Reciprocal regulation of TREK-1 channels by arachidonic acid and CRH in mouse corticotropes

Arachidonic acid (AA) is generated in the anterior pituitary gland upon stimulation by the ACTH secretagogue, CRH. Using the patch clamp technique, we examined the action of AA on the excitability of single pituitary corticotropes obtained from a transgenic mouse strain that expresses the enhanced green fluorescent protein driven by the proopiomelanocortin promoter. CRH evoked depolarization, but AA caused hyperpolarization. Under voltage clamp condition, AA caused a rapid inhibition of the delayed rectifier K(+) current and then increased a background K(+) current. Inhibition of AA metabolism did not prevent the activation of the K(+) current by AA, suggesting a direct action of AA. The sensitivity of the AA-activated K(+) current to fluoxetine, chlorpromazine, extracellular acidification, diphenylbutylpiperidine antipsychotics, and the membrane permeable cAMP analog [8-(4-chlorophenylthio)-cAMP] suggest that the current is mediated via TWIK-related K(+) channel (TREK)-1 channels. Activation of the CRH receptors that are coupled to the adenylate cyclase pathway suppressed the activation of TREK-1 current by AA and reversed the AA-mediated hyperpolarization. Intracellular acidification (pH 7.0) increased the basal amplitude of TREK-1 current and resulted in hyperpolarizaton. CRH suppressed the basal TREK-1 current in cells with intracellular acidification and caused depolarization. Our finding indicates that TREK-1 channels are important in setting the resting potential in corticotropes. The opposing actions of CRH and AA on the excitability of corticotropes raise the possibility that AA may act as a negative feedback regulator to reduce the stimulatory action of CRH and thus prevent excessive ACTH release during chronic stress.

Meal pattern analysis in neural-specific proopiomelanocortin-deficient mice

The central melanocortin system, consisting of melanocortin peptides, agouti gene related peptide and their receptors plays a critical role in the homeostatic control of energy balance. Loss of function mutations in the genes encoding proopiomelanocortin or melanocortin MC(4) receptors cause profound obesity and hyperphagia. However, little is known about the functional relationship of melanocortin neurocircuits to the temporal organization of meal-taking behavior. We used an operant paradigm that combined lever pressing for food pellet deliveries with free water intake monitored by lickometers to quantify meal patterns in mutant mice that selectively lack proopiomelanocortin expression in hypothalamic neurons (nPOMCKO). Compared to wildtype siblings, nPOMCKO mice consumed 50% more food and water daily and exhibited a more stereotyped feeding pattern characterized by reduced inter-meal and inter-mouse variations. Average meals were larger in size but shorter in duration, with no change in meal number. Consequently, intermeal intervals were prolonged in nPOMCKO mice. Similar patterns were observed in pre-obese juvenile and frankly obese adult mice suggesting that neither age nor degree of obesity was responsible for the altered phenotypes. Spontaneous locomotion and wheel running were decreased in nPOMCKO mice, but circadian variations in locomotor and feeding activity were conserved. These data show that hyperphagia in male nPOMCKO mice is due to increased meal size but not meal number, and this pattern is established by age of 5weeks. The combination of larger, more rapidly consumed meals and prolonged intermeal intervals suggests that proopiomelanocortin peptides are necessary for normal meal termination, but not the maintenance of satiety.

The estrogen receptor α colocalizes with proopiomelanocortin in hypothalamic neurons and binds to a conserved motif present in the neuron-specific enhancer nPE2

The gene encoding the prohormone proopiomelanocortin (POMC) is mainly expressed in two regions in vertebrates, namely corticotrophs and melanotrophs in the pituitary and a small population of neurons in the arcuate nucleus of the hypothalamus. In this latter region, POMC-derived peptides participate in the control of energy balance and sensitivity to pain. Neuronal expression of POMC is conferred by two enhancers, nPE1 and nPE2, which are conserved in most mammals, but no transcription factors are yet known to bind to these enhancers. In this work, by means of a one-hybrid screening, we identify that nPE2 possesses an element recognized by transcription factors of the nuclear receptor superfamily. This element, named NRBE, is conserved in all known nPE2 enhancers and is necessary to confer full enhancer strength to nPE2-driven reporter gene expression in transgenic mice assays, indicating that the phylogenetic conservation of the element is indicative of its functional importance. In a search for candidate nuclear receptors that might control POMC we observed that estrogen receptor alpha (ESR1) - a known regulator of energy balance at the hypothalamic level - can bind to the NRBE element in vitro. In addition we observed by immunofluorescence that ESR1 is coexpressed with POMC in around 25-30% of hypothalamic neurons of males and females during late embryonic stages and adulthood. Thus, our results indicate that hypothalamic expression of POMC is controlled by nuclear receptors and establish ESR1 as a candidate regulator of POMC.

