Postnatal development of caudate input neurons in the cat

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Abstract
Lectin‐bound horseradish peroxidase (WG‐HRP) was pressure‐injected into the caudate nucleus (Cd) of neonatal (< 24 hours of age) and adult cats in order to assess the postnatal development of monosynaptic Cd input neurons. Tissue was processed for peroxidase activity with a benzidine dihydrochloride chromagen. The injection of WG‐HRP produced relatively similar labelled zones of marker uptake in the caudate nuclei of both neonates and adults. Similar axonal projections were also labelled in both age groups. While many characteristics of retrogradely labelled Cd input neurons were apparently constant throughout postnatal life, each of these features had a particular developmental modification. (1) Regardless of age, neuronal somata that projected to the Cd were located in the neocortex, thalamus, substantia nigra, mesencephalic raphe nuclei, and globus pallidus. In each of these brain sites, labelled Cd input neurons appeared to migrate postnatally. (2) The Cd afferent axons originated from the same neuronal lines in neonates and adults‐small‐to‐medium‐sized cortical neurons and medium‐sized‐to‐large fusiform cells in all other brain sites. In each of the brain sites, labelled neurons displayed marked postnatal somatic growth. (3) In both age groups, there was a characteristic intrasomatic reaction product density in the labelled neurons located in each brain site (substantia nigra > thalamus = raphe = globus pallidus > cortex). In each of these brain sites, the intrasomatic reaction product density was less in neonates than in adults.