Effect of Morphine on Changes in Cutaneous Blood Flow Induced by Antidromic Stimulation of Primary Afferent Fibers in the Hind Instep of Rats

Abstract

The effects of morphine on the release of immunoreactive substance P (iSP) into the subcutaneous perfusate and the changes in cutaneous blood flow (CBF) elicited by antidromic stimulation of sectioned sciatic nerve were investigated in the instep of the hind paw of rats. Antidromic stimulation of the sectioned sciatic nerve induced a marked increase in iSP release into the subcutaneous perfusate and a biphasic flow response consisting of an initial transient decrease followed by an increase. Both the iSP release and the increase of the CBF evoked by antidromic stimulation (the second phase) were significantly inhibited by intra-arterial (i.a.) infusion of morphine (30 μmol/kg). These inhibitory effects of morphine were antagonized by pretreatment with naloxone (2 mg/kg, i.p.). The i.a. infusion of SP (0.25 μmol/kg) induced a biphasic flow response similar to that elicited by antidromic stimulation of the sectioned sciatic nerve. Neither phase induced by i.a. infusion of SP was affected by preinfusion of morphine (10 or 30 μmol/kg, i.a.). We suggest that morphine applied locally mainly acts on the peripheral endings of small-diameter afferent fibers, not on blood vessels, and that activation of this site is involved in the regulation of the microcirculatory hemodynamics of cutaneous tissue through inhibition of SP release.