Cells in the posterior thalamus project to both amygdala and temporal cortex: A quantitative retrograde double-labeling study in the rat

Auditory information from the posterior thalamus reaches the lateral nucleu s of the amygdala (LA) by way of two pathways: a direct thalamo-amygdala pr ojection and a polysynaptic thalamo-cortico-amygdala projection. However, t he quantitative extent of thalamic neurons that project to the LA or to the auditory association cortex (AAC) is not known. Furthermore, the extent an d topographical distribution of bifurcating cells that project to both LA a nd AAC are also unknown. Therefore, separate tracers were injected into LA and either into all of AAC or within discrete regions of AAC, such as tempo ral areas TE3 or perirhinal cortex (PRh), and quantitative analyses were pe rformed on labeling within the subregions of the auditory thalamus in rats. Following LA injections, retrogradely labeled cells were most numerous in the posterior intralaminar nucleus (PIN; 48.0% of all labeled thalamic cell s), whereas labeled cells following injections of the entire AAC were most numerous in the dorsal division of the medial geniculate nucleus (MGd; 32.9 % of all labeled thalamic cells). Following AAC injections localized to onl y TE3, the MGd again had the majority of labeled cells (35.9%), whereas fol lowing AAC injections localized to PRh, the PIN had the most labeled cells (32.8%). Double-labeled cells were found in all the thalamic regions studie d and were most commonly observed in the PIN (43.7% of all double-labeled c ells following injections into LA and throughout the AAC). The percentage o f double-labeled cells as a proportion of either LA-projecting or AAC-proje cting cells varied among the thalamic nuclei studied, ranging from 2.9% up to 42.4%. The topographic distribution of double-labeled cells in the thala mic nuclei resembled that of single-labeled cells following LA injections m ore than single-labeled cells following AAC injection. These findings sugge st that double-labeled cells contribute substantially to many of the direct thalamo-amygdala and indirect thalamo-AAC-amygdala projections. Among othe r functions, these bifurcating cells may help regulate the processing of in put to the LA arriving from these two pathways to allow for certain types o f plasticity in the LA during fear conditioning. J. Comp. Neurol. 425: 257- 274, 2000. (C) 2000 Wiley-Liss, Inc.