Increased spontaneous unit activity and appearance of spontaneous negative potentials in the goldfish tectum during refinement of the optic projection.

Spontaneous (not retinally driven) postsynaptic activity was examined during activity-dependent refinement of optic fibers in the goldfish tectum. Unit recordings in vivo and in vitro demonstrated that spontaneous tectal activity increased to 150% of normal during refinement at 1-2 months after optic nerve crush and subsequently returned to baseline over the next month. This increase was not mimicked by long-term denervation indicating an effect specifically influenced by regenerating fibers. Loss of optic input was also found to induce spontaneous negative potentials (SNPs) rapidly in the tectum. SNPs were negative, monophasic potentials of 70-120 msec duration and -0.15 to -1.5 mV amplitude. SNPs occurred with no apparent periodicity at a frequency of approximately 0.3-0.6 Hz. Multiple electrode recordings and depth analysis showed that SNPs were localized events occurring in columnar domains of tectum a few hundred micrometers wide. Cross-correlation analysis revealed that SNPs were strongly correlated with local unit bursting, suggesting SNPs are generated by the summed synaptic and spike currents of coactive cells in small regions of the tectum. SNPs were suppressed by a low concentration of APV indicating they were regulated by NMDA receptors. During regeneration, the number and size of SNPs reached a peak during refinement and subsequently decreased, eventually disappearing. This temporal association with refinement suggests that these patterns of postsynaptic activity may have functional relevance. It is hypothesized that SNPs or the underlying activity that produces them increases the excitability of target cells, allowing the weak, less-convergent input from regenerating axons to drive target groups of cells in the tectum during refinement.