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In amphibians, thyrotropin (TSH), corticotropin (ACTH) and prolactin (PRL) are regarded as the major pituitary hormones involved in metamorphosis, their releasing factors being corticotropin-releasing factor (CRF), arginine vasotocin (AVT), and thyrotropin-releasing hormone (TRH), respectively. It is also known that thyrotropes and corticotropes are equipped with CRF type-2 receptor and AVT V1b receptor, respectively. As for PRL cells, information about the type of receptor for TRH (TRHR) through which the action of TRH is mediated to induce the release of PRL is lacking. In order to fill this gap, an attempt was made to characterize the TRHR subtype existing in the PRL cells of the anterior pituitary gland of the bullfrog, Rana catesbeiana . We cloned cDNAs for three types of bullfrog TRHRs, namely TRHR1, TRHR2 and TRHR3, and confirmed that all of them are functional receptors for TRH by means of reporter gene assay. Analyses with semi-quantitative reverse transcription-PCR and in situ hybridization revealed that TRHR3 mRNA is expressed in the anterior lobe and that the signals reside mostly in the PRL cells. It was also noted that the expression levels of TRHR3 mRNA in the anterior pituitary as well as in the PRL cells of metamorphosing tadpoles elevate as metamorphosis progresses. Since the pattern of changes in TRHR3 mRNA levels in the larval pituitary is almost similar to that previously observed in the pituitary PRL mRNA and plasma PRL levels, we provide a view that TRHR3 mediates the action of TRH on the PRL cells to induce the release of PRL that is prerequisite for growth and metamorphosis in amphibians. [ABSTRACT FROM AUTHOR]

Abstract: Dopamine receptors in mammals are known to consist of two D1-like receptors (D1 and D5) and three D2-like receptors (D2, D3 and D4). The aim of this study was to determine the dopamine receptor subtype that mediates the inhibitory action of dopamine on the release of prolactin (PRL) from the amphibian pituitary. Distal lobes of the bullfrog (Rana catesbeiana) were perifused and the amount of PRL released in the effluent medium was measured by means of a homologous enzyme-immunoassay. TRH stimulated the release of PRL from perifused pituitaries. Dopamine suppressed TRH-induced elevation of PRL release. Quinpirole (a D2 receptor agonist) also suppressed the stimulatory effect of TRH on the release of PRL, whereas SKF-38393 (a D1 receptor agonist) exhibited no such an effect. The inhibitory action of dopamine on TRH-induced PRL release from the pituitary was nullified by the addition of L-741,626 (a selective D2 receptor antagonist) to the medium, but not by the addition of SCH-23390 (a selective D1 receptor antagonist). These data indicate that the inhibitory effect of dopamine on TRH-evoked PRL release from the bullfrog pituitary gland is mediated through D2 dopamine receptors. [ABSTRACT FROM AUTHOR]

Abstract: The cDNA encoding D2 dopamine receptor was cloned from the distal lobe of the bullfrog pituitary. The deduced amino acid sequence of the bullfrog D2 dopamine receptor (bfD2A) spanned 444 amino acids and exhibited typical features of those of D2 dopamine receptors cloned in other animals to date. It showed a high similarity of 75–87% with rat, turkey, Xenopus and tilapia counterparts. Further analysis of nucleotide sequence of the cDNA revealed the presence of putative truncated D2 dopamine receptor isoforms, bfD2B and bfD2C, of which nucleotide sequences lacked 12 and 99 nucleotides of the coding region for bfD2A, respectively. The alignment analysis indicated that putative bfD2C isoform was close to D2S subtype cloned in mammals and birds, whereas bfD2A and putative bfD2B isoforms were close to mammalian and avian D2L subtype and homologous to two isoforms of Xenopus. This is the first report of the presence of mRNAs for two D2L-like isoforms and one D2S-like isoform in a single species. The amino acid sequence responsible for producing isoforms is present in the third intracellular loop, which has been shown to play an important role in the coupling with G protein. Accordingly, differences in the mode of coupling with G protein among three isoforms were suggested. The expression of three isoforms mRNA in organs and tissues was analyzed by RT-PCR. In the brain, pars distalis and pars neurointermedia, mRNAs for three isoforms were invariably expressed, whereas only putative bfD2C mRNA was expressed in peripheral organs and tissues. [Copyright &y& Elsevier]