Connections of the insular cortex in kittens: an anatomical demonstration with wheatgerm agglutinin conjugated to horseradish peroxidase technique.

The postnatal development of the afferent and efferent connections of the feline insular cortex was investigated using wheat germ agglutinin conjugated to horseradish peroxidase method. The overall pattern of connections was found to be substantially the same as that observed in adults. In the cortex, anterograde and retrograde labeling was found in the presylvian sulcus, cingulate gyrus, cruciate sulcus, medial prefrontal area, lateral suprasylvian cortex, and posterior rhinal sulcus. Subcortical regions containing label included the lateralis medialis-suprageniculate nuclear complex, ventral medial thalamic nucleus, claustrum, lateral amygdaloid nucleus, hypothalamus, and raphe nuclei. In addition, axonal labeling was seen in the striatum, superior colliculus, and pontine nuclei. In contrast, the insular associational projections in kittens differed markedly from those found in adults. In the early postnatal period (1-14 days), dense terminal labeling was found in the cortical layers I, V and VI as well as in the underlying white matter, whereas only moderate to sparse labeling was observed in layers II, III and IV. By four-weeks, an adult-like distribution of terminals was present in each cortical areas: labeling was found mainly in layers I, II, III and IV, with less in layers V and VI. The present results suggest that the basic framework of the insular connections is formed prenatally and that the fine tuning of the axonal terminals and the formation of synapses occurs mainly during the first four postnatal weeks.