Sergio Dominguez-Lopez

Genetic Deletion of Fatty Acid Amide Hydrolase Alters Emotional Behavior and Serotonergic Transmission in the Dorsal Raphe, Prefrontal Cortex, and Hippocampus

Pharmacological blockade of the anandamide-degrading enzyme, fatty acid amide hydrolase (FAAH), produces CB1 receptor (CB1R)-mediated analgesic, anxiolytic-like and antidepressant-like effects in murids. Using behavioral and electrophysiological approaches, we have characterized the emotional phenotype and serotonergic (5-HT) activity of mice lacking the FAAH gene in comparison to their wild type counterparts, and their response to a challenge of the CB1R antagonist, rimonabant. FAAH null-mutant (FAAH−/−) mice exhibited reduced immobility in the forced swim and tail suspension tests, predictive of antidepressant activity, which was attenuated by rimonabant. FAAH−/− mice showed an increase in the duration of open arm visits in the elevated plus maze, and a decrease in thigmotaxis and an increase in exploratory rearing displayed in the open field, indicating anxiolytic-like effects that were reversed by rimonabant. Rimonabant also prolonged the initiation of feeding in the novelty-suppressed feeding test. Electrophysiological recordings revealed a marked 34.68% increase in dorsal raphe 5-HT neural firing that was reversed by rimonabant in a subset of neurons exhibiting high firing rates (33.15% mean decrease). The response of the prefrontocortical pyramidal cells to the 5-HT2A/2C agonist (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane ((±)-DOI) revealed desensitized 5-HT2A/2C receptors, likely linked to the observed anxiolytic-like behaviors. The hippocampal pyramidal response to the 5-HT1A antagonist, WAY-100635, indicates enhanced tonus on the hippocampal 5-HT1A heteroreceptors, a hallmark of antidepressant-like action. Together, these results suggest that FAAH genetic deletion enhances anxiolytic-like and antidepressant-like effects, paralleled by altered 5-HT transmission and postsynaptic 5-HT1A and 5-HT2A/2C receptor function.

Promotion of Non-Rapid Eye Movement Sleep and Activation of Reticular Thalamic Neurons by a Novel MT2 Melatonin Receptor Ligand

Melatonin activates two brain G-protein coupled receptors, MT1 and MT2, whose differential roles in the sleep–wake cycle remain to be defined. The novel MT2 receptor partial agonist, N-{2-[(3-methoxyphenyl) phenylamino] ethyl} acetamide (UCM765), is here shown to selectively promote non-rapid eye movement sleep (NREMS) in rats and mice. The enhancement of NREMS by UCM765 is nullified by the pharmacological blockade or genetic deletion of MT2 receptors. MT2, but not MT1, knock-out mice show a decrease in NREMS compared to the wild strain. Immunohistochemical labeling reveals that MT2 receptors are localized in sleep-related brain regions, and notably the reticular thalamic nucleus (Rt). Microinfusion of UCM765 in the Rt promotes NREMS, and its systemic administration induces an increase in firing and rhythmic burst activity of Rt neurons, which is blocked by the MT2 antagonist 4-phenyl-2-propionamidotetralin. Since developing hypnotics that increase NREMS without altering sleep architecture remains a medical challenge, MT2 receptors may represent a novel target for the treatment of sleep disorders.

Anatomical and cellular localization of melatonin MT1 and MT2 receptors in the adult rat brain

The involvement of melatonin in mammalian brain pathophysiology has received growing interest, but information about the anatomical distribution of its two G‐protein‐coupled receptors, MT1 and MT2, remains elusive. In this study, using specific antibodies, we examined the precise distribution of both melatonin receptors immunoreactivity across the adult rat brain using light, confocal, and electron microscopy. Our results demonstrate a selective MT1 and MT2 localization on neuronal cell bodies and dendrites in numerous regions of the rat telencephalon, diencephalon, and mesencephalon. Confocal and ultrastructural examination confirmed the somatodendritic nature of MT1 and MT2 receptors, both being localized on neuronal membranes. Overall, striking differences were observed in the anatomical distribution pattern of MT1 and MT2 proteins, and the labeling often appeared complementary in regions displaying both receptors. Somadendrites labeled for MT1 were observed for instance in the retrosplenial cortex, the dentate gyrus of the hippocampus, the islands of Calleja, the medial habenula, the suprachiasmatic nucleus, the superior colliculus, the substantia nigra pars compacta, the dorsal raphe nucleus, and the pars tuberalis of the pituitary gland. Somadendrites endowed with MT2 receptors were mostly observed in the CA3 field of the hippocampus, the reticular thalamic nucleus, the supraoptic nucleus, the inferior colliculus, the substantia nigra pars reticulata, and the ventrolateral periaqueductal gray. Together, these data provide the first detailed neurocytological mapping of melatonin receptors in the adult rat brain, an essential prerequisite for a better understanding of melatonin distinct receptor function and neurophysiology.

