**3. Animal models of SCS for inflammatory pain**

The injection of a chemical inflammatory agent in a skin dermatome has been effectively used for the study of the mechanism of acute and persistent inflammatory pain [62]. Such agents include solutions of formalin, complete Freund's adjuvant, or carrageenan (CAR). The CAR model has been adapted to study the effect of SCS in inflammatory pain in the limbs of rodents. Cui et al. [63] reported the first account of the utilization of a carrageenan-based model of chronic nociceptive pain coupled to SCS. The pain model was adapted from Woolf and Doubell [64] and consisted of the injection, under standard halothane anesthesia, of 0.15 mL solution of carrageenan lambda (in 0.9% saline to a concentration of 1%) in the mid plantar part of the hind paw. The inflammatory agent induces an edema at the site of injection and decreases the threshold for paw withdrawal or vocalization to mechanical stimuli in the affected area. Adult male Sprague-Dawley rats epidurally implanted, under anesthesia, with a monopolar cathode (2 x 3 mm2 ) placed retrograde in the L1-L3 vertebral region via a laminectomy in T11. The anode was placed subcutaneously in

the supravertebral region adjacent to the cathode. Rats were allowed to recover for 3 days post-surgery before any experimentation. Current-controlled SCS was applied using a pulsed signal with a width of 200 μs at a frequency of 50 Hz. The intensity was set to 67% of the MT and was on average 1.0 (± 0.3) mA. These parameters correspond to those used clinically during conventional SCS therapy. Treatment was applied for 30 minutes at 3 h, 1, 3, 5, 7 and 9 days after injection of the CAR solution. SCS was also applied to another group of animals 3 days (30 minutes/day) before injection besides the timepoints after injection. The study also included control animals that were injected with CAR but were not implanted with the SCS system, and animals that were subjected to SCS but were not injected with CAR. The extent of local inflammation was determined by measuring the circumference of the metatarsal region and mechanical sensitivity was measured by recording paw withdrawal or vocalization upon applying pressure to the affected paw. A pressure gauge measured the force (in g) that elicited the withdrawal or vocalization response.

The mean circumference of the edema at the paw was around 30 mm before injection (no edema) and increased to a maximum at around 43 mm 3 hours after injection. The edema gradually decreased back to baseline at day 7-9 post-injection. The mean paw withdrawal/vocalization threshold was 215-220 g before CAR injection and decreased to 77 g at 3 hour after CAR injection. Mechanical hypersensitivity reduced gradually and reached baseline at around day 7 post-injection. The reduction in mechanical hypersensitivity correlated well with the reduction in the edema (**Figure 6**). The application of 30 min of SCS at every time point produced a significant increase of the size of the edema until day 5 post-injection (**Figure 7**). However, mechanical sensitivity was only significantly increased by SCS at the 3h point after CAR injection. At day 3, mechanical sensitivity was reduced significantly relative to the pre-stimulation value and was similar at days 7 and 9 postinjection. Authors reported that application of SCS pre-emptively did not provide a beneficial effect. Application of SCS in the absence of the inflammatory insult did not produce significant changes in circumference size and mechanical sensitivity.

Authors concluded that this model is a representation of subacute pain between the third day post CAR injection and the 14th day, which is concomitant to the invasion of different types of inflammatory cells, while the stage previous to the

#### **Figure 6.**

*Correlation between edema size resulting from CAR injection as measured by the mean circumference of the metatarsal level of the paw and the change in the threshold force (g) that elicits paw withdrawal or vocalization. Labels by every post-injection point indicates the time point. The values at days 7 (7d) and 9 (9d) post-injection are the same as the pre-injection values. Values obtained from reference [63].*

**Figure 7.**

*Change in the edema size (left) and mechanical hypersensitivity (right) as a result of 30 minutes/day of SCS. A positive change in withdrawal threshold is equivalent to a reduction in hypersensitivity. \* indicates a significant difference (p < 0.05). Values obtained from reference [63].*

third day post-injection represents an acute pain phase. It was surprising to observe that the SCS signal applied in this study increased the size of the edema as well as mechanical sensitivity in the early stage of the acute phase. However, the increase in hypersensitivity was not likely causal to the edema, and rather may be due to plasticity changes in the dorsal horn as a result of the stimulating signal and the acute inflammatory process. Once the inflammatory processes have settled in during the subacute phase, authors hypothesized that SCS provides an analgesic effect due to the inhibition of neuronal hyperexcitability due to A fiber-mediated wind up.

