Figure 3: Cartoon to depict changes in receptor expression in the PAG at different stages of the oestrous cycle. receptors are expressed mainly by GABAergic interneurones where they are extrasynaptically located and mediate tonic currents. The excitability of output neurons from the PAG is limited by spontaneous activity in GABAergic interneurones. (a) When expression of receptors is low during proestrus (Pro), oestrus (O), and early dioestrus (ED), high levels of activity in the interneurone population limit the excitability of the output neurons. (b) When progesterone levels fall during late dioestrus (LD), increased expression of receptors leads to an increase in tonic current carried by GABAergic cells, thus limiting their on-going activity. The output neurones therefore become intrinsically more excitable, and their threshold for activation is lowered. The figure is adapted from [38].