Microglial activation elicits a negative affective state through prostaglandin-mediated modulation of striatal neurons.

Microglia are activated in many neurological diseases and have been suggested to play an important role in the development of affective disorders including major depression. To investigate how microglial signaling regulates mood, we used bidirectional chemogenetic manipulations of microglial activity in mice. Activation of microglia in the dorsal striatum induced local cytokine expression and a negative affective state characterized by anhedonia and aversion, whereas inactivation of microglia blocked aversion induced by systemic inflammation. Interleukin-6 signaling and cyclooxygenase-1 mediated prostaglandin synthesis in the microglia were critical for the inflammation-induced aversion. Correspondingly, microglial activation led to a prostaglandin-dependent reduction of the excitability of striatal neurons. These findings demonstrate a mechanism by which microglial activation causes negative affect through prostaglandin-dependent modulation of striatal neurons and indicate that interference with this mechanism could milden the depressive symptoms in somatic and psychiatric diseases involving microglial activation.

Interferon-É£ mediated signaling in the brain endothelium is critical for inflammation-induced aversion.

Systemic inflammation elicits malaise and a negative affective state. The mechanism underpinning the aversive component of inflammation include cerebral prostaglandin synthesis and modulation of dopaminergic reward circuits, but the messengers that mediate the signaling between the peripheral inflammation and the brain have not been sufficiently characterized. Here we investigated the role of interferon-É£ (IFN-É£) in the aversive response to systemic inflammation induced by a low dose (10µg/kg) of lipopolysaccharide (LPS) in mice. LPS induced IFN-É£ expression in the blood and deletion of IFN-É£ or its receptor prevented the development of conditioned place aversion to LPS. LPS induced expression of the chemokine Cxcl10 in the striatum of normal mice, but this induction was absent in mice lacking IFN-É£ receptors or Myd88 in blood brain barrier endothelial cells. Furthermore, inflammation-induced aversion was blocked in mice lacking Cxcl10 or its receptor Cxcr3. Finally, mice with a selective deletion of the IFN-É£ receptor in brain endothelial cells did not develop inflammation-induced aversion, demonstrating that the brain endothelium is the critical site of IFN-É£ action. Collectively, these findings show that circulating IFN-É£ that binds to receptors on brain endothelial cells and induces Cxcl10, is a central link in the signaling chain eliciting inflammation-induced aversion.

Inflammation-induced anorexia and fever are elicited by distinct prostaglandin dependent mechanisms, whereas conditioned taste aversion is prostaglandin independent.

Systemic inflammation evokes an array of brain-mediated responses including fever, anorexia and taste aversion. Both fever and anorexia are prostaglandin dependent but it has been unclear if the cell-type that synthesizes the critical prostaglandins is the same. Here we show that pharmacological inhibition or genetic deletion of cyclooxygenase (COX)-2, but not of COX-1, attenuates inflammation-induced anorexia. Mice with deletions of COX-2 selectively in brain endothelial cells displayed attenuated fever, as demonstrated previously, but intact anorexia in response to peripherally injected lipopolysaccharide (10µg/kg). Whereas intracerebroventricular injection of a cyclooxygenase inhibitor markedly reduced anorexia, deletion of COX-2 selectively in neural cells, in myeloid cells or in both brain endothelial and neural cells had no effect on LPS-induced anorexia. In addition, COX-2 in myeloid and neural cells was dispensable for the fever response. Inflammation-induced conditioned taste aversion did not involve prostaglandin signaling at all. These findings collectively show that anorexia, fever and taste aversion are triggered by distinct routes of immune-to-brain signaling.

Calcitonin gene related peptide α is dispensable for many danger-related motivational responses.

Calcitonin gene related peptide (CGRP) expressing neurons in the parabrachial nucleus have been shown to encode danger. Through projections to the amygdala and other forebrain structures, they regulate food intake and trigger adaptive behaviors in response to threats like inflammation, intoxication, tumors and pain. Despite the fact that this danger-encoding neuronal population has been defined based on its CGRP expression, it is not clear if CGRP is critical for its function. It is also not clear if CGRP in other neuronal structures is involved in danger-encoding. To examine the role of CGRP in danger-related motivational responses, we used male and female mice lacking αCGRP, which is the main form of CGRP in the brain. These mice had no, or only very weak, CGRP expression. Despite this, they did not behave differently compared to wildtype mice when they were tested for a battery of danger-related responses known to be mediated by CGRP neurons in the parabrachial nucleus. Mice lacking αCGRP and wildtype mice showed similar inflammation-induced anorexia, conditioned taste aversion, aversion to thermal pain and pain-induced escape behavior, although it should be pointed out that the study was not powered to detect any possible differences that were minor or sex-specific. Collectively, our findings suggest that αCGRP is not necessary for many threat-related responses, including some that are known to be mediated by CGRP neurons in the parabrachial nucleus.

