The development and neuromodulation of motor control systems in pro-metamorphic Xenopus laevis frog tadpoles
Abstract
My thesis has accomplished 3 significant contributions to neuroscience. Firstly, I
have discovered a novel example of vertebrate deep-brain photoreception.
Spontaneously generated fictive locomotion from the isolated nervous system of
pro-metamorphic Xenopus tadpoles is sensitive to the ambient light conditions,
despite input from the classical photoreceptive tissues of the retina and pineal
complex being absent. The photosensitivity is found to be tuned to short
wavelength UV light and is localised to a small region of the caudal diencephalon.
Within this region, I have discovered a population of neurons immuno-positive for
a UV-specific opsin protein, suggesting they are the means of phototransduction.
This may be a hitherto overlooked mechanism linking environmental luminance to
motor behaviour. Secondly, I have advanced the collective knowledge of how both
nitric oxide and dopamine contribute to neuromodulation within motor control
systems. Nitric oxide is shown to have an excitatory effect on the occurrence of
spontaneous locomotor activity, representing a switch in its role from earlier in
Xenopus development. Moreover, this excitatory effect is found to be mediated in
the brainstem despite nitric oxide being shown to depolarise spinal neurons.
Thirdly, I have developed a new preparation for patch-clamp recording in pro-metamorphic Xenopus tadpoles. My data suggest there are several changes to the
cellular properties of neurons in the older animals compared with the embryonic
tadpole; there appears to be an addition of Ih and K[sub](Ca) channels and the presence
of tonically active and intrinsically rhythmogenic neurons. In addition, I have
shown that at low doses dopamine acts via D2-like to hyperpolarise the membrane
potential of spinal neurons, while at higher doses dopamine depolarises spinal
neurons. These initial data corroborate previously reported evidence that
dopamine has opposing effects on motor output via differential activation of
dopamine receptor subtypes in Xenopus tadpoles.
Type
Thesis, PhD Doctor of Philosophy
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