Responses of the Optic Tectum to Telencephalic Stimulation in Catfish

Abstract

In order to test physiologically for cerebrotectal connections in a fish, averaged evoked potentials and unit responses were recorded from the optic tectum following electrical stimulation applied to the telencephalon in the siluroid teleost Ictalurus nebulosus. A single shock applied to the area dorsalis centralis (Dc) of the telencephalon, and only to this area, elicits a sequence of deflections in the ipsilateral optic tectum: an initial negative peak at about 8 ms, (= N8), a larger N25 and a slow P50-N95. The configurations, depth profiles, latencies and susceptibility to repetitive stimulation, together with the known tectal anatomy, suggest that the first wave is due to the afferent fibers from the telencephalon and that N25 is due to deep tectal neurons. Telencephalic input exerts a conditioning effect on the field potentials and unit responses evoked by direct optic nerve shock. Such a shock elicits, in the contralateral tectum, small negative, optic tract axon peaks followed by a large N6, believed to be postsynaptic, and a still later P12. As a first approximation it is argued that the telencephalic input and the retinal input are activating different sets of neuronal elements in the optic tectum, since the configuration and depth profile of the telencephalic and optic nerve shock-elicited potentials are different. A conditioning Dc stimulus has a long-lasting effect on the form of the optic nerve field potential, maximally when the pallial shock precedes the optic by about 90 ms. The effect, observed by subtracting the conditioned from the unconditioned tectal response to optic nerve shock, is a difference wave with N11 and P20. The unit activity from deep tectal laminae is either activated or accelerated following Dc stimulation, while superficially located neurons are not affected. In another group of tectal units, the optic nerve shock-induced response is depressed by a preceding pallial dorsalis centralis stimulus. The evidence is compatible with the assumption of direct projections from Dc to the deep layers of the tectum, but the timing could also permit indirect pathways. In any case, the influence is not simple or identical for different tectal cell classes.

Experimental study of the connections of the telencephalon in the rainbow trout (Oncorhynchus mykiss). II: Dorsal area and preoptic region

“…In Ictalurus, both the "medial zone of the dorsal telencephalon" (which in fact corresponds to Dc) and the entopeduncular nucleus have been reported as candidates for the origin of the telencephalo-tectal projections (Northcutt, 1982;Schlussman et al, 1990). Functional and electrophysiological studies also support the involvement of Dc and Dl in visual roles (Lee and Bullock, 1990;Saidel et al, 2001). Our results in trout show that the telencephalic afferents to the optic tectum are labeled only after DiI application to Dc and that this projection is restricted to the rostral half of the optic tectum, as indicated by our reciprocal experiments.…”

Afferent connectivity to different functional zones of the optic tectum in goldfish

“…A projection from the telencephalon to the optic tectum has been identified in different species of fish in both anatomical (Ito & Kishida, 1977;Grover & Sharma, 1981;Luiten, 1981;Northcutt, 1982;Smeets, 1982;Fiebig et al, 1983;Schlussman et al, 1990;Yamane et al, 1996;Correa et al, 1998) and physiological studies (Lee & Bullock, 1990). A noticeable result is that the telencephalic dorsocentral region of goldfish projected to the anteromedial tectal zone.…”

Afferent Connections of the Optic Tectum in Lampreys: An Experimental Study

“…Direct projections from the striatum to the OT were experimentally demonstrated in holostean [Northcutt, 1982] and teleostean fishes [Ito and Kishida, 1977;Grover and Sharma, 1981;Lee and Bullock, 1990;Schlussman et al, 1990], and in amphibians [Finkenstädt et al, 1983;Rettig, 1988;. In amphibians, the striatal afferents terminate primarily within the periventricular layers (4-8) of both sides of the OT, with the contralateral fibers decussating in the posterior and tectal commissures .…”