BASAL FOREBRAIN AND ANTERIOR THALAMIC CONTRIBUTIONS TO ACETYLCHOLINESTERASE ACTIVITY IN GRANULAR RETROSPLENIAL CORTEX OF RATS

Histochemical studies demonstrate that granular retrosplenial cortex (cortical areas 29b and c) of the adult rat displays a characteristic laminar pattern of acetylcholinesterase (AChE) activity. While some AChE-positive axons are found in all cortical layers, most intense staining occurs in two bands that correspond to layers I and III. The present studies were directed toward identifying the neural systems underlying this AChE activity. Unilateral electrolytic or excitatory amino acid induced lesions of the basal forebrain, including the nucleus of the diagonal band, result in reductions of AChE staining throughout ipsilateral granular retrosplenial cortex; particularly noteworthy are the reductions in layer I and the deeper cortical layers. AChE staining remains in superficial layer I and in layer III. Placement of lesions in the anterior thalamus, including all of the anterior dorsal nucleus, results in reduction of AChE histochemical staining in the outer part of layer I and especially in layer III. Staining remains in much of layer I and in the deepest band of layer III. Placement of electrolytic lesions in the hypothalamus or the midbrain tegmentum produce no detectable change in the pattern of AChE in retrosplenial cortex. These results indicate that AChE activity in granular retrosplenial cortex is found primarily within afferent axons from the basal forebrain system and from anterior dorsal thalamus, and these two systems of afferents display distinct laminar patterns of termination.