Impact of Serotonin Deficiency on Circadian Dopaminergic Rhythms.

Physiology and behavior are structured temporally to anticipate daily cycles of light and dark, ensuring fitness and survival. Neuromodulatory systems in the brain-including those involving serotonin and dopamine-exhibit daily oscillations in neural activity and help shape circadian rhythms. Disrupted neuromodulation can cause circadian abnormalities that are thought to underlie several neuropsychiatric disorders, including bipolar mania and schizophrenia, for which a mechanistic understanding is still lacking. Here, we show that genetically depleting serotonin in Tph2 knockout mice promotes manic-like behaviors and disrupts daily oscillations of the dopamine biosynthetic enzyme tyrosine hydroxylase (TH) in midbrain dopaminergic nuclei. Specifically, while TH mRNA and protein levels in the Substantia Nigra (SN) and Ventral Tegmental Area (VTA) of wild-type mice doubled between the light and dark phase, TH levels were high throughout the day in Tph2 knockout mice, suggesting a hyperdopaminergic state. Analysis of TH expression in striatal terminal fields also showed blunted rhythms. Additionally, we found low abundance and blunted rhythmicity of the neuropeptide cholecystokinin (Cck) in the VTA of knockout mice, a neuropeptide whose downregulation has been implicated in manic-like states in both rodents and humans. Altogether, our results point to a previously unappreciated serotonergic control of circadian dopamine signaling and propose serotonergic dysfunction as an upstream mechanism underlying dopaminergic deregulation and ultimately maladaptive behaviors.

Age-Related Changes in D-Aspartate Oxidase Promoter Methylation Control Extracellular D-Aspartate Levels and Prevent Precocious Cell Death during Brain Aging.

The endogenous NMDA receptor (NMDAR) agonist D-aspartate occurs transiently in the mammalian brain because it is abundant during embryonic and perinatal phases before drastically decreasing during adulthood. It is well established that postnatal reduction of cerebral D-aspartate levels is due to the concomitant onset of D-aspartate oxidase (DDO) activity, a flavoenzyme that selectively degrades bicarboxylic D-amino acids. In the present work, we show that d-aspartate content in the mouse brain drastically decreases after birth, whereas Ddo mRNA levels concomitantly increase. Interestingly, postnatal Ddo gene expression is paralleled by progressive demethylation within its putative promoter region. Consistent with an epigenetic control on Ddo expression, treatment with the DNA-demethylating agent, azacitidine, causes increased mRNA levels in embryonic cortical neurons. To indirectly evaluate the effect of a putative persistent Ddo gene hypermethylation in the brain, we used Ddo knock-out mice (Ddo(-/-)), which show constitutively suppressed Ddo expression. In these mice, we found for the first time substantially increased extracellular content of d-aspartate in the brain. In line with detrimental effects produced by NMDAR overstimulation, persistent elevation of D-aspartate levels in Ddo(-/-) brains is associated with appearance of dystrophic microglia, precocious caspase-3 activation, and cell death in cortical pyramidal neurons and dopaminergic neurons of the substantia nigra pars compacta. This evidence, along with the early accumulation of lipufuscin granules in Ddo(-/-) brains, highlights an unexpected importance of Ddo demethylation in preventing neurodegenerative processes produced by nonphysiological extracellular levels of free D-aspartate.

The Small GTP-Binding Protein Rhes Influences Nigrostriatal-Dependent Motor Behavior During Aging.

BACKGROUND: Here we aimed to evaluate: (1) Rhes mRNA expression in mouse midbrain, (2) the effect of Rhes deletion on the number of dopamine neurons, (3) nigrostriatal-sensitive behavior during aging in knockout mice. METHODS: Radioactive in situ hybridization was assessed in adult mice. The beam-walking test was executed in 3-, 6- and 12-month-old mice. Immunohistochemistry of midbrain tyrosine hydroxylase (TH)-positive neurons was performed in 6- and 12-month-old mice. RESULTS: Rhes mRNA is expressed in TH-positive neurons of SNpc and the ventral tegmental area. Moreover, lack of Rhes leads to roughly a 20% loss of nigral TH-positive neurons in both 6- and 12-month-old mutants, when compared with their age-matched controls. Finally, lack of Rhes triggers subtle alterations in motor performance and coordination during aging. CONCLUSIONS: Our findings indicate a fine-tuning role of Rhes in regulating the number of TH-positive neurons of the substantia nigra and nigrostriatal-sensitive motor behavior during aging.

