Project Abstract Synaptic transmission has been studied for decades, revealing many of the biochemical cascades that trigger and regulate neurotransmitter release, molecules that are used for neurotransmission and neuromodulation, and the postsynaptic consequences of binding these signaling molecules to their receptors. Nevertheless, many aspects of neurotransmission in the mammalian brain are poorly understood and linking the properties of synapses to the function of circuits, and eventually to behavior, has been difficult. Our studies in the last few years have revealed many surprising aspects of neurotransmission including that many neuromodulatory neurons, including midbrain dopaminergic and basal forebrain cholinergic ones, release GABA and sometimes use atypical methods of GABA handling. Furthermore, somatostatin neurons of the entopeduncular nucleus release both GABA and glutamate whereas cholinergic interneurons of cortex are actually a subclass of vasoactive intestinal peptide GABAergic interneurons. Thus, there are many mysteries surrounding the neurotransmitter identity of neurons in the mammalian brain as well as how these unexpected features provide mammalian circuits with specialized functions. Here we propose to use a variety of approaches to understand the identity of neurons in the mouse brain and how their specialized properties endow the brain with unique features. The proposed studies build on approximately 20 years of NINDS funded research into the determinants of the structure and function of mammalian synapses.