Years later, the same group reported [49] on using the CAR-based inflammatory pain model to assess the effect of signal rate at low intensities in the acute stage of the model. This was motivated by a shift in SCS paradigm resulting from the clinical introduction of a SCS therapy that provided pain relief at intensities below the perception threshold, thus removing the need for paresthesia overlap of the painful dermatome required by conventional SCS which operates at low frequency (50 Hz), in contrast to the high frequency (10 kHz) used by the novel therapy. The authors utilized signals at 50 Hz (conventional frequency), and 10 kHz, which is referred to as high frequency SCS (HF SCS). In the same report, the authors also studied the effects on HF SCS (500 Hz, 1 kHz, and 10 kHz) on the SNI model, which was allowed to develop more chronically (2 weeks after nerve injury) before SCS intervention. In this work, authors used male adult Wistar rats, which were injected with 0.15 mL of a 1% saline solution of CAR in the hind paw as previously described [63]. Animals were implanted with a SCS system consisting of a paddle lead with four circular poles (0.9-1.0 mm diameter) spaced by 1.8-2.0 mm, which was introduced epidurally via laminectomy at the T13 vertebral level and placed anterograde to cover the T10-T12 levels. This was a variation from the previous report [63]. Animals were left to recover from surgery for 48 hours before any additional experimental intervention. Pain-like behavior was tested before injection (baseline, before stimulation and after 120 minutes of SCS at days 1, 2 and 3 after CAR injection. The test consisted of applying force progressively with clamping forceps (algometer) terminated in a blunt tip in the affected paw until the animal withdraws the paw. The algometer was equipped with a pressure gauge that reads the force exerted (in g) at the threshold of paw withdrawal. In contrast to the previous study by this group, the circumference of the metatarsal level of the paw was not measured, so the effect of SCS frequency on the edema was not determined. SCS was distributed to the four contacts in the paddle lead using adjacent bipoles (+-+-, rostral to caudal). Conventional SCS (50 Hz) used monophasic pulses 200 μs wide and current-controlled intensity set to 80% of the MT, corresponding to intensities

in the 0.48 to 0.64 mA range. The HF-SCS (10 kHz) monophasic pulses were 24 μs wide with intensities set in the range 0.3-0.4 mA, which correspond to 40-50% of the MT, which were defined as subparesthetic based on observation of behavioral responses. Untreated animals served as control. Mean paw withdrawal threshold was 72 g before injection and decreased to around 19 g 1 day after CAR injection. As expected, for this model, the mean withdrawal threshold gradually increased reaching around 30 g and 56 g at days 2 and 3 post-injection, respectively (**Figure 8**).

Neither conventional SCS nor HF-SCS provided a significant improvement of acute inflammatory pain over the course of 3 days, although it is worth mentioning that, in contrast, their previous work reported a significant difference at 3 days post CAR injection. The authors did not comment on their counter results, but is plausible that the position of the lead, which was reported to be a differing factor may have influenced the outcome. A lumbar location may modulate neural circuits of the hind paw more effectively than a thoracic location used in this study. The findings for the acute inflammatory pain were similar to what was found when healthy animals were subjected to SCS and a test of acute pain (pinch force with a pointy tip in the algometer), but in contrast to the results observed in the neuropathic chronic pain model, in which 120 minutes of both conventional and HF-SCS treatments reduced mechanical hypersensitivity significantly. Thus, it can be concluded from the study that neither HF-SCS nor conventional SCS provide relief from acute inflammatory pain (as well as acute nociceptive pain), in agreement with previous clinical and well-controlled observations [65–67] and had led to the establishment of a segmental mechanism of action in which SCS works by modulating conduction of dorsal column fibers within a particular segmental circuitry that has reached