beta-Endorphin expression in the mouse retina

Evidence showing expression of endogenous opioids in the mammalian retina is sparse. In the present study we examined a transgenic mouse line expressing an obligate dimerized form of Discosoma red fluorescent protein (DsRed) under the control of the pro-opiomelanocortin promoter and distal upstream regulatory elements to assess whether pro-opiomelanocortin peptide (POMC), and its opioid cleavage product, beta-endorphin, are expressed in the mouse retina. Using double label immunohistochemistry we found that DsRed fluorescence was restricted to a subset of GAD-67-positive cholinergic amacrine cells of both orthotopic and displaced subtypes. About 50% of cholinergic amacrine cells colocalized DsRed and a large fraction of DsRed-expressing amacrine cells was positive for beta-endorphin immunostaining, whereas beta-endorphin-immunoreactive neurons were absent in retinas of POMC null mice. Our findings contribute to a growing body of evidence demonstrating that opioid peptides are an integral component of vertebrate retinas, including those of mammals.

A tale of two carboxypeptidases

Proopiomelanocortin (Pomc) neurons play a central role in energy homeostasis. Despite the complexity of Pomc posttranslational processing, regulation of Pomc gene expression often takes center stage. Complementary papers that zero in on distinct carboxypeptidases (Plum et al., 2009; Wallingford et al., 2009) now refocus the spotlight on regulated peptide cleavage.

Proopiomelanocortin expression in both GABA and glutamate neurons

Proopiomelanocortin (POMC) neurons have been intensively studied because of their essential role in regulating energy balance and body weight. Many effects of POMC neurons can be attributed to their release of cognate neuropeptides from secretory granules in axon terminals. However, these neurons also synaptically release non-peptide neurotransmitters. The aim of this study was to settle the controversy whether there are separate populations of POMC neurons that release GABA or glutamate. Transgenic mice expressing a red fluorescent protein [Discosoma red (DsRed)] driven by Pomc neuronal regulatory elements (POMC-DsRed) were crossed to mice that expressed green fluorescent protein (gfp) in GABAergic neurons (GAD67-gfp). Approximately 40% of POMC neurons in the arcuate nucleus of the double-transgenic mice expressed the GAD67-gfp transgene. In vitro neurotransmitter release was detected using whole-cell electrophysiologic recordings in cultured GAD67-gfp-positive and GAD67-gfp-negative POMC neurons that had formed recurrent synapses (autapses). Autapses from GAD67-gfp-positive neurons were uniformly GABAergic. In contrast, autapses from the GAD67-gfp-negative POMC neurons exclusively exhibited postsynaptic currents mediated by glutamate. Together, these results indicate that there are two subpopulations of POMC neurons in the arcuate nucleus differentiated by their amino acid neurotransmitter phenotype. Whole-cell voltage-clamp recordings from POMC neurons in live brain slices indicated that GABAergic and glutamatergic POMC neurons are under similar presynaptic and postsynaptic regulation, although the GABAergic POMC neurons are smaller and have higher input resistance. GABAergic and glutamatergic POMC neurons may mediate distinct aspects of POMC neuron function, including the regulation of energy homeostasis.

Hypothalamic orexin, OX1, alphaMSH, NPY and MCRs expression in dopaminergic D2R knockout mice

In 5-month-old male and female dopamine receptor 2 (D2R) knockout mice food intake per animal was unaltered while food per g BW was increased. We wished to evaluate the effect of D2R disruption on different components of energy balance and food intake regulation. We determined hypothalamic orexin precursor (PPO) expression, its receptor OX1, serum leptin levels, hypothalamic leptin receptor (OBR), circulating and pituitary alpha MSH levels, as well as central MC3 and MC4 receptors and NPY mRNA in wildtype and D2R knockout mice (KO). Loss of D2R caused a marked increase in serum prolactin levels, to higher levels in females compared to male KO mice. On the other hand, it produced a female-specific increase in circulating alphaMSH, and hypothalamic alphaMSH content, while neurointermediate alphaMSH content was decreased in both sexes. No differences were found in hypothalamic NPY, MC3R or MC4R concentration. Hypothalamic PPO mRNA expression was significantly decreased only in female KOs, while OX1 mRNA was not different between genotypes. Serum leptin levels were also similar in both genotypes. Our results show that in female and not in male mice disruption of the D2R produces two potentially anorexigenic events: an increase in serum and hypothalamic alphaMSH, and a decrease in hypothalamic orexin expression. Very high prolactin levels, which are orexigenic, probably counterbalance these effects, so that food intake is slightly altered. In males, on the other hand, hypothalamic PPO, and serum or hypothalamic alphaMSH are not modified, and increased prolactin levels may account for increased food intake per g BW. These results suggest a sexually dimorphic participation of the D2R in food intake regulation.

