NovelsAlex Chapter 1433 The Monkey's Pain Test
Chapter 1433 The Monkey's Pain Test
Chapter 1433 The Monkey's Pain Test
In the third week after surgery, M7's recovery entered a critical period.
Based on the patterns Eva observed in her previous mouse experiments, this week should be the end of the "silent period." The progenitor cells, after activation, are slowly maturing, and while no apparent functional improvement is visible, dramatic changes are occurring at the tissue level. Scarring begins, CTGF expression gradually increases, and FG-3019 should be starting to take effect during this week.
However, M7 is not a mouse; its damage is greater, its spinal cord structure is more complex, and its recovery period may be completely different.
“We may need to recalibrate the time window,” Eva said at the morning meeting on the seventeenth day after surgery, her laptop displaying M7’s daily activity log. “Look at this metric: the frequency of spontaneous movement in M7’s right leg started to rise slowly from the tenth day after surgery, but stopped for the past three days.”
She zoomed in on the chart, and everyone could see the curve that climbed up and entered the platform.
"What does this mean?" Tang Shun asked.
“This suggests that the progenitor cells may have been activated, but the scar tissue has started to ‘lock’ them in,” Eva said. “Normal neural plasticity requires axons to pass through the injury area, but if the scar tissue becomes too dense, the axons can’t pass through, and function stagnates. In mice, this stagnation period occurs from the end of the third week to the beginning of the fourth week, lasting about seven to ten days, followed by a burst of recovery. If our inference is correct, M7 should show a similar jump after two weeks.”
"Two weeks?" Mainstein frowned. "That means we have to wait another fourteen days to know if we're on the right track?"
“Unless we can see histological changes without euthanizing the animals,” Lina said, “but that would require live imaging, and we don’t have that equipment yet.”
The meeting room was quiet for a while. Yang Ping looked at the chart. The curve that had entered a plateau phase was like a slow-motion diver, suspended in mid-air, not knowing when it would fall.
“We don’t need to wait for histological changes,” he suddenly said. “What we need is another indicator.”
"What metrics?"
"pain!"
Everyone was watching him. Weber leaned forward slightly, like an old wolf that had caught the scent of its prey.
“After spinal cord injury, neuropathic pain is one of the earliest sensory functions to recover,” Yang Ping said. “This is because the pain pathway is more plastic than the motor pathway and responds more quickly to changes in the microenvironment. If the protocellular activation strategy is really working, M7 should recover pain first, then touch and proprioception, and finally motor function.”
"So if motor function jumps in two weeks, does that mean pain and sensation should have recovered much earlier?" Mainstein asked.
"Yes! But we have never systematically evaluated the sensory function of M7 because we assumed that primates' responses to painful stimuli are too complex and difficult to standardize, unlike humans, whose responses to pain can be quantified to a certain extent."
“Perhaps we can standardize it,” Fritz suddenly spoke up, his voice still soft, but this time everyone fell silent to listen to him. “M7 is afraid of chili peppers, so let’s use chili peppers to test M7’s pain.”
"What?" Tang Shun turned his head.
“Once I brought a chili pepper back from the cafeteria and accidentally dropped it next to a dog’s cage. After smelling it, the dog took two steps back and stared at the chili pepper for a long time, refusing to come closer. The next day when I brought the chili pepper closer to it again, it still avoided it.”
“That’s the smell, not the pain,” Lina said.
"Fritz insisted, "I've specifically studied this. When you cut a chili pepper open and put it on sensitive skin, it makes the animal wince in pain. Later, after rigorous research, I found that chili peppers can be used for pain assessment in primates. I even created my own quantitative assessment form."
Yang Ping looked at Fritz for a few seconds, then laughed: "Frittz, you are the strangest scientist I have ever met. Others use fluorescent proteins, you use chili peppers."
Fritz blushed again, but he didn't lower his head. Instead, he looked at Yang Ping and said seriously, "Science doesn't necessarily have to use complicated things. Sometimes, things from the kitchen will do, as long as they are useful."
Weber let out a short laugh.
"Fritz is right. Using chili peppers to test pain is unconventional, but logically sound. The question is, is your quantification method mature? It's like a monkey."
