The pedunculopontine nucleus (PPN) is a brainstem structure involved in motor control, sleep and arousal. The boundaries of the PPN are defined by its cholinergic neurons, but it also contains GABAergic, glutamatergic and calcium-binding protein- positive neurons. To further understand the physiological roles of the PPN it is necessary to understand which neuronal subtypes are present in the PPN and how they are connected with other regions of the brain in normal and pathological conditions. In order to address these issues, the total numbers, distributions and neurochemical phenotypes of neurons, positive for the calcium-binding proteins calbindin and calretinin, were studied in the rat PPN. Sagittal, perfuse-fixed rat brain sections were double or triple-immunolabelled to reveal the cholinergic marker choline acetyltransferase (ChAT) with calbindin and/or calretinin. A stereological approach revealed that calbindin- and calretinin-positive neurons account for a large proportion of PPN neurons, but they rarely eo-express ChAT. A combination of immunolabelling for calbindin or calretinin with in situ hybridisation for GAD65/67 or VGluT2 mRNAs revealed that about one third of the calbindin- and calretinin-expressing neurons are GABAergic and preferentially located in the rostral PPN, whereas approximately two thirds are glutamatergic and principally located in the caudal PPN. Additionally, retrograde tracer injections in the subthalamic nucleus (STN) and the gigantocellular nucleus (GiN) showed that the majority of PPN neurons, projecting to one or both of these nuclei, were not cholinergic (70-90%). Less than 10% of STN-projecting neurons expressed calbindin or calretinin and 5% of the GiN-projecting neurons contained calretinin but none contained calbindin. Finally, the expression of the immediate early gene, Egrl, a marker of neuronal activation, was evaluated in STN- and GiN-projecting neurons of the PPN in control and 6-0HDA lesioned animals. No statistically significant differences, in the number of Egr l-positive neurons, were observed between control and 6- OHDA lesioned animals. These findings show that calbindin- and calretinin-positive neurons are abundant in the PPN, heterogeneously distributed and display a GABAergic or glutamatergic phenotype. Additionally, calbindin- and calretinin-positive neurons represent only a minority of the PPN neurons projecting to either the STN, GiN or both nuclei. Results also suggest that the hyperactivity seen in the PPN in the 6-0HDA model of Parkinson's disease may not necessarily be due to the neurons projecting to the STN and/or GiN. Overall, this thesis supports the notion that the PPN is composed of a rich diversity of neuronal cell-types, which are heterogeneously distributed along its rostro-caudal axis. The heterogeneous neurochemistry, connectivity and physiology of these neurons allow the PPN to influence a wide range of brain regions through a variety of pathways presumably underlying its various functional roles.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:559803 |
| Date | January 2012 |
| Creators | Martínez González, Cristina |
| Contributors | Bolam, J. Paul ; Mena-Segovia, Juan |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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