Two interconnected functional systems in the amygdala of amniote vertebrates

The amygdala shows ventropallial and lateropallial derivatives that can be compared among vertebrates according to their topological position, either superficial (cortical amygdala) or deep (basolateral amygdala and amygdalo-hippocampal area), connections and histochemical features. On the other hand, the subpallial amygdala, also called extended amygdala, is composed of medial and central divisions. In mammals, both divisions consist of an intra-amygdaloid portion and a part of the bed nucleus of the stria terminalis. In non-mammals, the intratelencephalic trajectory of the stria terminalis is short and both poles of the extended amygdala are close together. Like its mammalian counterpart, the medial extended amygdala of non-mammals receives an olfactory input (reduced in birds), projects to the medial hypothalamus and shows a sexually dimorphic vasotocinergic (vasopressinergic) cell group. Thus, the medial extended amygdala is part of the forebrain circuitry for the expression of defensive and reproductive behaviours. In turn, the central extended amygdala of amniotes shows a prominent CGRP innervation and a medially located CRF/neurotensin-expressing cell group, and projects to the lateral hypothalamus and to the midbrain and brainstem centres involved in fear/anxiety expression. The projections from the pallial amygdala to the central and medial extended amygdala are similarly organized in the mammals and non-mammals. These circuits, which have not changed substantially in birds despite the disappearance of the vomeronasal system, delineate two functional divisions within the amygdala that, together, orchestrate the expression of species-specific behaviours with a strong emotional component.