Structural And Chemical Organization Of The Rat Interpeduncular Nucleus
The structural and chemical organization of the rat interpeduncular nucleus (IPN) was examined employing wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) and immunohistochemistry at the light and electron microscope level. The topographical and ultrastructural arrangements of habenular efferents innervating the IPN were studied using stereotaxic placements of WGA-HRP. The medial habenula provided a prominent afferent input to the many IPN subnuclei, with the lateral and rostral subnuclei receiving a predominantly ipsilateral input, and the intermediate, central and dorsomedial subnuclei receiving a symmetrical projection. Habenular terminals formed distinct axodendritic synapses in the rostral, lateral, intermediate and central subnuclei.;Projections of the fasciculus retroflexus (FR) were stained immunohistochemically for choline acetyltransferase (ChAT) or substance P (SP). These inputs were topographically organized among the IPN subnuclei, such that ChAT-positive axons innervated the rostral, intermediate and central subnuclei, and SP-positive inputs terminated in the lateral, dorsomedial and rostral subnuclei. Characteristic axodendritic synapses of the intermediate, central and rostral subnuclei were composed of ChAT-positive terminals, and those in the lateral, rostral and dorsomedial subnuclei were composed of SP-positive terminals.;A prominent population of neurons and an extensive plexus of axons and terminals stained immunohistochemically for glutamic acid decarboxylase (GAD) were present in the rostral, lateral, central and intermediate subnuclei. Axodendritic and axosomatic synapses possessing GAD immunoreactivity pre - and/or postsynaptically were evident in these subnuclei. GAD-immunoreactive dendrites in the intermediate, central and rostral subnuclei also received ChAT-positive terminals, and those in the lateral subnucleus received SF-positive terminals. SP-positive dendrites in the rostral subnucleus were also in receipt of ChAT-positive terminals.;The IPN is considered to be an important link between the septohabenular circuit of the limbic forebrain and several raphe and tegmental centres of the hindbrain. Neurons of the IPN are in receipt of descending habenular projections which provide putative excitatory inputs, as well as intrinsic, putative inhibitory inputs. The integration of habenular inputs with those arising from intrinsic sources may play an important role in the direct and transsynaptic modulation of projection neurons of the IPN which provide afferent inputs to the raphe and tegmental centres of the hindbrain.
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