Histochemical strategies in the study of neural evolution

The use of histochemical methods, including enzyme histochemical, immunohistochemical and pathway tracing methods, as tools for testing hypotheses about the mechanism of neural evolution is reviewed. Many hypotheses have been proposed to explain aspects of neural evolution, including the occurrence of encephalization, quantitative changes in homologous cell populations, changes in the alignment, orientation and parcellation of neuronal populations into cytoarchitectonic units, and the appearance of new neuronal phenotypes. It is argued here that neural evolution must be viewed as a set of coordinated changes in neural circuits, rather than as a set of discrete events. For illustrative purposes the present discussion focuses on the use of histochemical methods in testings hypotheses about neural evolution in the basal ganglia system of amniotes. Three problem areas are considered: the bases for (1) differences in the alignment and organization of catecholaminergic and noncatecholaminergic neurons into cytoarchitectonic fields within the substantia nigra; (2) differences in the relative size and neurotransmitter organization of the caudal nigral cell groups and (3) differences in the organization of pallidal efferent systems, including the organization of pallidal-pretectal and pallidal-thalamic projection systems. Furthermore, the role of comparative data in framing hypotheses concerning the adaptive significance of such changes is also considered.