Enkephalins modulate the responsiveness of rat atria in vitro to norepinephrine

Potential interactions between opiate peptides and catecholamines in mammalian heart were examined using isolated spontaneously beating rat atria as a test system. Methionine-enkephalin (ME), leucine-enkephalin (LE), phe-met-arg-phe amide (FMRFamide), D-ala2, N-methyl-phe4, met (O)5-ol-enkephalin (FK 33-834), methionine-enkephalin arg6 arg7 (ME arg6 arg7) and beta-endorphin had no effect on basal beating rate of isolated atria at all concentrations up to 10(-5) M. The positive chronotropic effect of norepinephrine (NE) on atrial rate is, however, significantly attenuated by enkephalin peptides. Thus, the maximal chronotropic effect of NE (an increase from 317 +/- 7.0 to 473 +/- 7.3 beats per minute (bpm) in 250 gm rats at a dose of 10(-5)M NE) is decreased by 42% in the presence of 10(-7)m ME. The action of ME is completely blocked by addition of 10(-7)M naloxone, which by itself has no effect on NE-induced positive chronotropy or basal beating rate. The dose-effect curve for ME attenuation of NE-induced positive chronotropy is bell-shaped, i.e., both 10(-8) M and 10(-5) M ME have no significant effect on NE positive chronotropy. Other enkephalin peptides acted in a similar manner to ME; LE (10(-7) M) and FK 33-834 (10(-8) M) decreased maximal NE-induced positive chronotropy 42 and 27%, respectively. The molluscan cardioexcitatory peptide FMRFamide (10(-7) M) also decreased maximal NE positive chronotropy, about 30%. In contrast, beta-endorphin did not significantly affect NE stimulation of atrial rate. We conclude that enkephalins can modulate the noradrenergic responsiveness of rat atria in vitro. The possible physiological relevance of this interaction is discussed.

Two chemically and immunologically distinct forms of luteinizing hormone-releasing hormone are differentially expressed in frog neural tissues

Immunoreactive luteinizing hormone-releasing hormone (IR-LRF) has been measured in several neural tissues of the frog Rana catesbeiana and characterized by bioassay, chromatography and immunological analysis. Frog brain contains the mammalian form of LRF (mLRF), whereas, sympathetic ganglia and adrenal glands contain a form of LRF (pLRF) chromatographically and immunologically similar to the IR-LRF found in the central nervous systems of several piscine species. Frog retina contains both mLRF and pLRF, in a ratio of about 1:2. The two forms of LRF are apparently equipotent in their ability to release LH from rat gonadotrophs in vitro. Immunological analysis of the pLRF found in the frog nervous system suggests that it is a decapeptide, like mLRF, with one or more amino acid substitutions in the mLRF molecule.

Recombinant DNA methods in neuroendocrinology: new answers to old questions

The development of methods for cloning DNA complementary to specific messenger RNAs encoding polypeptide hormones has opened new experimental avenues in the neurosciences. Recombinant DNA techniques have been used to answer questions about the structure of polypeptide hormone precursors, the regulation of peptide hormone production, and the location of peptide-producing cells in nervous and endocrine systems. Here, the application of cDNA cloning to the quantitation, structural elucidation and tissue localization of mRNAs coding for neuroendocrine peptides is reviewed.

