Tissue- and treatment-specific usage of multiple preproenkephalin transcriptional start sites

Principles of motivation revealed by the diverse functions of neuropharmacological and neuroanatomical substrates underlying feeding behavior

Circuits that participate in specific subcomponents of feeding (e.g., gustatory perception, peripheral feedback relevant to satiety and energy balance, reward coding, etc.) are found at all levels of the neural axis. Further complexity is conferred by the wide variety of feeding-modulatory neurotransmitters and neuropeptides that act within these circuits. An ongoing challenge has been to refine the understanding of the functional specificity of these neurotransmitters and circuits, and there have been exciting advances in recent years. We focus here on foundational work of Dr. Ann Kelley that identified distinguishable actions of striatal opioid peptide modulation and dopamine transmission in subcomponents of reward processing. We also discuss her work in overlaying these neuropharmacological effects upon anatomical pathways that link the telencephalon (cortex and basal ganglia) with feeding-control circuits in the hypothalamus. Using these seminal contributions as a starting point, we will discuss new findings that expand our understanding of (1) the specific, differentiable motivational processes that are governed by central dopamine and opioid transmission, (2) the manner in which other striatal neuromodulators, specifically acetylcholine, endocannabinoids and adenosine, modulate these motivational processes (including via interactions with opioid systems), and (3) the organization of the cortical-subcortical network that subserves opioid-driven feeding. The findings discussed here strengthen the view that incentive-motivational properties of food are coded by substrates and neural circuits that are distinguishable from those that mediate the acute hedonic experience of food reward. Striatal opioid transmission modulates reward processing by engaging frontotemporal circuits, possibly via a hypothalamic-thalamic axis, that ultimately impinges upon hypothalamic modules dedicated to autonomic function and motor pattern control. We will conclude by discussing implications for understanding disorders of "non-homeostatic" feeding.

Pharmacological characterization of high-fat feeding induced by opioid stimulation of the ventral striatum

Nucleus accumbens mu-opioid stimulation causes marked increases in the intake of highly palatable foods, such as a high-fat diet. However, to date there has been little examination of how other striatal neurotransmitters may mediate opioid-driven feeding of palatable foodstuffs. In the current study, free feeding rats with bilateral cannulae aimed at the nucleus accumbens received intra-accumbens pretreatment with antagonists for dopamine D-1 (SCH23390; 0 microg or 1 microg/0.5 microl/side), dopamine D-2 (raclopride; 0 microg or 2.0 microg/0.5 microl/side), AMPA (LY293558; 0 microg, 0.01 microg or 0.10 microg/0.5 microl/side), muscarinic (scopolamine 0 microg, 0.1, 1.0, or 10 microg/0.5 microl/side) or nicotinic (mecamylamine; 0 microg, 10 microg/0.5 microl/side) receptors, immediately prior to infusions of the mu-receptor agonist D-Ala2, NMe-Phe4, Glyol5-enkephalin (DAMGO; 0.25 microg/0.5 microl) or vehicle. The effects of these pretreatments on 2 hr fat intake was compared to pretreatment with a general opioid antagonist (naltrexone; 0 microg or 20 microg/0.5 microl/side). DAMGO-induced feeding was unaffected by prior antagonism of dopamine, glutamate, or nicotinic receptors. As expected, naltrexone infusions blocked DAMGO-elicited fat intake. Antagonism of muscarinic acetylcholine receptors reduced feeding in both the DAMGO and vehicle-treated conditions. In an additional experiment, cholinergic receptor stimulation alone did not affect intake of the fat diet, suggesting that nucleus accumbens cholinergic stimulation is insufficient to alter feeding of a highly palatable food. These data suggest that the feeding effects caused by striatal opioid stimulation are independent from or downstream to the actions of dopamine and glutamate signaling, and provide novel insight into the role of striatal acetylcholine on feeding behaviors.

