Formalin Test
Overview
The formalin test is a tonic chemical pain model in which a small volume (20-50 μL for mice, 50-100 μL for rats) of dilute formalin (1-5% formaldehyde in saline) is injected subcutaneously into the plantar surface of one hindpaw. The injection produces a characteristic biphasic pain response: Phase 1 (0-5 minutes) reflects direct activation of C-fiber nociceptors through TRPA1 channel stimulation, while Phase 2 (15-40 minutes) involves central sensitization of dorsal horn neurons, ongoing inflammatory input, and peripheral edema. The interphase quiescent period (5-15 minutes) between the two phases is mediated by active inhibitory mechanisms including descending serotonergic and noradrenergic pathways, and drugs that enhance descending inhibition selectively prolong this quiescent period.
Pain behavior is quantified by scoring the amount of time the animal spends licking, biting, or flinching the injected paw within consecutive time bins (typically 5-minute intervals across a 60-minute observation). The weighted pain score (0 = normal posture, 1 = favoring the paw, 2 = lifting the paw, 3 = licking/biting/flinching) provides graded nociceptive assessment. Analgesics acting at peripheral nociceptors (local anesthetics, NSAIDs) preferentially suppress Phase 1, while centrally acting drugs (morphine, NMDA antagonists, gabapentinoids) predominantly attenuate Phase 2, making the formalin test a powerful tool for distinguishing peripheral from central analgesic mechanisms.
ConductMaze automates formalin test scoring using ventral camera placement beneath a transparent observation platform, enabling continuous paw-tracking without manual scoring. The software detects paw lifting, licking bouts, and flinching events using machine-learning classifiers trained on annotated formalin behavior, and generates time-binned pain scores with Phase 1 and Phase 2 area-under-curve calculations. Automated scoring eliminates inter-rater variability and allows simultaneous testing of multiple animals in adjacent chambers, substantially increasing throughput for analgesic screening studies.
Trial Flow
Baseline Observation
Place animal in clear observation chamber on elevated platform; habituate for 15-30 min
Formalin Injection
Inject dilute formalin (20-50 μL) subcutaneously into the plantar surface of one hindpaw
Phase 1 Recording
Record pain behaviors (licking, biting, flinching) during minutes 0-5 post-injection
Interphase Period
Monitor quiescent period (minutes 5-15) with reduced nociceptive behavior
Phase 2 Recording
Record inflammatory-phase pain behaviors during minutes 15-40 post-injection
Behavior Classification
Classify each time bin: licking duration, flinch count, paw elevation time, weighted pain score
Phase Quantification
Calculate total licking time, flinch count, and AUC for Phase 1, interphase, and Phase 2
Session End
Return animal to home cage; measure paw edema with calipers if required
Parameters
| Parameter | Type | Default | Description |
|---|---|---|---|
| Formalin Concentration | float | 2.5 | Percent formaldehyde in saline (typically 1-5%) |
| Injection Volume | float | 20 | Volume injected into the hindpaw in microliters (20 μL mice, 50 μL rats) |
| Observation Duration | duration | 3600 | Total observation period in seconds (standard 60 min) |
| Time Bin Size | integer | 300 | Duration of each scoring bin in seconds (standard 5 min) |
| Phase 1 Window | duration | 300 | Time window for Phase 1 scoring in seconds (0-5 min) |
| Phase 2 Start | duration | 900 | Start of Phase 2 window in seconds post-injection (15 min) |
| Phase 2 End | duration | 2400 | End of Phase 2 window in seconds post-injection (40 min) |
| Injected Paw | enum | left-hind | Which hindpaw receives formalin injection: left-hind or right-hind |
| Species | enum | mouse | Species: mouse (20 μL, 2.5%) or rat (50 μL, 5%) |
Metrics
| Metric | Unit | Description |
|---|---|---|
| Phase 1 Licking Time | seconds | Total time spent licking or biting the injected paw during Phase 1 (0-5 min) |
| Phase 2 Licking Time | seconds | Total time spent licking or biting the injected paw during Phase 2 (15-40 min) |
| Phase 1 Flinch Count | count | Number of rapid paw flinches during Phase 1 |
| Phase 2 Flinch Count | count | Number of rapid paw flinches during Phase 2 |
| Interphase Duration | seconds | Duration of the quiescent period between Phase 1 and Phase 2 |
| Weighted Pain Score AUC | au | Area under the curve of the composite weighted pain score across the full session |
| Paw Edema | mm | Post-test paw thickness measured with digital calipers |
| Phase 2/Phase 1 Ratio | ratio | Ratio of Phase 2 to Phase 1 licking time, indexing central sensitization relative to nociception |
Sample Data
| Subject | Treatment | Phase 1 Lick (s) | Phase 2 Lick (s) | Phase 1 Flinch | Phase 2 Flinch | Interphase (s) | Paw Edema (mm) |
|---|
Representative data for illustration purposes. Actual values will vary by species, strain, and experimental conditions.
Applications
- 1Analgesic mechanism classification — distinguishing peripheral (Phase 1) from central (Phase 2) analgesic actions
- 2Central sensitization research — studying NMDA-dependent windup and spinal plasticity using Phase 2 as a readout
- 3Anti-inflammatory screening — evaluating COX inhibitors and cytokine antagonists on the inflammatory pain component
- 4TRPA1 pharmacology — assessing TRPA1 agonists and antagonists given formalin acts primarily through this channel
- 5High-throughput pain screening — automated scoring enables simultaneous multi-animal testing for drug discovery pipelines
Related Protocols
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