Connections of the amygdala of the rat. IV: Corticoamygdaloid and intraamygdaloid connections as studied with axonal transport of horseradish peroxidase.

The corticoamygdaloid and intraamygdaloid projections of the rat were studied by the use of retrograde transport of horseradish peroxidase (HRP). Observations based on anterograde transport of the enzyme were exploited to determine the course of the intrinsic connections. The HRP was injected stereotactically by means of iontophoresis. Most of the amygdaloid nuclei were selectively injected, and all but a few were reached by more than one approach. The vast majority of corticoamygdaloid fibers was found to originate in cortical areas defined as allocortical (Stephan, '75). From the medial frontal cortex the central amygdaloid nucleus (AC) receives a hitherto undescribed projection originating in the tenia tecta; and both the AC and the lateral amygdaloid nucleus (AL) receive fibers from the prelimbic and infralimbic areas. The anterior cingulate area entertains a weak connection with the basolateral amygdaloid nucleus (BL). As to the insular cortex, the posterior agranular insular area projects to all amygdaloid subdivisions; the BL, AC, and the anterior cortical nucleus (COa) receive, in addition, fibers from the ventral agranular area. The prepyriform cortex connects with the entire amygdala except the medial nucleus (Am). The amygdala receives afferents from a transitional area between the amygdala and the entorhinal area. The entorhinal area proper is related to the amygdala via projections from the ventral part of the lateral entorhinal area to the AL and from the dorsal part of the lateral entorhinal area to the BL. The former nucleus also receives fibers from the perirhinal region. Additional amygdalopetal connections from the hippocampal region include a previously undescribed projection from the temporal two-thirds of CA1 to the AL and BL and to the posterior cortical nucleus (COp) with the adjacent periamygdaloid cortex (PAC). The subiculum projections to the AL, and more modestly to other amygdaloid nuclei. There is an extensive network of intraamygdaloid connections, the Am and AC being the only nuclei not giving rise to the intrinsic fibers.