Localization of amino acids, neuropeptides and cholinergic neurotransmitter markers in identified projections from the mesencephalic tegmentum to the mammillary nuclei of the rat.

Retrograde labelling has been combined with immunohistochemistry to localize neurons containing GABA, glutamate, choline acetyltransferase, leu-enkephalin, neurotensin and substance P-like immunoreactivity in the projection pathways from the midbrain tegmental nuclei to the mammillary nuclei in the rat. Injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the medial mammillary nucleus resulted in retrogradely labelled neurons in the ventral tegmental nucleus of Gudden, whereas injections into the lateral mammillary nucleus resulted in large numbers of retrogradely labelled neurons in the ipsilateral dorsal tegmental nucleus of Gudden and in the laterodorsal tegmental nucleus. In the ventral tegmental nucleus, moderate to small numbers of retrogradely labelled neurons were also immunolabelled for GABA and approximately ten to 18 WGA-HRP-labelled neurons per section were immunoreactive for leu-enkephalin. In addition, small numbers of WGA-HRP-labelled neurons in the principal subnucleus of the ventral tegmental nucleus were immunoreactive for Glu whereas small numbers of retrogradely labelled neurons in the compact subnucleus of the central superior nucleus displayed neurotensin-like immunoreactivity. In the ventral subnucleus of the dorsal tegmental nucleus, moderate to small numbers of retrogradely labelled neurons were also GABA-immunoreactive and approximately ten to 14 WGA-HRP labelled neurons per section were immunoreactive for leu-enkephalin. The ventral subnucleus of the dorsal tegmental nucleus also contained small numbers of retrogradely labelled neurons that displayed either glutamate or substance P-like immunoreactivity. In addition, moderate to small numbers of WGA-HRP-labelled neurons (five to 20 per section) in the laterodorsal tegmental nucleus were immunoreactive for choline acetyltransferase. These results are compatible with the possibility that tegmentomammillary projection neurons use several different neurochemicals as neurotransmitter(s) and/or neuromodulator(s).