**Figure 8.**

*Effect of SCS treatments on acute inflammatory pain. Values obtained from reference [49].*

#### *Animal Pain Models for Spinal Cord Stimulation DOI: http://dx.doi.org/10.5772/intechopen.96403*

a level of central sensitization during the establishment of chronic pain due to a peripheral injury, such as in the SNI model of neuropathic pain tested.

Recently, Sato et al. [46] reported the utilization of a CAR-based model to study the effect of conventional SCS on joint inflammatory pain. This group had previously found [68] that the unilateral intraarticular anterior injection of 0.1 mL of a 3% solution of lambda carrageenan (type IV, dissolved in 0.9% saline) into the left knee joint of rats induces pain-like behavior that manifests as increased thermal (hot) and mechanical hyperalgesia in the ipsilateral hind paw and knee. The model can be used to evaluate acute or chronic onset of inflammatory pain. In their recent work, they implemented this joint inflammatory pain model to test the effect of short doses (15 min/day) of conventional SCS (60 Hz, 250 μs PW, intensity at 90% of the MT) on thermal and mechanical hyperalgesia using an acute stage of the model. The SCS system had been previously described [48, 69] and consisted of a stimulation lead epidurally introduced, under anesthesia, via laminectomy at the T13 vertebral level and positioned rostrally. The authors did not provide details on the lead design and final position of it relative to spinal levels within the epidural space. Lead wires were tunneled to an internal neurostimulator (Interstim iCon, model 3058, Medtronic Inc., Minneapolis, MN) implanted subcutaneously in the left flank. This allowed animals to roam freely in their cages while being stimulated. Animals were tested for both paw and knee withdrawal to noxious mechanical stimuli before CAR injection, and 30 min before and after SCS (15 min) on days 1, 2, 3, and 4 after injection. Paw withdrawal thresholds were obtained using von Frey filaments with bending force in the range 1-402 mN applied to the plantar surface of the paw ipsilateral to the affected joint. Measurements in the contralateral paw served as internal controls. Knee withdrawal thresholds were measured by compressing the affected extended knee with a pair of calibrated forceps (30 mm2 tip) until the knee was withdrawn due to the applied force. Mean paw and knee withdrawal thresholds are shown in **Figure 9**.

An acute application of conventional SCS, as applied in this work, improved mean withdrawal thresholds significantly relative to the pre-SCS state, implying that conventional SCS may be used to treat acute inflammatory pain. The study did not address the effects of a chronic inflammatory state, which is achievable with this CRA model. Similar to what was reported for effects on the SNI neuropathic pain model, the effect is reversible and reproducible over the different days of treatment. It has been established that SCS modulates inflammatory processes in the stimulated area of the spinal cord that contain neural circuits associated with the painful areas. These

#### **Figure 9.**

*Effect of SCS treatments on acute inflammatory joint pain as reflected in the ipsilateral paw and knee. All post-SCS values are significantly increased (p < 0.05) relative to Pre-SCS values. Pre-SCS values are significantly reduced relative to pre-injection. There is no significant difference between treatment days. Values obtained from reference [46].*

neural circuits contain neurons and other abundant non-neuronal cells that are highly involved in the establishment and chronification of pain, even at early stages. It is quite interesting to note that the effect of SCS sets in as early as 1 day to alleviate hyperalgesia associated with acute knee inflammation, which was not observed by Linderoth and coworkers when inflammation was elicited in the hind paw [49, 63]. It is plausible that the circuits operating in the thoracic region for inflammation of the hind paw are not as effective for SCS, in contrast to treat inflammation of the knee joint.
