3H-nicotine- and 125I-alpha-bungarotoxin-labeled nicotinic receptors in the interpeduncular nucleus of rats. II. Effects of habenular deafferentation.

The cholinergic innervation of the interpeduncular nucleus (IPN) is wholly extrinsic and is greatly attenuated by bilateral habenular destruction. We describe changes in the labeling of putative nicotinic receptors within this nucleus at 3, 5, or 11 days after bilateral habenular lesions. Adjacent tissue sections of the rat IPN were utilized for 3H-nicotine and 125I-alpha-bungarotoxin (125I-BTX) receptor autoradiography. Compared to sham-operated controls, habenular destruction significantly reduced autoradiographic 3H-nicotine labeling in rostral (-25%), intermediate (-13%), and lateral subnuclei (-36%). Labeling in the central subnucleus was unchanged. Loss of labeling was maximal at the shortest survival time (3 days) and did not change thereafter. In order to establish whether this loss was due to a reduction in the number or the affinity of 3H-nicotine-binding sites, a membrane assay was performed on microdissected IPN tissue from rats that had received surgery 3 days previously. Bilateral habenular lesions produced a 35% reduction of high-affinity 3H-nicotine-binding sites, with no change in binding affinity. Bilateral habenular lesions reduced 125I-BTX labeling in the intermediate subnuclei, and a slight increase occurred in the rostral subnucleus. In the lateral subnuclei, 125I-BTX labeling was significantly reduced (27%) at 3 days but not at later survival times. In view of the known synaptic morphology of the habenulointerpeduncular tract, it is concluded that a subpopulation of 3H-nicotine binding sites within the IPN is located on afferent axons and/or terminals. This subpopulation, located within rostral, intermediate, and lateral subnuclei, may correspond to presynaptic nicotinic cholinergic receptors. Sites that bind 125I-BTX may include a presynaptic subpopulation located in the lateral and possibly the intermediate subnuclei.