Dopaminergic (DA) neurons exhibit a short-latency, phasic response to unexpected biologically salient stimuli, including rewards. Despite extensive research on this DA signal, very little is known about the sources of sensory information reaching DA neurons. Previous research has identified the superior colliculus (SC) as the primary, if not exclusive route of short latency visual input to DA neurons. However, more recent research has suggested that the phasic DA response comprises two components; a short latency (50-110 ms), stimulus insensitive component, and a longer latency component (110-260 ms) that can reflect complex stimulus characteristics including reward value – more complex than might arise from intrinsic collicular processing. A solution to this apparent paradox may be suggested by recent studies that have demonstrated longer latency colour related responses in SC neurons. As the SC does not receive direct retinal input from colour sensitive cells, but it does receive input from a wide range of cortical structures, it is possible that cortical activation might underlie longer latency responses in the SC, which may in turn underlie longer latency responses in DA neurons. The aim of the research presented in this thesis was to investigate whether the cortex was capable of modulating the activity of DA neurons, and whether the SC was the relay for this cortical influence. In the anaesthetised rat, single pulse electrical stimulation of the barrel field of the primary somatosensory cortex (S1Bf) produced a short latency, short duration response in the SC, but DA neurons were largely insensitive to the stimulus. After disinhibition of the SC with the GABAA antagonist bicuculline, responses in the SC to S1Bf stimulation were enhanced, and DA neurons became responsive to S1Bf stimulation, suggesting that the SC is the route of cortical input to DA neurons. This was confirmed in the subsequent experiment. Responses were produced in DA neurons without the need for SC disinhibition by stimulating S1Bf with a high frequency train of pulses. This response in DA neurons was suppressed or eliminated by suppressing SC activity. Finally, the contribution of cortical and subcortical input to DA neuron responses was examined by stimulating the trigeminal nucleus. Trigeminal stimulation produced responses in the SC comparable to multiwhisker deflection, and produced responses in almost all DA neurons. Disinhibition of the SC differentially modulated phases of the SC response previously demonstrated to be produced by trigeminal and cortical input, and differential changes were seen in initial and later components of DA neuron responses, which were often associated with changes in the SC response. The results of these studies suggest that cortical inputs to the SC could provide a mechanism through which responses are produced in DA neurons that can reflect complex stimulus attributes. However, research in this thesis and elsewhere suggests that the activity of DA neurons is insufficiently discriminatory to reflect the full range of potentially rewarding stimuli, and hence it is suggested that DA neurons provide a salience signal, which can be biased by a pre-saccadic estimate of previously established reward value, but which does not communicate reward value per se.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:557483 |
| Date | January 2011 |
| Creators | Bertram, Craig |
| Contributors | Overton, P. G. ; Redgrave, P. |
| Publisher | University of Sheffield |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://etheses.whiterose.ac.uk/2262/ |
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