Muscarinic M4 Receptors on Cholinergic and Dopamine D1 Receptor-Expressing Neurons Have Opposing Functionality for Positive Reinforcement and Influence Impulsivity.

The neurotransmitter acetylcholine has been implicated in reward learning and drug addiction. However, the roles of the various cholinergic receptor subtypes on different neuron populations remain elusive. Here we study the function of muscarinic M4 receptors (M4Rs) in dopamine D1 receptor (D1R) expressing neurons and cholinergic neurons (expressing choline acetyltransferase; ChAT), during various reward-enforced behaviors and in a "waiting"-impulsivity test. We applied cell-type-specific gene deletions targeting M4Rs in D1RCre or ChATCre mice. Mice lacking M4Rs in D1R-neurons displayed greater cocaine seeking and drug-primed reinstatement than their littermate controls in a Pavlovian conditioned place preference (CPP) paradigm. Furthermore, the M4R-D1RCre mice initiated significantly more premature responses (PRs) in the 5-choice-serial-reaction-time-task (5CSRTT) than their littermate controls, indicating impaired waiting impulse control. In contrast, mice lacking M4Rs in cholinergic neurons did not acquire cocaine Pavlovian conditioning. The M4R-ChATCre mice were also unable to learn positive reinforcement to either natural reward or cocaine in an operant runway paradigm. Immediate early gene (IEG) expression (cFos and FosB) induced by repeated cocaine injections was significantly increased in the forebrain of M4R-D1RCre mice, whereas it remained normal in the M4R-ChATCre mice. Our study illustrates that muscarinic M4Rs on specific neural populations, either cholinergic or D1R-expressing, are pivotal for learning processes related to both natural reward and drugs of abuse, with opposing functionality. Furthermore, we found that neurons expressing both M4Rs and D1Rs are important for signaling impulse control.

Motivational valence is determined by striatal melanocortin 4 receptors.

It is critical for survival to assign positive or negative valence to salient stimuli in a correct manner. Accordingly, harmful stimuli and internal states characterized by perturbed homeostasis are accompanied by discomfort, unease, and aversion. Aversive signaling causes extensive suffering during chronic diseases, including inflammatory conditions, cancer, and depression. Here, we investigated the role of melanocortin 4 receptors (MC4Rs) in aversive processing using genetically modified mice and a behavioral test in which mice avoid an environment that they have learned to associate with aversive stimuli. In normal mice, robust aversions were induced by systemic inflammation, nausea, pain, and κ opioid receptor-induced dysphoria. In sharp contrast, mice lacking MC4Rs displayed preference or indifference toward the aversive stimuli. The unusual flip from aversion to reward in mice lacking MC4Rs was dopamine dependent and associated with a change from decreased to increased activity of the dopamine system. The responses to aversive stimuli were normalized when MC4Rs were reexpressed on dopamine D1 receptor-expressing cells or in the striatum of mice otherwise lacking MC4Rs. Furthermore, activation of arcuate nucleus proopiomelanocortin neurons projecting to the ventral striatum increased the activity of striatal neurons in an MC4R-dependent manner and elicited aversion. Our findings demonstrate that melanocortin signaling through striatal MC4Rs is critical for assigning negative motivational valence to harmful stimuli.

Prostaglandin-mediated inhibition of serotonin signaling controls the affective component of inflammatory pain.

Pain is fundamentally unpleasant and induces a negative affective state. The affective component of pain is mediated by circuits that are distinct from those mediating the sensory-discriminative component. Here, we have investigated the role of prostaglandins in the affective dimension of pain using a rodent pain assay based on conditioned place aversion to formalin injection, an inflammatory noxious stimulus. We found that place aversion induced by inflammatory pain depends on prostaglandin E2 that is synthesized by cyclooxygenase 2 in neural cells. Further, mice lacking the prostaglandin E2 receptor EP3 selectively on serotonergic cells or selectively in the area of the dorsal raphe nucleus failed to form an aversion to formalin-induced pain, as did mice lacking the serotonin transporter. Chemogenetic manipulations revealed that EP3 receptor activation elicited conditioned place aversion to pain via inhibition of serotonergic neurons. In contrast to their role in inflammatory pain aversion, EP3 receptors on serotonergic cells were dispensable for acute nociceptive behaviors and for aversion induced by thermal pain or a κ opioid receptor agonist. Collectively, our findings show that prostaglandin-mediated modulation of serotonergic transmission controls the affective component of inflammatory pain.

