Connections of some auditory-responsive posterior thalamic nuclei putatively involved in activation of the hypothalamo-pituitary-adrenocortical axis in response to audiogenic stress in rats: an anterograde and retrograde tract tracing study combined with Fos expression.

Prior studies in our laboratory demonstrated that part of the thalamus is necessary for activating the hypothalamo-pituitary-adrenocortical (HPA) axis in response to audiogenic stress in rats. The present studies were designed to determine how the auditory-responsive thalamic nuclei might activate the HPA axis. Both retrograde [Fluoro-Gold (FG)] and anterograde [Phasoleus vulgaris-leucoagglutinin (PHA-L) and biotinylated dextran amines (BDA)] tracers were employed to study the putative connectivity between the thalamus and the medial parvocellular region of the hypothalamic paraventricular nucleus (PAmp). In addition, rats receiving FG in the PAmp were subjected to audiogenic stress, and the distribution of both FG and the protein product of the immediate-early gene c-fos, Fos, were determined by double immunohistochemistry, to help assess putative functional links between the auditory-responsive thalamic nuclei and PAmp. The results of PAmp FG placement indicated retrogradely labeled cells in several areas, including the bed nucleus of the stria terminalis, hypothalamic regions, the supramammillary nucleus, some thalamic regions, and importantly, a few multisensory nuclei of the thalamus, including the parvicellular division of the subparafascicular and posterior intralaminar nuclei. Injections of the tracers PHA-L or BDA into these auditory-responsive posterior thalamic nuclei provided further evidence of projections to the PAmp. In addition, several forebrain areas were observed to receive moderate to heavy innervation. These areas included most of the regions described above, which, in turn, project to the PAmp. Because cells in the multisensory thalamic nuclei, hypothalamic, and forebrain areas were double labeled with FG and Fos, the results suggest that either direct projections from the thalamus to PAmp neurons, or indirect projections from the thalamus to stress-responsive forebrain areas projecting to the PAmp, might mediate activation of the HPA axis by audiogenic stress.