Are there chemicals that cause pain by acting directly on pain receptors?

Time for a very morbid question.

I know there's capsaicin. Are there other chemicals that cause pain receptors to go crazy not by causing damage but by 'lighting up' the receptors more directly? If a person received an intravenous dose of capsaicin, would pain receptors throughout the body activate? What about another chemical? Again, the key here is that the pain is caused by activating the receptor, not by doing damage, unless activating the receptor is inherently damaging.