Simple SummaryThe cocaine- and amphetamine-regulated peptide (CART), widely distributed in the mammalian brain, may play an important role in addiction, pain, rewards, learning and memory, cognition, sleep, reproduction and development, behavioral modulation, and the regulation of the autonomic nervous system. There is little information in the available literature on the presence of CART in the claustrum—a structure of the brain that is considered a relay station due to its anatomical connections with almost all cortical areas of the brain. The claustrum is divided into two main parts: the dorsal segment (CL) and the ventral segment (EN). In this article, we describe the occurrence of CART in the chinchilla claustrum, and also refer to the co-existence of CART with other neurotransmitters/neuromodulators such as somatostatin (SOM), neuropeptide Y (NPY), and parvalbumin (PV). We observed the immunoreactivity for the used substances in neuronal cell bodies distributed throughout the anterior–posterior extent of the chinchilla claustrum. Studying the distribution of the chemical code of neurons in the chinchilla claustrum will probably complement missing data as well as provide a better understanding of similarities and differences compared to other species of rodents.AbstractAlthough for many years, researchers have been working on understanding the function of the cocaine- and amphetamine-regulated transcript (CART) peptide at the central- and peripheral-nervous-system level, data describing the presence of CART in the claustrum are still missing. Therefore, the aim of the present study was to immunohistochemically investigate the CART expression in the claustrum neurons in chinchillas as well as the CART co-localization with somatostatin (SOM), parvalbumin (PV), and neuropeptide Y (NPY) using double-immunohistochemical staining. The claustrum is divided into two main parts: the dorsal segment (CL), which is located above the rhinal fissure, and the ventral segment (EN), located below the rhinal fissure. The presence of HU C/D-IR CART-IR-positive neurons was detected in both the insular claustrum (CL) and the endopiriform nucleus (EN). The vast majority of CART-IR neurons were predominantly small and medium in size and were evenly scattered throughout the claustrum. CART co-localization with selected neurotransmitters/neuromodulators (SOM, NPY, and PV) showed the presence of a CART-IR reaction only in the neurons, while the nerve fibers were, in all cases, devoid of the CART-IR response. Our research supplements missing knowledge about the distribution and co-localization pattern of CART with SOM, NPY, and PV in the chinchilla claustrum, and also provides a better understanding of the similarities and differences compared to other species of rodents and other mammals.