Most of the studies described in this monograph were carried out in the Department of Anatomy, Histology and Embryology at Thracian University in Stara Zagora, Bulgaria. Some of the in situ hybridization experiments were performed at the Department of Anatomy and Cell Biology, University ofUlm, Germany and the tracer experiments were conducted at the Department ofPhysi ology, Leiden University, The Netherlands. In 1992, we started elucidating the ultrastructural organization and synaptic connections of the mesencephalic trigeminal nu of its neuronal po- cleus as well as the related chemical coding 1ation, the putative neurotransmitters utilized by MTN neurons and their possible sources. In the years to come, painstaking research was focused on this subject and we were able to identify certain neuroactive substances in this unique nucleus, including classical and peptide transmitters. I spent the period of 1995-1997 in Germany as a fellow of the Alexander von Humboldt Foundation, at the department headed by Prof. Dr. Christof Pilgrim. During this stretch of time, I ac quired the in situ hybridization technique. It allowed me to expand my investigations and detect the expression of dopamine receptors and newly synthesized neuropeptides in MTN neurons after peripheral injury of their axons, not only at protein but also at mRNA levels.

1 Introduction

2 Materials and Methods
2.1. Animals
2.2 Electron Microscopy
2.3 Immunocytochemical Experiments
2.4 In Situ Hybridization Histochemistry
2.5. Peripheral Nerve Axotomy
2.6 Data and Image Analysis
3 Results
3.1 Normal Morphology of the Mesencephalic Trigeminal Nucleus
3.2 Neurochemistry of the Mesencephalic Trigeminal Nucleus
4 Discussion
4.1 Structural and Synaptic Organization of the Mesencephalic Trigeminal Nucleus
4.2 Connections of the Mesencephalic Trigeminal Nucleus
4.3 Neurochemistry of the Mesencephalic Trigeminal Nucleus
4.4 Neurochemical Plasticity of Mesencephalic Trigeminal Nucleus Neurons
4.5 Functional Considerations
4.6 Conclusions and Future Directions
5 Summary
References.