Differential regulation of cholinergic and peptidergic development in the rat striatum in culture

The development of substance P, somatostatin, and choline acetyltransferase activity was examined in embryonic rat striatum in vivo and in culture. The study was undertaken to help define mechanisms by which diverse neurotransmitter phenotypes may be regulated within the same structure in the brain. Choline acetyltransferase (CAT) was present in striatum before gestational Day 13.5 (E13.5), and enzyme levels increased continually between E13.5 and birth. By contrast, substance P (SP) and somatostatin (SS) did not develop in vivo until E15, and peptide levels fluctuated between E15 and birth, indicating that striatal peptidergic and cholinergic development were regulated differently. To define mechanisms mediating the differential regulation of striatal peptidergic and cholinergic neurons, neurotransmitter development was examined in embryonic striatum in vitro. Cultured striatal neurons from E13.5 embryos expressed substance P and somatostatin de novo after several days in culture, and peptide levels and CAT activity increased significantly in vitro. Each transmitter phenotype was regulated in vitro by a different constellation of environmental factors, and many factors differentially influenced SP, SS, and CAT development. For example, coculture of striatum with a target tissue, the ventral mesencephalon (substantia nigra), increased CAT activity and SP levels but had no significant effect on levels of SS. Moreover, there were widely differing effects on CAT, SP, and SS development of medium conditioned by exposure to a variety of cell types, indicating that the three transmitter systems were regulated by different soluble factors. Potassium-induced membrane depolarization also exerted different effects on the different transmitter traits, elevating CAT activity but decreasing SP and SS. Finally, insulin was required for the survival of SP-containing neurons, but not for the survival of SS- or CAT-containing neurons, indicating that the survival of different populations of striatal neurons was dependent upon different factors. Our observations suggest that different populations of neurons in the striatum are regulated by different mechanisms, so that alterations in the environment may produce strikingly diverse responses in the development of different phenotypic traits within the same structure.