Glutamate decarboxylase immunoreactivity in the rat interpeduncular nucleus: a light and electron microscope...

The distribution of immunohistochemically demonstrable glutamate decarboxylase, the synthetic enzyme for GABA, was examined in the rat interpeduncular nucleus at the light and electron microscope levels. Immunoreactive perikarya were distributed in a characteristic pattern among the subnuclear divisions. The rostral, ventral and caudal portions of the nucleus possessed numerous immunoreactive perikarya, while few immunoreactive somata were observed in the subnuclei of the dorsal aspect. A dense field of immunostained axons and terminals was also present throughout. Ultrastructural examination of glutamate decarboxylase immunoreactivity revealed numerous labelled somata, dendritic processes, axons and boutons. Axodendritic and axosomatic synapses with immunoreactive postsynaptic profiles were numerous throughout those subnuclei with large numbers of immunoreactive somata. Immunostained terminals in contact with both immunoreactive and non-immunoreactive somatic and dendritic profiles were also present. An abundance of immunostained terminals was observed in the subnuclei that possessed a sparse population of immunoreactive somata. Immunoreactive myelinated axons of unknown origin were also present. This investigation demonstrates that the rat interpeduncular nucleus possesses a large population of glutamate decarboxylase-immunoreactive neurons coextensive with a plexus of immunostained axons and terminals. The results suggest that the immunoreactive neurons give rise to axons which contribute to an intrinsic circuit interconnecting the different subnuclear divisions. These immunoreactive neurons are in receipt of non-immunoreactive afferent inputs of variable morphology, as well as projections from intrinsic immunoreactive neurons.