Anterior thalamic afferents from the mamillary body and the limbic cortex were studied by using single and double retrograde transport methods in the rat. The medial mamillary nucleus was divided on the basis of the cytoarchitecture into four subnuclei: the pars medialis centralis, pars medialis dorsalis, pars lateralis, and pars basalis. Extensive connections were seen between each of these subdivisions of the mamillary body and the anterior thalamic nuclei, topographically organized so that the anteromedial thalamic nucleus receives projections exclusively from the pars medialis centralis, while the anteroventral thalamic nucleus receives projections from the pars medialis dorsalis and pars lateralis. Nuclei in the dorsal half of these two mamillary subdivisions project predominantly to the medial half of the anteroventral thalamic nucleus, and those in the ventral half to the lateral half of the nucleus. The pars basalis was found to have numerous projections to the magnocellular part of the anteroventral nucleus. All limbic cortical areas send projections bilaterally to all regions of the anteromedial nucleus as well as to the parvicellular parts of the anteroventral thalamic nucleus, while the anterodorsal nucleus receives ipsilateral projections originating exclusively from the preagranular, anterior limbic, and cingular regions. The magnocellular part of the anteroventral nucleus, however, receives only ipsilateral projections from all of the limbic cortex. Some neurons in the infralimbic region also project bilaterally to all of the anterior thalamic nuclei except the anterodorsal nucleus. All of these cortical projections to the anterior thalamus originate in layers V and VI of the limbic cortex.

We investigated the direct projections from the paraventricular hypothalamic nucleus (PVH) to the spinal cord. When Phaseolus vulgaris leucoagglutinin (PHA-L) was injected into the PVH, labeled descending fibers were observed running bilaterally through three pathways. The first pathway ran into the dorsal longitudinal fasciculus and projected to the central gray matter, Edinger-Westphal nucleus, pedunculopontine tegmental nucleus, nucleus of the locus ceruleus and parabrachial nucleus. The second and third pathways coursed through the medial forebrain bundle, ventral tegmental area, and ventral part of the medulla oblongata. At the medulla oblongata, the second pathway curved dorsolaterally and joined Rexed's lamina V of C1 after giving many projections to the nucleus ambiguus, nucleus of the solitary tract, dorsal motor nucleus of the vagus, and a few to the area postrema. The fibers descended through lamina V until C5, and coursed through lamina I from C6 to the upper coccygeal segments. They gave collateral projections to lamina I from the cervical through the upper coccygeal segments. The third pathway coursed laterally and descended through the lateral funiculus after giving projections to the lateral reticular nucleus and the marginal layer of the spinal trigeminal nucleus. These fibers gave off many projections to the intermediolateral cell column of the thoracic cord and the sacral parasympathetic nucleus. Lamina X received many projections from the fibers of the lateral funiculus at C5 through the b-segment of sacral spinal cord. These results indicate that the PVH may integrate directly with the medullary and spinal autonomic regulatory nuclei, including the vagus complex, sympathetic intermediolateral cell column, laminae I and X, and sacral parasympathetic nucleus.

We examined the regions projecting to the supramammillary nucleus of the rat with retrograde transport of WGA-HRP and WGA, and anterograde transport of Phaseolus vulgaris leucoagglutinin. The supramammillary nucleus receives major descending afferents from the infralimbic cortex, the dorsal peduncular cortex, the nucleus of the diagonal band of Broca, the medial and lateral preoptic nuclei, bilaterally. The major ascending afferents come from the pars compacta of the nucleus centralis superior, the ventral tegmental nucleus, and the laterodorsal tegmental nucleus. The supramammillary nucleus also receives a few (but distinct) fibers from the anterior and lateral hypothalamic nuclei, the ventral premammillary nucleus, the interpeduncular nucleus, the cuneiform nucleus, the dorsal raphe nucleus, the incertus nucleus, and the C3 region including the prepositus hypoglossi nucleus. All descending fibers run through the medial forebrain bundle. Almost all ascending fibers from the pars compacta of the nucleus centralis superior and the laterodorsal tegmental nucleus run through the mammillary peduncle, and terminate throughout the supramammillary nucleus. A few fibers from the laterodorsal tegmental nucleus and the C3 region run through the fasciculus longitudinalis dorsalis and terminate in the dorsal part of the supramammillary nucleus including the supramammillary decussation.

Collateral axonal branching from the medial or lateral mammillary nuclei to the anterior thalamus, Gudden's tegmental nuclei, the nucleus reticularis tegmenti pontis, and the medial pontine nucleus was studied using the fluorescent retrograde double-labeling method. One day after injection of Fast Blue into the anterior thalamic nuclei or Gudden's tegmental nuclei, Nuclear Yellow was injected into Gudden's tegmental nuclei or the nucleus reticularis tegmenti pontis and the medial pontine nucleus. Following 1 day survival, single- and double-labeled neurons were examined in the mammillary nuclei. The lateral mammillary nucleus contains neurons whose collateral fibers project to both the dorsal tegmental nucleus of Gudden and the ipsilateral or contralateral anterodorsal thalamic nucleus, to both the medial pontine nucleus and the anterodorsal thalamic nucleus, and to both the dorsal tegmental nucleus of Gudden and the medial pontine nucleus. The pars medianus and pars medialis of the medial mammillary nucleus contain neurons whose collateral fibers project to both the anteromedial thalamic nucleus and the ventral tegmental nucleus of Gudden, to both the anteromedial thalamic nucleus and the medial part of the nucleus reticularis tegmenti pontis, and to both the ventral tegmental nucleus of Gudden and the medial part of the nucleus reticularis tegmenti pontis. The dorsal half of the pars posterior of the medial mammillary nucleus contains a few neurons whose collateral fibers project to both the anteromedial thalamic nucleus and the rostral part of the ventral tegmental nucleus of Gudden, and to both the caudal part of the anteroventral thalamic nucleus and the rostral part of the ventral tegmental nucleus of Gudden, while the pars lateralis of the medial mammillary nucleus contains no double-labeled neurons and projects only to the anteroventral thalamic nucleus.

