Direct projections to the rat pineal gland via the stria medullaris thalami. An anterograde tracing study by use of horseradish peroxidase.

The possible presence of a direct nervous projection from the paraventricular nucleus (PVN) of the hypothalamus to the pineal gland of the rat was investigated by means of the anterograde neuron-tracing method using horseradish peroxidase. The tracer was injected unilaterally into the PVN and the animals were allowed to survive between 12 and 26 h. Numerous peroxidase-positive fibers were observed, ipsilateral to the injection site, in the stria medullaris thalami and could be followed into the medial habenular nucleus and the habenular commissure. From there, fibers penetrated into the deep pineal gland (lamina intercalaris), and further into the pineal stalk. These data support results of previous investigations describing retrograde labeling of the PVN following intrapineal injections of horseradish peroxidase and are in accordance with recent experiments demonstrating an influence of the PVN on electrical and biochemical activity of the pineal gland.