Inverse agonist

I found a very nice and clear description of what an inverse agonist is - right in the Wikipedia (just one example of how things are described there sometimes clearer and better than anywhere else.... Go ahead and google "inverse agonist"! Here is a nice graphic and the descriptive text"

"....an inverse agonist is an agent that binds to the same receptor as anagonist but induces a pharmacological response opposite to that agonist.

A prerequisite for an inverse agonist response is that the receptor must have a constitutive (also known as intrinsic or basal) level activity in the absence of any ligand. An agonist increases the activity of a receptor above its basal level while an inverse agonist decreases the activity below the basal level. A neutral antagonist has no activity in the absence of an agonist or inverse agonist but can block the activity of either.

The efficacy of a full agonist is by definition 100%, a neutral antagonist has 0%, while an inverse agonist has < 0% (i.e., negative) efficacy...."

NB" I noted with some pleasure that the first author quoted in the references is a Terry Kenakin: he was student in my department at the U. of Alberta back in 1970/72 or so and has made a big name for himself when it comes to classical analysis of dose response curves (although coming up with hypothesis most have trouble following or seeing the relevance of with respect to medicine...).