The introgression of a functional nuclear gene from Poa to Festuca ovina.

In sheep's fescue, Festuca ovina, genes coding for the cytosolic enzyme phosphoglucose isomerase, PGIC, are not only found at the standard locus, PgiC1, but also at a segregating second locus, PgiC2. We have used PCR-based sequencing to characterize the molecular structure and evolution of five PgiC1 and three PgiC2 alleles in F. ovina. The three PgiC2 alleles were complex in that they carried two gene copies: either two active genes or one active and one pseudogene. All the PgiC2 sequences were very similar to each other but highly diverged from the five PgiC1 sequences. We also sequenced PgiC genes from several other grass species. Phylogenetic analysis of these sequences indicates that PgiC2 has introgressed into F. ovina from the distant genus Poa. Such an introgression may, for example, follow from a non-standard fertilization with more than one pollen grain, or a direct horizontal gene transfer mediated by a plant virus.

Prostaglandin-dependent modulation of dopaminergic neurotransmission elicits inflammation-induced aversion in mice.

Systemic inflammation causes malaise and general feelings of discomfort. This fundamental aspect of the sickness response reduces the quality of life for people suffering from chronic inflammatory diseases and is a nuisance during mild infections like common colds or the flu. To investigate how inflammation is perceived as unpleasant and causes negative affect, we used a behavioral test in which mice avoid an environment that they have learned to associate with inflammation-induced discomfort. Using a combination of cell-type­specific gene deletions, pharmacology, and chemogenetics, we found that systemic inflammation triggered aversion through MyD88-dependent activation of the brain endothelium followed by COX1-mediated cerebral prostaglandin E2 (PGE2) synthesis. Further, we showed that inflammation-induced PGE2 targeted EP1 receptors on striatal dopamine D1 receptor­expressing neurons and that this signaling sequence induced aversion through GABA-mediated inhibition of dopaminergic cells. Finally, we demonstrated that inflammation-induced aversion was not an indirect consequence of fever or anorexia but that it constituted an independent inflammatory symptom triggered by a unique molecular mechanism. Collectively, these findings demonstrate that PGE2-mediated modulation of the dopaminergic motivational circuitry is a key mechanism underlying the negative affect induced by inflammation.

Land-bridge calibration of molecular clocks and the post-glacial Colonization of Scandinavia by the Eurasian field vole Microtus agrestis.

Phylogeography interprets molecular genetic variation in a spatial and temporal context. Molecular clocks are frequently used to calibrate phylogeographic analyses, however there is mounting evidence that molecular rates decay over the relevant timescales. It is therefore essential that an appropriate rate is determined, consistent with the temporal scale of the specific analysis. This can be achieved by using temporally spaced data such as ancient DNA or by relating the divergence of lineages directly to contemporaneous external events of known time. Here we calibrate a Eurasian field vole (Microtus agrestis) mitochondrial genealogy from the well-established series of post-glacial geophysical changes that led to the formation of the Baltic Sea and the separation of the Scandinavian peninsula from the central European mainland. The field vole exhibits the common phylogeographic pattern of Scandinavian colonization from both the north and the south, however the southernmost of the two relevant lineages appears to have originated in situ on the Scandinavian peninsula, or possibly in the adjacent island of Zealand, around the close of the Younger Dryas. The mitochondrial substitution rate and the timescale for the genealogy are closely consistent with those obtained with a previous calibration, based on the separation of the British Isles from mainland Europe. However the result here is arguably more certain, given the level of confidence that can be placed in one of the central assumptions of the calibration, that field voles could not survive the last glaciation of the southern part of the Scandinavian peninsula. Furthermore, the similarity between the molecular clock rate estimated here and those obtained by sampling heterochronous (ancient) DNA (including that of a congeneric species) suggest that there is little disparity between the measured genetic divergence and the population divergence that is implicit in our land-bridge calibration.

Origin and timing of the horizontal transfer of a PgiC gene from Poa to Festuca ovina.

A segregating second locus, PgiC2, for the enzyme phosphoglucose isomerase (PGIC) is found in the grass sheep's fescue, Festuca ovina. We have earlier reported that a phylogenetic analysis indicates that PgiC2 has been horizontally transferred from the reproductively separated grass genus Poa. Here we extend our analysis to include intron and exon information on 27 PgiC sequences from 18 species representing five genera, and confirm our earlier finding. The origin of PgiC2 can be traced to a group of closely interrelated, polyploid and partially asexual Poa species. The sequence most similar to PgiC2 is found in Poa palustris with a divergence, based on synonymous substitutions, of only 0.67%. This value suggests that the transfer took place less than 600,000 years ago (late Pleistocene), at a time when most extant Poa and Festuca species already existed.

