Early in the development of Xenopus laevis tadpoles, as the animals become capable of actively navigating their environment, individual neurons in the optic tectum become cross-modal. That is, they receive information both from the eye, via retinal ganglion cell axons, and also from mechanosensory nuclei in the hindbrain. At present it is unknown how, or even whether, these two modalities interact in these young tectal neurons and in the tectal circuit, generally. To begin to address these questions, I here utilize an isolated-brain preparation to stimulate these afferent pathways and record, from single cells, either the excitatory and inhibitory synaptic inputs each receives, or output that each generates upon activation with cross-modal stimulus combinations, as well as with uni-modal (within-modality) combinations. Additionally, to investigate how these relationships might change over a developmental epoch characterized by extensive experience-dependent plasticity, I collect data from two groups: stages 44–46 and stages 48–49. My results show that cross-modal stimuli do indeed interact in individual neurons of the developing tectum, such that the magnitude (i.e., total number) and onset latency of responses are both dependent on inter-stimulus interval. Furthermore, the data show a selective sensitivity of these responses for cross-modal combinations, which is developmentally-regulated. Critically, although the pharmacological blockade of inhibition abolishes this differential integration of cross-modal and uni-modal combinations, no differences are seen between cross-modal and uni-modal effectiveness in the enhancement of synaptic inhibition, or excitation, at any stage of development. Additional experiments show a developmentally-regulated increase in the extent of the recurrent, intra-tectal connections that are activated by cross-modal combinations, however. These results thus indicate a mechanism for cross-modal sensitivity that is more nuanced than the simple balance between excitation and inhibition, and illustrate important roles for synapse location and dendritic integration.
Felch, Daniel L.,
"Cross-Modal Interactions in the Optic Tectum of Xenopus laevis Tadpoles"
(2015).
Neuroscience Theses and Dissertations.
Brown Digital Repository. Brown University Library.
https://doi.org/10.7301/Z05719DH
