Substance P antagonist-induced spinal cord vasoconstriction: Effects of thyrotropin-releasing hormone and substance P agonists

Local spinal cord vasomotor effects of 3 substance P (SP) antagonists were studied in the rat following intrathecal (IT) administration. Each SP antagonist (3.3 nmol) increased spinal cord vascular resistance and reduced blood flow. A LH-RH antagonist analog (10 nmol) of similar molecular weight and which also contained multiple D-Trp residues did not cause spinal cord vasoconstriction. The vasoconstrictor action of the SP antagonist, [D-Arg¹, D-Pro², D-Trp^7,9, Leu¹¹]-SP ([D-Arg]-SP) was unaffected by pretreatment with a stable SP receptor agonist (5 nmol IT). Given evidence for a cerebral vasodilator action of TRH agonists, the effects of TRH (IV) and a stable TRH analog (MK-771, IT) on [D-Arg]-SP-induced vasoconstriction were also assessed. Neither TRH nor MK-771 prevented the [D-Arg]-SP-induced vasoconstriction. However, TRH (IV) but not MK-771 (IT) partially opposed [D-Arg]-SP-induced reduction in thoracic spinal cord blood flow. Thus, SP antagonists cause spinal cord vasoconstriction by a non-SP receptor mediated phenomenon. In addition, the attenuation of SP-antagonist-induced neuropathological changes previously reported with IV. TRH administration is likely due to less severe consequences of vasoconstriction in the presence of a higher initial baseline blood flow rather than direct prevention of the vasoconstriction.