Pituitary adenylate cyclase-activating polypeptide (PACAP) stimulates release of somatolactin (SL)-α and SL-β from cultured goldfish pituitary cells via the PAC₁ receptor-signaling pathway, and affects the expression of SL-α and SL-β mRNAs

Different pituitary action of NK3Ra and NK3Rb in grass carp

In mammals, NK3R is the specific receptor for NKB, which played an important role in reproduction. Recently, two NK3R isoforms, namely NK3Ra and NK3Rb, have been identified in fish. However, little is known about the pituitary actions of the two NK3R isoforms in fish. In this study, both NK3Ra and NK3Rb were isolated from grass carp pituitary. Although their sequence similarity was only 61.6%, the two NK3R isoforms displayed similar ligand selectivity and binding affinity to TAC3 gene products (NKBa, NKBRPa and NKBRPb). In addition, both NK3Ra and NK3Rb displayed similar signaling pathways, including PKA, PKC, MAPK and Ca²⁺ cascades. Tissue distribution indicated that both NK3Ra and NK3Rb were highly detected in grass carp pituitary. Further study found that NK3Ra was mainly located in pituitary LHβ cells, while NK3Rb was only detected in pituitary SLα cells. Furthermore, NK3Ra and NK3Rb activation could induce LHβ and SLα promoter activity, respectively. These results suggested that the two NK3R isoforms displayed different pituitary actions in fish. Using grass carp pituitary cells as model, we found that PACAP could significantly reduce NK3Ra, but induce NK3Rb mRNA expression coupled with cAMP/PKA and PLC/PKC pathways. Interestingly, PACAP could also significantly inhibit LHβ, but stimulate SLα mRNA expression in grass carp pituitary cells. Furthermore, NK3R antagonist could not only inhibit LHβ mRNA expression, but also block PACAP-induced SLα mRNA expression in grass carp pituitary cells. These results suggested that NK3Ra and NK3Rb could mediate PACAP-reduced LHβ and -induced SLα mRNA expression in grass carp pituitary, respectively.

Estrogen mediates sex differences in preoptic neuropeptide and pituitary hormone production in medaka

The preoptic area (POA) is one of the most evolutionarily conserved regions of the vertebrate brain and contains subsets of neuropeptide-expressing neurons. Here we found in the teleost medaka that two neuropeptides belonging to the secretin family, pituitary adenylate cyclase-activating polypeptide (Pacap) and vasoactive intestinal peptide (Vip), exhibit opposite patterns of sexually dimorphic expression in the same population of POA neurons that project to the anterior pituitary: Pacap is male-biased, whereas Vip is female-biased. Estrogen secreted by the ovary in adulthood was found to attenuate Pacap expression and, conversely, stimulate Vip expression in the female POA, thereby establishing and maintaining their opposite sexual dimorphism. Pituitary organ culture experiments demonstrated that both Pacap and Vip can markedly alter the expression of various anterior pituitary hormones. Collectively, these findings show that males and females use alternative preoptic neuropeptides to regulate anterior pituitary hormones as a result of their different estrogen milieu.

Distribution of pituitary adenylate cyclase-activating polypeptide 2 in zebrafish brain

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a multipotent neuropeptide with an amino acid sequence that is well conserved among vertebrates. In teleosts, including zebrafish, the PACAP gene (adcyap1) has been duplicated to yield adcyap1a (coding PACAP1) and adcyap1b (coding PACAP2). This study aims to determine the distribution of these PACAPs and their mRNAs in zebrafish. We generated a zebrafish PACAP2-specific antibody. Using real-time PCR, we observed that adcyap1b mRNA was primarily localized in the brain, with the highest level in the telencephalon, followed by the diencephalon. Using immunostaining of brain tissue samples, PACAP2 immunoreactivity was observed mainly in the telencephalon, hypothalamus, and cerebellum, and the immunopositive fibers formed a line to the habenula. PACAP2-immunopositive cells were observed in the ventral and dorsal regions of the telencephalon and in the hypothalamic nucleus of the diencephalon in the colchicine-injected brain. This distribution of PACAP2 suggests its involvement in higher brain functions in teleosts, such as learning and cognition, as well as instinctive behaviors such as feeding and emotional regulation.

Why goldfish? Merits and challenges in employing goldfish as a model organism in comparative endocrinology research

Goldfish has been used as an unconventional model organism to study a number of biological processes. For example, goldfish is a well-characterized and widely used model in comparative endocrinology, especially in neuroendocrinology. Several decades of research has established and validated an array of tools to study hormones in goldfish. The detailed brain atlas of goldfish, together with the stereotaxic apparatus, are invaluable tools for the neuroanatomic localization and central administration of endocrine factors. In vitro techniques, such as organ and primary cell cultures, have been developed using goldfish. In vivo approaches using goldfish were used to measure endogenous hormonal milieu, feeding, behaviour and stress. While there are many benefits in using goldfish as a model organism in research, there are also challenges associated with it. One example is its tetraploid genome that results in the existence of multiple isoforms of endocrine factors. The presence of extra endogenous forms of peptides and its receptors adds further complexity to the already redundant multifactorial endocrine milieu. This review will attempt to discuss the importance of goldfish as a model organism in comparative endocrinology. It will highlight some of the merits and challenges in employing goldfish as an animal model for hormone research in the post-genomic era.

Mechanisms for PACAP-induced prolactin gene expression in grass carp pituitary cells

In mammals, pituitary adenylate cyclase-activating polypeptide (PACAP) is a pleiotropic hormone with diverse functions but its role in prolactin (PRL) regulation is highly controversial. To shed light on Prl regulation by PACAP in fish model, grass carp pituitary cells was used as a model to examine the receptor specificity and signal transduction for PACAP modulation of prl gene expression in the carp pituitary. Using RT-PCR, PACAP-selective PAC1 receptor was detected in carp lactotrophs. In carp pituitary cells, nanomolar doses of PACAP, but not VIP, could elevate Prl secretion and protein production with concurrent rise in prl mRNA and these stimulatory effects were blocked by PACAP antagonist but not VIP antagonist. PACAP-induced prl mRNA expression could be mimicked by activating adenylate cyclase (AC), increasing cAMP level by cAMP analog, or increasing intracellular Ca²⁺ ([Ca²⁺]i) by Ca²⁺ ionophore/voltage-sensitive Ca²⁺ channel (VSCC) activator. PACAP-induced prl gene expression, however, was attenuated/abolished by suppressing cAMP production, inhibiting PKA activity, blocking [Ca²⁺]i mobilization and VSCC activation, calmodulin (CaM) antagonism, and inactivation of JNK and CaM Kinase II (CaMK-II). Similar sensitivity to CaM, JNK, and CaMK-II blockade was also noted by substituting cAMP analog for PACAP as the stimulant for prl mRNA expression. These results, as a whole, provide evidence for the first time that (i) PACAP activation of PAC1 receptor expressed in carp lactotrophs could induce Prl synthesis and secretion, and (ii) Prl production induced by PACAP was mediated by upregulation of prl gene expression, presumably via functional coupling of cAMP/PKA-, Ca²⁺/CaM-, and MAPK-dependent cascades.