NEUROPEPTIDE-Y IMMUNOREACTIVE AXONS IN THE CORPUS-CALLOSUM OF THE CATDURING POSTNATAL-DEVELOPMENT

Many immunocytochemical studies have identified different types of neu rotransmitters localized in the corpus callosum (CC) axons in the adul t mammal. Few studies have looked at the development of different neur ochemically identified CC systems. Previous studies on the development of cat CC axons have indicated that a large number of transitory CC a xons project to the cortex during early postnatal development. The pre sent study focuses on the development of one neurochemically identifie d group of CC axons in the cat, labeled with an antibody against neuro peptide Y (NPY), to determine if this group participates in transitory CC axonal growth. Cats at specified ages from birth to adulthood were studied with a routine method of immunocytochemistry for antiserum to NPY. NPY-immunoreactive (ir) CC axons were detected at all stages exa mined, from newborn to adult; the peak density occurred during postnat al weeks (PNW) 3-4. During PNW 1-2, the denisty of NPY-ir CC axons inc reased gradually; some NPY-ir axons at this age had growth cones locat ed within the CC bundle between the cerebral hemispheres. The density of the NPY-ir CC axons decreased gradually during PNW 5-7, and from PN W 8 to maturity only a few NPY-ir CC axons were observed. These result s indicate that at least two types of NPY-ir CC axons (i.e., transitor y and permanent) exist during development, and that most of these axon s are eliminated or only express NPY-ir for a short period during deve lopment. The results also indicate that neurochemical subsets of CC ax ons participate in the extensive transitory growth observed by means o f the membrane tracer DiI but they may follow unique developmental tim etables.