Retrograde transport of nerve growth factor from hippocampus and amygdala to trkA messenger RNA expressing neurons in paraventricular and reuniens nuclei of the thalamus.

We previously reported the presence of trkA messenger RNA expressing non-cholinergic neurons in the paraventricular anterior and reuniens nuclei, which are located in the thalamic midline. In the present study, retrograde labeling with iodinated (125I) nerve growth factor was used to identify the innervation target of these cells. Neurons in the paraventricular anterior and reuniens nuclei were labeled following injection of iodinated nerve growth factor into amygdala and hippocampus, but not into nucleus accumbens and entorhinal cortex, the two other main areas receiving strong innervation from the thalamic midline. Target ablation of hippocampus or amygdala failed to down-regulate trkA messenger RNA expression in the two thalamic nuclei, thus suggesting a role for nerve growth factor different from a critical survival factor. The thalamic paraventricular anterior and reuniens nuclei are part of the reticular formation which plays a role in general cortical activation, behavioral arousal and control of awareness. Retrograde transport of nerve growth factor by trkA messenger RNA expressing neurons in these nuclei suggests a physiological role of this trophic factor in the function of these cells.