The pedunculopontine tegmental nucleus (PPTg) has an important anatomical position connecting basal ganglia and limbic systems with motor execution structures in the pons and spinal cord. It receives glutamatergic and GABAergic input and has additional reciprocal connections with mesencephalic dopaminergic neurons, suggesting that the PPTg plays a key role in frontostriatal information processing. In vivo microdialysis in freely moving rats, in combination with behavioral analysis, was used in this study to investigate whether the dopaminergic input can be modulated at the level of the PPTg via N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) or GABA(B) receptors. Stimulation of the GABA(B) receptor decreased dopamine release in the PPTg while that of the AMPA and NMDA receptors increased it. A time-related comparison of the effects of NMDA (0.75 and 1 mM) and AMPA (50 and 25 microM) revealed a more long-lasting effect after AMPA stimulation than after NMDA. However, only the infusion of the GABA(B) receptor agonist baclofen (100 and 200 microM) stimulated stereotyped behavior (e.g. sniffing, digging or head movements) and contralateral circling. This study clearly demonstrates that GABAergic as well as glutamatergic terminals in the PPTg are critically involved in the modulation of the dopamine system. Moreover, a decrease in PPTg dopamine via GABA(B) receptor stimulation seems to be behaviorally relevant.

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SummaryThe The reaction proceeded in excellent radiochemical yields of up to 94% within 5 min while showing good compatibility to many functional groups.

Over the last two decades, numerous attempts have been made to develop (11)C- and (18)F-labeled radiotracers in order to study glucocorticoid receptor (GR)-mediated abnormalities of the hypothalamus-pituitary-adrenocortical (HPA) axis function and regulation in vivo by means of positron emission tomography (PET). The present review addresses the research efforts dealing with the design, radiosynthesis and radiopharmacological evaluation of PET radiotracers for brain GR imaging. The underlying problems such as metabolic instability, insufficient blood-brain-barrier penetration and/or high non-specific binding will be discussed.