The projection of the supramammillary nucleus to the hippocampal formation: an immunohistochemical and anterograde transport study with the lectin PHA-L in the rat.

The organization and possible neurotransmitter specificity of a projection from the lateral supramammillary nucleus to the hippocampal formation has been examined with immunohistochemical and axonal transport methods in the adult male rat. Experiments with the retrograde tracer true blue indicate that neurons throughout the rostrocaudal extent of the nucleus are labeled after injections in either dorsal parts of the dentate gyrus and Ammon's horn, or the entorhinal area, although cells labeled by the entorhinal injections tended to occupy more ventral parts of the nucleus. Combined immunohistochemical-retrograde transport studies showed that a small number (less than 5%) of cholecystokinin-immunoreactive neurons in the caudal tip of the supramammillary nucleus project to the hippocampal formation, as do some (5-10%) vasoactive intestinal polypeptide (VIP)-immunoreactive neurons throughout the nucleus. Anterograde transport studies with the lectin phaseolus vulgaris leucoagglutinin (PHA-L) indicate that fibers from the supramammillary nucleus innervate all parts of the hippocampal formation. Many varicose fibers with terminal boutons were observed in the granular and molecular layers of the dentate gyrus, throughout the molecular layer of field CA3 of Ammon's horn, and in the pyramidal layer and stratum oriens of subfield CA3a. Only scattered fibers were found in fields CA1 and CA2. Apparent terminal fields were also observed in superficial parts of the molecular layer, and deep parts of the pyramidal layer, of the subiculum, in the deepest layer of the presubiculum and parasubiculum, and in all layers of the entorhinal area.