Postnatal development of striatal neurotensin immunoreactivity in relation to clusters of substance P immunoreactive neurons and the "dopamine islands" in the rat.
Conventional immunoperoxidase preparations of the coronally sectioned brains of rats killed at various times during the early postnatal period revealed the distributions of tyrosine hydroxylase, substance P, and neurotensin immunoreactivities. At birth, patches of dense tyrosine hydroxylase immunoreactivity were present across the breadth of the rostral striatum, whereas patches displaying substance P immunoreactivity were present only in its lateral half, appearing in its medial half by about postnatal day 3. Neuronal neurotensin immunoreactivity was absent in the rostral striatum at birth, although some neurotensin immunoreactive cells were present in the tail of the caudate-putamen. Rostrally, neurotensin immunoreactive cells appeared first along the lateral margin of the caudate-putamen on postnatal day 3, became numerous there about day 5, spread medially into the striatum by day 7, and achieved their medialmost distribution by about day 10. Their numbers and those of substance P immunoreactive neurons diminished thereafter. Substance P immunoreactive patches, which contained numerous labeled neurons and "puncta," shared coextensive distributions with patches of dense tyrosine hydroxylase immunoreactivity, but interdigitated with neurotensin immunoreactive cell clusters. The neurotensin immunoreactive cell clusters lacked puncta, the light microscopic representation of axon terminals, or swellings. It is concluded that the patchy infrastructure of the striatum, which is established prior to birth, is substrate for the progression of separate "waves" of elevated neuronal peptide content, one reflecting substance P and a later one reflecting neurotensin. These proceed along rostromedialward trajectories to involve interdigitating neuronal domains.