Organization of projections to the lateral amygdala from auditory and visual areas of the thalamus in the rat

Projections to the amygdala from the auditory thalamus have been implicated in the associative conditioning of fear responses to acoustic stimuli. Tha lamo-amygdala auditory projections enter the amygdala via the lateral nucle us (LA). It is well documented that these projections originate in the medi al division (MGm) of the medial geniculate nucleus (MGN), the posterior int ralaminar nucleus (PIN), and the suprageniculate nucleus (Sg). It is not kn own, however, whether these thalamic projections terminate in a topographic fashion within the LA. We therefore used several retrograde tract tracing techniques to determine whether the terminations of thalamo-amygdala fibers have a topographic organization within the LA. These tracers were injected into various locations within the LA, and the distribution of the retrogra dely labeled cells throughout the thalamus was analyzed. In general, rostra l to caudal distinctions in the thalamus are maintained in the LA, such tha t projections from throughout the MGN terminate in the anterior part of the LA, whereas the caudal part of the MGN projects to the caudal part of the LA. Furthermore, the density of cells that give rise to thalamo-amygdala pr ojections varies within each thalamic nucleus along the rostro-caudal axis. The patterns of thalamo-amygdala connectivity observed support previous pa rcellation schemes that segregate the LA into dorsal, medial, and lateral a reas, and suggest that the LA should be further divided into anterior and p osterior parts. In addition to the well-known projections to the LA origina ting from PIN, MGN, and Sg, we also found substantial projections from the dorsal portion of the MGN (MGd) and the lateral posterior thalamic nucleus (LP). These findings suggest that some of the functional segregation in the thalamus may be preserved in the LA, and that the role of the MGd and LP i n thalamo-amygdala transmission should be reconsidered. (C) 1999 Wiley-Liss , Inc.