Ontogeny of the striatal neurons expressing neuropeptide genes in the human fetus and neonate.

The distribution patterns of neurons expressing mRNAs for four neuropeptides in the human striatum were studied during ontogeny by the use of in situ hybridization. The results of our study demonstrate that somatostatin, enkephalin, dynorphin, and substance P mRNAs are present in striatal neuronal populations from week 12 of fetal life. Each neuronal population undergoes a specific differentiation. Neurons containing somatostatin mRNA are scattered throughout the caudate-putamen up until birth. Neurons containing enkephalin, dynorphin, or substance P mRNAs evolve throughout fetal life in relation to caudate-putamen and patch-matrix compartmentalization. Neurons containing enkephalin mRNA (distinct from those containing substance P or dynorphin mRNAs) are present in the matrix from week 12 of fetal life. These neurons are preferentially distributed in the matrix and, at birth, display higher enkephalin mRNA content in the matrix than in the patches. Dynorphin mRNA is found in the caudate and putamen, preferentially in the patch neurons; nevertheless, a low level of dynorphin mRNA is also present in neurons of the caudate matrix. Substance P mRNA is initially restricted to caudate neurons. At birth, both substance P and dynorphin mRNAs are expressed at high levels in the patches. These results demonstrate that each neuropeptide gene is expressed during human fetal life in neurons with a specific topology and pace of development in relation to caudate-putamen and patch-matrix differentiation. These results also contribute evidence that neurochemical evolution of the striatal neuronal populations is not complete at birth in humans.