I use this story to introduce today’s topic - there is so much more to the capsaicin in chili peppers than just heat and pain. We learned last week how capsaicin and other TRPV1 agonists can combat obesity and damage from stroke, but these abilities were related to the TRPV1 function in thermoregulation. Today let’s look at some functions that are so far outside normal TRPV1/capsaicin function that they can be considered exceptional – like jazz piano from the RHCP.
We have explained the mechanism of hair cell action in hearing before. The inner hair cells are bent by the sound wave in the cochlea and converted to a neural impulse to be detected as sound. The outer hair cells work to amplify the wave so that the sound is louder and can register a signal in the inner hair cells.
Scientists know that TRPV1 is expressed on the inner and outer hair cells, and works in the cochlear amplification system particularly. However, we don’t know its exact function(s) there. On the down side, we know that TRPV1 must work in a narrow range, because drugs and agonists that excite TRPV1 can lead to hearing problems.
Acoustic injury (acute or chronic loud noise), gentimicin (an antibiotic), and cisplatin (a cancer drug) can all cause hearing damage. A 2009 study of gentimicin damage to the cochlea showed that it increased TRPV1 expression in the outer hair cells. A 2008 study of cisplatin showed that its damage to the hair cells could be suppressed if you decreased TRPV1 activity.
A 2013 study indicates that TRPV1 is important for the uptake of the drugs into the hair cells, and then they do their damage. An older study shows that even without drugs to be taken up, increased TRPV1 expression is the cause of acoustic injury tinnitus (ringing) and hearing loss. Another paper showed that capsaicin itself blocks the action potential in the outer hair cells and dampens the cochlear amplification system. Why anyone would stick a chili pepper in his ear is beyond me.
So we see that a little TRPV1 activity is good, but too much is bad. This is not to say that eating a lot of peppers will harm your hearing; dietary capsaicin never gets to your cochlea unless you’re a horribly messy eater. We will return to this idea with TRPV1 and capsaicin; it can be an angel or a devil.
Memory and learning
Neural plasticity (the ability to change neural connections, building some, losing others) is important in learning and memory. We have discussed long-term potentiation (LTP) in terms of memory, where a reinforcement of action potentials between two neurons or in a pathway lead to a strong connection and a new item learned or remembered.
The flip side is long-term depression (LTD). This mechanism is what allows you to weaken and lose connections between neurons that aren’t being reinforced. You can’t learn something as well unless you keep reinventing the connections, including losing some. However, don’t get the idea that you have to forget some thing in order to remember something new, it is much more complicated than that.
TRPV1 acts at the level of the hippocampus (important for memory) through an endocannabinoid called anandamide. We already know that anandamide is a TRPV1 agonist. A 2008 study indicated that there was no LTD activity in mice that had no TRPV1 channels. This study found that TRPV1 was “necessary and sufficient” for LTD, meaning that it was needed and only it was needed for the effect.
Another 2008 study points out how this is important for us. Stress brings too much LTD and not enough LTP. When you’re stressed it’s harder to learn new things and easier to lose established knowledge. Amazingly, TRPV1 agonists like capsaicin can regulate this in stress. Stress + TRPV1 agonists led to reduced LTD and improved LTP. It seems that capsaicin and other TRPV1 agonists regulate synaptic plasticity in both directions, keeping us on balance and always learning.
Put simply, without capsaicin channels none of us are here to read this great stuff. TRPV1 activity, through various agonists, is important for sperm motility and fertilization of the egg. The sperm meets the egg and here we go - is that the whole picture? Nope. It takes a lot to arrange their introduction, and some of it involves TRPV1 channels.
You ever wonder why the testicles are kept outside the trunk of the body? It’s to keep the sperm cool – heat kills sperm. TRPV1 receptors sense high heat and institutes local reactions to cool the sperm. A 2008 study showed that TRPV1 knockout mice had much higher levels of sperm death in the testicle when the temperature was raised. It didn’t make the mice sterile, but it was close.