Striatal muscarinic receptor antagonism reduces 24-h food intake in association with decreased preproenkephalin gene expression

Cholinergic interneurons of the striatum respond to motivationally relevant stimuli and are involved in appetitive learning. However, there has been relatively little inquiry into the role of striatal acetylcholine in food motivation. Here we show in rats that a single infusion of the muscarinic receptor antagonist scopolamine (0, 5.0 or 10.0 microg/0.5 microL bilaterally) potently reduced 24-h food intake following injections into either the ventral or dorsal striatum, without affecting water intake. Furthermore, muscarinic receptor blockade induced reliable and widespread reductions in striatal preproenkephalin, but not preprodynorphin, mRNA expression. These data suggest a novel role for striatal acetylcholine in modulating feeding behavior via its effects on enkephalin gene expression. As prior research indicates a critical role for striatal enkephalin in consummatory behaviors and palatability, we hypothesize that cholinergic interneurons assist in translating hypothalamic energy state signals into food-directed behaviors via their regulation of striatal opioid peptides.

Transcription of actin, cyclophilin and glyceraldehyde phosphate dehydrogenase genes: tissue- and treatment-specificity

Studies involving RNA transcription, in varying biological systems, usually necessitate a term of transcriptional reference. Traditionally, the transcription of the gene of interest was compared to a constitutively expressed 'control' gene. Run-on transcription analysis was undertaken to evaluate and compare the transcription of three frequently used 'control genes' (beta-actin, cyclophilin and glyceraldehyde-3-phosphate dehydrogenase), in nine rat tissues. Similarities, but also clear and highly significant differences, were found in the transcription profiles of these three genes. There was significantly greater transcription for uterine glyceraldehyde-phosphate dehydrogenase compared to all other tissues tested, while both cyclophilin and glyceraldehyde-phosphate dehydrogenase were significantly elevated in the adrenal cortex. Upon cholinergic agonist treatment, both beta-actin and glyceraldehyde-phosphate dehydrogenase RNA expression were greatly induced in the adrenal medulla (41- and 94-fold, respectively), while cyclophilin transcription was not altered. In another treatment paradigm, surgical ovariectomy, only uterine glyceraldehyde-phosphate dehydrogenase transcription was significantly reduced. While, all three of these genes are assumed to be constitutively expressed throughout the body and hence used as normalization controls, the current study questions these accepted terms of reference. As cyclophilin transcription was not affected in both treatment paradigms, it should be considered more seriously as a RNA normalization control.

Novel transcriptional mechanisms are involved in regulating preproenkephalin gene expression in vivo

For the dissection of the temporal and spatial patterns of cell- and tissue-specific gene expression an understanding of the contributing regulating mechanisms is required. We now confirm that there are novel mechanisms regulating preproenkephalin gene expression in basal as well as cholinergic agonist treated rats. Moreover, we demonstrate that these novel transcriptional mechanisms are consistent with RNA intragenic elongation pausing, alternate promoter usage, and small sense and antisense RNA transcription from the preproenkephalin gene locus. We report that while basal striatal and olfactory bulb proenkephalin RNA transcripts are initiated from the "normal" proximal promoter, in cerebellum de novo RNA transcription appears to be initiated from the distal so-called "germ-cell" promoter. Furthermore, "normally" initiated olfactory bulb proenkephalin RNA transcripts appear to be down-regulated by the time the RNA polymerase II complex reaches the first preproenkephalin intron, in a way that is consistent with RNA elongation pausing. As the pattern of small sense and antisense transcripts found associated with this gene's expression is tissue-specific, we suggest that they may also play a role in regulating gene expression. The understanding of this gene's regulation should have widespread importance, not only to those interested in opioid gene expression, but also to those interested in gene regulation, in general.

Characterization of the mouse Cyp1B1 gene. Identification of an enhancer region that directs aryl hydrocarbon receptor-mediated constitutive and induced expression

