Development of visual input to superior colliculus in fetal rats : in vivo optical imaging.

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The purpose of the present experiments is to study the functional development of neurons in the superior colliculus (SC)in fetal rats. For this purpose, the opticaI imaging system using a voltage-sensitive dye was employed in elucidating spatio- temporaI patterns of in vivo neural activity of the SC in fetal rats which were still connected with the dam by the intact umbilical cord. In the present experiments, signal propagation in the fetal SC caused by focal stimulation of the rostral SC was examined using the optical imaging system. Two to three fetal rats (embryonic day 21- 22 day) in each pregnant rat (Sprague-Dawley, 3 months) were used for each experiment. The fetal head was fixed to a conventional stereotaxic apparatus by the previously developed method (Nakamura and Sakaguchi, 1990 ; Sakata et al., 1998). For electrical stimulation of the SC, a bipolar stimulating electrode was implanted in the rostral part of the SC.Stimulation was given with single (1 msec duration) or ten train pulses (100 msec duration). Solution of a voltage-sensitive dye RH795 was applied to the surface of the SC for 1 hour. SC stimulation produced propagation of excitation rostro- caudally from the stimulating site. Inhibitory signal propagated diffusely in the SC.These signal propagation totally disappeared after application of TTX (3 mu M) to the surface of the SC.CNQX(non-NMDA receptor antagonist) blocked an early phase of excitatory propagation induced by focal SC stimulation, while the NMDA receptor antagonist AP-5 attenuated a late phase of excitation without affecting the early phase of excitation. The inhibitory propagation was remarkably attenuated by application of bicuculline.These results suggest that at late stages of the prenatal period in the rat SC, excitatory synaptic transmission occurs through both non-NMDA and NMDA receptors, while inhibitory synaptic transmission is mediated by GABA receptors.