Proopiomelanocortin peptides are not essential for development of ethanol-induced behavioral sensitization

BACKGROUND

Behavioral sensitization is a result of neuroadaptation to repeated drug administration and is hypothesized to reflect an increased susceptibility to drug abuse. Proopiomelanocortin (POMC) derived peptides including beta-endorphin and alpha-melanocyte stimulating hormone have been implicated in development of behavioral sensitization and the reinforcing effects of alcohol and other drugs of abuse. This study used a genetically engineered mouse strain that is deficient for neural POMC to directly determine if any POMC peptides are necessary for the development of ethanol-induced locomotor sensitization.

METHODS

Adult female mice deficient for POMC in neurons only (Pomc(-/-)Tg/Tg, KO) and wildtype (Pomc(+/+)Tg/Tg, WT) littermates were injected once daily with either saline or ethanol (i.p.) for 12 to 13 days. On ethanol test day (day 13 or 14) all mice from both treatment groups received an i.p. injection of ethanol immediately before a 15-minute analysis of locomotor activity. Blood ethanol concentration (BEC) was measured on ethanol test day immediately following the test session. Baseline locomotor activity was measured for 15 minutes after a saline injection 2 days later in both groups.

RESULTS

There was no significant difference in BEC between genotypes (WT = 2.11 +/- 0.06; KO = 2.03 +/- 0.08 mg/ml). Both WT and nPOMC-deficient mice treated repeatedly with ethanol demonstrated a significant increase in locomotor activity on test day when compared to repeated saline-treated counterparts. In addition, mice of both genotypes in the repeated saline groups showed a significant locomotor stimulant response to acute ethanol injection.

CONCLUSIONS

Central POMC peptides are not required for either the acute locomotor stimulatory effect of ethanol or the development of ethanol-induced locomotor sensitization. While these peptides may modulate other ethanol-associated behaviors, they are not essential for development of behavioral sensitization.

Somatostatin secreted by islet delta-cells fulfills multiple roles as a paracrine regulator of islet function

OBJECTIVE

Somatostatin (SST) is secreted by islet delta-cells and by extraislet neuroendocrine cells. SST receptors have been identified on alpha- and beta-cells, and exogenous SST inhibits insulin and glucagon secretion, consistent with a role for SST in regulating alpha- and beta-cell function. However, the specific intraislet function of delta-cell SST remains uncertain. We have used Sst(-/-) mice to investigate the role of delta-cell SST in the regulation of insulin and glucagon secretion in vitro and in vivo.

RESEARCH DESIGN AND METHODS

Islet morphology was assessed by histological analysis. Hormone levels were measured by radioimmunoassay in control and Sst(-/-) mice in vivo and from isolated islets in vitro.

RESULTS

Islet size and organization did not differ between Sst(-/-) and control islets, nor did islet glucagon or insulin content. Sst(-/-) mice showed enhanced insulin and glucagon secretory responses in vivo. In vitro stimulus-induced insulin and glucagon secretion was enhanced from perifused Sst(-/-) islets compared with control islets and was inhibited by exogenous SST in Sst(-/-) but not control islets. No difference in the switch-off rate of glucose-stimulated insulin secretion was observed between genotypes, but the cholinergic agonist carbamylcholine enhanced glucose-induced insulin secretion to a lesser extent in Sst(-/-) islets compared with controls. Glucose suppressed glucagon secretion from control but not Sst(-/-) islets.

CONCLUSIONS

We suggest that delta-cell SST exerts a tonic inhibitory influence on insulin and glucagon secretion, which may facilitate the islet response to cholinergic activation. In addition, delta-cell SST is implicated in the nutrient-induced suppression of glucagon secretion.

