Intramedullary blood vessels of the spinal cord express V(1a) vasopressin receptors: Visualization by a biotinylated ligand

The neurohypophysial peptide hormone [Arg⁸]vasopressin (AVP) has well documented presser effects in the periphery. These are mediated by vasopressin receptors (VPRs) of the V(1a) subtype, expressed by vascular smooth muscle cells, which induce vascular contraction when activated. AVP also has effects on the vasculature of the brain, where it has been reported to induce both vasodilation and vasoconstriction. The responsiveness of blood vessels of the spinal cord, however, has received little attention. To determine the morphology and distribution of blood vessels within the spinal cord, vessels were vizualised using a mouse anti-rat smooth muscle α actin IgG as primary antibody and fluorescein isothiocyanate-conjugated anti-mouse IgG secondary antibodies. A complementary vizualisation strategy which detected the endogenous peroxidase activity of red blood cells within vessels was also utilised. The characteristics of the structures observed using both visualisation strategies were typical of blood vessels. VPRs were localized using a recently characterized high affinity biotinylated analogue of AVP (PhAcAL(Btn)VP), which is selective for the V(1a) subtype of VPR. PhAcAL(Btn)VP:VPR complexes were subsequently visualized by avidin-Texas red. The pharmacological characteristics of these sites were established using selective analogues of vasopressin and oxytocin. This confirmed that V(1a) receptors were indeed being visualized. The structures observed following visualization of VPRs had the same morphology as the vasculature revealed by the anti smooth muscle α-actin antibody. It can therefore be concluded that the blood vessels of the spinal cord express VPRs and are potentially responsive to AVP. Furthermore, VPRs were detected on capillaries of the microvasculature. As these capillaries are devoid of smooth muscle, VPRs must be expressed by endothelial cells as well as by smooth muscle cells. This distribution of VPRs would enable AVP to regulate local blood flow. The source of the AVP could be the general circulation, or perhaps more likely, to be local release from vasopressinergic hypothalamic neurones which are known to innervate specific regions of the spinal cord.