Are there metaphoric expressions to express the frustration you feel when you are not allowed to speak up before people?

Solution 1:

In English, we use several metaphors to express this feeling which arises from being forbidden to speak.

• pent-up anger, trapped like an animal in a pen
• bottled up anger, trapped like a fluid in a sealed container
• stifled or smothered anger, stopped from healthy flow like a person who is suffocating

Bridle? Maybe. Literally, bridle, n., is the device for controlling a horse's head. Bridle, v., can mean to control a person or to control one's own visible reactions, as a rider controls a horse using a bridle (trans.). Or, it can mean to react with anger (not necessarily hidden) when one's will is interfered with (intrans.). The problem, then, is that bridled anger may be misunderstood to mean the act of bottling up an emotion, when what you meant was the resulting feeling.

Responding to the follow-up question about where we locate stifled anger: emotional reactions are located in the stomach not because of a cultural metaphor but because of a physical cause. Release of the stress hormone cortisol stimulates the body's “fight or flight” systems and disrupts other systems which might compete for resources, notably digestion. Extreme ongoing emotion can even cause evacuation of the digestive tract and bladder (the person may vomit, urinate, and defecate uncontrollably). This is thought to be an adaptation which enhances survival when one's life is threatened by an attacker.

In English, we often describe the experience of cortisol's effects as a visceral feeling, butterflies in the stomach, knot in the stomach, or sinking feeling in the pit of the stomach.

Several emotions can trigger the stress reaction, such as fear, anger, worry, and excitement. In the case of pent-up anger, the stress is continuous because the situation is unresolved, so we are likely to feel a knot in the stomach.

Solution 2:

We don't have such a colorful expression, but we do use the verb bridle to describe our reaction to being controlled or stifled.

bridle 2 [ no obj. ] show one's resentment or anger, esp. by throwing up the head and drawing in the chin: ranchers have bridled at excessive federal control. [NOAD]

Now, as a noun, a bridle is "headgear used to control a horse" and the first sense of the verb reveals the figurative sense in entry 2 above:

bridle verb
1 [ with obj. ] (usu. be bridled) put a bridle on (a horse). • bring (something) under control; curb: the fact that he was their servant bridled his tongue.

So when we bridle at something we are metaphorically pulling at (and resenting) some restraint.

Solution 3:

It looks like in Japanese culture Anger is (Fluid) Pressure, whereas in English Anger is Heat.

These are congruent -- not contradictory -- metaphors since heat produces steam, whence the steam engine, etc. But they have different implications.

• Pressure, once built up, must be released (the "emergency valve"). See Javanese amok.
• Heat, however, can be turned down rapidly -- at least in our modern experience, which includes far fewer fires and far more electric and gas heaters than previous generations.

So if somebody gets "hot under the collar", they'll cool down if you wait a while.

This is quite different from the Japanese scenario presented in the question.

Solution 4:

People are often said to fume (feel or show resentment or vexation) when they are angry but not able to express this overtly.

The metaphoric space surrounding fire is well colonised in English idiomatic usages relating to anger. We get fired up (expand Word Origin & History at the end of that link) and blaze with anger, for example. If we can give voice, we vent our frustration/anger, but just like a suffocated fire unable to burn freely, we fume if we can't express our feelings openly.

Solution 5:

In English we do have similar metaphors, like someone's "stomach churning" or how his "guts were twisted in a knot" when he is angry, upset, or frightened. But these are not restricted to cases of being forcibly silenced. It could be anger, frustration, etc, about anything.

We have numerous terms to describe the action: the metaphorical "muzzle" that Gnawme mentions, plus the literal "censor", "silence", etc. But metaphors for the resulting emotion? I can't think of anything specific to that experience.