Role of dopamine D1-like receptors in methamphetamine locomotor responses of D2 receptor knockout mice

Behavioral sensitization to psychostimulants manifests as an increased locomotor response with repeated administration. Dopamine systems are accepted to play a fundamental role in sensitization, but the role of specific dopamine receptor subtypes has not been completely defined. This study used the combination of dopamine D2 receptor-deficient mice and a D1-like antagonist to examine dopamine D1 and D2 receptor involvement in acute and sensitized locomotor responses to methamphetamine. Absence of the dopamine D2 receptor resulted in attenuation of the acute stimulant effects of methamphetamine. Mutant and wild-type mice exhibited sensitization that lasted longer within the time period of the challenge test in the mutant animals. Pretreatment with the D1-like receptor antagonist SCH 23390 produced more potent reductions in the acute and sensitized locomotor responses to methamphetamine in D2 receptor-deficient mice than in wild-type mice; however, the expression of locomotor sensitization when challenged with methamphetamine alone was equivalently attenuated by previous treatment with SCH 23390. These data suggest that dopamine D2 receptors play a key role in the acute stimulant and sensitizing effects of methamphetamine and act in concert with D1-like receptors to influence the acquisition of methamphetamine-induced behavioral sensitization, traits that may influence continued methamphetamine use.

Contributions of dopamine D1, D2, and D3 receptor subtypes to the disruptive effects of cocaine on prepulse inhibition in mice

Deficits in prepulse inhibition (PPI) of startle, an operational measure of sensorimotor gating, are characteristics of schizophrenia and related neuropsychiatric disorders. Previous studies in mice demonstrate a contribution of dopamine (DA) D(1)-family receptors in modulating PPI and DA D(2) receptors (D2R) in mediating the PPI-disruptive effects of amphetamine. To examine further the contributions of DA receptor subtypes in PPI, we used a combined pharmacological and genetic approach. In congenic C57BL/6 J wild-type mice, we tested whether the D1R antagonist SCH23390 or the D2/3R antagonist raclopride would attenuate the effects of the indirect DA agonist cocaine (40 mg/kg). Both the D1R and D2/3R antagonists attenuated the cocaine-induced PPI deficit. We also tested the effect of cocaine on PPI in wild-type and DA D1R, D2R, or D3R knockout mice. The cocaine-induced PPI deficit was influenced differently by the three DA receptor subtypes, being absent in D1R knockout mice, partially attenuated in D2R knockout mice, and exaggerated in D3R knockout mice. Thus, the D1R is necessary for the PPI-disruptive effects of cocaine, while the D2R partially contributes to these effects. Conversely, the D3R appears to inhibit the PPI-disruptive effects of cocaine. Uncovering neural mechanisms involved in PPI will further our understanding of substrates of sensorimotor gating and could lead to better therapeutics to treat complex cognitive disorders such as schizophrenia.

Differential activation and trafficking of micro-opioid receptors in brain slices

The activation of G protein-coupled receptors results in a cascade of events that include acute signaling, desensitization, and internalization, and it is thought that not all agonists affect each process to the same extent. The early steps in opioid receptor signaling, including desensitization, have been characterized electrophysiologically using brain slice preparations, whereas most previous studies of opioid receptor trafficking have been conducted in heterologous cell models. This study used transgenic mice that express an epitope-tagged (FLAG) micro-opioid receptor (FLAGMOR) targeted to catecholamine neurons by regulatory elements from the tyrosine hydroxylase gene. Brain slices from these mice were used to study tagged MOR receptors in neurons of the locus ceruleus. Activation of the FLAGMOR with [Met5]enkephalin (ME) produced a hyperpolarization that desensitized acutely to the same extent as native MOR in slices from wild-type mice. A series of opioid agonists were then used to study desensitization and receptor trafficking in brain slices, which was monitored with a monoclonal antibody against the FLAG epitope (M1) conjugated to Alexa 594. Three patterns of receptor trafficking and desensitization were observed: 1) ME, etorphine, and methadone resulted in both receptor desensitization and internalization; 2) morphine and oxymorphone caused significant desensitization without evidence for internalization; and 3) oxycodone was ineffective in both processes. These results show that two distinct forms of signaling were differentially engaged depending on the agonist used to activate the receptor, and they support the hypothesis that ligand-specific regulation of opioid receptors occurs in neurons maintained in brain slices from adult animals.

