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.

This thesis explored whether interpretations of the reinforcing effect of stimulation trains used in the self-administration of brain-stimulation (SABS) paradigm were artifacts of the reinforcement schedule chosen or whether it represented a genuine attempt by animals to maintain optimal levels of reward. Results demonstrate that stimulation trains used in SABS are reinforcing and that animals regulate pulse frequency to optimize the level of reward. The thesis then explored whether pedunculopontine tegmental nucleus (PPTg) lesions blocked the acquisition or maintenance of SABS, and the acquisition of eight-arm radial maze learning. Results showed that lesions confined to the PPTg block acquisition and maintenance of SABS, suggesting that the PPTg mediates the positive reinforcing effects of BSR. Further, PPTg lesions blocked win-shift and win-stay radial maze learning. However, results indicate that animals were not impaired in "shifting" or "staying" behavior. It is speculated that PPTg lesions block the reinforcing effects of food, which produce inefficient performance on both memory tasks.

The role of the pedunculopontine tegmental nucleus (PPTg) in motivation and cognitive functions is controversial. In order to clarify the involvement of this nucleus in learning, rats with N-methyl-D-Aspartate (NMDA) lesions to the PPTg were tested on the acquisition of a delayed non-matching to position task (DNMP) performed in a T-maze. Unlike sham-lesioned rats, animals with PPTg lesions did not learn the task. Analysis of the behavior displayed by the lesions animals, however, suggested that these rats suffered from elevated emotionality or arousal, rather than from learning deficits. This hypothesis was confirmed by demonstrating that the anxiolytic compound diazepam (1 mg/kg) normalized the performance of the PPTg-lesioned rats on the DNMP task and reduced the indices of anxiety displayed by animals with PPTg lesions when tested on the elevated plus maze. / These results suggested the possibility that the motivational impairments reported to be induced by PPTg lesions, could also be an artifact of lesion-induced elevation of anxiety or arousal. Thus, in order to verify this hypothesis, it was tested whether diazepam would modify the expression of conditioned place preference (CPP) to morphine and amphetamine in animals with NMDA-induced lesions to the PPTg. Diazepam reversed the effects of the lesion on a morphine CPP but not on an amphetamine CPP. A series of experiments, aimed at characterizing the effects of diazepam on morphine and amphetamine reinforcement in normal rats, showed that diazepam, either systemic or injected in the nucleus accumbens, blocks the reinforcing effects of amphetamine but has no effect on the reinforcing effects of morphine. These results suggest that impairments in CPP learning caused by PPTg lesions do not result from motivational deficits, but are caused by elevated emotionality or abnormal arousal.

No description available.

No description available.