The paleostriatal system of Caiman crocodilus

Abstract

The organization and projections of the presumed homologue of the mammalian corpus striatum, the ventrolateral area (VLA) of the telencephalon, were investigated in the reptile Caiman crocodilus. The caiman VLA was divided into two major cell fields on the basis of cytoarchitectonic criteria: a rostromedial small celled field (VLA s.c.) and a large celled field occupying the dorsal, lateral and ventrocaudal portions of the VLA (VLA l.c.). Histochemical results indicate that the VLA s.c. contains high levels of both cholinesterase and catecholamine (CA) activity. An intense lacy plexus of CA-containing axons and terminals was found in the VLA s.c. Far less CA activity was found within the VLA l.c. CA activity within the VLA appears to be derived primarily from axons of cells located within a large field of the midbrain tegmentum which is called in this report nucleus tegmenti pedunculo pontinus (TP). In the caiman the VLA also receives projections from the CA-positive cells of the locus ceruleus and from serotonin containing cells of the midline raphe system. Anatomical experiments indicate that the VLA l.c. receives projections from neurons in the VLA s.c. and projects upon these subtelencephalic cell groups: the anterior and posterior entopeduncular nuclei (ENa and ENp), the ventral lateral and ventral medial thalamic areas (Avl and Avm), the dorsal nucleus of the posterior commissure (nDCP), and TP. The VLA s.c. projects upon TP. ENa neurons project upon cells in the VLA l.c. Cells of Avl and Avm receive both paleostriatal and cerebellar projections; Avl neurons project upon portions of the rostral telencephalon external to the VLA. NDCP neurons project upon the optic tectum. The organization and projections oleostriatal complex of the pigeon (Karten and Dubbeldam, '73; Brauth, Ferguson and Kitt, '78) and to the mammalian basal ganglia. The following paleostriatal characteristics appear to be common features in these species and may represent retained characteristics derived from the common ancestor: (1) an ascending catecholaminergic system derived from neurons in the midbrain tegmentum; (2) projections from the basal striatum upon the catecholamine containing neurons of the midbrain tegmentum; (3) an intrinsic cholinergic system; (4) projections upon thalamic cell groups which are also in receipt of cerebellar projections and which project upon telencephalic regions external to the paleostriatum; (5) reciprocal connections between the paleostriatum and the ventral or subthalamus; and (6) projections upon cell groups which project to the optic tectum. The results are discussed in terms of the overall role of the basal ganglia in the neural control of behavior.