A number of SRIs were under development that had auxiliary interactions with other receptors. Particularly notable were agents behaving as co-joint SSRIs with additional antagonist activity at 5-HT1A receptors. 5-HT1A receptors are located presynaptically as well as post-synaptically. It is the presynaptic receptors that are believed to function as autoreceptors (cf. studies done with pindolol). These agents were shown to elicit a more robust augmentation in the % elevation of extracellular 5-HT relative to baseline than was the case for SSRIs as measured by in vivo microdialysis.