Reversible Morphological Remodeling of Prefrontal and Hippocampal Serotonergic Fibers by Fluoxetine.

Serotonin-releasing fibers depart from the raphe nuclei to profusely innervate the entire central nervous system, displaying in some brain regions high structural plasticity in response to genetically induced abrogation of serotonin synthesis. Chronic fluoxetine treatment used as a tool to model peri-physiological, clinically relevant serotonin elevation is also able to cause structural rearrangements of the serotonergic fibers innervating the hippocampus. Whether this effect is limited to hippocampal-innervating fibers or extends to other populations of axons is not known. Here, we used confocal imaging and three-dimensional (3-D) modeling analysis to expand our morphological investigation of fluoxetine-mediated effects on serotonergic circuitry. We found that chronic treatment with a behaviorally active dose of fluoxetine affects the morphology and reduces the density of serotonergic axons innervating the medial prefrontal cortex, a brain region strongly implicated in the regulation of depressive- and anxiety-like behavior. Axons innervating the somatosensory cortex were unaffected, suggesting differential susceptibility to serotonin changes across cortical areas. Importantly, a 1-month washout period was sufficient to reverse morphological changes in both the medial prefrontal cortex and in the previously characterized hippocampus, as well as to normalize behavior, highlighting an intriguing relationship between axon density and an antidepressant-like effect. Overall, these results further demonstrate the bidirectional plasticity of defined serotonergic axons and provide additional insights into fluoxetine effects on the serotonergic system.

Perturbation of Cortical Excitability in a Conditional Model of PCDH19 Disorder.

PCDH19 epilepsy (DEE9) is an X-linked syndrome associated with cognitive and behavioral disturbances. Since heterozygous females are affected, while mutant males are spared, it is likely that DEE9 pathogenesis is related to disturbed cell-to-cell communication associated with mosaicism. However, the effects of mosaic PCDH19 expression on cortical networks are unknown. We mimicked the pathology of DEE9 by introducing a patch of mosaic protein expression in one hemisphere of the cortex of conditional PCDH19 knockout mice one day after birth. In the contralateral area, PCDH19 expression was unaffected, thus providing an internal control. In this model, we characterized the physiology of the disrupted network using local field recordings and two photon Ca2+ imaging in urethane anesthetized mice. We found transient episodes of hyperexcitability in the form of brief hypersynchronous spikes or bursts of field potential oscillations in the 9-25 Hz range. Furthermore, we observed a strong disruption of slow wave activity, a crucial component of NREM sleep. This phenotype was present also when PCDH19 loss occurred in adult mice, demonstrating that PCDH19 exerts a function on cortical circuitry outside of early development. Our results indicate that a focal mosaic mutation of PCDH19 disrupts cortical networks and broaden our understanding of DEE9.

D-aspartate oxidase gene duplication induces social recognition memory deficit in mice and intellectual disabilities in humans.

The D-aspartate oxidase (DDO) gene encodes the enzyme responsible for the catabolism of D-aspartate, an atypical amino acid enriched in the mammalian brain and acting as an endogenous NMDA receptor agonist. Considering the key role of NMDA receptors in neurodevelopmental disorders, recent findings suggest a link between D-aspartate dysmetabolism and schizophrenia. To clarify the role of D-aspartate on brain development and functioning, we used a mouse model with constitutive Ddo overexpression and D-aspartate depletion. In these mice, we found reduced number of BrdU-positive dorsal pallium neurons during corticogenesis, and decreased cortical and striatal gray matter volume at adulthood. Brain abnormalities were associated with social recognition memory deficit at juvenile phase, suggesting that early D-aspartate occurrence influences neurodevelopmental related phenotypes. We corroborated this hypothesis by reporting the first clinical case of a young patient with severe intellectual disability, thought disorders and autism spectrum disorder symptomatology, harboring a duplication of a chromosome 6 region, including the entire DDO gene.

