| Projections to the amygdala from the auditory thalamus have been implicated in the associative conditioning of fear responses to acoustic stimuli. In rats, auditory information from the thalamus reaches the lateral nucleus of the amygdala (LA) via two pathways: a direct thalamo-amygdala projection and a polysynaptic thalamo-cortico-amygdala projection. However, many details about the nature of these two pathways are not well understood.; First, it is not known whether these thalamo-LA projections terminate in a topographic fashion within LA. In the first study, therefore, we injected several retrograde tract tracers into various locations within LA to determine whether the terminations of thalamo-amygdala fibers have a topographic organization within LA. Results indicate that some of the functional segregation in the thalamus may be preserved in LA, with particular distinctions between projections to anterior vs. posterior LA. Furthermore, surprisingly substantial projections were found between certain thalamic nuclei and LA, which suggests that the roles of these nuclei in thalamo-amygdala transmission should be reconsidered.; Second, the quantitative extent of thalamic neurons that project to LA or to cortex is not known. In addition, the extent and topographical distribution of bifurcating cells that project to both LA and cortex are also unknown. In the second study, therefore, separate tracers were injected into LA and cortex and quantitative analyses were performed on labeling within subregions of the auditory thalamus. Findings indicate the quantitative extent of various thalamo-LA and thalamo-cortex projections, and furthermore that double-labeled cells contribute substantially to many of the direct thalamo-amygdala and indirect thalamo-AAC-amygdala projections. These bifurcating cells may help regulate the processing of input to LA arriving from these two pathways to allow for certain types of plasticity in LA during fear conditioning.; Third, little is known about the physiology of the auditory cortical areas that project to LA. In the third study, therefore, electrophysiological recordings were made from single units in regions of the rat cortex. The existence of multiple auditory cortical fields was verified and response properties were characterized, particularly from primary auditory cortex and the posterior field. Findings were consistent with auditory cortical properties in other mammals. |
