Sources of olfactory inputs to opossum mediodorsal nucleus identified by horseradish peroxidase and autoradiographic methods.

Some sources of olfactory input to the opossum mediodorsal thalamic nucleus (MD) were identified by retrograde horseradish peroxidase and anterograde autoradiographic methods. One major source originated from the olfactory tubercle and a narrow strip of piriform cortex bordering the tubercle. The tubercle-MD projection exhibited a definite spatial organization and included all except the most medial part of MD. The fact that the projection reached the most lateral and ventral extent of MD abutting the intralaminar complex suggests that the entire opossum MD may correspond to only the medial, magnocellular division in the primate and that the equivalents of both the parvocellular and paralamellar divisions may be absent.