Simultaneous demonstration of choline acetyltransferase and glutamic acid decarboxylase immunoreactivity in the rat interpeduncular nucleus.

Double antigen immunohistochemistry was employed to simultaneously examine the distribution of choline acetyltransferase (ChAT) and glutamic acid decarboxylase (GAD) immunoreactivity in the rat interpeduncular nucleus (IPN) at the subnuclear and ultrastructural level. ChAT-immunoreactive axons of the fasciculus retroflexus (FR) innervated specific subnuclear divisions of the IPN that possessed GAD-immunoreactive somata and a high density of GAD-immunoreactive axons and terminals. At the ultrastructural level, each of the cholinoceptive subnuclei possessed a characteristic axodendritic synaptic contact. These morphologically distinct synapses were composed of terminals of ChAT-positive FR axons forming asymmetric contacts with dendritic profiles of GAD-positive neurons. An array of symmetric axodendritic contacts with GAD immunoreactivity located pre- and/or postsynaptically was also present in the cholinoceptive subnuclear divisions. The present study provides direct evidence for synaptic interactions between ChAT-immunoreactive FR axons and dendritic processes of GAD-immunoreactive neurons in the rat IPN. Also, GAD-positive terminals arising from possible intrinsic projections contact dendritic profiles of GAD-immunoreactive neurons in receipt of ChAT-positive FR terminals. These results reveal that putative cholinergic afferent inputs and GABAergic intranuclear projections simultaneously innervate a subpopulation of IPN neurons that possess GAD immunoreactivity.