Normal development and effects of early deafferentation on choline acetyltransferase, substance P and serotonin-like immunoreactivity in the interpeduncular nucleus.

The normal postnatal development and response to neonatal fasciculus retroflexus (FR) lesions of serotonin, substance P (SP), and choline acetyltransferase (ChAT) distribution are described for the rat interpeduncular nucleus (IPN). Serotonin-, SP- and ChAT-containing axons differed in development, distribution, and response to deafferentation. Serotonergic axons and cell bodies were present at birth. SP was present in the FR and in the lateral subnuclei by 3 days of age but did not appear in the rostral or dorsal subnuclei until 7-14 days. Intrinsic SP perikarya were not seen until 17 days of age. The development of ChAT was late, appearing only during the second week of life and not reaching adult patterns and density until after 21 days of age. The pattern of development of cytochrome oxidase and Bodian silver staining are also described. Both cytochrome oxidase and Bodian staining paralleled the patterns of localization and development of ChAT staining. Bilateral neonatal FR lesions resulted in a permanent loss of ChAT and cytochrome oxidase staining throughout the IPN and of SP in the lateral and rostral subnuclei. No changes were seen in the serotonergic system. Following unilateral lesions, the pattern of SP loss and replacement paralleled that seen after adult lesions. The pattern of replacement of ChAT differed from that after adult lesions in that there was partial replacement in the ipsilateral intermediate subnucleus following neonatal lesions. This result suggests that late developing cholinergic axons can innervate the contralateral intermediate nucleus to a much greater extent following infant lesions than following adult lesions.