Left-right pairing at the crest synapses of rat interpeduncular nucleus

Synaptic terminals of the habenulo-interpeduncular axons in rat interpeduncular nucleus are demonstrated by electron-microscopic autoradiography following [3H]leucine injection into the habenula. The endings are of two types, S and crest synapses. Previous descriptions of the S synapses based on indirect identification were confirmed. The crest synapses in the interpeduncular nucleus consist of two habenular afferents contacting a narrowed dendrite. Injection of both habenulae with radiolabeled amino acid resulted in autoradiographic labeling of both axons at 48% of individual crest synapses, and of one axon at 39%. However, when only one habenula was injected, no more than one axon was labeled at individual crest synapses; this occurred at 59%. Interpretation of unlabeled endings is possible using the "detection rate" of the method (per cent ratio of labeled to total endings). The detection rate, which can be determined for the bilateral injection case, allows recalculation of the raw data for the unilateral injection case. Left-right pairing of habenular afferents actually occurs at approximately 90% of crest synapses. The use of detection rates in this way, additionally exemplified by analysis of an electron-microscopic degeneration study, is also important in many other applications of both of these experimental neuroanatomical methods, especially when comparisons between two sets of data are made. The conclusions of studies which have not taken this problem into account must be interpreted with caution.