No abstract

The structure and location of the dorsal and ventral tegmental nuclei of Gudden were studied in brains of 12 species of mammals. The ventral tegmental nucleus of Gudden is composed of oval or polygonal, medium-sized, well-stained cells, and includes the pars principalis situated ventral to the fasciculus longitudinalis medialis, and the pars suprafascicularis in the central gray matter dorsal to the fasciculus longitudinalis medialis (in the golden hamster, mouse, vole, and house shrew). The dorsal tegmental nucleus of Gudden is situated within the central gray matter. This nucleus includes the pars ventralis, composed of oval or triangular cells, medium-sized but smaller than those of the ventral tegmental nucleus of Gudden, and the pars dorsalis, composed of round, small, and lightly stained cells, and often encapsulated by fibers of the fasciculus longitudinalis dorsalis. The isthmus between the dorsal and ventral tegmental nuclei of Gudden is distinct. In the cat, dog, rat, gerbil, and rabbit, the isthmus stands at the dorsal edge of the fasciculus longitudinalis medialis; in the golden hamster, mouse, vole, and house shrew, within the central gray matter; and in the guinea pig, at the ventral edge of the fasciculus longitudinalis medialis. In the green monkey and man, we could not find a ventral tegmental nucleus of Gudden, and in the house shrew and man, the pars ventralis and pars dorsalis of the dorsal tegmental nucleus of Gudden could not be separated.

The regions projecting to Gudden's tegmental nuclei were examined by retrograde transport of horseradish peroxidase or wheat-germ-agglutinin-conjugated horseradish peroxidase. Gudden's tegmental nuclei in the rabbit can be divided into a pars principalis of the ventral tegmental nucleus (TVP), a pars ventralis of the dorsal tegmental nucleus (TDV), and a pars dorsalis of the dorsal tegmental nucleus (TDD). The TVP receives many fibers from the medial division of the ipsilateral medial mammillary nucleus and bilaterally from the lateral habenular nucleus, and additionally some fibers from the posterior nucleus of the interpeduncular complex. The TDV receives many fibers from the ipsilateral lateral mammillary nucleus, from the ipsilateral prepositus hypoglossi nucleus, bilaterally from the lateral habenular nucleus, from the central and paramedian nuclei of the interpeduncular complex, from the bilateral gray matter along the floor of the fourth ventricle, and from the contralateral supragenual nucleus. The TDD receives a projection from the lateral habenular nucleus of both sides and from the central and paramedian nuclei of the interpeduncular complex, and a minor projection from the ipsilateral lateral mammillary nucleus, the posterior nucleus of the interpeduncular complex, the prepositus hypoglossi nucleus, and the contralateral supragenual nucleus.

The synaptic organization of projections from the lateral mammillary neurons within the dorsal tegmental nucleus of Gudden is studied in the rat with the aid of anterograde transport of horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP) and visualized with tetramethylbenzidine. The dorsal tegmental nucleus consists of the pars ventralis (TDV) and the pars dorsalis (TDD). The normal neuropil of the dorsal tegmental nucleus contains three classes of axodendritic terminals, that is, terminals containing round, flat, and pleomorphic vesicles. They make up 44%, 5%, and 51%, respectively, of all axodendritic terminals in the TDV, and 62%, 1%, and 37% in the TDD. Injection of WGA-HRP into the lateral mammillary nucleus permits ultrastructural recognition of many anterograde labeled terminals within both the TDV and TDD. In the TDV, 81% of the labeled terminals contain round synaptic vesicles and make asymmetric synaptic contacts. A few of the labeled terminals contain pleomorphic vesicles and make symmetric synaptic contacts. More than 50% of the labeled terminals contact intermediate dendrites (1-2 microns diameter). In the TDD, almost all labeled terminals are small, contain round vesicles, and make asymmetric synaptic contacts. These terminals mainly contact intermediate as well as distal (less than 1 micron diameter) dendrites. There are only a few labeled terminals with pleomorphic vesicles and no terminals with flat vesicles. The termination pattern of the lateral mammillary neurons in the TDV is similar to that in the TDD. Anterograde labeled axon terminals often contact retrograde labeled dendrites in the TDV. No reciprocal connections are present in the TDD. These results suggest that the TDV and the TDD receive mainly excitatory and a few inhibitory inputs from the lateral mammillary nucleus. The TDV neurons also have direct reciprocal connections with the lateral mammillary neurons.