Microglia Morphological Changes in the Motor Cortex of hSOD1G93A Transgenic ALS Mice.

Amyotrophic lateral sclerosis (ALS) is characterized by the progressive degeneration of spinal motor neurons as well as corticospinal (CSN) large pyramidal neurons within cortex layer V. An intense microglia immune response has been associated with both upper and lower motor neuron degeneration in ALS patients, whereas microgliosis occurrence in the motor cortex of hSOD1G93A mice-the best characterized model of this disease-is not clear and remains under debate. Since the impact of microglia cells in the neuronal environment seems to be crucial for both the initiation and the progression of the disease, here we analyzed the motor cortex of hSOD1G93A mice at the onset of symptoms by the immunolabeling of Iba1/TMEM119 double positive cells and confocal microscopy. By means of Sholl analysis, we were able to identify and quantify the presence of presumably activated Iba1/TMEM119-positive microglia cells with shorter and thicker processes as compared to the normal surveilling and more ramified microglia present in WT cortices. We strongly believe that being able to analyze microglia activation in the motor cortex of hSOD1G93A mice is of great importance for defining the timing and the extent of microglia involvement in CSN degeneration and for the identification of the initiation stages of this disease.

The striatal-enriched protein Rhes is a critical modulator of cocaine-induced molecular and behavioral responses.

Previous evidence pointed out a role for the striatal-enriched protein Rhes in modulating dopaminergic transmission. Based on the knowledge that cocaine induces both addiction and motor stimulation, through its ability to enhance dopaminergic signaling in the corpus striatum, we have now explored the involvement of Rhes in the effects associated with this psychostimulant. Our behavioral data showed that a lack of Rhes in knockout animals caused profound alterations in motor stimulation following cocaine exposure, eliciting a significant leftward shift in the dose-response curve and triggering a dramatic hyperactivity. We also found that Rhes modulated either short- or long-term motor sensitization induced by cocaine, since lack of this protein prevents both of them in mutants. Consistent with this in vivo observation, we found that lack of Rhes in mice caused a greater increase in striatal cocaine-dependent D1R/cAMP/PKA signaling, along with considerable enhancement of Arc, zif268, and Homer1 mRNA expression. We also documented that lack of Rhes in mice produced cocaine-related striatal alterations in proteomic profiling, with a differential expression of proteins clustering in calcium homeostasis and cytoskeletal protein binding categories. Despite dramatic striatal alterations associated to cocaine exposure, our data did not reveal any significant changes in midbrain dopaminergic neurons as a lack of Rhes did not affect: (i) DAT activity; (ii) D2R-dependent regulation of GIRK; and (iii) D2R-dependent regulation of dopamine release. Collectively, our results strengthen the view that Rhes acts as a pivotal physiological "molecular brake" for striatal dopaminergic system overactivation induced by psychostimulants, thus making this protein of interest in regulating the molecular mechanism underpinning cocaine-dependent motor stimulatory effects.

RGS9-2 rescues dopamine D2 receptor levels and signaling in DYT1 dystonia mouse models.

Dopamine D2 receptor signaling is central for striatal function and movement, while abnormal activity is associated with neurological disorders including the severe early-onset DYT1 dystonia. Nevertheless, the mechanisms that regulate D2 receptor signaling in health and disease remain poorly understood. Here, we identify a reduced D2 receptor binding, paralleled by an abrupt reduction in receptor protein level, in the striatum of juvenile Dyt1 mice. This occurs through increased lysosomal degradation, controlled by competition between ß-arrestin 2 and D2 receptor binding proteins. Accordingly, we found lower levels of striatal RGS9-2 and spinophilin. Further, we show that genetic depletion of RGS9-2 mimics the D2 receptor loss of DYT1 dystonia striatum, whereas RGS9-2 overexpression rescues both receptor levels and electrophysiological responses in Dyt1 striatal neurons. This work uncovers the molecular mechanism underlying D2 receptor downregulation in Dyt1 mice and in turn explains why dopaminergic drugs lack efficacy in DYT1 patients despite significant evidence for striatal D2 receptor dysfunction. Our data also open up novel avenues for disease-modifying therapeutics to this incurable neurological disorder.