Subchronic Peripheral Neuregulin-1 Increases Ventral Hippocampal Neurogenesis and Induces Antidepressant-Like Effects

Background Adult hippocampal neurogenesis has been implicated in the mechanism of antidepressant action, and neurotrophic factors can mediate the neurogenic changes underlying these effects. The neurotrophic factor neuregulin-1 (NRG1) is involved in many aspects of brain development, from cell fate determination to neuronal maturation. However, nothing is known about the influence of NRG1 on neurodevelopmental processes occurring in the mature hippocampus. Methods Adult male mice were given subcutaneous NRG1 or saline to assess dentate gyrus proliferation and neurogenesis, as well as cell fate determination. Mice also underwent behavioral testing. Expression of ErbB3 and ErbB4 NRG1 receptors in newborn dentate gyrus cells was assessed at various time points between birth and maturity. The phenotype of ErbB-expressing progenitor cells was also characterized with cell type-specific markers. Results The current study shows that subchronic peripheral NRG1β administration selectively increased cell proliferation (by 71%) and neurogenesis (by 50%) in the caudal dentate gyrus within the ventral hippocampus. This pro-proliferative effect did not alter neuronal fate, and may have been mediated by ErbB3 receptors, which were expressed by newborn dentate gyrus cells from cell division to maturity and colocalized with SOX2 in the subgranular zone. Furthermore, four weeks after cessation of subchronic treatment, animals displayed robust antidepressant-like behavior in the absence of changes in locomotor activity, whereas acute treatment did not produce antidepressant effects. Conclusions These results show that neuregulin-1β has pro-proliferative, neurogenic and antidepressant properties, further highlight the importance of peripheral neurotrophic factors in neurogenesis and mood, and support the role of hippocampal neurogenesis in mediating antidepressant effects.

Selective melatonin MT2 receptor ligands relieve neuropathic pain through modulation of brainstem descending antinociceptive pathways.

Abstract Neuropathic pain is an important public health problem for which only a few treatments are available. Preclinical studies show that melatonin (MLT), a neurohormone acting on MT1 and MT2 receptors, has analgesic properties, likely through MT2 receptors. Here, we determined the effects of the novel selective MLT MT2 receptor partial agonist N-{2-([3-bromophenyl]-4-fluorophenylamino)ethyl}acetamide (UCM924) in 2 neuropathic pain models in rats and examined its supraspinal mechanism of action. In rat L5–L6 spinal nerve ligation and spared nerve injury models, UCM924 (20-40 mg/kg, subcutaneously) produced a prolonged antinociceptive effect that is : (1) dose-dependent and blocked by the selective MT2 receptor antagonist 4-phenyl-2-propionamidotetralin, (2) superior to a high dose of MLT (150 mg/kg) and comparable with gabapentin (100 mg/kg), but (3) without noticeable motor coordination impairments in the rotarod test. Using double staining immunohistochemistry, we found that MT2 receptors are expressed by glutamatergic neurons in the rostral ventrolateral periaqueductal gray. Using in vivo electrophysiology combined with tail flick, we observed that microinjection of UCM924 into the ventrolateral periaqueductal gray decreased tail flick responses, depressed the firing activity of ON cells, and activated the firing of OFF cells; all effects were MT2 receptor–dependent. Altogether, these data demonstrate that selective MT2 receptor partial agonists have analgesic properties through modulation of brainstem descending antinociceptive pathways, and MT2 receptors may represent a novel target in the treatment of neuropathic pain.

