Neocortex provides direct synaptic input to interstitial neurons of the intermediate zone of kittens and white matter of cats: a light and electron microscopic study.

The existence of direct synaptic input from the neocortex to intermediate zone and white matter interstitial neurons was examined in both neonate and adult cats. This projection was studied by injecting the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) into the neocortex and examining whether cortical efferent axons formed synapses in the intermediate zone or white matter. Anterogradely labeled boutons establishing synapses in the intermediate zone and white matter were found at the electron microscopic level after injecting PHA-L into the primary visual, somatosensory, and suprasylvian cortex. Although labeled synapses were found in the intermediate zone of kittens injected at postnatal days 2 and 6, their morphological features appeared immature compared to those found in kittens aged 3 weeks or in adults. Postsynaptic targets of efferent cortical axons were studied in serial sections and shown to be dendritic shafts and spines. This paper shows that cortical efferent axons contribute synapses to interstitial neurons located in the intermediate zone of kittens and white matter of adults. The functional role of the corticointermediate zone/white matter projection remains to be determined.