Transcriptional activation of the Cyp1B1 gene in rodents is stimulated by both polycyclic hydrocarbons and cAMP. The mouse Cyp1B1 gene structure contains three exons, of which the second nucleotide of exon 2 is the translation start site. Primer extension analysis identified a transcription start domain defining an exon 1 of 371 base pairs. The sequence 1.075 kilobases upstream of the transcription start site showed 11 xenobiotic-responsive elements (XRE) (TnGCGTG or GCGTG) that are putative aryl hydrocarbon receptor (AhR)-binding sites and three steroidogenic factor-1 motifs that are associated with cAMP-mediated transcriptional activation of genes. A transiently transfected Cyp1B1-luciferase construct, composed of exon 1 and 1.075 kilobases of 5'-flanking region, was induced by 2,3, 7,8-tetrachlorodibenzo-p-dioxin (TCDD; 10.0 +/- 3.0-fold, n = 6) in C3H10T1/2 cells, which exclusively express Cyp1B1. The 90-base pair basal promoter contains two SP-1 sites, one SF-1 site, and a TATA-like box. TCDD induction and basal expression were dependent on positive regulatory elements present between -1075 and -810. Five XRE motifs localized in the enhancer region were completely conserved between mouse and human CYP1B1 sequences. Similar inductions were seen in Hepa-1 cells, which express Cyp1A1 but not Cyp1B1. However, basal Cyp1B1 promoter activities were 4-10-fold higher in C3H10T1/2 cells providing the enhancer region was present, partially reproducing the in vivo cell-specific expression of Cyp1B1. Gel shift experiments established that TCDD stimulates AhR binding to the downstream XRE in the enhancer region. However, oligonucleotides that encompass two other XREs show a high affinity complex of similar size that is evident even without TCDD treatment and that does not contain either the AhR or AhR nuclear translocator. The fourth XRE is immediately adjacent to an E-box, and this oligonucleotide formed a smaller complex that was dependent on this E-box sequence. Negative regulatory sequences have been located between the promoter and TCDD-responsive enhancer regions. Constitutive expression of the Cyp1B1 gene was lost in AhR-deficient cells and was restored by transfected AhR cDNA. Reporter constructs function in a parallel manner, demonstrating the key role of the AhR in constitutive as well as TCDD-induced expression of Cyp1B1 in mouse embryo fibroblasts.

Guinea pig preprodynorphin mRNA: primary structure and regional quantitation in the brain

We isolated and sequenced genomic and cDNA clones of the guinea pig preprodynorphin (ppDyn) mRNA. The sequence of ppDyn mRNA was deduced from a combination of genomic and cDNA clones: The primary structure of two coding exons was derived from a genomic clone and 5' and 3' untranslated sequences were obtained using rapid amplification of cDNA ends (RACE). The predicted mRNA of 2,350 nucleotides coincides well with the size of transcripts in Northern blot analyses of RNA from different brain regions. The deduced amino acid sequence of guinea pig ppDyn shares 70%, 68%, and 61% identity to porcine, human, and rat ppDyn, respectively. The 5' untranslated sequences of guinea pig hippocampal and adrenal ppDyn mRNA are identical; both contain sequences of exon I and, like porcine mRNA, lack an exon (exon II) present in human and rat mRNA. Quantitative solution hybridization RNase protection analysis of total RNA from selected guinea pig brain regions was performed. The nucleus accumbens was found to have the greatest abundance of ppDyn mRNA, followed by caudate putamen, hippocampus, hypothalamus, amygdala, frontal cortex, olfactory bulb, and pons/medulla.

Molecular characterization of immune derived proenkephalin mRNA and the involvement of the adrenergic system in its expression in rat lymphoid cells

Proenkephalin (PENK), a classically defined opioid gene, was originally thought to be expressed almost exclusively in the mature nervous and neuroendocrine systems. In the last few years, it was demonstrated, however, that significant levels of PENK mRNA and PENK-derived peptides are transiently expressed in cells of the immune system. Very little is known about the molecular mechanisms regulating this transient expression. In order to investigate those mechanisms, we examined the in vivo expression of PENK mRNA in mesenteric lymph nodes after exposing rats to lipopolysaccharide. In the present study we demonstrate that: (i) promoter usage and splicing of PENK mRNA function similarly in mesenteric lymph nodes as in neural cells; (2) PENK expression in mesenteric lymph nodes is modulated by adrenaline via adrenergic receptors; and (3) the adrenergic system participates in the modulation of the LPS induced PENK mRNA expression. These results provide more evidence for the involvement of opioids in neuro-immune interactions.

