Demonstration of a reciprocal connection between the periaqueductal gray matter and the reticular nucleus of the thalamus.

Lectin-conjugated horseradish peroxidase was either injected or implanted in crystalline form in various parts of the periaqueductal gray substance in the cat. After survival times ranging between 24 and 48 h the animals were fixed, and the mesencephalon and thalamus were sectioned and processed for peroxidase histochemistry, using tetramethylbenzidine as the chromogen. Light microscopic examination of the sections revealed that there exists a prominent reciprocal connection between the ventral and lateral parts of the periaqueductal gray matter on one hand, and the reticular nucleus of the thalamus on the other. The connections are mainly ipsilateral, and involve the entire rostrocaudal extent of the thalamic reticular nucleus, but mainly the ventrolateral sector of its caudal two thirds. There is a differential labelling within the thalamic reticular nucleus.