Distribution and neuronal circuit of spexin 1/2 neurons in the zebrafish CNS

Abstract

Spexin (SPX) is a highly conserved neuropeptide that is widely expressed in mammalian brain and peripheral tissue. In teleost, SPX1 is mainly expressed in the brain and ovary, and is involved in reproduction and food intake. A second form of SPX, SPX2, was recently identified in chick, Xenopus, and zebrafish. The expression pattern and roles of SPX2 are unknown. SPX (spx1) is highly expressed in the vertebrate brain, but its distribution, circuits, and interactions with its putative receptor are unknown. Here, we observed expression of spx1 in the midbrain and hindbrain, and spx2 in the hypothalamic preoptic area using in situ RNA hybridization in zebrafish. Analysis of transgenic reporter zebrafish revealed that hindbrain SPX1 neurons are PAX2+ inhibitory interneurons and project to the spinal cord, where they interact with galanin receptor 2b (GALR2b) neurons, suggesting that hindbrain SPX1 neurons are reticulospinal neurons. spx1 mRNA and SPX1 reporter expression were observed in dorsal habenula (dHb). SPX1 neurons in the dHb project to the interpeduncular nucleus (IPN), where GALR2a and GALR2b expression was also observed, suggesting that habenula SPX1 neurons may interact with GALR2a/2b in the IPN.

Spexin and a Novel Cichlid-Specific Spexin Paralog Both Inhibit FSH and LH Through a Specific Galanin Receptor (Galr2b) in Tilapia

“…In tilapia, the two spexin paralogs exhibited wide differential tissue distribution patterns, where Spx1a was primarily expressed in the midbrain (containing the hypothalamus in our preparations), and Spx1b was primarily expressed in the anterior brain, which contains the preoptic area. Similarly, in a detailed report of spexin neuron circuitry in zebrafish, Spx1 expression was restricted to the midbrain tegmentum and the hindbrain and Spx2 expression was found in the preoptic area, however neither was detected in the hypothalamus (53). In goldfish, Spx1 was identified in the hypothalamus, thalamus, and medial longitudinal fasciculus, however immunoreactivity was detected using heterologous antibodies that may have recognized both Spx1 and Spx2 (10).…”

Evolution of Structural and Functional Diversity of Spexin in Mammalian and Non-mammalian Vertebrate Species

“…Schematic drawing of the representative localization of SPX1, GALR2a/2b, and GARL3 in the sagittal sections of the brain of various vertebrate species. SPX1 has only localized in rodent (2, 25, 26), goldfish (7), and zebrafish (27, 28). GALR2a/2b was not localized in goldfish.…”

“…SPX1 positive cells are also reported in the ventromedial thalamic nucleus and medial longitudinal fasciculus of the goldfish (7), and in the zebrafish (27). Recently, Kim et al (28) reported the detailed distribution of both SPX1 and SPX2 in the brain of the zebrafish. This study is the first to report the localization of SPX2 in non-mammalian vertebrate.…”

“…On the other hand, SPX2 is localized in the hypothalamic preoptic area. The discrepancy in the localization of SPX1 reported by Kim et al (28) and Zheng et al (27) may be due to the use of different detection methods; in-situ RNA hybridization (28), and immunofluorescence using polyclonal antibody raised against human SPX1 (27).…”

“…In addition, SPX is upregulated in the amygdala following treatment with a atypical antipsychotics, chlorpromazine in rats (64). Interestingly, SPX in the medial habenula of mice (26) and its ortholog dorsal habenula in zebrafish (28) provide further evidence on the potential role of SPX in regulating depression and anxiety. As the medial habenula projects to the interpeduncular nucleus, which subsequently projects to the raphe nucleus where the serotonergic neurons are located (65), a potential interaction between SPX and the serotonergic system is plausible.…”