Ontogeny of preproenkephalin mRNA expression in the rat retina

Endogenous opioid systems (i.e. opioid peptides and opioid receptors) modulate developmental events in the neonatal mammalian retina. In the present study, the mRNA encoding preproenkephalin A (PPE), the prohormone for the opioid growth factor (OGF), [Met5]-enkephalin, was studied in the developing and the adult retinas of rats. Northern analysis indicated the presence of a 1.4-kb message in the developing and adult retinas corresponding to rat PPE mRNA. Quantitation showed that PPE message was present on postnatal day 1 at 5% of the adult level, and increased during development until the adult quantity was reached by postnatal day 27. In situ hybridization experiments first detected the presence of PPE mRNA in retinal tissues during late gestation. In late prenatal and neonatal retinas, PPE message was associated with areas of the developing retina containing proliferating neuroblasts and postmitotic cells. Later in development, message appeared to be located primarily within the inner retina, with abundant PPE mRNA associated with putative horizontal cells of the inner nuclear layer (INL). The adult retina showed a similar pattern of PPE gene expression in the cells of the INL. These findings document that the gene expression in the retina for PPE begins in the fetus, continues during retinal development, and coincides with the presence of a PPE mRNA derivative ([Met5]-enkephalin) that regulates DNA synthesis during retinal ontogeny. Our results are also the first to show the presence of PPE message in the adult mammalian retina, suggesting transcription of an opioid gene in the mature visual system.

Gene-peptide relationships in the developing rat brain: the response of preproenkephalin mRNA and [Met5]-enkephalin to acute opioid antagonist (naltrexone) exposure

[Met5]-enkephalin, encoded by the preproenkephalin (PPE) gene, serves as a growth factor during brain development in addition to its role as a neurotransmitter. This study examined the relationship of gene and peptide expression in the developing (postnatal day 6) rat brain by disrupting peptide-receptor interaction with either a brief (4-6 h) or continuous opioid receptor blockade using a single injection of 1 or 50 mg/kg naltrexone (NTX), respectively; such perturbations result in growth inhibition or acceleration, respectively. In the caudate putamen, an area that has completed neurogenesis by postnatal day 6 and has an abundance of PPE mRNA and enkephalins in adulthood, NTX did not influence PPE mRNA in either NTX group, or the enkephalin levels in the 1 mg/kg NTX group. [Met5]-enkephalin values in the neostriatum, however, were 67-183% greater than controls in rats given 50 mg/kg NTX, beginning 5 min after drug injection. In the cerebellum, PPE mRNA expression was depressed from 5 min to 4 h in the 1 mg/kg NTX group, and was normal thereafter; mRNA levels in the 50 mg/kg NTX group were markedly subnormal for 24 h. Enkephalin levels were significantly depressed within 5 min of drug injection and remained so for 4 h in the 1 mg/kg NTX group, but were elevated to approximately 135% of control values at 8, 16, and 24 h. Enkephalin levels were not changed in the cerebellum of the 50 mg/kg NTX group, or in the plasma of either NTX group. These data suggest that a single exposure to NTX can affect transcriptional and translational mechanisms related to PPE mRNA and opioid peptide expression in a rapid and sustained manner, and that this treatment elicits a specific pattern of alterations dependent upon the brain region sampled, drug dosage, and/or the duration of opioid receptor blockade. Additionally, our results indicate that the decreased DNA synthesis in external germinal cells occurring after opioid receptor blockade as recorded earlier may be related to an increase in the potent opioid growth factor, [Met5]-enkephalin.

Androgen modulation of multiple transcription start sites of the mitochondrial aspartate aminotransferase gene in rat prostate

Mitochondrial aspartate aminotransferase (mAAT) is one of two key enzymes in the pathway of citrate production in prostate. Expression of mAAT is modulated by testosterone and prolactin in prostate. We cloned the promoter and 5'-flanking region of the rat mAAT gene and sequenced 2.0 kilobases of the DNA. This fragment contains the 5'-regulatory promoter region that lacks a TATA and a CCAAT box but is G+C rich. The 5'-upstream flanking region contains sequences that have high homology with the consensus glucocorticoid response element/androgen response element (ARE) and a reported ARE sequence that is different from the consensus sequence. Functional transcription studies showed that a 481-base region containing the two ARE sequences was sufficient for androgen-regulated gene expression. There are multiple transcription start sites that are regulated by testosterone in prostate. In liver, on the other hand, castration did not affect transcription from any of the start sites. Therefore, these data provide evidence that transcriptional regulation of the rat pmAAT gene occurs through an ARE located in the 5'-region. In addition, not only is gene expression modulated by testosterone, but the effect of testosterone on transcription is cell specific.

