EVOLUTION OF SPONTANEOUS ACTIVITY IN THE DEVELOPING RAT SUPERIOR COLLICULUS

During the first 10 days after birth in the rat there are a succession of major developmental stages in the retinotectal pathway. During mos t of this time, the only recordable event in the superior colliculus i s spontaneous activity. We studied and characterized this spontaneous activity, hypothesizing that it could play an important role in pathwa y development. The spontaneous discharges are detectable on postnatal day 5 (P5). After P5, the number of spontaneously active cells per pen etration increases up to P10, after which they decrease to adult-like levels by P14-P15. Between P5 and P10, the spontaneous discharges exhi bit several patterns of activity, from constant firing to intermittent bursts with periods of quiescence, without any bearing to age. We iso lated the retina and superior colliculus by injecting xylocaine onto t he optic nerve and found no change in collicular activity. While this suggests that the spontaneous activity in the colliculus is independen t of the retina at the ages studied, the opposite experiment, i.e., el ectrically stimulating the optic nerve, resulted in increased firing b y collicular neurons, perhaps via nonclassical synaptic transmission. Finally, we compared interval histograms for spontaneously active cell s between P5 and P15. The histograms suggest that at certain ages, spo ntaneous firing is more regular; moreover, these ages precede major fu nctional advances, e.g., onset of numerous spontaneously firing cells at P6, the first response to optic nerve stimulation at P10, and the f irst light-evoked response at P12-P13. Our results support the hypothe sis that spontaneous activity in the neonatal superior colliculus has a role in development of the retinotectal pathway, but the data also i ndicate that classical synaptic transmission is not involved.