Morphological and electrophysiological evidence for habenular influence on the guinea-pig pineal gland

The central innervation of the guinea-pig pineal gland was investigated by histological and electrophysiological methods: Staining the pineal gland and the epithalamus, a double route of central innervation could be shown in the anterior part of the organ: (a) Fibres from the habenular nuclei, mainly from the lateral part, penetrate the organ via the pineal stalk. (b) Other fibres join the striae medullares and running in the habenulae reach the organ more dorsally. The fibres end in the intercellular space where they form a dense network. In 15 male guinea-pigs under urethane anesthesia, two series of unit recording experiments were performed: (a) Recordings were made from 128 units in the posterior and anterior part of the pineal gland and the effects of electrical stimulation of the habenular nuclei were observed. Lateral habenular stimulation influenced 44% of the units. 80% of these were excited and 20% were inhibited. (b) Recordings were made from 42 units in the lateral habenular nucleus. Twelve units (29%) responded with an augmentation of spontaneous activity following pineal gland stimulation. No inhibition response was observed.