Tolerance and cross-tolerance to stress-induced increases in plasma Met-enkephalin in rats with adaptively increased resting secretion

Influence of prenatal transportation stress-induced differential DNA methylation on the physiological control of behavior and stress response in suckling Brahman bull calves

The objective of this experiment was to examine potential differential methylation of DNA as a mechanism for altered behavioral and stress responses in prenatally stressed (PNS) compared with nonprenatally stressed (Control) young bull calves. Mature Brahman cows (n = 48) were transported for 2-h periods at 60 ± 5, 80 ± 5, 100 ± 5, 120 ± 5, and 140 ± 5 d of gestation (Transported group) or maintained as nontransported Controls (n = 48). From the offspring born to Transported and Control cows, a subset of 28-d-old intact bulls (n = 7 PNS; n = 7 Control) were evaluated for methylation of DNA of behavior and stress response-associated genes. Methylation of DNA from white blood cells was assessed via reduced representation bisulfite sequencing methods. Because increased methylation of DNA within gene promoter regions has been associated with decreased transcriptional activity of the corresponding gene, differentially methylated (P ≤ 0.05) CG sites (cytosine followed by a guanine nucleotide) located within promoter regions (n = 1,205) were used to predict (using Ingenuity Pathway Analysis software) alterations to canonical pathways in PNS compared with Control bull calves. Among differentially methylated genes (P ≤ 0.05) related to behavior and the stress response were OPRK1, OPRM1, PENK, POMC, NR3C2, TH, DRD1, DRD5, COMT, HTR6, HTR5A, GABRA4, GABRQ, and GAD2. Among altered (P < 0.05) signaling pathways related to behavior and the stress response were Opioid Signaling, Corticotropin-Releasing Hormone Signaling, Dopamine Receptor Signaling, Dopamine-DARPP32 Feedback in cAMP Signaling, Serotonin Receptor Signaling, and GABA Receptor Signaling. Alterations to behavior and stress response-related genes and canonical pathways supported previously observed elevations in temperament score and serum cortisol through weaning in the larger population of PNS calves from which bulls in this study were derived. Differential methylation of DNA and predicted alterations to behavior and stress response-related pathways in PNS compared with Control bull calves suggest epigenetic programming of behavior and the stress response in utero.

m-Trifluoromethyl-diphenyl Diselenide Regulates Prefrontal Cortical MOR and KOR Protein Levels and Abolishes the Phenotype Induced by Repeated Forced Swim Stress in Mice

The present study aimed to investigate the m-trifluoromethyl-diphenyl diselenide [(m-CF₃-PhSe)₂] effects on prefrontal cortical MOR and KOR protein levels and phenotype induced by repeated forced swim stress (FSS) in mice. Adult Swiss mice were subjected to repeated FSS sessions, and after that, they performed the spontaneous locomotor/exploratory activity, tail suspension, and splash tests. (m-CF₃-PhSe)₂ (0.1 to 5 mg/kg) was administered to mice 30 min before the first FSS session and 30 min before the subsequent repeated FSS. (m-CF₃-PhSe)₂ abolished the phenotype induced by repeated FSS in mice. In addition, a single FSS session increased μ but reduced δ-opioid receptor contents, without changing the κ content. Mice subjected to repeated FSS had an increase in the μ content when compared to those of naïve group or subjected to single FSS. Repeated FSS induced an increase of δ-opioid receptor content compared to those mice subjected to single FSS. However, the δ-opioid receptor contents were lower than those found in the naïve group. The mice subjected to repeated FSS showed an increase in the κ-opioid receptor content when compared to that of the naïve mice. (m-CF₃-PhSe)₂ regulated the protein contents of μ and κ receptors in mice subjected to repeated FSS. These findings demonstrate that (m-CF₃-PhSe)₂ was effective to abolish the phenotype induced by FSS, which was accompanied by changes in the contents of cortical μ- and κ-opioid receptors.

Stress and opioids: role of opioids in modulating stress-related behavior and effect of stress on morphine conditioned place preference

