Fine structural survey of Gudden's tegmental nuclei in the rat: cytology and axosomatic synapses

The head direction signal: origins and sensory-motor integration

Navigation first requires accurate perception of one's spatial orientation within the environment, which consists of knowledge about location and directional heading. Cells within several limbic system areas of the mammalian brain discharge allocentrically as a function of the animal's directional heading, independent of the animal's location and ongoing behavior. These cells are referred to as head direction (HD) cells and are believed to encode the animal's perceived directional heading with respect to its environment. Although HD cells are found in several areas, the principal circuit for generating this signal originates in the dorsal tegmental nucleus and projects serially, with some reciprocal connections, to the lateral mammillary nucleus --> anterodorsal thalamus --> PoS, and terminates in the entorhinal cortex. HD cells receive multimodal information about landmarks and self-generated movements. Vestibular information appears critical for generating the directional signal, but motor/proprioceptive and landmark information are important for updating it.

Fine structure of the lateral mammillary projection to the dorsal tegmental nucleus of Gudden in the rat

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.

Ultrastructure of the mammillotegmental projections to the ventral tegmental nucleus of Gudden in the rat

This study examines the termination pattern of axons from the medial mammillary nucleus within the ventral tegmental nucleus of Gudden (TV) in rats by using anterograde transport of horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP) and visualized with tetramethylbenzidine. The neuropil of TV contains three classes of axodendritic terminals, that is, terminals containing round, flat, and pleomorphic synaptic vesicles. These types make up 55.6%, 26.1%, and 18.3%, respectively, of all normal axodendritic terminals. Injection of WGA-HRP into the medial mammillary nucleus permits ultrastructural recognition of anterogradely labeled terminals within the TV. More than 80% of the labeled terminals contain round synaptic vesicles and form asymmetric synaptic contacts, whereas about 16% contain flat synaptic vesicles with symmetric synaptic contacts. There are a few labeled terminals with pleomorphic vesicles and only a few axosomatic terminals. Almost all labeled terminals are small, having diameters of less than 1.5 microns. Compared with the distributions of normal and labeled terminals with round vesicles, there is an increase of the percentage of labeled terminals with round vesicles on the intermediate dendrites (1-2 microns diameter) and a decrease on the distal dendrites (less than 1 micron diameter). Anterogradely labeled axon terminals often contact retrogradely labeled dendrites. These results suggest that the medial mammillary neurons send mainly excitatory as well as a few inhibitory inputs to the dendrites of TV and have direct reciprocal contacts with the TV neurons.

Retrograde double-labeling study of the mammillothalamic and the mammillotegmental projections in the rat

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.