Molecular cloning and characterization of the hamster preproenkephalin A cDNA

The cDNA for hamster preproenkephalin A (ENK) was cloned from an adrenal gland cDNA library constructed in the lambda ZapII vector. A nearly full-length cDNA was obtained and its 5' end region was completed using the technique of rapid amplification of cDNA ends (RACE). The coding and 3' untranslated regions of the hamster ENK cDNA share a high sequence identity with the rat, human, and bovine cDNAs, whereas the sequence identity is lower for the 5' untranslated region. Southern blot analysis of genomic DNA digests showed that a single copy of the ENK gene is present in the hamster haploid genome. Northern blot analysis of poly(A)+RNA from various hamster tissues indicated the following rank order for ENK messenger RNA abundance: adrenal glands > right atrium > brain > left atrium > right ventricle > ventricular septum > left ventricle, whereas primer extension analysis showed a single, identical transcriptional initiation site for the ENK mRNA in all these tissues. The sequence of the 5' untranslated region of the heart ENK cDNA was found to be identical to that from adrenal glands. This rules out the possibility that structural divergences in the 5' untranslated region of the heart ENK mRNA could decrease its translation efficiency and contribute to the very low level of enkephalin-containing peptides in the heart, compared to the adrenal glands.

Preproenkephalin mRNA expression in the developing and adult rat brain

[Met5]-Enkephalin is derived from the protein precursor, proenkephalin A, which in turn is encoded by the preproenkephalin (PPE) gene. [Met5]-Enkephalin is not only a putative neuromodulatory substance, but also serves as a growth factor (= opioid growth factor, OGF). OGF exerts an inhibitory influence on the developing nervous system and is especially targeted to cell proliferative and differentiative events. This study examined the relationship of PPE mRNA expression to late prenatal and postnatal rat brain development. Northern blot analysis of the whole brain and cerebellum showed that message is present in the fetal nervous system on prenatal day 15 (the earliest timepoint examined), is expressed at relatively similar levels within each tissue during the first 2 postnatal weeks, and reaches adult levels by the beginning of the 3rd postnatal week. In situ hybridization methodology revealed that PPE mRNA was prominent in areas associated with cell generation. Message was found in sites of primary (i.e., ventricular region) and secondary (e.g., external germinal layer of the cerebellum) cellular replication, as well as in discrete foci of cell proliferation (e.g., medullary layer of the cerebellum). PPE mRNA was also present for varying periods of time in postmitotic cells. During development, a number of patterns (decrease, increase, and no perceptible change) of PPE mRNA could be detected in relationship to the fetal/neonatal period. Given the strong evidence (e.g., regulation of cell proliferation and differentiation, temporal and spatial patterns of peptide and zeta opioid receptor) that enkephalin immunoreactivity is associated with proliferating and differentiating neurons and glia, these results suggest that the source of [Met5]-enkephalin is both autocrine and paracrine in nature.

Cholinergic regulation of rat preproenkephalin RNA in the adrenal medulla

Expression of the rat preproenkephalin (ppENK) gene involves transsynaptic cholinergic mechanisms. We evaluated the effects of cholinergic agonist treatments in vivo on the expression of adrenomedullary ppENK RNA. Cholinergic treatment with nicotinic + muscarinic receptor agonists resulted in a synergistic 100-fold rise in steady-state ppENK messenger RNA levels, but only a 30- to 35-fold rise in initiation of steady-state ppENK RNA transcripts. The levels of initiated ppENK steady-state RNA peaked at two days, whereas mature (1.45 kb) ppENK mRNA levels continued to rise, peaking at four days. This suggested that other transcriptional (attenuation or alternative splicing) or post-transcriptional (RNA stabilization) regulatory mechanisms must be operative. As multiple ppENK RNA start sites exist, we examined how usage of multiple sites was altered by cholinergic treatments. The predominant start site changed from E2 in the basal state, to E4 after primary cholinergic stimulation, to E3 after re-treatment. This represents novel example of differential usage of multiple RNA initiation start sites in vivo. Differences in initiated and mature transcripts are consistent with at least four mechanisms involved in control of cholinergic-induced ppENK RNA expression: (i) simply new initiation of RNA transcripts, (ii) differential usage of the multiple RNA start sites, (iii) stabilization of mRNA transcripts, and (iv) attenuation and/or alternative RNA splicing of RNA transcripts.