Presynaptic and postsynaptic determinants of claustro-cortical connectivity

The claustrum (CLA) is a thin and elongated brain structure that is located between the insula and lateral striatum and is implicated in a wide range of behaviors. It is characterized by its extensive synaptic connectivity with multiple cortical regions. While CLA projection neurons are glutamatergic, several studies have shown an inhibitory impact of CLA on its cortical targets, suggesting the involvement of inhibitory cortical interneurons. We employed both in vivo and ex vivo electrophysiology and optogenetics in mice to dissect the synaptic organization of projections from the CLA to the anterior cingulate cortex (ACC), a robust claustro-cortical pathway implicated in cognitive functions. Optogenetic stimulation of CLA neurons expressing calcium/calmodulin-dependent protein kinase II alpha (CaMKIIa) or vesicular glutamate transporter-2 (VGLUT2) evoked distinct multiphasic excitatory and inhibitory responses in the ACC that depended on the stimulated CLA neuron population and the recipient layer in the ACC. Stimulation of CLA-ACC axons evoked monosynaptic responses in pyramidal neurons and four types of molecularly defined interneurons. Synaptic responses were uniquely determined by the presynaptic CLA projection type, the recipient cortical layer, and the type of postsynaptic neuron, with interneurons displaying the highest specificity. This intricate organization of the CLA-ACC pathway explains the diverse and complex influence of CLA on cortical activity.