Signal transducer and activator of transcription-3 is required in hypothalamic agouti-related protein/neuropeptide Y neurons for normal energy homeostasis

Signal transducer and activator of transcription (Stat)-3 signals mediate many of the metabolic effects of the fat cell-derived hormone, leptin. In mice, brain-specific depletion of either the long form of the leptin receptor (Lepr) or Stat3 results in comparable obese phenotypes as does replacement of Lepr with an altered leptin receptor locus that codes for a Lepr unable to interact with Stat3. Among the multiple brain regions containing leptin-sensitive Stat3 sites, cells expressing feeding-related neuropeptides in the arcuate nucleus of the hypothalamus have received much of the focus. To determine the contribution to energy homeostasis of Stat3 expressed in agouti-related protein (Agrp)/neuropeptide Y (Npy) arcuate neurons, Stat3 was deleted specifically from these cells, and several metabolic indices were measured. It was found that deletion of Stat3 from Agrp/Npy neurons resulted in modest weight gain that was accounted for by increased adiposity. Agrp/Stat3-deficient mice also showed hyperleptinemia, and high-fat diet-induced hyperinsulinemia. Stat3 deletion in Agrp/Npy neurons also resulted in altered hypothalamic gene expression indicated by increased Npy mRNA and decreased induction of suppressor of cytokine signaling-3 in response to leptin. Agrp mRNA levels in the fed or fasted state were unaffected. Behaviorally, mice without Stat3 in Agrp/Npy neurons were mildly hyperphagic and hyporesponsive to leptin. We conclude that Stat3 in Agrp/Npy neurons is required for normal energy homeostasis, but Stat3 signaling in other brain areas also contributes to the regulation of energy homeostasis.

In vivo evidence for inverse agonism of Agouti-related peptide in the central nervous system of proopiomelanocortin-deficient mice

OBJECTIVE

Melanocyte-stimulating hormone (MSH) peptides processed from proopiomelanocortin (POMC) regulate energy homeostasis by activating neuronal melanocortin receptor (MC-R) signaling. Agouti-related peptide (AgRP) is a naturally occurring MC-R antagonist but also displays inverse agonism at constitutively active melanocortin-4 receptor (MC4-R) expressed on transfected cells. We investigated whether AgRP functions similarly in vivo using mouse models that lack all neuronal MSH, thereby precluding competitive antagonism of MC-R by AgRP.

RESEARCH DESIGN AND METHODS

Feeding and metabolic effects of the MC-R agonist melanotan II (MTII), AgRP, and ghrelin were investigated after intracerebroventricular injection in neural-specific POMC-deficient (Pomc(-/-)Tg/+) and global POMC-deficient (Pomc(-/-)) mice. Gene expression was quantified by RT-PCR.

RESULTS

Hyperphagic POMC-deficient mice were more sensitive than wild-type mice to the anorectic effects of MTII. Hypothalamic melanocortin-3 (MC3)/4-R mRNAs in POMC-deficient mice were unchanged, suggesting increased receptor sensitivity as a possible mechanism for the heightened anorexia. AgRP reversed MTII-induced anorexia in both mutant strains, demonstrating its ability to antagonize MSH agonists at central MC3/4-R, but did not produce an acute orexigenic response by itself. The action of ghrelin was attenuated in Pomc(-/-)Tg/+ mice, suggesting decreased sensitivity to additional orexigenic signals. However, AgRP induced delayed and long-lasting modifications of energy balance in Pomc(-/-)Tg/+, but not glucocorticoid-deficient Pomc(-/-) mice, by decreasing oxygen consumption, increasing the respiratory exchange ratio, and increasing food intake.

CONCLUSIONS

These data demonstrate that AgRP can modulate energy balance via a mechanism independent of MSH and MC3/4-R competitive antagonism, consistent with either inverse agonist activity at MC-R or interaction with a distinct receptor.

Ancient exaptation of a CORE-SINE retroposon into a highly conserved mammalian neuronal enhancer of the proopiomelanocortin gene

The proopiomelanocortin gene (POMC) is expressed in the pituitary gland and the ventral hypothalamus of all jawed vertebrates, producing several bioactive peptides that function as peripheral hormones or central neuropeptides, respectively. We have recently determined that mouse and human POMC expression in the hypothalamus is conferred by the action of two 5′ distal and unrelated enhancers, nPE1 and nPE2. To investigate the evolutionary origin of the neuronal enhancer nPE2, we searched available vertebrate genome databases and determined that nPE2 is a highly conserved element in placentals, marsupials, and monotremes, whereas it is absent in nonmammalian vertebrates. Following an in silico paleogenomic strategy based on genome-wide searches for paralog sequences, we discovered that opossum and wallaby nPE2 sequences are highly similar to members of the superfamily of CORE-short interspersed nucleotide element (SINE) retroposons, in particular to MAR1 retroposons that are widely present in marsupial genomes. Thus, the neuronal enhancer nPE2 originated from the exaptation of a CORE-SINE retroposon in the lineage leading to mammals and remained under purifying selection in all mammalian orders for the last 170 million years. Expression studies performed in transgenic mice showed that two nonadjacent nPE2 subregions are essential to drive reporter gene expression into POMC hypothalamic neurons, providing the first functional example of an exapted enhancer derived from an ancient CORE-SINE retroposon. In addition, we found that this CORE-SINE family of retroposons is likely to still be active in American and Australian marsupial genomes and that several highly conserved exonic, intronic and intergenic sequences in the human genome originated from the exaptation of CORE-SINE retroposons. Together, our results provide clear evidence of the functional novelties that transposed elements contributed to their host genomes throughout evolution.

