To identify neuropeptides in the rodent PVT, we used in situ hybridization data from the Allen Brain Atlas (Allen Institute, 2004). We used gray matter as a contrast and an expression threshold of 0.1, to analyze approximately 14,000 genes expressed in the PVT of the mouse brain. We identified 41 unique neuropeptides in this list, and used the Allen Brain Reference Atlas, Mouse Coronal v1 (Allen Institute, 2008); Allen Brain Reference Atlas, Mouse P56, Coronal (Allen Institute, 2011); and the Paxinos and Franklin mouse brain atlas (Paxinos and Franklin, 2004) to verify their location in the PVT. From this, we examined the gene expression of every neuropeptide in the coronal brain slices provided. The number of slices containing the PVT varied for each gene, ranging from 7 to 20, with an average of 11.5 slices per neuropeptide. All slices were from adult male C57Bl/6 mice. Of the 41 neuropeptides that we identified in the PVT, we classified 24 as being related to behavior (Table 1), 14 as related to cell function (Table 2), and three as related to reproduction (Table 3). For the neuropeptides that were specifically associated with affective and motivated behavior, we then searched the published literature to determine if and how these neuropeptides were previously described in the mouse and rat. In the following sections, we describe the distribution of nine of these neuropeptides, identified in the PVT and associated with affective and motivated behavior, and we also address their known or likely roles in this nucleus. For comparison, we describe the distribution of markers for glutamate and GABA in the PVT.
Neuropeptides in the paraventricular nucleus of the thalamus (PVT) with known roles in behavior, as identified with the Allen Brain Atlas.
Gene is the gene symbol; peptide is the known peptide(s) derived from the gene; fold change vs. gray matter is the degree of neuropeptide expression compared to overall gray matter in the brain; signal transduction pathway is the putative signaling activated by ligand binding to its receptor(s), as indicated by the International Union of Basic and Clinical Pharmacology (IUPHAR) and the British Pharmacological Society (BPS; Armstrong et al., 2019).
Neuropeptides in the PVT with known roles in cell function, as identified with the Allen Brain Atlas.
Gene is the gene symbol; peptide is the known peptide(s) derived from the gene; fold change vs. gray matter is the degree of neuropeptide expression compared to overall gray matter in the brain.
Neuropeptides in the PVT with known roles in reproduction, as identified with the Allen Brain Atlas.
Tachykinin 2 (Tac2) is very highly expressed in the PVT, being found in numerous cells in this nucleus, albeit at low-to-moderate levels in those cells (Allen Institute, 2004). While generally consistent across the antero-posterior axis of the PVT, tachykinin 2 expression is somewhat higher in the posterior PVT, where it is denser in the medial part of this subregion (Allen Institute, 2004). Prior research using in situ hybridization in rats had reported that only a few cells express this gene in the PVT (Lucas et al., 1992) and noted that expression was similar between rats and mice in the thalamus as a whole (Duarte et al., 2006). It may be that the lower gene expression in individual cells was below the threshold of detection in these earlier studies. Work with immunohistochemistry has consistently identified moderate levels of the derived peptide, neurokinin B, in fibers of the PVT and in the overall thalamus of both rats and mice (Lucas et al., 1992; Marksteiner et al., 1992; Duarte et al., 2006).
Neurokinin B, the peptide encoded by Tac2, is a member of the larger tachykinin family, which also includes substance P and neurokinin A, which are derived from Tac1. Neurokinin B acts preferentially at the neurokinin receptor 3 and is perhaps best known for its role in growth and reproduction, which occur primarily via the hypothalamic-pituitary-gonadal axis (Zhang et al., 2020). While no prior research has examined the function of neurokinin B in the PVT, a body of work has examined the effects of neurokinin receptor 3 manipulations through injections into the lateral ventricles or periphery. In both rats and mice, stimulation of neurokinin receptor 3 induces positive hedonic motivation. Injection of a neurokinin receptor 3 agonist into the lateral ventricles of rats can, on its own, induce conditioned place preference (Ciccocioppo et al., 1998). In mice, it increases time spent and the number of entries into the open arms of an elevated plus-maze (Ribeiro and De Lima, 1998), and in rats, systemic injection of a neurokinin receptor 3 agonist increases time spent in the center of an open field (Schäble et al., 2010), which together suggest that neurokinin B is anxiolytic. Similarly, systemic injection in rats of a neurokinin receptor 3 agonist reduces immobility time in a forced swim test (Schäble et al., 2010), suggesting that it is also anti-depressive. Perhaps due to the rewarding effects of neurokinin receptor 3 stimulation, agonists of this receptor have consistently been found to reduce ethanol drinking, when injected into the lateral ventricles of selectively-bred alcohol-preferring rats (Perfumi et al., 1991; Ciccocioppo et al., 1994, 1998). In light of the higher expression of tachykinin 2 in the posterior and medial PVT, and the known ability of posterior PVT stimulation to affect anxiety-like behavior (Barson et al., 2020) and ethanol drinking (Pandey et al., 2019), as well as the involvement of the PVT in depression-like behavior (Kasahara et al., 2016; Kato et al., 2019), we hypothesize that tachykinin 2/neurokinin B in cells in the PVT could promote a positive affective state and reduce the intake of drugs of abuse.
Although the overall gene expression of galanin is lower than that of tachykinin 2 in the PVT, it occurs at higher levels in individual cells of this nucleus (Allen Institute, 2004). With galanin cells being most dense in the anterior PVT and becoming progressively less dense across the antero-posterior axis, galanin expression in the most anterior PVT is found more in the lateral part of this subregion but becomes more medially restricted in posterior portions of the PVT (Allen Institute, 2004). Prior research in mice using in situ hybridization similarly reported more abundant expression of galanin in the anterior compared to posterior PVT and in more medial parts of the PVT (Gao et al., 2020), although work with immunohistochemistry reported that galanin peptide-expressing neurons could be found at moderate levels throughout the antero-posterior axis of the PVT (Perez et al., 2001). No published research has identified galanin in the PVT of the rat.
There is substantial prior research on the behavioral role of galanin, using injections into the lateral ventricles or global gene knockdown or overexpression, but very recent research on galanin has examined it specifically in cells of the PVT itself. This work has found that galanin-containing PVT neurons in the mouse decrease their activity, as measured by calcium transients, during the transition from NREM sleep to wakefulness, and that chemogenetic activation of these cells decreases wakefulness (Gao et al., 2020). Thus, galanin in cells of the PVT appears to be important in suppressing arousal. It is possible, however, that these cells also affect valence and motivated behavior. Injection of galanin into the lateral ventricles in mice increases time spent in the open arms of an elevated zero maze (Rajarao et al., 2007), indicating that it is anxiolytic. On the other hand, galanin does not appear to play a role in depression-like behaviors, as injection of galanin into the lateral ventricles does not affect behavior in a mouse tail suspension test or rat forced swim test (Rajarao et al., 2007). Galanin does, however, have clear effects on fo
