Postnatal synaptogenesis in the rat interpeduncular nucleus.

The rat interpeduncular nucleus (IPN) was studied by electron microscopy from the day of birth to 33 days of age. The habenulointerpeduncular (H-IPN) axons, the principal afferents to IPN, were prominent at birth, occurring in large groups. They formed occasional S synapses en passant at this time. S synapses subsequently increased progressively in number and maturity until 28 days of age. Crest synapses, also formed by the H-IPN axons, appeared at eight days without a recognized formative stage. They exhibited their diverse adult morphology by 14 days of age. F synapses, endings of uncertain origin that contain flattened vesicles, were seen at 21 days, although endings containing small vesicles at 14 days represented the same population. Axosomatic synapses were first seen at four days and remained infrequent subsequently. Dendritic growth cones had their maximal prevalence at birth, decreased at four days and were absent by six days of age. Thus each of the four synaptic types previously described in the adult rat IPN exhibited a characteristic time of appearance and pattern of development in normal rat pups. This well-ordered sequence of synaptogenesis makes the H-IPN system well suited for a study of synaptic plasticity.