TRPV1 and its agonist anandamide (same as in LTD) are important late in capacitation. They prepare the sperm for transfer of the DNA to the egg.
TRPV1 and endocannabinoid receptors are also responsible for the increase in sperm motility after capacitation. The sperm swim toward the egg, but how do they know where she is? There are many signals, including osmolarity differences, hormones, temperature changes, pH changes, and liquid currents. We have seen that several of these (pH, temp., hormones, osmosis) are sensed by TRPV1 channels.
It’s been shown that TRPV1 on the sperm post-acrosomal and head regions are important for sperm motility. A 2013 study showed that inhibiting TRPV1 slowed down fish sperm. And in a 2013 study of infertile men, the motility of the sperm in men with low levels of anandamide was decreased.
Strangely, even though capacitated sperm are thermotactic (swim toward higher temperatures), this is one time where it DOESN’T involve TRPV1/capsaicin receptors. It still involves calcium, but the flux is through a different receptor system.
Once the sperm finds the egg, it undergoes the acrosome reaction. This makes it possible for the sperm - only one mind you - to enter the egg. A 2009 study indicated that anandamide and TRPV1 are crucial for the acrosome reaction as well.
From inside the male, to inside the female, to inside the egg, capsaicin receptors are crucial for making tiny humans. It doesn’t get more important than that. What I don’t know is if eating peppers affects any of this. Would a habanero make you more fertile or less?
In cancer we see the dual nature of TRPV1 again. In some cases it prevents or stops cancer, while in others, capsaicin might contribute to cancer.
Prostate cancer has been studied a lot with respect to TRPV1. Capsaicin, in particular, has an effect on prostatic cancer cells. It kills the cells and reduces tumor size in mice. However, in humans, little work as been done. One epidemiology study found that Nigerians that eat more peppers have a lower rate and later onset of prostate cancer.
In addition, a single study followed a man whose prostate cancer returned after several disease free years. His PSA value (prostate specific antigen, a sign of cancerous growth) started to double weekly, so he started taking 2 ml habanero sauce once or twice week. His PSA value increases slowed by half. He stopped and they sped up. He started daily habanero sauce and the PSA dropped. Sounds like a treatment to me.
The mechanisms of capsaicin action on prostate cancer cells has been determined in in vitro work. They are both TRPV1 dependent and TRPV1 independent. (studies) Capsaicin increases ROS, induces apoptosis, destabilizes membranes, throws off ion balances…basically everything capsaicin stops when used for protective hypothermia (from last week).
It isn’t just prostate cancer where capsaicin may help. In a mouse model of lung cancer, giving capsaicin prevent the lung tumors from forming by increasing apoptosis in the cancerous cells. Whether this is TRPV1 dependent or independent was not discussed.
But there is a darker side to capsaicin and TRPV1 in prostate cancer. The agonists have a small window of effectiveness. Low levels actually promote cancer growth, but higher levels cause cell cycle arrest and apoptosis. In one study, 10 µM capsaicin actually created prostate tumors in mice.
One epidemiologic study showed that US counties with higher Mexican American, Cajun, and Creole populations have higher rates of these cancers, and it just so happens that these are the folks that eat more chili peppers. Some skin cancers are also associated with capsaicin. This is a bit disconcerting considering the number of topical capsaicin pain creams on the market.
In squamous cell skin cancer, capsaicin can sometimes stunt the growth of tumors via apoptosis, but the mechanism seems to be TRPV1 independent, since a TRPV1 antagonist brings the same effect. The take home message is that TRPV1/capsaicin effects may be diet, individual, cancer specific. This will make it hard to develop therapeutics.
Next week - now its time to start talking about cool receptors. Like how the cigarette companies use "cool" to sell smokes.
For more information or classroom activities, see:
Cochlear amplification –
Sperm capacitation –
Acrosome reaction –