Serotonergic Signaling Controls Input-Specific Synaptic Plasticity at Striatal Circuits.

Monoaminergic modulation of cortical and thalamic inputs to the dorsal striatum (DS) is crucial for reward-based learning and action control. While dopamine has been extensively investigated in this context, the synaptic effects of serotonin (5-HT) have been largely unexplored. Here, we investigated how serotonergic signaling affects associative plasticity at glutamatergic synapses on the striatal projection neurons of the direct pathway (dSPNs). Combining chemogenetic and optogenetic approaches reveals that impeding serotonergic signaling preferentially gates spike-timing-dependent long-term depression (t-LTD) at thalamostriatal synapses. This t-LTD requires dampened activity of the 5-HT4 receptor subtype, which we demonstrate controls dendritic Ca2+ signals by regulating BK channel activity, and which preferentially localizes at the dendritic shaft. The synaptic effects of 5-HT signaling at thalamostriatal inputs provide insights into how changes in serotonergic levels associated with behavioral states or pathology affect striatal-dependent processes.

Serotonin depletion causes valproate-responsive manic-like condition and increased hippocampal neuroplasticity that are reversed by stress.

Abnormal hippocampal neural plasticity has been implicated in behavioural abnormalities and complex neuropsychiatric conditions, including bipolar disorder (BD). However, the determinants of this neural alteration remain unknown. This work tests the hypothesis that the neurotransmitter serotonin (5-HT) is a key determinant of hippocampal neuroplasticity, and its absence leads to maladaptive behaviour relevant for BD. Depletion of brain 5-HT in Tph2 mutant mice resulted in reduced behavioural despair, reduced anxiety, marked aggression and lower habituation in novel environments, reminiscent of bipolar-associated manic behaviour. Treatment with valproate produced a substantial improvement of the mania-like behavioural phenotypes displayed by Tph2 mutants. Brain-wide fMRI mapping in mutants revealed functional hippocampal hyperactivity in which we also observed dramatically increased neuroplasticity. Importantly, remarkable correspondence between the transcriptomic profile of the Tph2 mutant hippocampus and neurons from bipolar disorder patients was observed. Chronic stress reversed the emotional phenotype and the hippocampal transcriptional landscape of Tph2 mutants. These changes were associated with inappropriate activation of transcriptional adaptive response to stress as assessed by gene set enrichment analyses in the hippocampus of Tph2 mutant mice. These findings delineate 5-HT as a critical determinant in BD associated maladaptive emotional responses and aberrant hippocampal neuroplasticity, and support the use of Tph2-/- mice as a new research tool for mechanistic and therapeutic research in bipolar disorder.

Perturbation of Serotonin Homeostasis during Adulthood Affects Serotonergic Neuronal Circuitry.

Growing evidence shows that the neurotransmitter serotonin (5-HT) modulates the fine-tuning of neuron development and the establishment of wiring patterns in the brain. However, whether serotonin is involved in the maintenance of neuronal circuitry in the adult brain remains elusive. Here, we use a Tph2fl°x conditional knockout (cKO) mouse line to assess the impact of serotonin depletion during adulthood on serotonergic system organization. Data show that the density of serotonergic fibers is increased in the hippocampus and decreased in the thalamic paraventricular nucleus (PVN) as a consequence of brain serotonin depletion. Strikingly, these defects are rescued following reestablishment of brain 5-HT signaling via administration of the serotonin precursor 5-hydroxytryptophan (5-HTP). Finally, 3D reconstruction of serotonergic fibers reveals that changes in serotonin homeostasis affect axonal branching complexity. These data demonstrate that maintaining proper serotonin homeostasis in the adult brain is crucial to preserve the correct serotonergic axonal wiring.

