Pre- and postsynaptic determinants of claustro-cortical connectivity

The claustrum (CLA) is characterized by its extensive synaptic connectivity with multiple cortical 23 regions. While CLA projection neurons are glutamatergic, several studies have shown an inhibitory 24 impact of CLA on its cortical targets, suggesting the involvement of inhibitory cortical 25 interneurons. We employed in vivo and ex vivo electrophysiology and optogenetics in mice to 26 dissect the synaptic organization of projections from CLA to the anterior cingulate cortex (ACC), 27 a robust claustro-cortical pathway implied in cognitive functions. Optogenetic stimulation of CLA 28 neurons expressing CaMKIIa or VGLUT2 evoked distinct multiphasic excitatory and inhibitory 29 responses in the ACC that depended on the stimulated CLA population and the recipient layer in 30 the ACC. Stimulation of CLA-ACC axons evoked monosynaptic responses in pyramidal neurons 31 and four types of molecularly defined interneurons. Synaptic responses were uniquely determined 32 by the presynaptic CLA projection type, the recipient cortical layer, and the type of postsynaptic 33 neuron, with interneurons displaying the highest specificity. This intricate organization of the CLA-34 ACC pathway explains the diverse and complex influence of CLA on cortical activity.