Multigenic and morphometric differentiation of ground squirrels (Spermophilus, Scuiridae, Rodentia) in Turkey, with a description of a new species.

This study reports on the molecular phylogenetics of ground squirrels, genus Spermophilus, in Turkey using cytochrome b (1140bp), part of the D-loop and flanking tRNAs (572bp), X chromosome (867-1051bp) and Y chromosome (983-989bp) DNA sequences. Individuals also were characterized by karyotype and with geometric morphometric analyses of mandibles and skulls. Two hundred fourteen individuals from 91 localities were studied. All the data support the recognition of a new species in SW Anatolia: the Taurus ground squirrel Spermophilus taurensis sp. nov. The new species has a small distribution in the Taurus Mountains in an area that is a hotspot for biodiversity. Molecular clock analysis suggests that the new species diverged from the European ground squirrel, Spermophilus citellus, about 2.5 million years ago and that the ancestor of these two species diverged from the widespread Anatolian ground squirrel, Spermophilus xanthoprymnus, about 5 million years ago. Morphometric differentiation in skull and mandible shape among the three species is incomplete, but statistically significant. S. xanthoprymnus is subdivided into five cytochrome b phylogroups and we use these data to infer the location of glacial refugia where the species lived during the last glacial maximum. This study illustrates the potential of combined molecular and morphometric studies to uncover new Anatolian species and to reconstruct their phylogeographic history. The new species is important for squirrel taxonomy and for understanding Eurasian mammal evolution.

Analysis of sex-linked sequences supports a new mammal species in Europe.

European mammals have been the focus of particularly detailed taxonomic studies by traditional morphological methods. However, DNA analyses have the potential to reveal additional, cryptic species. We describe two highly divergent evolutionary lineages within a small Eurasian mammal, the field vole (Microtus agrestis). We show that the two lineages can be detected not only with maternally (mitochondrial DNA), but also with paternally (Y chromosome) and biparentally (X chromosome) inherited DNA sequences. Reciprocal monophyly of all genealogies and their congruent geographical distributions is consistent with reproductive isolation. Our results suggest that the field vole should be reclassified as two separate species.

MITOCHONDRIAL DNA VARIATION IN THE FIELD VOLE (MICROTUS AGRESTIS): REGIONAL POPULATION STRUCTURE AND COLONIZATION HISTORY.

Restriction fragment length polymorphism analysis of mitochondrial DNA (mtDNA) was used to examine the genetic structure among field voles (Microtus agrestis) from southern and central Sweden. A total of 57 haplotypes was identified in 158 voles from 60 localities. Overall mtDNA diversity was high, but both haplotype and nucleotide diversity exhibited pronounced geographic heterogeneity. Phylogenetic analyses revealed a shallow tree with seven primary mtDNA lineages separated by sequence divergences ranging from 0.6% to 1.0%. The geographic structure of mtDNA diversity and lineage distribution was complex but strongly structured and deviated significantly from an equilibrium situation. The extensive mtDNA diversity observed and the recent biogeographic history of the region suggests that the shallow mtDNA structure in the field vole cannot be explained solely by stochastic lineage sorting in situ or isolation by distance. Instead, the data suggest that the genetic imprints of historical demographic conditions and vicariant geographic events have been preserved and to a large extent determine the contemporary geographic distribution of mtDNA variation. A plausible historical scenario involves differentiation of mtDNA lineages in local populations in glacial refugia, a moving postglacial population structure, and bottlenecks and expansions of mtDNA lineages during the postglacial recolonization of Sweden. By combining the mtDNA data with an analysis of Y-chromosome variation, a specific population unit was identified in southwestern Sweden. This population, defined by a unique mtDNA lineage and fixation of a Y-chromosome variant, probably originated in a population bottleneck in southern Sweden about 12,000 to 13,000 calendar years ago.

A CONTACT ZONE WITH NONCOINCIDENT CLINES FOR SEX-SPECIFIC MARKERS IN THE FIELD VOLE (MICROTUS AGRESTIS).

