Fos expression in afferents to the rat midline thalamus following immobilization stress

The paraventricular thalamic nucleus (PVT), the most dorsal component of the thalamic midline, is known to be strongly activated following a variety of stressors and thus might be suggested to play a role as a relay for stress-related information targeted for viscerolimbic areas in the brain. This thalamic midline nucleus, however, lacks significant direct connections with the paraventricular hypothalamic nucleus (PVH), which is a key player in the hypothalamic-pituitary-adrenal (HPA) axis whose activation and subsequent glucocorticoid secretion are clearly crucial for homeostasis under 'stressful' conditions. The present study was designed to identify afferents of the PVT, which are activated by an immobilization stress, one type of the 'neurogenic' stress paradigms, using combined Fos immunohistochemistry and retrograde tracing experiments with cholera toxin B subunit. Dual immunohistochemistry revealed that immobilization stress induced expression of Fos immunoreactive nuclei was constantly observed in many regions of the neuraxis. Dually-labeled neurons in the cerebral cortex were mainly observed in the hippocampus, exclusively in the pyramidal layer of the caudal part of the ventral subiculum. In diencephalons a small number of dually labeled neurons was observed in the rostromedial zona incerta. In the midbrain, many of the retrogradely labeled neurons in the dorsal raphe nucleus were also immunoreactive for Fos protein. Mesencephalic periaqueductal gray contained a substantial number of dually labeled neurons. In the pons, the parabrachial nuclei, locus ceruleus, Barrington's nucleus and raphe nucleus contained only small numbers of dually labeled neurons. Within the medulla, nearly all of the retrogradely labeled neurons in the caudal part of the ventrolateral medulla were also immunoreactive for Fos antigen. Dually labeled neurons in the medulla were also observed in the nucleus of the solitary tract, exclusively in its commissural part. Given the known fact that most of the regions mentioned above provide important inputs to the HPA axis, our results suggest that a diencephalic network, presumably implicated in behavioral responses to given stress, might be activated by the parallel projection system that activate the HPA axis and might add some important insights to the understanding of animal and human stress-related HPA pathology.