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Transient and Permanent Patterns of Expression of the Low-Affinity Neurotrophin Receptor in the Interpeduncular Nucleus of the Rat

There is a prominent cholinergic projection from the medial habenular to the interpeduncular nucleus (IPN) which develops postnatally. In this study we examined the developmental course of expression of the low-affinity neurotrophin receptor (LANR), a receptor that binds to all members of the neurotrophin family, in the IPN. Three systems express LANR in the IPN. The cholinergic habenular axons that project to the intermediate and central subnuclei of the IPN are immunoreactive only during the time that the axons are growing and forming characteristic crest synapses in the intermediate subnuclei. The dorsomedial (DM) subnuclei, which are neither cholinergic targets nor contain cholinergic neurons, develop LANR immunoreactivity postnatally and the expression remains high in the adult. The walls of the prominent system of arterioles and venules that penetrate the IPN and ascend through the intermediate subnuclei are strongly immunoreactivity during the time of active angiogenesis and retain detectable but diminished levels of immunoreactivity in the adult. The LANR immunoreactivity seen in the vessels is likely to be associated with the peripheral sympathetic axons that innervate the smooth muscle in the vessels. These three systems within the same nucleus, which differ in phenotype, develop neurotrophin receptor immunoreactivity contemporaneously but their levels of expression differ in the adult, suggesting that they are regulated in different ways, possibly by different members of the neurotrophin family.