Head-Twitch Response (HTR)
The head twitch response, usually shortened to HTR, is a rapid movement of the head back and forth that takes place in rodents after the 5-HT2A receptor has been activated. A variety of compounds lead to an HTR in rodents that do not necessarily lead to a similar effect in humans; however, there is usually a correlation between an HTR in mice and an HTR in humans. One of the most common examples is LSD, which leads to an HTR in humans and mice. Even though 5-HT2A receptors are commonly associated with an HTR, triggering the 5-HT2C receptors can lead to a similar response. Some of the other compounds that bind to these receptors include atropine, yohimbine, and phencyclidine.