Location of rat liver iodothyronine deiodinating enzymes in the endoplasmic reticulum

The iodothyronine-deiodinating enzymes (iodothyronine-5- and 5′-deiodinase) of rat liver were found to be located in the parenchymal cells. Differential centrifugation of rat liver homogenate revealed that the deiodinases resided mainly in the microsomal fraction. The subcellular distribution pattern of these enzymes correlated best with glucose-6-phosphatase, a marker enzyme of the endoplasmic reticulum. Plasma membranes, prepared by discontinuous sucrose gradient centrifugation, were found to contain very little deiodinating activity. Analysis of fractions obtained during the course of plasma membrane isolation showed that the deiodinases correlated positively with glucose-6-phosphates (r >/ 0.98) and negatively with the plasma membrane marker 5′-nucleotidase (r ranging between ?0.88 and ?0.97). It is concluded that the iodothyronine-deiodinating enzymes of rat liver are associated with the endoplasmic reticulum.     

Location of rat liver iodothyronine deiodinating enzymes in the endoplasmic reticulum.

The iodothyronine-deiodinating enzymes (iodothyronine-5- and 5'-deiodinase) of rat liver were found to be located in the parenchymal cells. Differential centrifugation of rat liver homogenate revealed that the deiodinases resided mainly in the microsomal fraction. The subcellular distribution pattern of these enzymes correlated best with glucose-6-phosphatase, a marker enzyme of the endoplasmic reticulum. Plasma membranes, prepared by discontinuous sucrose gradient centrifugation, were found to contain very little deiodinating activity. Analysis of fractions obtained during the course of plasma membrane isolation showed that the deiodinases correlated positively with glucose-6-phosphatase (r larger than or equal to 0.98) and negatively with the plasma membrane marker 5'-nucleotidase (r ranging between -0.88 and -0.97). It is concluded that the iodothyronine-deiodinating enzymes of rat liver are associated with the endoplasmic reticulum.     

Serotonin and motor activity

The activity of brain serotonergic neurons in both the pontine-mesencephalic and medullary groups is positively correlated with the level of behavioral arousal and/or the behavioral state. This, in turn, appears to be related to the level of tonic motor activity, especially as manifested in antigravity muscles and other muscle groups associated with gross motor activity. In addition, a subset of serotonergic neurons displaysa further increase in activity in association with repetitive, central pattern generator mediated responses. Accumulating evidence indicates that this relation to motor activity is related both to the co-activation of the sympathetic nervous system and to the modulation of afferent inputs.     

Environmental salinity selectively modifies the outer-ring deiodinating activity of liver, kidney and gill in the rainbow trout

We here analyzed the effect of a mild hyperosmotic challenge on the activities of deiodinases type I (D1) and II (D2) in the trout liver, and D1 in kidney and gill, two organs involved in osmoregulation. FW-adapted immature rainbow trout were transferred to 5 per thousand SW and killed 0.5, 1, 2, 4, 8 12, 24 and 48 h post-transfer (PT). Fish maintained in FW served as controls. Hepatic, renal and branchial D1 and hepatic D2 activities were assessed as well as circulating levels of T(3), T(4) and cortisol. Hyperosmotic challenge elicited significant and sustained decreases in kidney D1 and liver D2 activities at 8 h PT, which returned to control values at 48 h PT. In contrast, liver and gill D1 activities exhibited no significant change throughout the study. Also, significant increases in circulating T(4) at 2-4 and 48 h PT were observed. Circulating T(3) remained unmodified until 24-48 h PT, when it rose sharply. Simultaneously, cortisol showed a trend towards increase during the initial 4 h PT, which attained significance at 48 h PT. The present findings demonstrate that a mild hypertonic challenge is sufficient to elicit responses in the trout thyroidal axis. Hormonal changes in the circulatory compartment are in accordance with those previously described for migratory salmonids. A novel aspect of our findings is the organ-specific differential response exhibited by ORD-enzymes when trout are exposed to a mildly different osmotic environment. Our findings further establish the uniqueness of fish thyroid physiology, and can be of value in further understanding the evolutionary aspects of this ORD family of deiodinases.     

An effect of diet on the activity of phosphofructokinase in rat heart

Phosphorylation of the inhibitory subunit of cardiac troponin (TN-I) occurs $\text{in vivo}$ after catecholamine intervention through adenylcyclase, cyclic AMP and cyclic AMP dependent protein kinase system. Also, TN-I and tropomyosin binding subunit of troponin (TN-T) are specifically hydrolyzed by calcium-activated neutral protease (CANP). In this study, we compared proteolysis of a set of TNs before and after phosphorylation by cyclic AMP dependent protein kinase plus cyclic AMP, using CANP from cardiac muscle. The initial rate of peptide release from both TNs was the same. After prolonged incubation, however, unphosphorylated TN degradation retarded, while phosphorylated TN proteolysis still continued. The amount of peptide release at the latter phase was dependent on the degree of phosphorylation. These results were confirmed by SDS polyacrylamide gel electrophoresis, and they suggest that a conformational change occurred in the whole TN molecule after phosphorylation of TN-I.     

