NUCLEUS ISTHMI IN GOLDFISH - INVITRO RECORDINGS AND FIBER-CONNECTIONSREVEALED BY HRP INJECTIONS

Recordings of field potentials in nucleus isthmi (NI) were obtained in an in vitro preparation of goldfish brain using a lateral approach. H orseradish peroxidase (HRP) was injected from recording electrodes to verify recordings within the nucleus and to label axonal pathways and cell bodies. Activity in NI was repetitive and could be elicited by st imulation of the optic nerve, tectum, pretectum, or tectobulbar tract. Spontaneous activity was present in some preparations and consisted o f bursts with intervening silent periods. Anatomical and electrophysio logical evidence indicated that the primary isthmotectal pathway is co mposed of fine fibers that exit NI rostrally and pass through pretectu m to enter tectum rostrally. An afferent pathway consisting of both fi ne- and large-diameter fibers entered NI ventromedially; the large dia meter axons have been previously reported in percomorph fishes, but we re not thought to be present in cyprinids such as goldfish. The large diameter axons arise from labeled cell bodies in the region of the lat eral thalamic nucleus. No labeled cell bodies were seen in ipsilateral nucleus pretectalis superficialis, pars magnocellularis, where they a re seen in percomorphs. The fine axons, which have not been reported i n percomorph fishes, were shown to arise from tectal bipolar (type VI) neurons. As in percomorphs, tectal type XIV neurons were also labeled . This and corroborating recordings from nucleus isthmi constitute the first demonstration of a tectoisthmic projection in a cyprinid fish.