Development of Serotonergic Fibers in the Post-Natal Mouse Brain.

Serotonin (5-HT)-synthetizing neurons, which are confined in the raphe nuclei of the rhombencephalon, provide a pervasive innervation of the central nervous system (CNS) and are involved in the modulation of a plethora of functions in both developing and adult brain. Classical studies have described the post-natal development of serotonergic axons as a linear process of terminal field innervation. However, technical limitations have hampered a fine morphological characterization. With the advent of genetic mouse models, the possibility to label specific neuronal populations allowed the rigorous measurement of their axonal morphological features as well as their developmental dynamics. Here, we used the Tph2GFP knock-in mouse line, in which GFP expression allows punctual identification of serotonergic neurons and axons, for confocal microscope imaging and we performed 3-dimensional reconstruction in order to morphologically characterize the development of serotonergic fibers in specified brain targets from birth to adulthood. Our analysis highlighted region-specific developmental patterns of serotonergic fiber density ranging from a linear and progressive colonization of the target (Caudate/Putamen, Basolateral Amygdala, Geniculate Nucleus and Substantia Nigra) to a transient increase in fiber density (medial Prefrontal Cortex, Globus Pallidus, Somatosensory Cortex and Hippocampus) occurring with a region-specific timing. Despite a common pattern of early post-natal morphological maturation in which a progressive rearrangement from a dot-shaped to a regular and smooth fiber morphology was observed, starting from post-natal day 28 serotonergic fibers acquire the region specific morphological features present in the adult. In conclusion, we provided novel, target-specific insights on the morphology and temporal dynamics of the developing serotonergic fibers.

A Tph2GFP Reporter Stem Cell Line To Model in Vitro and in Vivo Serotonergic Neuron Development and Function.

Modeling biological systems in vitro has contributed to clarification of complex mechanisms in simplified and controlled experimental conditions. Mouse embryonic stem (mES) cells can be successfully differentiated toward specific neuronal cell fates, thus representing an attractive tool to dissect, in vitro, mechanisms that underlie complex neuronal features. In this study, we generated and characterized a reporter mES cell line, called Tph2GFP, in which the vital reporter GFP replaces the tryptophan hydroxylase 2 (Tph2) gene. Tph2GFP mES cells selectively express GFP upon in vitro differentiation toward the serotonergic fate, they synthesize serotonin, possess excitable membranes, and show the typical morphological, morphometrical, and molecular features of in vivo serotonergic neurons. Thanks to the vital reporter GFP, we highlighted by time-lapse video microscopy several dynamic processes such as cell migration and axonal outgrowth in living cultures. Finally, we demonstrated that predifferentiated Tph2GFP cells are able to terminally differentiate, integrate, and innervate the host brain when grafted in vivo. On the whole, the present study introduces the Tph2GFP mES cell line as a useful tool allowing accurate developmental and dynamic studies and representing a reliable platform for the study of serotonergic neurons in health and disease.

Decreased Rhes mRNA levels in the brain of patients with Parkinson's disease and MPTP-treated macaques.

In rodent and human brains, the small GTP-binding protein Rhes is highly expressed in virtually all dopaminoceptive striatal GABAergic medium spiny neurons, as well as in large aspiny cholinergic interneurons, where it is thought to modulate dopamine-dependent signaling. Consistent with this knowledge, and considering that dopaminergic neurotransmission is altered in neurological and psychiatric disorders, here we sought to investigate whether Rhes mRNA expression is altered in brain regions of patients with Parkinson's disease (PD), Schizophrenia (SCZ), and Bipolar Disorder (BD), when compared to healthy controls (about 200 post-mortem samples). Moreover, we performed the same analysis in the putamen of non-human primate Macaca Mulatta, lesioned with the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Overall, our data indicated comparable Rhes mRNA levels in the brain of patients with SCZ and BD, and their respective healthy controls. In sharp contrast, the putamen of patients suffering from PD showed a significant 35% reduction of this transcript, compared to healthy subjects. Interestingly, in line with observations obtained in humans, we found 27% decrease in Rhes mRNA levels in the putamen of MPTP-treated primates. Based on the established inhibitory influence of Rhes on dopamine-related responses, we hypothesize that its striatal downregulation in PD patients and animal models of PD might represent an adaptive event of the dopaminergic system to functionally counteract the reduced nigrostriatal innervation.