Adolescent amphetamine exposure elicits dose-specific effects on monoaminergic neurotransmission and behaviour in adulthood

Despite the growing non-medical consumption of amphetamine (Amph) during adolescence, its long-term neurobiological and behavioural effects have remained largely unexplored. The present research sought to characterize the behavioural profile and electrophysiological properties of midbrain monoaminergic neurons in adult rodents after Amph exposure during adolescence. Adolescent rats were administered vehicle, 0.5, 1.5, or 5.0 mg/kg.d Amph from postnatal day (PND) 30-50. At adulthood (PND 70), rats were tested in an open-field test (OFT) and elevated plus maze (EPM), paralleled by in-vivo extracellular recordings of serotonin (5-HT), dopamine (DA) and norepinephrine (NE) neurons from the dorsal raphe nucleus, ventral tegmental area, and locus coeruleus, respectively. 5-HT firing in adulthood was increased in rats that had received Amph (1.5 mg/kg.d) during adolescence. At this regimen, DA firing activity was increased, but not NE firing. Conversely, the highest Amph dose regimen (5.0 mg/kg.d) enhanced NE firing, but not DA or 5-HT firing rates. In the OFT, Amph (1.5 mg/kg.d) significantly increased the total distance travelled, while the other doses were ineffective. In the EPM, all three Amph doses increased time spent in the open arms and central platform, as well as the number of stretch-attend postures made. Repeated adolescent exposure to Amph differentially augments monoaminergic neuronal firing in a dose-specific fashion in adulthood, with corresponding alterations in locomotion, risk assessment (stretch-attend postures and central platform occupancy) and risk-taking behaviours (open-arm exploration). Thus, adolescent Amph exposure induces long-lasting neurophysiological alterations that may have implications for drug-seeking behaviour in the future.

Characterization of serotonin neurotransmission in knockout mice: implications for major depression

Abstract The interaction between genes and environment plays a significant role in the pathogenesis of major depression and mood disorders. Preclinical and clinical studies have established that a dysfunction of serotonin (5-HT) neurotransmission is a common hallmark in major depression and drugs acting on the 5-HT system have antidepressant properties. In the past 15 years, the development of knockout mice showing a depressive-like or resilience-like phenotype have allowed us to better understand the complex relationship between genes, behaviour and the 5-HT system in mood disorders. The present review revises several knockout mice genotypes with ‘mood’ alteration and analyses how 5-HT firing activity, measured with electrophysiological techniques, is impaired after a gene manipulation. The behavior and electrophysiology data from 5-HT transporter (5HTT), 5-HT1A, 5-HT4, the neurokinin 1 (NK1) receptor, fatty acid amide hydrolase (FAAH) and the TWIK-1 related K+ (TREK-1) channel knockout mice are here analysed. Interestingly, a correlation between 5-HT firing rate and depressive/resilience phenotypes can be established in these different knockouts. Furthermore, findings in knockout mice have been successfully translated to humans, and findings from human studies have helped to design and generate knockout mice to explore new hypotheses of the etiology of human depression. The correlation of 5-HT activity and behavior could be a predictor factor for understanding the role of receptors, channels and enzymes in depression, and could be used also to assess the potential antidepressive effects of novel drugs.

Short-term effects of melatonin and pinealectomy on serotonergic neuronal activity across the light–dark cycle

Melatonin (MLT) and serotonin (5-HT) are two biosynthetically related compounds implicated in several common physiological functions and the etiology of mood disorders. How they interact, though, is not yet fully understood. In this study, single-unit extracellular recordings were used to monitor dorsal raphe nucleus (DR) 5-HT neuronal activity in anesthetized rats, under basal conditions (CTRL), in response to MLT administration, and after pinealectomy (PX) across the light–dark cycle. Under basal conditions, the number of spontaneously active 5-HT neurons and their firing rate were both significantly lower in the dark phase. In the light phase, administration of MLT at low doses (0.5–1 mg/kg, i.v.) decreased 5-HT firing activity. This inhibitory effect of MLT was completely blocked by the MT1/MT2 receptor antagonist luzindole, but not by the selective MT2 receptor antagonist 4P-PDOT, the selective 5-HT1A receptor antagonist WAY100635, or by the α2 adrenoceptor antagonist idazoxan. In the opposite experiment, PX increased 5-HT firing activity in the dark phase, and this was reversed by MLT administration (1 mg/kg, i.v.). Finally, in a forced swim test, MLT (1 mg/kg, i.p.) increased immobility time and decreased swimming behavior. Together, these results suggest that nocturnal MLT secretion imposes tonic inhibitory control over a sub-population of DR 5-HT neurons. This MLT-induced decrease in 5-HT neurotransmission may represent a biological mechanism underlying mood disorders characterized by increased MLT secretion, such as seasonal affective disorder.