Characteristics of protein carboxyl methylation in the rat hypothalamus

The formation of methyl-labeled S-adenosylmethionine (AdoMet) and methyl esters of endogenous methyl-acceptor proteins (MAPs) was studied in a synaptosomal preparation from the rat hypothalamus labeled with L-[methyl-3H]methionine. Incubation of synaptosomes with L-[methyl-3H]methionine resulted in a rapid labeling of the AdoMet pool and a less rapid formation of 3H-methyl-MAPs. Accumulation of 3H-methyl-MAPs was linear over a 30-min period. The effects of various inhibitors of Ado-Met-dependent transmethylation reactions on the formation of carboxylmethylated MAPs were examined. When hypothalamic synaptosomes were preincubated with L-[methyl-3H]methionine and subsequently incubated for 30 min in the presence of S-adenosyl-L-homocysteine (AdoHcy, 100 microM), 3H-methyl-MAP formation was inhibited by approximately 70%, 100 microM-L-homocysteine thiolactone (HTL) as well as 100 microM-3-deazaadenosine (c3Ado) also caused a 60--70% inhibition of 3H-methyl-MAP formation; the combination of both c3Ado and HTL produced a slightly but not significantly greater inhibition than either agent alone. 10 microM-adenosine or 10 microM-HTL each produced an approximately 40% inhibition of 3H-methyl-MAP formation: the inhibitory effect of the two agents in combination was additive. Sinefungin and A9145C, potent inhibitors of bovine adrenomedullary protein carboxyl methylase, had no effect on 3H-methyl-MAP formation in hypothalamic synaptosomes at concentrations up to 1 mM. However, these compounds were potent inhibitors of 3H-methyl-MAP formation in lysed synaptosomes incubated with [3H-methyl]AdoMet. These results demonstrate that hypothalamic synaptosomes are capable of methionine activation and protein carboxyl methylation.

Distribution of vasoactive intestinal polypeptide (VIP) in the rat brain stem nuclei

The distribution of vasoactive intestinal polypeptide (VIP) in the individual nuclei of the rat brain stem was examined. Highest VIP concentrations in brain stem (greater than 1 ng/mg protein) are confined to dorsal regions, including periaqueductal gray and structures under the fourth ventricle. VIP concentrations are moderately high (0.8-1 ng/mg protein) in gracile nucleus, area postrema, nucleus of the solitary tract, motor nucleus of the XIIth, locus ceruleus and dorsal tegmental nucleus. Cerebellum and pontine nuclei contained only very low levels (less than 0.2 ng/mg protein) of VIP.

Extrahypothalamic distributions and functions of hypothalamic peptide hormones

The isolation and chemical identification of the hypothalamic hormones somatostatin, luteinizing hormone-releasing factor (LRF), and thyrotropin-releasing factor (TRF) within the last decade have allowed the chemical synthesis of these compounds and the development of radioimmunoassay and immunohistochemical methods for them. As a result, these "hypothalamic" hormones have been detected in many extrahypothalamic locations, including brain and gastroenteropancreatic tissues. The hypothalamoneurohypophysial hormones vasopressin and oxytocin have also been localized in extrahypothalamic neuronal pathways. It has become clear that these neuropeptides subserve multiple endocrine and neurotransmitter functions outside the hypothalamus. The information now available about the extrahypothalamic distributions and possible physiological roles of somatostatin, TRF, LRF, vasopressin, and oxytocin is reviewed.

Acute thyroid hormone increases noradrenergic responsiveness of rat atria in vitro

The effect of acute (10-75 min) exposure of isolated spontaneously beating rat atria to L-triiodothyronine (T3)(12.9-1290 nM) on the positive chronotropic response to (-)-norepinephrine (NE) 10(-10)-10(-4)M) was examined. Preincubation (50 min) with 129 nM T3 increased the maximal response to (-)-NE from 128 +/- 10 to 176 +/- 11 beats/min. This effect was temporally biphasic, being maximal at 50 min pre-exposure. The results demonstrate an effect of acute in vitro exposure to T3 on the noradrenergic responsiveness of the heart.

Chemical and photooxidation of thiothixene (Navane): structure of the thiothixene fluorophor

The spontaneous photooxidation of thiothixen (Navane) to 2-(N,N-dimethyl-sulfonamido)-9-thioxanthone is described. The corresponding sulfoxide is demonstrated to be the fluorescent species obtained upon permanganate oxidation of thiothixene in the fluorometric assay procedure of Mjörndal and Oreland.