Adrenergic innervation of forebrain neurons that project to the paraventricular thalamic nucleus in the rat

The paraventricular thalamic nucleus (PVT) lies in a pivotal position between the sensorium and a neural network involved in viscerolimbic integration. The aim of this study was to identify pathways used by adrenergic afferents to influence the outflow of the PVT. Potential disynaptic adrenergic projections to the PVT were investigated in chloral hydrate-anesthetized male Sprague-Dawley rats. PVT afferents were retrogradely labeled with cholera toxin B subunit on tissues processed with phenylethanolamine N-methyltransferase (PNMT) immunohistochemistry for displaying putative adrenergic innervation. In several regions of subcortical forebrain, PNMT-immunoreactive terminal-like varicosities were found to be closely associated with the soma and proximal dendritic segments of neurons retrogradely labeled from the PVT. These cell groups formed two topographically organized projection systems. The lateral telencephalic system was composed of a cell continuum formed by the central nucleus of amygdala, sublenticular substantia innominata and bed nucleus of the stria terminalis. The medial diencephalic system included the lateral hypothalamic area, perifornical nucleus, dorsomedial and periventricular hypothalamic nuclei. Adrenergic neurons in the medulla oblongata may modulate the activity of midline thalamic circuit neurons implicated in behavior.