These groups were studying the responses of large pyramidal projection neurons to GABA, the main inhibitory neurotransmitter in the central nervous system. GABA is typically released by small interneurons in many regions of the brain, where its job is to inhibit the activity of primary neurons, such as the CA1 pyramidal neurons of the hippocampus or the Purkinje cells of the cerebellum. Activation of GABA receptors on these cells, whether they are ionotropic or metabotropic, typically results in the influx of chloride ions into that target cell. This build-up of negative charge from the chloride ions results in the hyperpolarization of the target cell, making it less likely to fire an action potential. Accordingly, any ionic current that hyperpolarizes a cell is called an inhibitory current.