Abstract
The suprachiasmatic nucleus (SCN) is the circadian pacemaker in mammals and contains a network of arginine-vasopressin-immunoreactive (AVP-ir) neurons. AVP-recipient cells contain the Vla class of receptors linked to phosphoinositol turnover and protein kinase C (PKC). The present study describes the localization of AVP and the four Ca 2+-dependent PKC-isoforms in the mouse and rabbit SCN. An estimate of the numerical density of AVP-ir neurons at the rostral, medial, and caudal level of the SCN revealed that the mouse SCN contains more than twice the number of AVP-ir neurons than the rabbit SCN. Neurons immunostained for AVP or PKC dominated in the dorsomedial and ventrolateral aspects of the mouse SCN, while the central area of the SCN revealed only weakly stained neurons. The rabbit SCN was characterized by a more homogeneous distribution of AVP-ir and PKC-ir neurons. PKCa was the most abundantly expressed isozyme in both species, whereas the presence of the other isoforms differed (mouse: PKCα > PKC,βI > PKCβI > PKCγ; rabbit: PKC SCN predominantly contained immunolabeled fiber tracts for this PKC isozyme. Astrocytes immunoreactive for each PKC isoform were frequently encountered in the rabbit SCN, but were absent in mice. Immunofluorescence double labeling showed that numerous AVP-recipient cells in the mouse SCN were immunopositive for PKCα, and that nearly all AVP-ir neurons express PKCα abundantly. These results substantiate the putative role for PKCα in vasopressinergic signal transduction in the SCN. The differential expression in degree and cell type of the Ca 2+-dependent PKC-isoforms in the mouse and rabbit SCN may be related to the differences observed in circadian timekeeping between the two species.
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