Brain-wide Mapping of Endogenous Serotonergic Transmission via Chemogenetic fMRI.

Serotonin-producing neurons profusely innervate brain regions via long-range projections. However, it remains unclear whether and how endogenous serotonergic transmission specifically influences regional or global functional activity. We combined designed receptors exclusively activated by designed drugs (DREADD)-based chemogenetics and functional magnetic resonance imaging (fMRI), an approach we term "chemo-fMRI," to causally probe the brain-wide substrates modulated by endogenous serotonergic activity. We describe the generation of a conditional knockin mouse line that, crossed with serotonin-specific Cre-recombinase mice, allowed us to remotely stimulate serotonergic neurons during fMRI scans. We show that endogenous stimulation of serotonin-producing neurons does not affect global brain activity but results in region-specific activation of a set of primary target regions encompassing corticohippocampal and ventrostriatal areas. By contrast, pharmacological boosting of serotonin levels produced widespread fMRI deactivation, plausibly reflecting the mixed contribution of central and perivascular constrictive effects. Our results identify the primary functional targets of endogenous serotonergic stimulation and establish causation between activation of serotonergic neurons and regional fMRI signals.

Generation of a Tph2 Conditional Knockout Mouse Line for Time- and Tissue-Specific Depletion of Brain Serotonin.

Serotonin has been gaining increasing attention during the last two decades due to the dual function of this monoamine as key regulator during critical developmental events and as neurotransmitter. Importantly, unbalanced serotonergic levels during critical temporal phases might contribute to the onset of neuropsychiatric disorders, such as schizophrenia and autism. Despite increasing evidences from both animal models and human genetic studies have underpinned the importance of serotonin homeostasis maintenance during central nervous system development and adulthood, the precise role of this molecule in time-specific activities is only beginning to be elucidated. Serotonin synthesis is a 2-step process, the first step of which is mediated by the rate-limiting activity of Tph enzymes, belonging to the family of aromatic amino acid hydroxylases and existing in two isoforms, Tph1 and Tph2, responsible for the production of peripheral and brain serotonin, respectively. In the present study, we generated and validated a conditional knockout mouse line, Tph2flox/flox, in which brain serotonin can be effectively ablated with time specificity. We demonstrated that the Cre-mediated excision of the third exon of Tph2 gene results in the production of a Tph2null allele in which we observed the near-complete loss of brain serotonin, as well as the growth defects and perinatal lethality observed in serotonin conventional knockouts. We also revealed that in mice harbouring the Tph2null allele, but not in wild-types, two distinct Tph2 mRNA isoforms are present, namely Tph2Δ3 and Tph2Δ3Δ4, with the latter showing an in-frame deletion of amino acids 84-178 and coding a protein that could potentially retain non-negligible enzymatic activity. As we could not detect Tph1 expression in the raphe, we made the hypothesis that the Tph2Δ3Δ4 isoform can be at the origin of the residual, sub-threshold amount of serotonin detected in the brain of Tph2null/null mice. Finally, we set up a tamoxifen administration protocol that allows an efficient, time-specific inactivation of brain serotonin synthesis. On the whole, we generated a suitable genetic tool to investigate how serotonin depletion impacts on time-specific events during central nervous system development and adulthood life.

Rhes influences striatal cAMP/PKA-dependent signaling and synaptic plasticity in a gender-sensitive fashion.

