DISTRIBUTION OF CALRETININ-IMMUNOREACTIVE SEPTAL AXONS IN THE NORMAL AND DEAFFERENTED MEDIAL HABENULA OF ADULT-RATS

To characterize the neural circuitry and plasticity of the septohabenu lar pathway, the present study analyzes the distribution of calretinin -immunoreactive fibers within the normal and deafferented medial haben ula (MHb) at the light and ultrastructural levels. In the adult rat, a dense plexus of calretinin-positive fibers was found throughout the e ntire MHb neuropil; these immunoreactive terminals formed asymmetric s ynaptic contacts with unstained dendritic profiles. Calretinin-positiv e axons that innervate the MHb originated from neurons of the ipsilate ral posterior septum, specifically those of the nucleus septofimbriali s and the nucleus triangularis. Unilateral deafferentation of the MHb resulted in the complete loss of calretinin-immunostained fibers withi n the ipsilateral MHb after 7 days; no reduction was apparent on the c ontralateral side. Four weeks after unilateral MHb deafferentation, ne w calretinin-immunoreactive fibers were found confined to the caudal r egions of the MHb; these axons again formed asymmetrical contacts with unstained dendritic profiles. No calretinin-positive axons, however, were found within the MHb at 4 weeks following bilateral deafferentati on, thus suggesting that the source of these new fibers within the lon g-term deafferented MHb arises from the contralateral septal neurons. Supporting this idea, injections of biotinylated dextran amine into th e 4-week deafferented MHb resulted in retrogradely labeled somata obse rved in the contralateral posterior septum. These data reveal that sep tal projections to the MHb, which are normally ipsilateral, respond to a unilateral deafferentation by extending contralateral fibers that c ross the midline at the habenular commissure and reinnervate the cauda l regions of the nucleus. (C) 1996 Wiley-Liss, Inc.