Horseradish peroxidase pellets implanted into infant neocortex: some technical considerations.

This study was undertaken to examine whether implanting pellets of horseradish peroxidase (HRP), rather than injecting an aqueous solution, would improve the sensitivity of the retrograde tracing method as applied to infant rat neocortex. From 1 to 10 pellets, each containing approximately 10 microgram of HRP, were implanted into somatosensory cortex of 6-day-old rats. Implantation of one pellet labeled 4 neuronal groups; 5 pellets, 37 groups. Higher doses of injected HRP (20--50 microgram and 200-400 microgram) are needed to label the same number of groups. Also, individual neurons of a group generally contain more granules/cell after pellets than following injections of much higher doses of HRP. The pellet implant technique offers a high degree of reproducibility and is technically simpler than injections. We conclude that HRP pellet implants offer advantages over injections in identifying potential afferents to immature neocortex.