Neurotransmitter characteristics of neurons projecting to the supramammillary nucleus of the rat

Retrograde labelling was combined with immunohistochemistry to localize neu rons containing choline acetyltransferase, gamma-aminobutyric acid (GABA), glutamate, serotonin, somatostatin, Leuenkephalin, neurotensin, and substan ce P-immunoreactivity in neurons projecting to the supramammillary nucleus in the rat. Injections of wheat germ agglutinin conjugated to horseradish p eroxidase (WGA-HRP) into the supramammillary nucleus resulted in retrograde ly labelled neurons in the medial septal nucleus, the nuclei of the diagona l band of Broca, the infralimbic cortex, the medial and lateral preoptic nu cleus, the subiculum, the laterodorsal tegmental nucleus, the compact subnu cleus of the central superior nucleus and the dorsal raphe nucleus. In the medial septal nucleus and in the nuclei of the diagonal band of Broca, 80-8 5% of WGA-HRP-labelled neurons (30-40 per section) were also immunoreactive for choline acetyltransferase and small numbers of WGA-HRP-labelled neuron s were immunoreactive for GABA, glutamate, neurotensin or substance P. In t he medial preoptic nucleus, 85-90% of retrogradely labelled neurons (25-30 per section) were immunoreactive for somatostatin and a few WGA-HRP-labelle d neurons displayed neurotensin-immunoreactivity. In the rostroventral part of the subiculum, small numbers of retrogradely labelled neurons were also immunoreactive for neurotensin or for glutamate. In the laterodorsal tegme ntal nucleus, 90% of WGA-HRP-labelled neurons (20-25 per section) were immu noreactive for choline acetyltransferase and small numbers of retrogradely labelled neurons also displayed substance P immunoreactivity. In the compac t subnucleus of the central superior nucleus, 50-60% of retrogradely labell ed neurons (15-20 per section) were also immunolabelled for GABA and approx imately 30-40% of WGA-HRP-labelled neurons (10-12 per section) were immunor eactive for Leu-enkephalin. The compact subnucleus of the central superior nucleus also contained small numbers of retrogradely labelled neurons that displayed neurotensin immunoreactivity. In the dorsal raphe nucleus, 80-85% of WGA-HRP-labelled neurons (30-40 per section) were also immunoreactive f or serotonin and small numbers of retrogradely labelled neurons displayed n eurotensin or glutamate immunoreactivity. These results suggest that the mu ltiple neurochemicals contained in ascending and descending projections to the SuM participate in complex interactions in the transmission process of SuM neurons.