Projection patterns from the amygdaloid nuclear complex to subdivisions of the dorsal raphe nucleus in the rat

The goal of the present study was to identify the projection from the subdivisions of the amygdaloid nuclear complex to specified subregions of the dorsal raphe (DR) nucleus and to attempt to compare the density of amygdaloid input to the DR with that of inputs from other limbic structures. Use of a retrograde tracer, gold-conjugated and inactivated wheatgerm agglutinin-horseradish peroxidase (WGA-apo-HRP-gold), demonstrated that amygdaloid input to midline DR subdivision originates mainly from the medial portion of the medial amygdaloid nucleus, whereas that to lateral wing subdivision derives from the region extending from the lateral portion of the medial amygdaloid nucleus to the commissural stria terminalis. Use of the retrograde tracer Fluorogold (FG) produced relatively large but circumscribed injection sites comprising midline DR as well as portions of lateral wing subdivision and confirmed that the medial amygdaloid nucleus provides the major input to the DR. We also demonstrated that although amygdaloid input was not as extensive as inputs from other limbic structures such as the medial prefrontal cortex or the lateral habenular nucleus, it was comparable to input from the lateral septal nucleus. Based on these observations, we suggest that the medial amygdaloid nucleus provides substantial input to the DR and may contribute an emotional influence on sleep–wakefulness cycle or pain–stress modulation. Furthermore, it seems that the medial amygdaloid–DR projection might be anatomically and functionally distinct from the well-characterized central amygdaloid–periaqeductal gray (PAG) circuit which is essential for conditioned fear.