Glutamate/aspartate and leu-enkephalin immunoreactivity in mammillothalamic projection neurons of the rat

We have used retrograde transport and immunohistochemistry to study glutama te, aspartate, and enkephalin-like immunoreactive pathways from the mammill ary nuclei to the anterior nuclei of the thalamus, Injections of wheat germ agglutinin conjugated to horseradish peroxidase into the anterodorsal thal amic nucleus resulted in retrogradely labelled cell bodies in the lateral m ammillary nucleus, bilaterally, whereas injections into the anteroventral t halamic nucleus resulted in retrogradely labelled neurons in the ipsilatera l medial mammillary nucleus. In three parallel series of sections immunorea cted for glutamate, aspartate, and enkephalin, respectively, 50-60% of the retrogradely labelled cell bodies were also immunolabelled for glutamate, 5 0-60% for aspartate, and 40-50% for enkephalin, The enkephalin-immunoreacti ve neurons may coincide with or constitute a separate population from the g lutamate/aspartate-containing neurons. These results are compatible with th e possibility that mammillothalamic projection neurons may use glutamate an d/or aspartate and enkephalin as neurotransmitters. (C) 1999 Elsevier Scien ce Inc.