Elsevier

Neuroscience

Volume 15, Issue 4, August 1985, Pages 1049-1086
Neuroscience

Research paper
Neurotransmitters of the hypothalamic suprachiasmatic nucleus: Immunocytochemical analysis of 25 neuronal antigens

https://doi.org/10.1016/0306-4522(85)90254-4Get rights and content

Abstract

An immunocytochemical analysis with 33 antisera was undertaken to investigate the localization of 25 different neurotransmitter-related antigens in the hypothalamic suprachiasmatic nucleus in the rat. To obtain estimates of relative densities of immunoreactive axons a stereological approach was used involving counting of intersections of immunoreactive axons with a superimposed semi-circle test grid.

All neurotransmitter-related antigens found in perikarya within the suprachiasmatic nucleus, including those stained with antisera against bombesin, gastrin-releasing peptide, neurophysin, vasopressin, somatostatin, γ-aminobutyrate, glutamate decarboxylase and vasoactive intestinal polypeptide were also found in axons within the nucleus. A greater number of these immunoreactive axons was found within the nucleus than in the adjacent anterior hypothalamus. The size of all immunoreactive axons in the suprachiasmatic nucleus was consistently small; immunoreactive axons were found ramifying widely in the nucleus, often ending with terminal boutons near perikarya immunoreactive for the same antigen. All neurotransmitter-related substances found in perikarya of the suprachiasmatic nucleus were also found in axons crossing over the midline to innervate the contralateral nucleus, providing an anatomical substrate for a high degree of communication between the paired nuclei. Axons immunoreactive for other putative transmitters including serotonin arising outside the nucleus were also found in high densities within the nucleus and crossing over the midline between the nuclei. Immunoreactivity for some transmitters was found in axons of similar densities within and outside the nucleus, including antisera against tyrosine hydroxylase; a small number of dopamine β-hydroxylase and a few phenylethanolamine N-methyltransferase-immunoreactive axons were found in the SCN, suggesting that dopamine, norepinephrine and epinephrine may occur in a limited number of axons in the nucleus.

Small numbers of axons immunoreactive with antisera raised against cholecystokinin, prolactin, substance P, thyrotropin-releasing hormone and choline acetyltransferase were found within the suprachiasmatic nucleus. Axons immunoreactive for luteinizing hormone-releasing hormone, adrenocorticotropic hormone, α-melanocyte-stimulating hormone and neurotensin were rarely found within the suprachiasmatic nucleus; axons immunoreactive for luteinizing hormone-releasing hormone, adreno-corticotropic hormone, cholecystokinin and tyrosine hydroxylase were found in both horizontal and coronal sections in the area between the left and right suprachiasmatic nuclei. Tyrosine hydroxylase-immunoreactive cell bodies were situated in the same midline area, as well as at the lateral and dorsal borders of the suprachiasmatic nuclei. Although axons immunoreactive for any one substance were not found exclusively in any one area of the nucleus, regional differences in axon density were found; bombesin- and gastrin-releasing peptide-immunoreactive axons, for instance, were more frequent in the ventral suprachiasmatic nucleus near immunoreactive perikarya, while vasopressin-neurophysin immunoreactivity appeared greater in the dorsomedial part of the nucleus. Efferent projections from the nucleus to hypothalamic and extrahypothalamic areas are discussed and the probable cross-reaction of different antisera with a single antigen is considered.

The finding of many different putative neurotransmitters within the suprachiasmatic nucleus serves to underscore the complex axonal organization of the nucleus.

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