Research studies have defined the important role of endogenous opioids in modulating stress-associated behavior. The release of β-endorphins in the amygdala in response to stress helps to cope with a stressor by inhibiting the over-activation of HPA axis. Administration of mu opioid agonists reduces the risk of developing post-traumatic stress disorder (PTSD) following a traumatic event by inhibiting fear-related memory consolidation. Similarly, the release of endogenous enkephalin and nociceptin in the basolateral amygdala and the nucleus accumbens tends to produce the anti-stress effects. An increase in dynorphin levels during prolonged exposure to stress may produce learned helplessness, dysphoria and depression. Stress also influences morphine-induced conditioned place preference (CPP) depending upon the intensity and duration of the stressor. Acute stress inhibits morphine CPP, while chronic stress potentiates CPP. The development of dysphoria due to increased dynorphin levels may contribute to chronic stress-induced potentiation of morphine CPP. The activation of ERK/cyclic AMP responsive element-binding (CREB) signaling in the mesocorticolimbic area, glucocorticoid receptors in the basolateral amygdala, and norepinephrine and galanin system in the nucleus accumbens may decrease the acute stress-induced inhibition of morphine CPP. The increase in dopamine levels in the nucleus accumbens and augmentation of GABAergic transmission in the median prefrontal cortex may contribute in potentiating morphine CPP. Stress exposure reinstates the extinct morphine CPP by activating the orexin receptors in the nucleus accumbens, decreasing the oxytocin levels in the lateral septum and amygdala, and altering the GABAergic transmission (activation of GABAA and inactivation of GABAB receptors). The present review describes these varied interactions between opioids and stress along with the possible mechanism.

Met5-enkephalin is localized within axon terminals in the subfornical organ: vascular contacts and interactions with neurons containing gamma-aminobutyric acid

Met5-enkephalin inhibits sodium and water excretion and antagonizes the central actions of angiotensin II in subfornical organ of rat brain. We examined the ultrastructural basis for enkephalin modulation in this circumventricular region. Additionally, we examined the possibility that there might be cellular sites for functional interactions involving Met5-enkephalin and gamma-aminobutyric acid (GABA), a known inhibitory transmitter throughout the central nervous system. Met5-enkephalin and GABA were identified in single coronal sections through the subfornical organ using immunoperoxidase and silver-enhanced immunogold labeling methods, respectively. Enkephalin-like immunoreactivity was most prominently localized within axon terminals. These were distributed primarily in the central, highly vascular, regions of the subfornical organ. Enkephalin-labeled terminals were apposed to the basement membranes of fenestrated capillaries and also formed symmetric, inhibitory type synapses with neurons. In terminals associated with either blood vessels or neurons, the enkephalin immunoreactivity was enriched in large (80-150 nm) dense core vesicles. The immunoreactive vesicles were usually located within portions of the axon in close proximity to astrocytic processes. In contrast, smaller vesicles in the same terminals were more often aggregated near the basement membrane of the capillaries and the active zone of the synapse. The targets of enkephalin-immunoreactive terminals were either unlabeled or GABA-labeled dendrites of local neurons. Enkephalin was also co-localized with GABA in perikarya and in axon terminals. Terminals containing only GABA were far more abundant than those containing enkephalin or enkephalin and GABA. GABA-immunoreactive terminals formed symmetric synapses on unlabeled dendrites some of which also received convergent input from terminals containing enkephalin. Additionally, the enkephalin-immunoreactive terminals were closely apposed to GABA-labeled and unlabeled terminals. These results suggest sites for nonsynaptic release of Met5-enkephalin from dense core vesicles in contact with astrocytes near blood vessels and synaptic complexes in the rat subfornical organ. Moreover, the observed dual localization and pre- and postsynaptic associations between neurons containing Met5-enkephalin and GABA indicate that inhibitory effects of opioids in the subfornical organ may be mediated or potentiated by GABA.

Nicotine-induced alterations in peripheral tissue concentrations of native and cryptic Met- and Leu-enkephalin

This study examined the peripheral tissue distribution of native and cryptic Met- and Leu-enkephalin, and regulation of tissue enkephalins by nicotine. Met- and Leu-enkephalin concentrations showed widespread variation in tissue concentration and degree of processing. HPLC characterization of homogenate of spleen revealed that both native and cryptic immunoreactive Met-enkephalin are comprised of two peaks, one representing authentic Met-enkephalin pentapeptide and the other its sulfoxide. Subacute repeated administration of nicotine 0.1 mg/kg ip, six times at 30 min intervals, increased native Met- and Leu-enkephalin in adrenal medulla without affecting cryptic Met- and Leu-enkephalin concentrations, consistent with increased processing of larger peptides to Met- and Leu-enkephalin. Subacute nicotine decreased splenic concentrations of native and cryptic Met-enkephalin and native Leu-enkephalin, consistent with increased release of Met- and Leu-enkephalin from spleen and decreased synthesis of proenkephalin A or inadequate processing of larger peptides to enkephalin pentapeptides in spleen to compensate for the increased release during this period. HPLC characterization revealed that nicotine-induced decrease in native Met-enkephalin in spleen resulted from reductions in both pentapeptide and its sulfoxide. Nicotine also increased native Met-enkephalin in jejunum, decreased cryptic Met-enkephalin in heart atrium, increased native Leu-enkephalin in anterior pituitary and decreased cryptic Leu-enkephalin in jejunum. Nicotine may produce some of its effects through alterations in release of enkephalins from peripheral tissues.