Modification of left‐right pairing during the development of individual crest synapses in the rat interpeduncular nucleus

The synaptic organization of the rat interpeduncular nucleus is highly ordered in the normal adult. By 90 days of age, 90% of crest synapses in its intermediate subnuclei are formed by two cholinergic endings, one from each medial habenula. Stereological calculation of the number of crest synapses per intermediate subnucleus, based on total samples of crest synapses in 3–4 sections through the subnucleus, allows comparisons of afferent pairing among ages without interference by other developmental changes. Between 21 and 90 days of age, the total number of crest synapses per intermediate subnucleus increases tenfold (p ≤ 10 −8 ), from 90,000 at 21 days of age, through 130,000 at 28 days, 440,000 at 45 days, to 1,000,000 at 90 days. The volume of the intermediate subnucleus increases fivefold during the same interval. Electron microscopic degeneration was used to estimate the pairing of left and right habenula afferents at crest synapses at the same ages. Through 21 days of age, only one‐third of crest synapses are formed with pairing of one left and one right medial habenula afferent, whereas two‐third have both afferent endings arising from the same medial habenula. At 28 days of age left‐right pairing has increased to 43%, and at 45 days of age 53%, or 240,000, are so paired. The number of same‐side paired crest synapses at 45 days, 210,000, is 3.5 times the number so paired at 21 days (p = .003). This indicates continued formation of considerable numbers of crest synapses with this transient mode of pairing as late as 45 days of age. Since the number with same‐side pairing in the adult, 100,000, is half the number at 45 days (p ≤ 10 −8 ), remodeling of afferent pairing must continue to be an important process in the development of interpeduncular nucleus throughout the period of synaptogenesis. Conversely, the data are inconsistent with the principal alternative, a marked change in synapse formation after 21 days of age such that newly forming crest synapses are initially left‐right paired. The remodeling may involve any of several modes of synapse elimination and/or altered pairing. It appears to be similar to remodeling observed in a number of other systems, although these have rarely addressed connectivity at the level of individual synapses. Since the cellular and molecular mechanisms of remodeling in general remain to be discovered, a system where these processes can be experimentally approached in terms of their level of action at individual synaptic sites may facilitate analysis.