“We can use two stimuli for comparison,” Lina immediately chimed in. “One is a harmless stimulus, like a banana; the other is a harmful chemical stimulus, like capsaicin. If M7 only responds to capsaicin, and the intensity of the response increases over time, then that would be easy to explain.”
“This design is excellent,” Eva said. “Add a mechanical stimulus, using Von Frey fibers to measure the mechanical pain threshold. Although pain testing in primates is not as standardized as in rodents, the M7 is already adapted to the operating environment, so we can try it.”
Yang Ping stood up and drew a table on the whiteboard, with three columns and three rows. The horizontal axis represented harmless stimuli, harmful chemical stimuli, and mechanical stimuli, while the vertical axis represented the preoperative baseline, early postoperative period, and current stage.
“We need three sets of data,” he said. “Baseline data to show that M7 had a normal response to capsaicin and Von Frey before the injury; data from the first three weeks after the surgery to show that the injury caused a loss of sensory function; and then data from the current stage, where if there is recovery, we should be able to see changes in the response threshold.”
“We didn’t test it before the operation,” Tang Shun said. “The M7 was trained by someone else and sent over; we don’t have its baseline data.”
“Then let’s use a control group,” Yang Ping said without hesitation. “Weber, can the lab find some other primates? They don’t need to be spinal cord injury models, just healthy individuals of the same age, to do a set of baseline tests as a reference.”
Weber thought for a moment: "Okay, do behavioral tests without harming them."
"No damage is needed; we only need to measure the normal response and establish a reference range."
"can!"
The testing program began that afternoon. Lena and Fritz bought a huge amount of stuff at the supermarket: red peppers, bananas, oranges, mustard, and peppermint oil. Tang Shun watched them pile up their lab bench like a market stall and couldn't help but shake his head.
"This is the first time I've seen people use things from supermarket shelves for neuroscience research."
“This is called low technology, high concept,” Lina said with a smile. “Professor Yang came up with the name.”
The results of the first round of testing surprised everyone.
M7 showed almost no reaction to banana and orange flavors, and a slight aversion to mint, but it did react to capsaicin. When stimulated with a red chili pepper, it let out a sharp cry, then retreated to the corner of the cage, curled up, and remained curled up for about twenty seconds before slowly relaxing.
More importantly, this reaction wasn't a one-off. Two hours later, Fritz touched M7 with chili peppers again, and it flinched again, but this time the cries were shorter, and the curl-up lasted only about ten seconds. "The reaction intensity has decreased," Lina said, staring at the video playback. "A two-hour interval makes the habituation effect too strong. This doesn't seem like pure pain; it's more like learning to anticipate."
“You’re right,” Eva nodded. “Primates have such strong cognitive abilities that they can learn. The first time they’re afraid, the second time they learn that the chili pepper won’t hurt them, so they’re not so afraid anymore. Using chili peppers to test pain is unreliable.”
Fritz's face darkened. He had thought his discovery would bring a breakthrough to the team, but reality was much more complicated than he had imagined. It seemed that new things matured more slowly than expected.
“But,” Eva changed the subject, “the fact that M7 reacted to chili peppers in the third week after surgery speaks volumes. Think about it, did we do a similar test on it in the first week after surgery?”
Fritz paused for a moment, then his eyes lit up: "I've done it! I put mustard near it, and it didn't react much."
“So the change is real,” Eva said. “From no response at all, to a response, and then to a decrease in the intensity of the response, this timeline perfectly matches the pattern of habituation after the pain sensation is restored. We don’t need to prove that M7 is in pain now, we just need to prove that it can feel pain now, whereas it couldn’t three weeks ago.”
Yang Ping stood by and listened without saying a word. He picked up the red chili pepper, examined it in his palm for a while, and then said something that surprised everyone: "If I were a reviewer, I wouldn't accept this evidence. The chili pepper test is too crude; it's qualitative rather than quantitative. We need a real mechanical pain threshold test."
“But how do you work with Von Frey fibers in macaques?” Mainstein asked. “The strength of those fibers is designed for rodents, and the skin thickness and nerve distribution in primates are completely different.”
“Then let’s make one ourselves,” Yang Ping said. “Use nylon filaments of different diameters to calibrate the bending force, from 0.1 grams to 100 grams, and create a gradient. Lina, can you design it?”
