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The earlier described age-related decreases in vasopressin innervation of extra-hypothalamic rat brain regions were found to coincide with a decrease in vasopressin expression in the cells where these fibers originated. A significant age-related decrease in the number of vasopressin-immunoreactive cell bodies was found in the medial amygdala and locus coeruleus of senescent Brown-Norway (BN/BiRij) rats (33 months) when compared to middle-aged (19 months) and young (3 months) rats. In addition, total testosterone plasma levels were significantly reduced in middle-aged and old rats as compared to young animals and the number of vasopressin-immunoreactive cells in both the medial amygdala and locus coeruleus correlated significantly with the decreased testosterone levels in a similar way as found earlier for vasopressin terminals.

Following two weeks of cerebroventricular administration of arginine-vasopressin (AVP) by Accurel implants, two types of binding sites for this peptide were immunocytochemically visualized in blood vessels in the brain of Brattleboro (di/di) rats: (1) endothelial cells of capillaries were stained with the highest density in hippocampus, striatum, and locus coeruleus (LC), whereas only few such stained cells were present in the septum and cerebral cortex. (2) Bound AVP was also present on endothelial cells and pericyte-like cells in larger blood vessels in striatum and the LC. Both types of vasopressin binding site staining on blood vessels were dose-dependent and could be further enhanced by additional in vitro preincubation of the fixed sections with AVP. This staining was not present in rats implanted with either oxytocin or alpha-melanocyte-stimulating hormone.