The role of Olfaction in MCH-regulated spontaneous maternal responses

The olfactory bulb: A neuroendocrine spotlight on feeding and metabolism

Olfaction is the most ancient sense and is needed for food-seeking, danger protection, mating and survival. It is often the first sensory modality to perceive changes in the external environment, before sight, taste or sound. Odour molecules activate olfactory sensory neurons that reside on the olfactory epithelium in the nasal cavity, which transmits this odour-specific information to the olfactory bulb (OB), where it is relayed to higher brain regions involved in olfactory perception and behaviour. Besides odour processing, recent studies suggest that the OB extends its function into the regulation of food intake and energy balance. Furthermore, numerous hormone receptors associated with appetite and metabolism are expressed within the OB, suggesting a neuroendocrine role outside the hypothalamus. Olfactory cues are important to promote food preparatory behaviours and consumption, such as enhancing appetite and salivation. In addition, altered metabolism or energy state (fasting, satiety and overnutrition) can change olfactory processing and perception. Similarly, various animal models and human pathologies indicate a strong link between olfactory impairment and metabolic dysfunction. Therefore, understanding the nature of this reciprocal relationship is critical to understand how olfactory or metabolic disorders arise. This present review elaborates on the connection between olfaction, feeding behaviour and metabolism and will shed light on the neuroendocrine role of the OB as an interface between the external and internal environments. Elucidating the specific mechanisms by which olfactory signals are integrated and translated into metabolic responses holds promise for the development of targeted therapeutic strategies and interventions aimed at modulating appetite and promoting metabolic health.

Sex differences and sex-specific regulation of motivated behavior by Melanin-concentrating hormone: a short review

Recent preclinical research exploring how neuropeptide transmitter systems regulate motivated behavior reveal the increasing importance of sex as a critical biological variable. Neuropeptide systems and their central circuits both contribute to sex differences in a range of motivated behaviors and regulate sex-specific behaviors. In this short review, we explore the current research of how sex as a biological variable influences several distinct motivated behaviors that are modulated by the melanin-concentrating hormone (MCH) neuropeptide system. First, we review how MCH regulates feeding behavior within the context of energy homeostasis differently between male and female rodents. Then, we focus on MCH's role in lactation as a sex-specific process within the context of energy homeostasis. Next, we discuss the sex-specific effects of MCH on maternal behavior. Finally, we summarize the role of MCH in drug-motivated behaviors. While these topics are traditionally investigated from different scientific perspectives, in this short review we discuss how these behaviors share commonalities within the larger context of motivated behaviors, and that sex differences discovered in one area of research may impact our understanding in another. Overall, our review highlights the need for further research into how sex differences in energy regulation associated with reproduction and parental care contribute to regulating motivated behaviors.

Loss of glutamatergic signalling from MCH neurons reduced anxiety-like behaviours in novel environments

Melanin-concentrating hormone (MCH) neurons within the hypothalamus are heterogeneous and can coexpress additional neuropeptides and transmitters. The majority of MCH neurons express vesicular transporters to package glutamate for synaptic release, and MCH neurons can directly innervate downstream neurons via glutamate release. Although glutamatergic signalling from MCH neurons may support physiological and behavioural roles that are independent of MCH (e.g., in glucose homeostasis and nutrient-sensing), it can also mediate similar roles to MCH in the regulation of energy balance. In addition to energy balance, the MCH system has also been implicated in mood disorders, as MCH receptor antagonists have anxiolytic and anti-depressive effects. However, the contribution of glutamatergic signalling from MCH neurons to mood-related functions have not been investigated. We crossed Mch-cre mice with floxed-Vglut2 mice to delete the expression of the vesicular glutamate transporter 2 (Vglut2) and disable glutamatergic signalling specifically from MCH neurons. The resulting Mch-Vglut2-KO mice showed Vglut2 deletion from over 75% of MCH neurons, and although we did not observe changes in depressive-like behaviours, we found that Mch-Vglut2-KO mice displayed anxiety-like behaviours. Mch-Vglut2-KO mice showed reduced exploratory activity when placed in a new cage and were quicker to consume food placed in the centre of a novel open arena. These findings showed that Vglut2 deletion from MCH neurons resulted in anxiolytic actions and suggested that the anxiogenic effects of glutamate are similar to those of the MCH peptide. Taken together, these findings suggest that glutamate and MCH may synergize to regulate and promote anxiety-like behaviour.

Regulation of Brain Primary Cilia Length by MCH Signaling: Evidence from Pharmacological, Genetic, Optogenetic, and Chemogenic Manipulations

The melanin-concentrating hormone (MCH) system is involved in numerous functions, including energy homeostasis, food intake, sleep, stress, mood, aggression, reward, maternal behavior, social behavior, and cognition. In rodents, MCH acts on MCHR1, a G protein-coupled receptor, which is widely expressed in the brain and abundantly localized to neuronal primary cilia. Cilia act as cells' antennas and play crucial roles in cell signaling to detect and transduce external stimuli to regulate cell differentiation and migration. Cilia are highly dynamic in terms of their length and morphology; however, it is not known if cilia length is causally regulated by MCH system activation in vivo. In the current work, we examined the effects of activation and inactivation of MCH system on cilia lengths by using different experimental models and methodologies, including organotypic brain slice cultures from rat prefrontal cortex (PFC) and caudate-putamen (CPu), in vivo pharmacological (MCHR1 agonist and antagonist GW803430), germline and conditional genetic deletion of MCHR1 and MCH, optogenetic, and chemogenetic (designer receptors exclusively activated by designer drugs (DREADD)) approaches. We found that stimulation of MCH system either directly through MCHR1 activation or indirectly through optogenetic and chemogenetic-mediated excitation of MCH-neuron, caused cilia shortening, detected by the quantification of the presence of ADCY3 protein, a known primary cilia marker. In contrast, inactivation of MCH signaling through pharmacological MCHR1 blockade or through genetic manipulations - germline deletion of MCHR1 and conditional ablation of MCH neurons - induced cilia lengthening. Our study is the first to uncover the causal effects of the MCH system in the regulation of the length of brain neuronal primary cilia. These findings place MCH system at a unique position in the ciliary signaling in physiological and pathological conditions and implicate MCHR1 present at primary cilia as a potential therapeutic target for the treatment of pathological conditions characterized by impaired primary cilia function associated with the modification of its length.

