Korean J Pain.  2023 Jan;36(1):51-59. 10.3344/kjp.22297.

Imbalance in the spinal serotonergic pathway induces aggravation of mechanical allodynia and microglial activation in carrageenan inflammation

  • 1Department of Anesthesiology and Pain Medicine, Chonnam National University Hospital, Chonnam National University Medical School, Gwangju, Korea
  • 2BioMedical Sciences Graduate Program (BMSGP), Chonnam National University Medical School, Hwasun, Korea


This study investigated the effect of an excess and a deficit of spinal 5-hydroxytryptamine (5-HT) on the mechanical allodynia and neuroglia activation in a rodent pain model of carrageenan inflammation.
Male Sprague–Dawley rats were implanted with an intrathecal (i.t.) catheter to administer the drug. To induce an excess or deficit of 5-HT in the spinal cord, animals were given either three i.t. 5-HT injections at 24-hour intervals or a single i.t. injection of 5,7-dihydroxytryptamine (5,7-DHT) before carrageenan inflammation. Mechanical allodynia was measured using the von Frey test for 0–4 hours (early phase) and 24–28 hours (late phase) after carrageenan injection. The changes in the activation of microglia and astrocyte were examined using immunofluorescence of the dorsal horn of the lumbar spinal cord.
Both an excess and a deficit of spinal 5-HT had no or a minimal effect on the intensity of mechanical allodynia during the early phase but prevented the attenuation of mechanical allodynia during the late phase, which was observed in animals not treated with i.t. 5-HT or 5,7-DHT. Animals with an excess or deficit of 5-HT showed stronger activation of microglia, but not astrocyte, during the early and late phases, than did normal animals.
Imbalance in the descending 5-HT pathway in the spinal cord could aggravate the mechanical allodynia and enhance the activation of microglia, suggesting that the spinal 5-HT pathway plays an essential role in maintaining the nociceptive processing in balance between facilitation and inhibition in inflammatory pain caused by carrageenan inflammation.


Carrageenan; Central Nervous System Sensitization; Inflammation; Microglia; Nociception; Pain; Rats; Serotonin; Spinal Cord Dorsal Horn


  • Fig. 1 Groups of animals according to the intrathecal (i.t.) treatment and the intraplantar injection. On day 1, intrathecal 5-hydroxytryptamine (5-HT), 5,7-dihydroxytryptamine (5,7-DHT) or vehicle (Veh) was administered to induce an excess, deficit or no change in the 5-HT concentration in the spinal cord, respectively. On day 3, those animals were given the intraplantar injection with carrageenan (CA) or saline. With these, the animals were classified into 6 groups: No inflammation (Veh/Saline, 5-HT/Saline, 5,7-DHT/Saline) and CA inflammation (Veh/CA, 5-HT/CA, 5,7-DHT/CA).

  • Fig. 2 Effects of 5-HT excess or deficit in the spinal cord on the mechanical allodynia in early and late phase of carrageenan (CA) inflammation. Time course of paw withdrawal threshold (PWT) in early (A) and late phase (B) is shown in upper panel, and the corresponding area under curve (AUC) is depicted in lower panel (C and D). No significant changes are seen in the PWT and AUC in group with no inflammation (Veh/Saline, 5-HT/Saline, 5,7-DHT/Saline), but significant decrease in PWT and AUC was found in animals with CA inflammation (group Veh/CA, 5-HT/CA, 5,7-DHT/CA). The error bars indicate mean ± standard deviation. 5-HT: 5-hydroxytryptamine, 5,7-DHT: 5,7-dihydroxytryptamine, Veh: vehicle. * indicates P < 0.001 vs. group Veh/Saline, #P < 0.05 vs. group Veh/CA.

  • Fig. 3 Effect of additional intrathecal (i.t.) treatment of 5-HT at 24 hours after carrageenan (CA) injection into the hind paw. Time course of paw withdrawal threshold (PWT) and its corresponding area under curve (AUC) in early (A, C) and late phase (B, D) is shown. No significant changes in mechanical allodynia was produced in animals with normal level of spinal 5-HT (group Veh/CA) as well as excess and deficit of 5-HT (group 5-HT/CA and 5,7-DHT/CA). The error bars indicate mean ± standard deviation. 5-HT: 5-hydroxytryptamine, 5,7-DHT: 5,7-dihydroxytryptamine, Veh: vehicle. * indicates P < 0.001 vs. group Veh/Saline, #P < 0.001 (group 5-HT/CA) or P = 0.004 (group 5,7-DHT/CA) vs. group Veh/CA.

  • Fig. 4 Immunofluorescence study for microglial activation in the early and late phase of carrageenan (CA) inflammation in the dorsal horn of lumbar spinal cord. The sections are acquired 3 (A, early phase) and 27 hours (B, late phase) after carrageenan injection, and the images represent the average fluorescent intensity obtained from 4 animals in each group. The percentage of the area of Iba-1(+) cell is presented in the lower panel (C, D). The error bars indicate mean ± standard deviation. 5-HT: 5-hydroxytryptamine, 5,7-DHT: 5,7-dihydroxytryptamine, Veh: vehicle. * indicates P < 0.001 vs. group Veh/Saline, #P < 0.001 vs. group Veh/CA. Scale bar: 400 μm.


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