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Ventral lateral geniculate nucleus efferents to the rat suprachiasmatic nucleus exhibit avian pancreatic polypeptide‐like immunoreactivity

The distribution of avian pancreatic polypeptide‐like (APP) immunoreactivity was investigated in the suprachiasmatic nucleus (SCN) of the rat hypothalamus with immunohistochemical methods. Specificity of the antisera was established by the absence of all immunoreactive staining in tissue incubated in antisera which had been preabsorbed with the pure APP antigen. In addition, the antisera exhibited no significant cross reactivity with vasoactive intestinal polypeptide, vasopressin, somatostatin, or secretin. Within the rat SCN, APP immunoreactivity is restricted to varicose axons in the ventral and lateral aspects of the nucleus; the dorsomedial component of the nucleus is totally devoid of immunoreactivity and immunoreactive perikarya are not present in any portion of the SCN. The immunoreactive axons in the ventrolateral portion of the nucleus form an extensive plexus which is distributed in a pattern closely corresponding to the distribution of retinal and ventral lateral geniculate (vLGN) efferents to the SCN. No immunoreactive perikarya are observed in the retina following immunoperoxidase staining for APP and neither unilateral nor bilateral enucleation causes an observable alteration in the pattern of APP axon distribution within the SCN, thus indicating that the fiber plexus is not of retinal origin. In contrast, APP immunoreactive neurons are present in the same area of the vLGN in which retrogradely filled neurons have been demonstrated following iontophoretic injection of HRP into the SCN. Bilateral electrolytic lesions of the vLGN result in a total loss of immunoreactive axons in both SCN. Unilateral vLGN lesions cause a loss of approximately 60 to 75% of the immunoreactive fibers in the ipsilateral SCN with a lesser contralateral loss. These observations provide further information on the organization of afferents to the rat SCN and demonstrate that the vLGN projection is chemically distinct from other SCN afferents.