Neuropeptide content and connectivity of the rat claustrum

The rat claustrum has a homogeneous distribution of the neuropeptides somatostatin (SOM), cholecystokinin (CCK) and vasoactive intestinal polypeptide (VIP) along its rostrocaudal axis. In general, neuropeptide levels are comparable to those of overlying pyriform cortex. Visualization of mRNA encoding SOM, CCK and VIP in cell bodies of the claustrum by in situ hybridization histochemistry demonstrates that all 3 neuropeptides are contained in intrinsic claustral neurons. Mid-coronal section of the claustrum itself, or interruption of potential rostral, caudal or medial connections between the claustrum and the rest of the brain did not significantly alter levels of VIP, SOM or CCK in claustrum, cerebral cortex, or basal ganglia. Isolation of the claustrum from the cerebral cortex immediately dorsal to it along its rostrocaudal aspect caused no change in peptide levels in claustrum indicating that VIP, SOM and CCK projections to claustrum do not arrive from dorsal cortical areas. Transection of the external capsule above the claustrum caused a 50-100% elevation of all 3 peptides on the contralateral side of the lesion, suggesting that VIP, SOM and CCK synthesis and/or release within the claustrum may be regulated by projections from the contralateral side. VIP, SOM and CCK are candidates for neurotransmitters contained in neurons whose cell bodies are within the claustrum and possibly also immediately overlying lateral neocortex, and have their terminals mainly within the claustrum itself.