A field vole (Microtus agrestis) population characterized by unique mitochondrial DNA (mtDNA) and Y chromosome markers occurs in southwest Sweden. A contact zone between this Lund (Lu) population and a standard (St) population was examined with two sex-specific genetic markers. The field vole mtDNA and Y chromosome clines display a remarkable lack of coincidence, rarely observed in contact zones. The cline width for both markers is about 50 km, but the two clines are displaced from each other: the mtDNA cline is found in the central part of the study area, whereas the cline for the Y chromosome is located in the eastern part. Thus, the absolute width of the Lu-St zone spans about 95-110 km. As a result of the cline shift, all male hybrids carry the Lu-Y chromosome and St-type mtDNA. The other possible male hybrid class is lacking. The distinct noncoincidence of the mtDNA and Y chromosome clines is most likely explained by selection against male hybrids with the St-Y chromosome and Lu-mtDNA. It is possible that incompatibilities between the maternal genome of Lu-type animals and the paternal genome of St-type individuals exist. However, alternative explanations based on neutrality or selective advantage cannot be totally dismissed.

Phylogeography of field voles (Microtus agrestis) in Eurasia inferred from mitochondrial DNA sequences.

In a distribution-wide phylogeographic survey of the field vole (Microtus agrestis), 75 specimens from 56 localities across Eurasia were examined for DNA sequence variation along the whole 1140 base pair (bp) mitochondrial (mt) cytochrome b gene. The species is subdivided into three main mtDNA phylogeographic groups - western, eastern and southern - with largely allopatric distributions. The western phylogeographical group is found in west and central Europe and spread most probably from a glacial refugium in the Carpathians. The eastern group covers a large range from Lithuania to central Asia, and probably originated from a southeast European source (e.g. the southern Urals or the Caucasus). The southern group occupies an area from Portugal to Hungary, with division into two distinct mtDNA sublineages that presumably derive from separate glacial refugia in the Iberian Peninsula. Molecular clock estimates suggest that the western and eastern field vole populations separated during the last glaciation, whereas the southern population dates back 0.5-0.9 Myr. High levels of mtDNA variation indicate relatively large population sizes and subdivisions within phylogeographic groups during the last glaciation. We report a possible new suture zone in east Europe.

Phylogeography of the common vole (Microtus arvalis) with particular emphasis on the colonization of the Orkney archipelago.

To investigate the human introduction of the common vole Microtus arvalis onto the Orkney islands, the complete cytochrome b gene was sequenced in 41 specimens from both Orkney (four localities) and elsewhere in their range (26 localities). Orkney voles belonged to the same phylogenetic lineage, 'Western', as individuals from France and Spain indicating southwestern Europe as the most likely source area for the islands. This result is of interest with respect to the movement and trading links of the Neolithic people who likely transported the voles. As well as the Western lineage, our phylogenetic trees revealed three other purely European lineages: the 'Italian' (single specimen from N. Italy), the 'Central' (Germany, Netherlands, Denmark) and the 'Eastern' (Hungary, Slovakia, Poland, Ukraine, Finland, European Russia). Individuals from European Russia, W. Siberia, Georgia, Ukraine and Armenia formed a fifth distinct lineage coinciding with the distribution of the 'obscurus' chromosomal form of M. arvalis. These phylogeographical data suggest that M. arvalis occupied multiple refugia during the last glaciation.

Molecular phylogeny of the speciose vole genus Microtus (Arvicolinae, Rodentia) inferred from mitochondrial DNA sequences.

Voles of the genus Microtus represent one of the most speciose mammalian genera in the Holarctic. We established a molecular phylogeny for Microtus to resolve contentious issues of systematic relationships and evolutionary history in this genus. A total of 81 specimens representing ten Microtus species endemic to Europe as well as eight Eurasian, six Asian and one Holarctic species were sequenced for the entire cytochrome b gene (1140 bp). A further 25 sequences were retrieved from GenBank, providing data on an additional 23, mainly Nearctic, Microtus species. Phylogenetic analysis of these 48 species generated four well-supported monophyletic lineages. The genus Chionomys, snow voles, formed a distinct and well-supported lineage separate from the genus Microtus. The subgenus Microtus formed the strongest supported lineage with two sublineages displaying a close relationship between the arvalis species group (common voles) and the socialis species group (social voles). Monophyly of the Palearctic pitymyid voles, subgenus Terricola, was supported, and this subgenus was also subdivided into two monophyletic species groups. Together, these groupings clarify long-standing taxonomic uncertainties in Microtus. In addition, the "Asian" and the Nearctic lineages reported previously were identified although the latter group was not supported. However, relationships among the main Microtus branches were not resolved, suggesting a rapid and potentially simultaneous radiation of a widespread ancestor early in the history of the genus. This and subsequent radiations discernible in the cytochrome b phylogeny, show the considerable potential of Microtus for analysis of historical and ecological determinants of speciation in small mammals. It is evident that speciation is an ongoing process in the genus and that the molecular data provides a vital insight into current species limits as well as cladogenic events of the past.