One of the most striking observations derived from the genomic era is the overwhelming contribution of transposed elements to mammalian genomes. For example, 45% of the human genome is derived from mobile element fragments. Although historically viewed as “junk DNA,” transposed elements could also contribute to novel advantageous functional elements in their host genomes, a process called exaptation. Functionally proven examples of exaptation derived from ancient retroposition events are rare. Using an in silico paleogenomic strategy, we unraveled the evolutionary origin of nPE2, a neuronal enhancer of the proopiomelancortin gene that participates in the production of hypothalamic peptides involved in feeding behavior and stress-induced analgesia. We demonstrate that nPE2 originated from the exaptation of a SINE retroposon in the lineage leading to mammals and remained under purifying selection for the last 170 million years. The difficulty in detecting nPE2 origin as an exapted retroposon illustrates the underestimation of this phenomenon and encourages the finding of the many thousands of retroposon-derived functional elements still hidden within the genomes. Their discovery will contribute to a better understanding of the dynamics of gene evolution and, at a larger scale, the origin of macroevolutionary novelties that lead to the appearance of new species, orders, or classes.

Transcriptional regulation of pituitary POMC is conserved at the vertebrate extremes despite great promoter sequence divergence

The stress response involves complex physiological mechanisms that maximize behavioral efficacy during attack or defense and is highly conserved in all vertebrates. Key mediators of the stress response are pituitary hormones encoded by the proopiomelanocortin gene (POMC). Despite conservation of physiological function and expression pattern of POMC in all vertebrates, phylogenetic footprinting analyses at the POMC locus across vertebrates failed to detect conserved noncoding sequences with potential regulatory function. To investigate whether ortholog POMC promoters from extremely distant vertebrates are functionally conserved, we used 5'-flanking sequences of the teleost fish Tetraodon nigroviridis POMCalpha gene to produce transgenic mice. Tetraodon POMCalpha promoter targeted reporter gene expression exclusively to mouse pituitary cells that normally express Pomc. Importantly, transgenic expression in mouse corticotrophs was increased after adrenalectomy. To understand how conservation of precise gene expression mechanisms coexists with great sequence divergence, we investigated whether very short elements are still conserved in all vertebrate POMC promoters. Multiple local sequence alignments that consider phylogenetic relationships of ortholog regions identified a unique 10-bp motif GTGCTAA(T/G)CC that is usually present in two copies in POMC 5'-flanking sequences of all vertebrates. Underlined nucleotides represent totally conserved sequences. Deletion of these paired motifs from Tetraodon POMCalpha promoter markedly reduced its transcriptional activity in a mouse corticotropic cell line and in pituitary POMC cells of transgenic mice. In mammals, the conserved motifs correspond to reported binding sites for pituitary-specific nuclear proteins that participate in POMC transcriptional regulation. Together, these results demonstrate that mechanisms that participate in pituitary-specific and hormonally regulated expression of POMC have been preserved since mammals and teleosts diverged from a common ancestor 450 million years ago despite great promoter sequence divergence.

The absence of endogenous beta-endorphin selectively blocks phosphorylation and desensitization of mu opioid receptors following partial sciatic nerve ligation

Phosphorylation of specific sites in the second intracellular loop and in the C-terminal domain have previously been suggested to cause desensitization and internalization of the mu-opioid receptor (MOP-R). To assess sites of MOP-R phosphorylation in vivo, affinity-purified, phosphoselective antibodies were raised against either phosphothreonine-180 in the second intracellular loop (MOR-P1) or the C-terminal domain of MOP-R containing phosphothreonine-370 and phosphoserine-375 (MOR-P2). We found that MOR-P2-immunoreactivity (IR) was significantly increased within the striatum of wild-type C57BL/6 mice after injection of the agonist fentanyl. Pretreatment with the antagonist naloxone blocked the fentanyl-induced increase. Furthermore, mutant mice lacking MOP-R showed only non-specific nuclear MOR-P2-IR before or after fentanyl treatment, confirming the specificity of the MOR-P2 antibodies. To assess whether MOP-R phosphorylation occurs following endogenous opioid release, we induced chronic neuropathic pain by partial sciatic nerve ligation (pSNL), which caused a significant increase in MOR-P2-IR in the striatum. pSNL also induced signs of mu opioid receptor tolerance demonstrated by a rightward shift in the morphine dose response in the tail withdrawal assay and by a reduction in morphine conditioned place preference (CPP). Mutant mice selectively lacking all forms of the beta-endorphin peptides derived from the proopiomelanocortin (Pomc) gene did not show increased MOR-P2-IR, decreased morphine antinociception, or reduced morphine CPP following pSNL. In contrast gene deletion of either proenkephalin or prodynorphin opioids did not block the effects of pSNL. These results suggest that neuropathic pain caused by pSNL in wild-type mice activates the release of the endogenous opioid beta-endorphin, which subsequently induces MOP-R phosphorylation and opiate tolerance.

