Differential intertectal delay between Rana pipiens and Xenopus laevis: Implications for species-specific visual plasticity

Abstract

In the frog Xenopus laevis, the isthmotectal projection, which relays input from the ipsilateral eye, exhibits anatomical reorganization following surgical eye rotation performed during tadpole stages while the isthmotectal projection in the frog Rana pipiens fails to show reorganization. This plasticity has been shown to be dependent upon activation of the N-methyl-D-aspartate (NMDA) receptor located on tectal cell dendrites. The reorganization process in Xenopus is hypothesized to employ a Hebbian mechanism requiring correlated firing of ipsilateral and contralateral inputs to a given tectal cell; when an ipsilateral axon synapses onto a tectal cell that receives input from a contralateral axon with a matching receptive-field location, the correlation in activity triggers stabilization of the ipsilateral synapse. However, in neither Xenopus nor Rana do ipsilateral and contralateral inputs begin to fire simultaneously in response to a given visual stimulus; the ipsilateral input is delayed because it reaches the tectum indirectly, through a polysynaptic relay via the opposite tectum and nucleus isthmi. The objective of this experiment was to test whether there is a significant difference in this intertectal delay between Xenopus laevis and Rana pipiens in order to determine whether intertectal delay could be a contributing factor in this species-specific ability to exhibit visual plasticity. We have found that intertectal delay is 26.16 ms longer in Rana pipiens (36.53 ms) than in Xenopus laevis (10.37 ms).

Neurotrophins, but not depolarization, regulate substance P expression in the developing optic tectum

“…Depolarization of tectal cells by non-NMDA receptors in response to retinal ganglion cell activity is important to the relay of tectal activity from one tectal lobe to the other through another midbrain structure, the nucleus isthmi (Scherer and Udin, 1995). The nucleus isthmi receives a projection from the ipsilateral tectal lobe, and in the adult frog, sends cholinergic projections to both the contralateral and ipsilateral tecta (Desan et al, 1987;Wallace et al, 1990).…”