Electrophysiological characterization of dopamine neuronal activity in the ventral tegmental area across the light-dark cycle.

Direct evidence that dopamine (DA) neurotransmission varies during the 24 h of the day is lacking. Here, we have characterized the firing activity of DA neurons located in the ventral tegmental area (VTA) using single‐unit extracellular recordings in anesthetized rats kept on a standard light–dark cycle. DA neuronal firing activity was measured under basal conditions and in response to intravenous administration of increasing doses of amphetamine (AMPH: 0.5, 1, 2, 5 mg/kg), apomorphine (APO: 25, 50, 100, 200 µg/kg) and melatonin (MLT: 0.1, 1, 10 mg/kg) at different time intervals of the light–dark cycle. DA firing activity peaked between 07:00 and 11:00 h (3.5 ± 0.3 Hz) and between 19:00 and 23:00 h (4.1 ± 0.7 Hz), with lowest activity occurring between 11:00 and 15:00 h (2.4 ± 0.2 Hz) and between 23:00 and 03:00 h (2.6 ± 0.2 Hz). The highest number of spontaneously active neurons was observed between 03:00 and 06:00 h (2.5 ± 0.3 neurons/track), whereas the lowest was between 19:00 and 23:00 h (1.5 ± 0.2 neurons/track). The inhibitory effect of AMPH on DA firing rate was similar in both phases. The inhibitory effect of low dose of APO (25 μg/kg, dose selective for D2 autoreceptor) was more potent in the dark phase, whereas APO effects at higher doses were similar in both phases. Finally, MLT administration (1 mg/kg) produced a moderate inhibition of DA cell firing in both phases. These experiments demonstrate the existence of an intradiurnal rhythmic pattern of VTA DA neuronal firing activity and a higher pharmacological response of D2 autoreceptors in the dark phase. Synapse 68:454–467, 2014.

Melancholic-Like behaviors and circadian neurobiological abnormalities in melatonin MT1 receptor knockout mice.

Background: Melancholic depression, described also as endogenous depression, is a mood disorder with distinctive specific psychopathological features and biological homogeneity, including anhedonia, circadian variation of mood, psychomotor activation, weight loss, diurnal cortisol changes, and sleep disturbances. Although several hypotheses have been proposed, the etiology of this disorder is still unknown. Methods: Behavioral, electrophysiological and biochemical approaches were used to characterize the emotional phenotype, serotonergic and noradrenergic electrical activity, and corticosterone in melatonin MT1 receptor knockout mice and their wild type counterparts, during both light and dark phases. Results: Melatonin MT1 receptor knockout mice have decreased mobility in the forced swim and tail suspension tests as well as decreased sucrose consumption, mostly during the dark/inactive phase. These mood variations are reversed by chronic treatment with the tricyclic antidepressant desipramine. In addition, MT1 receptor knockout mice exhibit psychomotor disturbances, higher serum levels of corticosterone the dark phase, and a blunted circadian variation of corticosterone levels. In vivo electrophysiological recordings show a decreased burst-firing activity of locus coeruleus norepinephrine neurons during the dark phase. The circadian physiological variation in the spontaneous firing activity of high-firing neuronal subpopulations of both norepinephrine neurons and dorsal raphe serotonin neurons are abolished in MT1 knockout mice. Conclusions: These data demonstrate that melatonin MT1 receptor knockout mice recapitulate several behavioral and neurobiological circadian changes of human melancholic depression and, for the first time, suggest that the MT1 receptor may be implicated in the pathogenesis of melancholic depression and is a potential pharmacological target for this mental condition.