Intrathecal Ultra-Low Dose Naloxone Enhances the Antinociceptive Effect of Morphine by Enhancing the Reuptake of Excitatory Amino Acids from the Synaptic Cleft in the Spinal Cord of Partial Sciatic Nerve–Transected Rats

Abstract

In this study, we examined the effects of ultra-low dose naloxone on the antinociceptive effect of morphine and on spinal cord dorsal horn glutamate transporter expression in rats with neuropathic pain. Neuropathic pain was induced in male Wistar rats by partial transection of the left sciatic nerve and an intrathecal catheter was implanted for drug administration; in some rats, an intrathecal microdialysis probe for cerebrospinal fluid (CSF) dialysate collection was also implanted. Nociception was assessed using the plantar test, a Hargreaves radiant heat apparatus, and by the von Frey test, using a dynamic plantar anesthesiometer. Glutamate transporter protein expression in the left spinal cord dorsal horn was examined by Western blotting and immunohistochemistry. Levels of the excitatory amino acids (EAAs) glutamate and aspartate in the CSF dialysate were measured using high-performance liquid chromatography. Reduced astrocyte expression of glutamate transporters (GLT-1 and GLAST levels were 55% and 53%, respectively, of that in sham-operated rats) in laminae I and II of the spinal cord dorsal horn ipsilateral to the partial sciatic nerve transection (PST), and hyperalgesia and allodynia in the PST hindlimb were observed. High-dose naloxone (15 μg) attenuated the antihyperalgesia and antiallodynia effects of the morphine (10 μg). In contrast, ultra-low dose (15 ng) naloxone enhanced the antinociceptive effect of morphine (10 μg), with an increase in the paw withdrawal threshold to thermal stimulus (from 19% to 35%) and to tactile stimulus (from 33% to 55%) compared with morphine treatment alone, and this was associated with restoration of GLAST and GLT-1 expression to control levels (102% and 114%, respectively) in the astrocytes of laminae I and II in the spinal cord dorsal horn ipsilateral to the PST hindlimb and a decrease in EAA levels in the CSF dialysate (glutamate: 10.0 μM; aspartate: 1.1 μM). Ultra-low dose naloxone enhanced the antinociceptive effect of morphine in PST rats, possibly by restoration of GLAST and GLT-1 expression in astrocytes, which inhibited the accumulation of EAAs in the synapses, resulting in a neuroprotective effect.