Changes in Response Behaviors to Noxious Heat and Mechanical Stimuli After Carrageenan-induced Inflammation in Mice Treated with Capsaicin 2 or 15 days After Birth

The aim of the present study was to investigate the cause of normal response behaviors to noxious heat despite the marked loss of small neurons in the dorsal root ganglion (DRG) as a result of neonatal capsaicin treatment. Capsaicin (50 mg/kg) was injected subcutaneously (s.c.) into mice on either day P2 or P15; control mice received a vehicle injection. Twenty days after the capsaicin injection, 2% carrageenan (20 μL) was injected into the right hind paw of each animal. Twenty-four hours after the carrageenan injection, behavioral tests using noxious heat stimuli (NHS; Hargreaves method) and noxious mechanical stimuli (NMS; von Frey method) were performed using the control and capsaicin-treated mice. Pre-carrageenan measurements were used as the baseline values for each group. After the experiments, the mice were perfused with a mixture of 4% paraformaldehyde and 0.2% picric acid in a 0.1-M phosphate buffer (pH 7.4). The L4 and L5 DRGs were extracted, sectioned using a cryostat, and immunostained using a TRPV1 antibody and reacted with isolectin IB4. The P2 capsaicin-injected mice exhibited a marked increase in their analgesic responses to NHS, compared with both their baseline values and the respective control mice. Both the capsaicin-treated and control animals exhibited significant hyperalgesia in response to NMS. Naturally, the control P15 mice exhibited a shorter response time to NHS than their baseline values, while the capsaicin-injected P15 mice exhibited increased algesia comparable to the baseline values of the control. The capsaicin-injected mice also exhibited algesia in response to NMS. The number of TRPV1-immunoreactive (ir) small neurons decreased by 90% in the P2 capsaicin-injected mice and 45% in the P15 capsaicin-injected mice, whereas an increase in IB4-positive neurons was seen in both the P2 and P15 capsaicin-injected mice. In association with the decrease in larger neurons, the numbers of smaller neurons were increased in both P2 and P15 capsaicin-injected mice. TRPV1-ir small neurons are closely correlated with inflammatory heat pain perception, suggesting the enhancement of an unknown acute noxious heat sensor in the present study. The present findings indicate that noxious mechanical stimuli can be sensed despite the presence or absence of TRPV1-ir or IB4-positive neurons.