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Absence of long-term potentiation in the retinotectal synaptic region of the adult rat superior colliculus

University of British Columbia

To answer whether the mammalian retinotectal pathway is modifiable in the adult, an attempt was made to induce long-term potentiation (LTP) in retinal synapes in the superior colliculus (SC) of the adult rat, in vivo. Extracellular field potentials were recorded in the primary retinotectal afferent zone of the rat superior colliculus while electrically stimulating the optic chiasm. Induction of LTP in this primary visual pathway was attempted using a wide range of stimulus parameters. However, LTP was not observed. Iontophoretic application of bicuculline methiodide, before and during trains of stimuli, did not facilitate LTP in the rat SC. The broad spectrum glutamatergic antagonist, kynurenic acid, greatly reduced the size of the field potentials. This supports suggestions that retinotectal neurotransmission may be mediated by excitatory amino acids. An N-methyl-D-aspartate (NMDA) glutamate receptor mediated contribution to synaptic transmission in the evoked field potential was not evident. Iontophoretic application of the NMDA receptor selective antagonist 2-amino-5-phosphonovaleric acid (APV) had no effect on the field potentials. Even in the presence of bicuculline, there was no evidence for an NMDA component in the field potential response. The non-NMDA glutamate receptor antagonist, 6-cyano-7-nitroquinoxa-line-2,3-dione (CNQX), did not affect the evoked potentials. These data suggest that LTP was not observed in the retinotectal pathway due to several factors that may include: a loss of visual plasticity in the adult rat following the critical period, absence of necessary modulation factors and insufficient NMDA receptor mediated synaptic transmission.

Text

2010-11-08

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http://hdl.handle.net/2429/29865

Neuroscience

University of British Columbia

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