Afferents to the central nucleus of the amygdala and functional subdivisions of the periaqueductal gray: neuroanatomical substrates for affective behavior

Evidence suggests the periaqueductal gray (PAG) is involved in the integrat ion of behavioral and autonomic components of affective behavior. Our labor atory has shown that electrical stimulation of the ventrolateral periaquedu ctal gray (vl FAG) versus the dorsolateral periaqueductal gray (dl FAG), in the rabbit, elicits two distinct behavioral/cardiorespiratory response pat terns. Furthermore, evidence suggests that the amygdaloid central nucleus ( ACe) may influence cardiovascular activity during emotional states. The pur pose of this study was to delineate the topography and determine the origin of forebrain projections to the FAG and the ACe, as well as commonalties a nd differences in the pattern of afferents. Examination of common afferents may lend insights into their function as components of a forebrain system regulating autonomic activity during emotional states. Separate retrograde tracers were injected into functional subdivisions of the FAG and the ACe i n rabbits. FAG injections led to neuronal labeling in numerous cortical reg ions including the ipsilateral medial prefrontal and insular cortices. Addi tionally, bilateral labeling was observed in several hypothalamic nuclei in cluding the paraventricular nucleus, the dorsomedial nucleus and the ventro medial nucleus as well as the region lateral to the descending column of th e fornix. Sparse labeling was also seen in various basal forebrain regions, thalamic nuclei and amygdaloid nuclei. Many of these regions were also lab eled following injections in the ACe. Although double-labeled cells were ne ver observed, afferents to the ACe were often proximal to FAG afferents. Im plications of these findings are discussed in terms of two functionally dis tinct behavioral/cardiovascular response patterns. (C) 2000 Elsevier Scienc e B.V. All rights reserved.