Apparent movement of optic terminals out of a local postsynaptically blocked region in goldfish optic tectum.

In goldfish tectum, alpha-bungarotoxin (alpha BTX) blocks postsynaptically generated field potentials elicited by either photic or electrical stimulation, but leaves presynaptic activity unaffected. To assess the chronic effects of blocking transmission on synaptic stability, small restricted areas of synaptic block were created by slow, low-pressure microinjection of toxin-Ringer's solution from a micropipette. Local transmission was blocked, but field potentials outside the injected region were unchanged. Radioautography of 125I-alpha BTX demonstrated that 6 days postinjection it was still highly localized and concentrated in discrete synaptic laminae of the tectal neuropil, in agreement with its known tight binding in goldfish brain (18). Electrophysiological maps made 6-8 days postinjection showed that few if any optic terminals could be recorded in the toxin-blocked zone, but there was no scotoma in the visual field. Terminals with receptive fields that would normally have corresponded to the region of injection were instead recorded from surrounding tectal areas. Since the normal units were also recorded there, this produced very large multiunit receptive fields. Apparently the optic terminals within the toxin-blocked area moved outward and innervated neighboring areas. In a minority of cases, no silent areas were noted and only the enlarged multiunit receptive fields were seen in the injection zone. All tecta injected with alpha BTX, but none injected with Ringer's, showed these disturbances in the map at the treated area at 6-8 days postinjection. By 2-3 wk, the maps in the alpha BTX-injected tecta had returned to normal. There were no silent areas and no enlarged receptive fields. The movement of the optic arbors was shown to be dependent on activity in the optic fibers. In fish receiving intraocular injections of tetrodotoxin (TTX) for the first 4-6 days, no changes were seen in the retinotectal maps recorded after the effects of the TTX wore off. When the entire surface of the tectum was blocked with multiple injections of alpha BTX, normal maps were recorded at 6-8 days postinjection. Thus in order to produce movements in optic terminals, the fibers require activity and an opportunity to make effective synapses in a nearby zone.