behavior have not been solved. Electrophysiological examination of the contralateral isthmotectal projection has proved that it contributes to binocular vision. On the other hand, there are very few physiological investigations of the ipsilateral isthmotectal projection. In this study, current source density (CSD) analysis was applied to contra- and ipsilateral isthmotectal projections. The contralateral projection produced monosynaptic sinks in superficial layers and in layer 8. The results confirmed former findings obtained by single unit recordings. The ipsilateral projection elicited a prominent monosynaptic sink in layer 8. Recipient neurons were located in layers 6-7. These results, combined with those from the former intracellular study, led to the following neuronal circuit. Afferents from the ipsilateral NI inhibit non-efferent pear shaped neurons in the superficial layers, and strongly excite large ganglionic neurons projecting to the descending motor regions. Thus feedback to the output neurons strengthens the visually elicited responses.

Unitary EPSPs evoked by visual stimuli and spatial distribution of activated synapses in tectal neurons of the frog

Using intracellular recording techniques, we recorded the responses of tectal neurons of the frog to moving visual configurations. Most of the responses sampled in this experiment originated from class 3 retinal ganglion cells. We classified EPSPs according to their shapes into two quantitative criterions, rise time and peak amplitude. Rise time histograms for observed EPSPs had a single peak. This implies the localization of activated synapses, which is consistent with the morphological findings in frogs optic tectum. Amplitude histograms had several peaks which are regularly spaced by 0.3 to 0.6 mV. Through such examination, we successfully identified unitary EPSPs evoked by class 3 retinal fibers. We also observed unitary IPSPs in the same manner. These results were mainly obtained from responses elicited by manual movement of black cardboard. In the experiment where a computer controlled stimulus (a moving light slit) was used, the relationship between the position of the stimulus and rise time of EPSPs was examined.