Triiodothyronine deiodinating activity in brown adipose tissue after short cold stimulation test in trained and untrained rats

Interscapular brown adipose tissue (IBAT) activity is controlled by sympathetic nervous system, and factors that influence thermogenesis appear to be centrally connected to the sympathetic outflow to IBAT. Cold exposure produces a rise in BAT temperature, which is associated with an increased thyroid activity, elevated serum levels of 3,5,3'-triiodothyronine (T3), and an increased rate of T3 production. This study evaluated the effect of swimming training on 5'-triiodothyronine deiodinase (5'-D) activity in IBAT under normal environmental conditions and after short (30 min) cold exposure (TST stimulation test). 5'-D activity is lower in trained rats at basal condition, and TST increases 5'-D in IBAT of both untrained and trained rats. However, this increase is lower in trained rats. Training reduces the deiodinating activity in normal environmental conditions as well as after short cold exposure. Probably, other compensatory mechanisms of heat production are active in trained rodents.     

Serotonin synthesis and its light-dark variation in the rat retina

Retinal circadian rhythms are driven by an intrinsic oscillator, using chemical signals such as melatonin, secreted by photoreceptor cells. The purpose of the present work was to identify the origin of serotonin, the precursor of melatonin, in the retina of adult rat, where no immunoreactivity for serotonin or tryptophan hydroxylase had ever been detected. To demonstrate local synthesis of serotonin in the rat retina, substrates of tryptophan hydroxylase, the first limiting enzyme in the serotonin pathway, have been used. Tryptophan, in the presence of an inhibitor of aromatic amino acid decarboxylase, enhanced 5-hydroxytryptophan levels, whereas alpha-methyltryptophan, a competitive substrate inhibitor, was hydroxylated into alpha-methyl-5-hydroxytryptophan. Tryptophan hydroxylase substrate concentration was higher in the dark period than in the light period, and formation of hydroxylated compounds was increased. The presence of tryptophan hydroxylase mRNA in the rat retina was confirmed by RT-PCR. Taken together, the results support the local synthesis of serotonin by tryptophan hydroxylation, this metabolic pathway being required more critically when 5-HT is used for melatonin synthesis.     

Serotonin immunoreactivity in the intermediate lobe of the rat pituitary.

Immunocytochemical staining for serotonin (5-HT) in paraffin-embedded sections of rat pituitary resulted in the localization of reactive nerve fibres and cell bodies in the intermediate lobe. Immunostaining was also found in the anterior and posterior lobes. Labelled nerve fibres appear to enter the intermediate lobe from the neural lobe through the interlobular spaces. These fibres are relatively scarce and lightly stained. Neuroglandular contacts were identified between varicose nerve endings containing serotonin and immunoreactive perykarion. It is not clear whether intermediate lobe cells produced 5-HT themselves or, alternatively, these cells take in 5-HT from serotoninergic nerve terminals.     

The effect of clofibrate feeding on the NADP-linked dehydrogenases activity in rat tissue

Administration of clofibrate to the rat increased several fold the activity of malic enzyme in the liver. Clofibrate treatment resulted also in an increased activity of the hepatic hexose monophosphate shunt dehydrogenases but was without effect on NADP-linked isocitrate dehydrogenase. The increased activity of malic enzyme in the liver resulting from the administration of clofibrate was inhibited by ethionine and puromycin, which suggests that de novo synthesis of the enzyme protein did occur as the result of the drug action. In contrast to the liver malic enzyme, the enzyme activity in kidney cortex increased only two-fold, whereas in the heart and skeletal muscle the activity was not affected by clofibrate administration.     

Serotonin phase-shifts the circadian rhythm of locomotor activity in the cockroach

Serotonin, a putative neurotransmitter in insects, was found to cause consistent phase shifts of the circadian rhythm of locomotor activity of the cockroach Leucophaea maderae when administered during the early subjective night as a series of 4-microliters pulses (one every 15 min) for either 3 or 6 hr. Six-hour treatments with dopamine also caused significant phase shifts during the early subjective night, but 3-hr treatments with dopamine had no phase-shifting effect. Other substances tested in early subjective night (norepinephrine, octopamine, gamma-aminobutyric acid, glutamate, carbachol, histamine, tryptophan, tryptamine, N-acetyl serotonin, or 5-hydroxyindole-3-acetic acid) did not consistently cause phase shifts. The phase-shifting effect of serotonin was found to be phase-dependent. The phase response curve (PRC) for serotonin treatments was different from the PRC for light. Like light, serotonin caused phase delays in the late subjective day and early subjective night, but serotonin did not phase-shift rhythms when tested at phases where light causes phase advances.     

Serotonin agonist quipazine induces photic-like phase shifts of the circadian activity rhythm and c-Fos expression in the rat suprachiasmatic nucleus

Nonphotic stimuli can reset and entrain circadian activity rhythms in hamsters and mice, and serotonin is thought to be involved in the phase-resetting effects of these stimuli. In the present study, the authors examined the effect of the serotonin agonist quipazine on circadian activity rhythms in three inbred strains of rats (ACI, BH, and LEW). Furthermore, they investigated the effect of quipazine on the expression of c-Fos in the mammalian circadian pacemaker, the suprachiasmatic nucleus (SCN). Quipazine reduced the amount of running wheel activity for 3 h after treatment, however, no long-term changes in tau and in the activity level were observed. More important, quipazine induced significant phase advances of the activity rhythm and c-Fos production in the SCN at the end of the subjective night (Circadian Time [CT] 22), whereas neither phase shifts nor c-Fos induction were observed during the subjective day. Quipazine injections also resulted in moderate phase delays at the beginning of the subjective night (CT 14). A similar phase-response characteristic typically can be observed for photic stimuli. By contrast, nonphotic stimuli normally produce phase advances during the subjective day. The present results suggest species differences between the hamster and the rat with respect to the serotonergic action on circadian timekeeping and indicate that serotonergic pathways play a role in the transmission of photic information to the SCN of rats.