In order to characterize each nucleus of the interpeduncular complex; the central nucleus (IPC), the paramedian nucleus (IPC), the outer division of the posterior nucleus (IPO), the inner division of the posterior nucleus (IPI), and the apical nucleus (IPA), the efferent fiber connections from these nuclei were examined both by Fink-Heimer's method and by the retrograde transport of horseradish peroxidase (HRP) using cats.
The efferent fibers from the IPC and IPP projected to the dorsal tegmental nucleus of Gudden via the pedunculotegmental tract and to the mediodorsal nucleus of the thalamus via the fasciculus retroflexus. The IPO projected to the ventral tegmental nucleus of Gudden via the pedunculotegmental tract, and projected to the anterior mamillary nucleus, the lateral hypothalamic nucleus, the preoptic area, the diagonal band of Broca and the septal area via the medial forebrain bundle. The IPA projected to the anterior mamillary nucleus, the lateral hypothalmic nucleus and the preoptic area.
Based on these results, the interpeduncular complex may be divided into a central group including the IPC and IPP, and a posterior group including the IPI, IPO and IPA.