Central dysregulation of the hypothalamic-pituitary-adrenal axis in neuron-specific proopiomelanocortin-deficient mice

Proopiomelanocortin (POMC) is synthesized predominantly in pituitary corticotrophs, melanotrophs, and arcuate hypothalamic neurons. Corticotroph-derived ACTH mediates basal and stress-induced glucocorticoid secretion, but it is uncertain whether POMC peptides produced in the brain also regulate the hypothalamic-pituitary-adrenal axis. To address this question, we generated neuron-specific POMC-deficient mice by transgenic (Tg) replacement of pituitary POMC in a global Pomc(-/-) background. Selective restoration of pituitary POMC prevented the adrenal insufficiency and neonatal mortality characteristic of Pomc(-/-) mice. However, adult Pomc(-/-)Tg/+ mice expressing the pituitary-specific transgene exhibited adrenal cortical hypertrophy, elevated basal plasma corticosterone, elevated basal but attenuated stress-induced ACTH secretion, and inappropriately elevated CRH expression in the hypothalamic paraventricular nucleus. In addition, Pomc(-/-)Tg/+, Pomc(+/-)Tg/+, and Pomc(+/-) mice, which all displayed varying degrees of elevated CRH, frequently developed melanotroph adenomas after 1 yr of age, whereas Pomc(-/-) mice, with maximal CRH expression and glucocorticoid disinhibition, developed corticotroph and melanotroph adenomas. These results indicate that neuronal POMC peptides are necessary to regulate CRH within physiological limits and that a chronic reduction or absence of hypothalamic POMC leads to trophic stimulation of pituitary cells directly or indirectly through elevated CRH levels.

Role of dopamine D1 and D2 receptors in CRF-induced disruption of sensorimotor gating

Corticotropin-releasing factor (CRF), a neuropeptide released during stress, has been reported to modulate startle behavior, including reducing the threshold for acoustic startle responding and reducing prepulse inhibition (PPI). The central mechanisms mediating CRF system regulation of startle and PPI are still unclear. Some antipsychotic drugs attenuate CRF-induced deficits in PPI in rats and mice. Here we tested the hypothesis that indirect activation of DA(1)-receptors (D(1)) and DA(2)-receptors (D(2)) contributes to the effects of CRF on PPI. We compared the effect of central administration of h/r-CRF (0.2-0.6 nmol) on PPI in mice with either a D(1) or D(2) receptor null mutation (knockout, KO) or in mice pretreated with D(1) or D(2) receptor antagonists SCH23390 (1 mg/kg) or haloperidol (1 mg/kg). D(1) and D(2) KO mice exhibited no significant differences in their sensitivity to CRF-induced disruptions of PPI. Similarly, neither SCH23390 nor haloperidol pretreatment altered the CRF-induced disruption in PPI, although both increased PPI at baseline. CRF-induced increases in startle also remained unchanged by any of the DA receptor manipulations. These results indicate that neither D(1)- nor D(2)-receptor activation is necessary for CRF to exert its effects on acoustic startle and PPI in mice.

The contribution of endogenous opioids to food reward is dependent on sex and background strain

