Quantitative and ultrastructural study of ascending projections to the medial mammillary nucleus in the rat

We analyzed the termination pattern of axons from the superior central nucleus and the ventral tegmental nucleus of Gudden within the medial mammillary nucleus (MM) in the rat. The neuropil of the MM consists of two classes of terminals, that is, terminals containing round synaptic vesicles and forming asymmetric synaptic contact, and terminals containing pleomorphic synaptic vesicles and forming symmetric synaptic contact. The number of axodendritic terminals with round vesicles is almost equal to that of terminals with pleomorphic vesicles. Almost all axosomatic terminals contain pleomorphic vesicles with symmetric synaptic contact. Injection of WGA-HRP into the central part of the superior central nucleus permitted ultrastructural recognition of many anterogradely labeled terminals within the median region of MM. The labeled terminals contacted mainly intermediate (1-2 microns diameter) and proximal dendrites (more than 2 microns diameter) as well as the neuronal somata. Serial ultrathin sections of neurons of the median region of the MM revealed that 37% of the axosomatic terminals were labeled anterogradely. The pars compacta of the superior central nucleus had reciprocal connections with the median region of MM. The axon terminals from this nucleus occupied 53% of axosomatic terminals, and contacted mainly intermediate dendrites. Following injection of WGA-HRP into the ventral tegmental nucleus, many labeled terminals were found in the medial and lateral regions of MM. They contacted mainly intermediate dendrites as well as neuronal somata. In the medial region, 78% of axosomatic terminals contacting retrogradely labeled neurons were labeled anterogradely. All labeled terminals from these nuclei contained pleomorphic vesicles, and made symmetric synaptic contact.