Expression of low affinity NGF (p75) receptors in rat superior colliculus: studies in vivo, in vitro, and in fetal tectal grafts.

The distribution and level of expression of the low affinity nerve growth factor receptor (LNGFR) in the rat visual system has been investigated under a number of experimental conditions. The aim was to determine the cellular location of the receptor and to study the factors which influence its expression. The monoclonal antibody 192-Ig and immunohistochemical techniques were used to examine LNGFR expression in (i) developing and adult rat superior colliculus (SC), (ii) fetal collicular tissue transplanted to the midbrain of newborn host rats, (iii) the SC of rats which had been unilaterally enucleated at birth, and (iv) mixed glial cell cultures from the neonatal SC. The effect of eye removal on LNGFR immunoreactivity in other normally retino-recipient areas was also assessed. Postnatal maturation of the rat SC was associated with an increase in LNGFR immunoreactivity. At birth, weak staining was seen ventral to the superficial gray layer. Staining gradually became located more dorsally until in the adult there was a dense band of immunoreactivity that extended 50-100 microns from the surface. Immunoreactive processes and cellular-like profiles were seen. Compared to adult host SC there was considerably more LNGFR immunoreactivity in transplanted tectal tissue, irrespective of whether the grafts were connected to the host. LNGFR expression in transplants was patchy and usually contiguous with the graft surface. Staining was not obviously related to the distribution of astrocytes or microglia and very few cells were LNGFR positive in tectal glial cell cultures. In SC and in tectal grafts it is probable that LNGFR immunoreactivity was primarily associated with intrinsic neurons. In support of this, LNGFR expression in the superficial SC was unaffected by neonatal eye removal; however, LNGFR staining in the pretectum and diencephalon was reduced or absent on the side contralateral to visual deafferentation. These conflicting sets of data suggest that (i) LNGFRs in central visual pathways are sometimes, but by no means always, associated with retinal innervation and (ii) LNGFR expression in visual target areas originates from diverse sources and is influenced and regulated by a variety of factors.