Background: The zebrafish has become a widely used model to study disease resistance and immunity. Although the genes encoding many components of immune signaling pathways have been found in teleost fish, it is not clear whether all components are present or whether the complexity of the signaling mechanisms employed by mammals is similar in fish.

SUMMARY The zebrafish genome contains ten genes that encode class II cytokine-like peptides, of which the two that are related most closely to mammalian interferon gamma (IFN-γ) were named IFN-γ1 and IFN-γ2. Although the zebrafish has become a popular model system to study immune mechanisms, and although interferons are central regulators of immunity, which zebrafish cytokines correspond functionally to mammalian IFN-γ has not been established. We used zebrafish embryos to assay the functions of IFN-γ1 and IFN-γ2, and have identified a subset of zebrafish homologs of the mammalian IFN-responsive genes as IFN-γ targets in the zebrafish embryo: these genes are upregulated in response to raised levels of either IFN-γ1 or IFN-γ2. Infection studies using two different pathogens show that IFN-γ signalling is required for resistance against bacterial infections in the young embryo and that the levels of IFN-γ need to be regulated tightly: raising IFN-γ levels sensitizes fish embryos against bacterial infection. Embryos injected with high doses of Escherichia coli are able to clear the bacteria within a day, and the γ-interferons are necessary for this defence reaction. The protective response to Yersinia ruckeri, a natural fish pathogen that is lethal at low doses, also requires IFN-γ. As in the induction of target genes, the two interferons act at least partly redundantly. Together with the previously demonstrated type III interferon response, these results show that the counterparts of the mammalian viral and bacterial interferon-dependent defence functions are in place in zebrafish embryos, and suggest that zebrafish IFN-γ1 and IFN-γ2 are functionally equivalent to mammalian IFN-γ.

The zebrafish genome contains a large number of genes encoding potential cytokine receptor genes as judged by homology to mammalian receptors. The sequences are too divergent to allow unambiguous assignments of all receptors to specific cytokines, and only a few have been assigned functions by functional studies. Among receptors for class II helical cytokines—i.e., IFNs that include virus-induced Ifns (Ifn-ϕ) and type II Ifns (Ifn-γ), together with Il-10 and its related cytokines (Il-20, Il-22, and Il-26)—only the Ifn-ϕ–specific complexes have been functionally identified, whereas the receptors for the two Ifn-γ (Ifn-γ1 and Ifn-γ2) are unknown. In this work, we identify conditions in which Ifn-γ1 and Ifn-γ2 (also called IFNG or IFN-γ and IFN-gammarel) are induced in fish larvae and adults. We use morpholino-mediated loss-of-function analysis to screen candidate receptors and identify the components of their receptor complexes. We find that Ifn-γ1 and Ifn-γ2 bind to different receptor complexes. The receptor complex for Ifn-γ2 includes cytokine receptor family B (Crfb)6 together with Crfb13 and Crfb17, whereas the receptor complex for Ifn-γ1 does not include Crfb6 or Crfb13 but includes Crfb17. We also show that of the two Jak2 paralogues present in the zebrafish Jak2a but not Jak2b is involved in the intracellular transmission of the Ifn-γ signal. These results shed new light on the evolution of the Ifn-γ signaling in fish and tetrapods and contribute toward an integrated view of the innate immune regulation in vertebrates.

The techniques of extracellular single cell recording and microiontophoresis were used to study the effects of serotonin (5‐HT) and of 5‐HT1A, 5‐HT2A/2C and 5‐HT3 receptor agonists on the spontaneous activity of amygdaloid neurons in rats anesthetized with urethane. The background discharge rate was modified by 5‐HT as well as by 5‐HT agonists in about two‐thirds of neurons tested in different nuclei of the amygdaloid complex. Whereas the 5‐HT2 and 5‐HT3 agonists significantly increased the neuronal discharge rate in nearly all subdivisions of the amygdala, the 5‐HT1A agonist significantly inhibited the firing rate. Co‐administration of bicuculline and 5‐HT receptor agonists prevented the 8‐OH‐DPAT‐induced increases in the firing rate in most cases tested, as well as the inhibitory effects of DOI or 2‐methyl‐5HT. Therefore, GABAergic interneurons seem to be involved in the mediation of serotonergic effects. The action of 5‐HT agonists on the neuronal discharge rate was blocked by different receptor‐specific antagonists. The results support the hypothesis that 5‐HT exerts control throughout the amygdala by acting at least on 5‐HT1A, 5‐HT2A/2C and 5‐HT3 receptors seemingly located both on projection and interneurons. Synapse 38:328–337, 2000. © 2000 Wiley‐Liss, Inc.