Developing the theory of the extended amygdala with the use of the cupric-silver technique

The amygdaloid complex is a crucial component of the basal forebrain that participates in the modulation of many homeostatic functions, emotional behaviors, and learning. These features require a widespread pattern of connections with several brain structures. In the past, the amygdaloid complex was divided into corticomedial and basolateral groups. The existence of a neuronal continuum linking the central amygdaloid nucleus to the lateral bed nucleus of stria terminalis through the subpallidal area was first revealed by José de Olmos (1932-2008) with the aid of his cupric-silver technique. This observation gave birth to the concept of the extended amygdala, a conceptual framework that is useful for understanding the anatomofunctional organization of the amygdaloid complex, with relevance for basic neuroscience and clinical interventions. Traditional tract-tracing staining methods were complicated and tedious to reproduce. Axonal terminal endings were lost among a myriad of normal fibers. The need to visualize these terminals drove de Olmos to develop cupric-silver methods that revealed disintegrating synaptic terminals, without staining normal fibers. In this article, we describe the historical events leading to the development of the cupric-silver technique that evolved into the amino-cupric-silver technique, which developed hand-in-hand over the years.