Mechanisms of gender-specific synaptic plasticity in the striatum, a brain region that controls motor, cognitive and psychiatric functions, remain unclear. Here we report that Rhes, a GTPase enriched in medium spiny neurons (MSNs) of striatum, alters the striatal cAMP/PKA signaling cascade in a gender-specific manner. While Rhes knockout (KO) male mice, compared to wild-type (WT) mice, had a significant basal increase of cAMP/PKA signaling pathway, the Rhes KO females exhibited a much stronger response of this pathway, selectively under the conditions of dopamine/adenosine-related drug challenge. Corticostriatal LTP defects are exclusively found in A2AR/D2R-expressing MSNs of KO females, compared to KO males, an effect that is abolished by PKA inhibitors but not by the removal of circulating estrogens. This suggests that the synaptic alterations found in KO females could be triggered by an aberrant A2AR/cAMP/PKA activity, but not due to estrogen-mediated effect. Consistent with increased cAMP signaling, D1R-mediated motor stimulation, haloperidol-induced catalepsy and caffeine-evoked hyper-activity are robustly enhanced in Rhes KO females compared to mutant males. Thus Rhes, a thyroid hormone-target gene, plays a relevant role in gender-specific synaptic and behavioral responses.

Generation of Pet1210-Cre transgenic mouse line reveals non-serotonergic expression domains of Pet1 both in CNS and periphery.

Neurons producing serotonin (5-hydroxytryptamine, 5-HT) constitute one of the most widely distributed neuronal networks in the mammalian central nervous system (CNS) and exhibit a profuse innervation throughout the CNS already at early stages of development. Serotonergic neuron specification is controlled by a combination of secreted molecules and transcription factors such as Shh, Fgf4/8, Nkx2.2, Lmx1b and Pet1. In the mouse, Pet1 mRNA expression appears between 10 and 11 days post coitum (dpc) in serotonergic post-mitotic precursors and persists in serotonergic neurons up to adulthood, where it promotes the expression of genes defining the mature serotonergic phenotype such as tryptophan hydroxylase 2 (Tph2) and serotonin transporter (SERT). Hence, the generation of genetic tools based on Pet1 specific expression represents a valuable approach to study the development and function of the serotonergic system. Here, we report the generation of a Pet1(210)-Cre transgenic mouse line in which the Cre recombinase is expressed under the control of a 210 kb fragment from the Pet1 genetic locus to ensure a reliable and faithful control of somatic recombination in Pet1 cell lineage. Besides Cre-mediated recombination accurately occurred in the serotonergic system as expected and according to previous studies, Pet1(210)-Cre transgenic mouse line allowed us to identify novel, so far uncharacterized, Pet1 expression domains. Indeed, we showed that in the raphe Pet1 is expressed also in a non-serotonergic neuronal population intermingled with Tph2-expressing cells and mostly localized in the B8 and B9 nuclei. Moreover, we detected Cre-mediated recombination also in the developing pancreas and in the ureteric bud derivatives of the kidney, where it reflected a specific Pet1 expression. Thus, Pet1(210)-Cre transgenic mouse line faithfully drives Cre-mediated recombination in all Pet1 expression domains representing a valuable tool to genetically manipulate serotonergic and non-serotonergic Pet1 cell lineages.

The GTP-binding protein Rhes modulates dopamine signalling in striatal medium spiny neurons.

Rhes is a small GTP-binding protein prominently localized in the striatum. Previous findings obtained in cell culture systems demonstrated an involvement of Rhes in cAMP/PKA signalling pathway, at a level proximal to the activation of heterotrimeric G-protein complex. However, its role in the striatum has been, so far, only supposed. Here we studied the involvement of Rhes in dopaminergic signalling, by employing mice with a null mutation in the Rhes gene. We demonstrated that the absence of Rhes modulates cAMP/PKA signalling in both striatopallidal and striatonigral projection neurons by increasing Golf protein levels and, in turn, influencing motor responses challenged by dopaminergic agonist/antagonist. Interestingly, we also show that Rhes is required for a correct dopamine-mediated GTP binding, a function mainly associated to stimulation of dopamine D2 receptors. Altogether, our results indicate that Rhes is an important modulator of dopaminergic transmission in the striatum.