The substance P‐containing striatotegmental path in reptiles: An immunohistochemical study

Immunohistochemical methods were used to (1) characterize the organization of descending striatotegmental pathways in two species of reptiles, thecrocodilian Caiman crocodilus and the turtle Chrysemys scripta , and (2) compare the organization of the Striatotegmental pathways in these species to those of mammalian and avian species. In turtles, numerous substance P‐containing neurons were found in the medial portions of the small‐celled subdivision of the basal ganglia (paleostriatal complex), termed area d (Riss et al., ′69), and paleostriatum augmentatum (PA). In caiman numerous substance P‐containing neurons were also located within medial portions of the basal ganglia, including the small‐celled portion of the ventrolateral area of the telencephalon (VLA s.c.) and the dorsomedial portion of the large‐celled component of the ventrolateral area of the telencephalon (VLA I.c.). In both caiman and turtle, substance P‐containing fibers could be followed from the medial small‐celled fields into the medial forebrain bundle. The substance P (SP) fibers of the medial forebrain bundle are continuous caudally with several substance P‐rich fiber plexuses in the midbrain tegmentum. The substance P‐rich tegmental fields include the area ventralis of Tsai (AVT) and the substantia nigra (SN) in turtle. In caiman, these regions are called the area ventralis of Tsai and the nucleus tegmenti pedunculopontinus (TP) 1 . Knife cuts placed in the course of the SP‐containing fibers of the medial fore‐brain bundle resulted in a loss (in the case of complete transections) or diminution (in the case of incomplete transections) of the substance P‐containing fibers in AVT and SN or TP. Both the SN of turtle and the TP of caiman can be subdivided into two portions. Each contains a zone of densely packed catecholaminergic neurons and a more ventrally situated cell‐poor zone containing no catecholamine neurons. The distribution of catecholamine neurons was determined by the use oftyrosine hydroxylase (TOH) immunohistochemistry. The celldense zones of SNand TP (called the pars compacta of SN, or SNc, and the pars compacta of TP, or TPc) contain rich substance P fiber plexuses; however, the cell‐poor portions (called the pars ventralis of SN or SNV and the pars ventralis of TP or TPv) contain the highest levels of substance P immunore‐activity and densest substance P fiber plexuses found in the brains of these reptilian species. Immunohistochemical methods indicate that the AVT of caiman and turtle also contains many neurons positive for tyrosine hydroxy‐lase immunoreactivity (i.e., catecholaminergic neurons) and a rich substance P fiber plexus. Immunohistochemical methods were also used to map the distribution of serotonin (5‐HT) within the caiman and turtle brains. The results show that the terminal fields of the substance P‐containing striatotegmental fibers in these species also contain rich plexuses of serotonergic fibers, as is the case in avian and mammalian species. The results of these studies in caiman and turtles show many parallels in the organization of the descending striatotegmental path of reptiles with that of mammals and birds. As in mammals and birds, striatotegmental substance P neurons are found in the medial small‐celled portions of the basal ganglia and project upon portions of the midbrain tegmentum containing catecholaminergic neurons. These catecholamine cell groups (the AVT, SN, and TP) have themselves been shown to project back to the striatum, supplying the rich catecholamine fiber plexuses present in the basal ganglia of birds, reptiles, and mammals. These similarities inbasal ganglia organization in species of three different classes of amniotevertebrates suggest that modern amniotes inherited their striatotegmental substance P systems from a common ancestor and that many features of basal ganglia organization such as the striatotegmental and tegmentostriate systems are ancient features of brain organization.