Lina thought for a moment and nodded: "Okay, give me three days."
Three days later, a set of handcrafted Von Frey fiber filaments appeared on the M7's testing bench. Lena spent a weekend in the laboratory measuring the bending force of each nylon filament one by one, repeatedly measuring with an analytical balance and vernier calipers to ensure that each strength level was accurate.
The mechanical pain threshold test for M7 was performed on Monday of the fourth week post-surgery.
The testing method was simple: Fritz took M7 out of its cage, placed it on a soft cushion, gently stabilized its body with one hand, and held the fiber filament in the other. Tang Shun recorded the video from the opposite side, while Eva recorded it from the side.
The smallest one, equivalent to 0.1 grams, was pressed onto the skin of M7's right sole. The fiber bent slightly, and then Tang Shun released it; M7 did not react.
0.5 gram, no reaction.
1 gram, no reaction.
With a 2-gram force, M7's right leg twitched slightly.
5 grams. M7 turned his head, looked at his right leg, and then turned back.
With a short cry, M7 pulled its right leg back.
“10 grams is the threshold,” Eva noted. “What about the baseline data? We need to know what the normal threshold is for healthy macaques.”
Weber thought for a moment: "We just tested those three elderly cynomolgus monkeys, and their leg retraction threshold is between 15 and 20 grams. M7's 10 grams is slightly below the normal range, but it's very close."
"What do you mean by 'below the normal range'?" Yang Ping asked.
“It could be hyperalgesia,” Weber said. “Post-injury central sensitization causes normally harmless stimuli to become harmful. This is common in spinal cord injury patients, but usually occurs in the chronic phase. The fact that hyperalgesia appeared four weeks after the M7 surgery indicates that its sensory pathways recovered very quickly.”
Yang Ping was silent for a few seconds before his gaze fell on M7. The monkey was sitting in its cage, its right leg slightly bent, its posture much more relaxed than a week ago. Its eyes were looking towards the testing platform, the pupils reflecting the fine glint of the nylon threads.
“Hyperalgesia is a double-edged sword,” Yang Ping said slowly. “It proves that the pathway is open, but it also means that the central nervous system is undergoing abnormal changes in excitability. If left untreated, it may develop into chronic neuropathic pain, which is more torturous than paralysis.”
Weber nodded. He had seen too many spinal cord injury patients, some of whom would rather be paralyzed again than endure the constant burning, electric shock, and needle-like pain.
“So we need another indicator,” Weber said, “that can demonstrate that the sensory pathway is recovering without causing pain to M7.”
“Touch,” Eva immediately chimed in, “the tactile pathway and the pain pathway are separate in the dorsal horn of the spinal cord, but their recovery time windows are close. If we can prove that touch is also recovering in M7 and that the recovery time points coincide, then we don’t need to rely on pain data.”
"How do you measure touch?" Lina asked.
“Use cotton,” Fritz said again.
This time, no one laughed, because everyone noticed that Fritz's expression was serious, even somewhat solemn, when he said this, not like he was joking.
"Cotton?" Yang Ping turned around and looked at him.
“Yes, cotton,” Fritz walked to M7’s cage and took a cotton ball out of his lab coat pocket. “Touch its skin gently. If it can feel it, it will have a very slight reaction. Its skin will twitch, or its muscles will have a tiny contraction. This reaction is so weak that it is difficult for the human eye to detect, but it can be captured by a high-speed camera.”
"How do you know all this?" Mainstein asked curiously.
Fritz lowered his head and said, "I've studied it..."
“Then use cotton,” Weber said.
The tactile testing began that afternoon.
Fritz placed M7 on a soft mat and covered its eyes with a soft towel to prevent it from seeing the source of the irritation. Then, using a thin cotton swab with a fluffy cotton ball at the tip, he started from the instep of M7's foot and moved upwards little by little, working his way up the instep, ankle, calf, thigh, waist, abdomen, and chest.
The M7's reaction was recorded simultaneously by two high-speed cameras, one from the side and the other from above. Weber set the frame rate to 1,000 frames per second so that no tiny muscle tremor would be missed.
As the experiments became more complex and the scope wider, Yang Ping needed more data, and he wanted to find something useful within that data.