Complex behaviors such as those associated with reward to unconditioned positive reinforcers are polygenic processes. In studies using genetically modified mice specific for the endogenous opioid systems an observed phenotype in a complex behavior is likely to be dependent on interacting genes which, in inbred mouse lines, influence that phenotype. To address this issue we examined operant responding for palatable food reinforcers in mice lacking the expression of beta-endorphin, enkephalin or both peptides congenic to two different genetic backgrounds; C57BL/6J and DBA/2J. These two inbred strains were chosen because their endogenous opioid states differ and they respond differently to exogenous opioids in many behavioral assays. We found that wildtype and mutant C57BL/6J mice acquired operant responding for food reinforcers faster than DBA/2J mice, regardless of their opioid genotype. Although wildtype DBA/2J mice had a significant deficit in acquisition of bar-pressing behavior to reach a pre-established performance criterion, no subsequent deficit was observed under two different schedules of reinforcement. Additionally, we found that mice lacking enkephalin had decreased motivation to bar press for palatable food reinforcers under a progressive ratio regardless of sex or background strain. In contrast, the only subset of beta-endorphin-deficient mice that had decreased motivation to bar press under a progressive ratio was males on the C57BL/6J background. Of the two classical endogenous opioid peptides with preferential activation of the mu opioid receptor, the knockout models would suggest that enkephalins play a more consistent role than beta-endorphin in mediating the motivation for food reward when tested under a progressive ratio.

Differential involvement of endogenous opioids in sucrose consumption and food reinforcement

Endogenous opioids within the central nervous system are postulated to mediate hedonic aspects of feeding behavior. To identify the relevant endogenous opioid receptor ligands, mice lacking one or two of the opioid peptide families beta-endorphin, enkephalins or dynorphins were tested for sucrose preference in a two-bottle free-choice drinking paradigm under drug-naïve conditions and following treatment with an opioid antagonist (1 mg/kg naloxone i.p.) or saline. Basal sucrose consumption was unaltered in all of the knockout genotypes compared to their congenic wild-type C57BL/6 littermates during 0.5 and 6 h access to a bottle containing 2, 4, 8, or 16% sucrose and a second bottle containing water. Moreover, all mutant genotypes and wildtype mice exhibited a similar compensatory decrease in overnight food intake following the extra caloric load from 6 h sucrose access. Although these basal responses to sucrose were unaffected by the knockout genotypes, naloxone reduced sucrose consumption by 50% compared to saline treatment during the first 0.5 h in wild-type and beta-endorphin knockout mice, but had no effect in enkephalin knockouts, beta-endorphin and enkephalin double knockouts, or dynorphin knockouts. These data suggest that naloxone reduces sucrose consumption in wild-type mice by blocking endogenous enkephalin and/or dynorphin signaling, but not beta-endorphin. Dynorphin knockouts in the current study had bar-pressing responses for a palatable food reinforcer in an operant procedure under free-feeding conditions similar to wild-type mice while we found in a previous study that beta-endorphin and enkephalin knockout mice had reduced motivation to respond [Hayward MD, Pintar JE, Low MJ. Selective reward deficit in mice lacking beta-endorphin and enkephalin. J Neurosci 2002;22:8251-8258.]. We conclude from these studies directly comparing three strains of opioid peptide knockout mice that enkephalin and dynorphin can modulate sucrose preference but are not necessary to support sucrose consumption. However, dynorphin was not necessary to support wildtype levels of operant responding suggesting that only enkephalin and beta-endorphin modulate conditioned food reinforcement.

Differential contributions of dopamine D1, D2, and D3 receptors to MDMA-induced effects on locomotor behavior patterns in mice

MDMA or 'ecstasy' (3,4-methylenedioxymethamphetamine) is a commonly used psychoactive drug that has unusual and distinctive behavioral effects in both humans and animals. In rodents, MDMA administration produces a unique locomotor activity pattern, with high activity characterized by smooth locomotor paths and perseverative thigmotaxis. Although considerable evidence supports a major role for serotonin release in MDMA-induced locomotor activity, dopamine (DA) receptor antagonists have recently been shown to attenuate these effects. Here, we tested the hypothesis that DA D1, D2, and D3 receptors contribute to MDMA-induced alterations in locomotor activity and motor patterns. DA D1, D2, or D3 receptor knockout (KO) and wild-type (WT) mice received vehicle or (+/-)-MDMA and were tested for 60 min in the behavioral pattern monitor (BPM). D1 KO mice exhibited significant increases in MDMA-induced hyperactivity in the late testing phase as well as an overall increase in straight path movements. In contrast, D2 KO mice exhibited reductions in MDMA-induced hyperactivity in the late testing phase, and exhibited significantly less sensitivity to MDMA-induced perseverative thigmotaxis. At baseline, D2 KO mice also exhibited reduced activity and more circumscribed movements compared to WT mice. Female D3 KO mice showed a slight reduction in MDMA-induced hyperactivity. These results confirm differential modulatory roles for D1 and D2 and perhaps D3 receptors in MDMA-induced hyperactivity. More specifically, D1 receptor activation appears to modify the type of activity (linear vs circumscribed), whereas D2 receptor activation appears to contribute to the repetitive circling behavior produced by MDMA.