DEVELOPMENTAL EXPRESSION OF GLYCINE IMMUNOREACTIVITY AND ITS COLOCALIZATION WITH GABA IN THE EMBRYONIC CHICK LUMBOSACRAL SPINAL-CORD

The development of immunoreactivity for the putative inhibitory amino acid neurotransmitter glycine was investigated in the embryonic and po sthatched chick lumbosacral spinal cord by using postembedding immunoc ytochemical methods. Glycine immunoreactive perikarya were first obser ved at embryonic day 8 (E8) both in the dorsal and ventral gray matter s. The number of immunostained neurons sharply increased by E10 and wa s gradually augmented further at later developmental stages. The gener al pattern of glycine immunoreactivity characteristic of mature animal s had been achieved by E12 and was only slightly altered afterward. Mo st of the immunostained neurons were located in the presumptive deep d orsal horn (laminae IV-VI) and lamina VII, although glycine-immunoreac tive neurons were scattered throughout the entire extent of the spinal gray matter. By using some of our previously obtained and published d ata concerning the development of gamma-aminobutyric acid (GABA)-ergic neurons in the embryonic chick lumbosacral spinal cord, we have compa red the numbers, sizes, and distribution of glycine- and GABA-immunore active spinal neurons at various develomental stages and found the fol lowing marked differences in the developmental characteristics of thes e two populations of putative inhibitory interneurons. (i) GABA immuno reactivity was expressed very early (E4), whereas immunoreactivity for glycine appeared relatively late (E8) in embryonic development. (ii) In the ventral horn, GABA immunoreactivity declined, whereas immunorea ctivity for glycine gradually increased from E8 onward in such a manne r that the sum of glycinergic and GABAergic perikarya remained constan t during the second half of embryonic development. (iii) Glycinergic a nd GABAergic neurons showed different distribution patterns in the spi nal gray matter throughout the entire course of embryogenesis as well as in the posthatched animal. When investigating the colocalization of glycine and GABA immunoreactivities, perikarya immunostained for both amino acids were revealed at all developmental stages from E8 onward, and the proportions of glycine- and GABA-immunoreactive neurons that were also immunostained for the other amino acid were remarkably const ant during development. The characteristic features of the development of the investigated putative inhibitory spinal interneurons are discu ssed and correlated with previous neuroanatomical and physiological st udies. (C) 1995 Wiley-Liss, Inc.