 Gama amino butyric acid GABA produced by inhibitory neurons calms symptoms of anxiety, irritability and aggression. Part of the receptors, called GABAA on the neurone surface, necessary for GABA to have its effect, cannot be made without the break-down products of progesterone. The occurrence of severe symptoms has been correlated with falling levels of progesterone metabolites. Therefore, progesterone could relieve the symptoms of PMS by preventing falling levels of progesterone metabolites and loss of GABAA enhancement. 