This collection presents the complete updates of a comprehensive three-part research series originally conducted at Max Planck Institute for Brain Research, Frankfurt (thesis 1981/thesis 1984), investigating chronic pain mechanisms using a novel tetanus toxin model. Part I establishes the behavioral and metabolic characteristics of localized tetany as a chronic pain model, demonstrating reversible muscle dysfunction with persistent pain behaviors that adapt over 2-3 weeks. Part II provides the first comprehensive temporal and regional analysis of Met- and Leu-enkephalin responses in six brain regions during chronic pain development and resolution. Results reveal rapid site-specific changes of endogenous opioid systems as the brain's primary analgesic response, with distinct functional roles for different enkephalin types. Part III examines further neurochemical adaptations through biogenic amine metabolism and cAMP concentrations in the same brain regions. Findings demonstrate coordinated multi-system responses that persist beyond behavioral recovery and primary enkephalin normalization, revealing extended phases of pain-related neurochemical adaptation. This research demonstrates that localized chronic pain produces widespread, specific bilateral brain changes across multiple neurotransmitter systems. The temporal analysis reveals distinct phases of neurochemical adaptation: immediate enkephalin responses, sustained biogenic amine changes, and persistent cAMP alterations. These findings remain relevant to current understanding of chronic pain as a complex neurobiological condition requiring multi-system therapeutic approaches. The series includes comprehensive literature integration spanning 50 years of pain research evolution, complete methodological documentation, and analysis of clinical implications for contemporary pain management and pharmaceutical development.