Dual effects of brain sparing opioid in newborn rats: Analgesia and hyperalgesia

Abstract

Effective pain management in neonates without the unwanted central nervous system (CNS) side effects remains an unmet need. To circumvent these central effects we tested the peripherally acting (brain sparing) opioid agonist loperamide in neonate rats. Our results show that: 1) loperamide (1 mg/kg, s.c.) does not affect the thermal withdrawal latency in the normal hind paw while producing antinociception in all pups with an inflamed hind paw. 2) A dose of loperamide 5 times higher resulted in only 6.9 ng/mL of loperamide in the cerebrospinal fluid (CSF), confirming that loperamide minimally crosses the blood-brain barrier (BBB). 3) Unexpectedly, sustained administration of loperamide for 5 days resulted in a hyperalgesic behavior, as well as increased excitability (sensitization) of dorsal root ganglia (DRGs) and spinal nociceptive neurons. This indicates that opioid induced hyperalgesia (OIH) can be induced through the peripheral nervous system. Unless prevented, OIH could in itself be a limiting factor in the use of brain sparing opioids in the neonate.

Keywords: Blood-brain barrier; Loperamide; Neonatal; Opiate induced hyperalgesia; Peripheral analgesia.

Fig. 1

Effect of a single dose of loperamide on thermal withdrawal latency, and systemic vs. central distribution. (A) Loperamide (1 mg/kg) or its vehicle were injected s.c. and 30 min later carrageenan (1% in 0.9% saline, 20 μl, intradermal) was injected in the left hind paw. The antinociceptive effect of loperamide was then monitored for the following 4 h (n = 10 for both groups) using the Hargreaves plantar test. P values are obtained after comparing Vehicle vs. Loperamide groups at each time point. (B) Concentrations of loperamide in the serum and CSF. Mass spectrometry showed that loperamide poorly penetrates the blood-brain barrier in neonates (P3). Six hours following 5 mg/kg, s.c., the concentration of loperamide in serum was 334 ng/mL, while in the CSF it was 6.9 ng/mL. P values are obtained by comparing the concentration of each treatment group with the 3 others. * P < .05, ** P < .01, *** P < .001.

Fig. 2

Effect of daily administration of loperamide (1 mg/kg, s.c.) for 5 days (P3 to P7) on nociceptive paw withdrawal latency and DRG neuron membrane conductance. (A) Behavioral hyperalgesia to a heat stimulus appeared on the 4th day (P6) of loperamide administration (P values are obtained by comparing Vehicle vs. Loperamide groups at each time point). (B) Patch clamp recordings of small diameter DRG neurons were done on the 5th day (P7) of loperamide administration. Neurons in the loperamide group showed lower rheobase (i) and membrane threshold (ii) compared with the vehicle group. Numerals in each column stand for the number of neurons recorded. n = 10 rats for both groups; * P < .05, ** P < .01, *** P < .001.

Fig. 3

Fos expression in the lumbar spinal cord following daily administration of loperamide (1 mg/kg, s.c.) for 5 days. Photomicrographs of representative sections of the superficial dorsal horn from vehicle-carrageenan (A), and loperamide-carrageenan (B) treated pups show greater number of Fos positive neurons for loperamide treated pups. Arrows in A and B point to Fos positive neurons. In C, the histogram shows the average number of Fos positive cells for individual treatment groups. Within each treatment group (Vehicle or Loperamide) the number of Fos cells after a saline vs. carrageenan stimulus is compared (white bars vs. red bars) and the p values are showed with stars (*). A second level of comparison is made between the Vehicle and Loperamide, and the p values are showed with hash tags (#). *** P < .001, ### P < .001. Data are from 8 animals per group. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)