Neural Contributions of the Hypothalamus to Parental Behaviour

Parental behaviour is a comprehensive set of neural responses to social cues. The neural circuits that govern parental behaviour reside in several putative nuclei in the brain. Melanin concentrating hormone (MCH), a neuromodulator that integrates physiological functions, has been confirmed to be involved in parental behaviour, particularly in crouching behaviour during nursing. Abolishing MCH neurons in innate MCH knockout males promotes infanticide in virgin male mice. To understand the mechanism and function of neural networks underlying parental care and aggression against pups, it is essential to understand the basic organisation and function of the involved nuclei. This review presents newly discovered aspects of neural circuits within the hypothalamus that regulate parental behaviours.

An N-terminal fusion allele to study melanin concentrating hormone receptor 1

Cilia on neurons play critical roles in both the development and function of the central nervous system (CNS). While it remains challenging to elucidate the precise roles for neuronal cilia, it is clear that a subset of G-protein-coupled receptors (GPCRs) preferentially localize to the cilia membrane. Further, ciliary GPCR signaling has been implicated in regulating a variety of behaviors. Melanin concentrating hormone receptor 1 (MCHR1), is a GPCR expressed centrally in rodents known to be enriched in cilia. Here we have used MCHR1 as a model ciliary GPCR to develop a strategy to fluorescently tag receptors expressed from the endogenous locus in vivo. Using CRISPR/Cas9, we inserted the coding sequence of the fluorescent protein mCherry into the N-terminus of Mchr1. Analysis of the fusion protein (^mCherry MCHR1) revealed its localization to neuronal cilia in the CNS, across multiple developmental time points and in various regions of the adult brain. Our approach simultaneously produced fortuitous in/dels altering the Mchr1 start codon resulting in a new MCHR1 knockout line. Functional studies using electrophysiology show a significant alteration of synaptic strength in MCHR1 knockout mice. A reduction in strength is also detected in mice homozygous for the mCherry insertion, suggesting that while the strategy is useful for monitoring the receptor, activity could be altered. However, both lines should aid in studies of MCHR1 function and contribute to our understanding of MCHR1 signaling in the brain. Additionally, this approach could be expanded to aid in the study of other ciliary GPCRs.

Oxytocin-MCH circuit regulates monosynaptic inputs to MCH neurons and modulates social recognition memory

Oxytocin regulates social behaviors and has been linked to the etiology of autism and schizophrenia. Oxytocin and another hypothalamic neuropeptide, melanin concentrating hormone (MCH), share several physiological actions such as emotion, social behavior and recognition, maternal care, sexual behavior and stress, which suggests that these two systems may interact, however, how they would do it is not known. Here, we study the interactions between the oxytocin and MCH systems in behaviors related to autism and schizophrenia. Specifically, we examined the synaptic inputs of the oxytocin-to the MCH neurons. We selectively deleted oxytocin receptors (OXTR) from MCH neurons (OXTR-cKO mice) using a Cre/loxP recombinase-technology, and used rabies-mediated circuit mapping technique to reveal the changes in the direct monosynaptic inputs to MCH neurons. We examined the behavioral responses of OXTR-cKO mice. Deletion of OXTR from MCH neurons induced a significant decrease in the primary inputs received by MCH neurons from the paraventricular nucleus and the lateral hypothalamus, and from the nucleus accumbens and ventral tegmental area. While OXTR-cKO mice exhibited similar social interactions as control mice, they displayed significantly impaired social recognition memory and increased stereotypic behavior. Our study identifies a selective role for the oxytocin-MCH pathway in social recognition memory and stereotyped behavior that are relevant to psychiatric disorders such as schizophrenia and autism, and warrant further investigation of this circuit to uncover potential benefit of targeting the oxytocin-MCH circuit as a novel therapeutic target for treatment of social recognition deficits in these two disorders.

Mating and parenting experiences sculpture mood-modulating effects of oxytocin-MCH signaling

The two hypothalamic neuropeptides oxytocin and melanin concentrating hormone (MCH) share several physiological actions such as the control of maternal care, sexual behavior, and emotions. In this study, we uncover the role for the oxytocin-MCH signaling pathway in mood regulation. We identify discrete effects of oxytocin-MCH signaling on depressive behavior and demonstrate that parenting and mating experiences shape these effects. We show that the selective deletion of OXT receptors from MCH neurons increases and decreases depressive behavior in sexually naïve and late postpartum female mice respectively, with no effect on sexually naïve male mice. We demonstrate that both parenting experience and mood-regulating effects of oxytocin-MCH are associated with synaptic plasticity in the reward and fear circuits revealed by the alterations of Arc expressions, which are associated with the depressive behavior. Finally, we uncover the sex-dependent effects of mating on depressive behavior; while the sexual activity reduces the basal levels of depressive behavior in male mice, it reduces in female mice evoked-depression only. We demonstrate that the oxytocin-MCH pathway mediates the effects of sexual activity on depressive behavior. Our data suggest that the oxytocin-MCH pathway can serve as a potential therapeutic target for the treatment of major depression and postpartum mood disorders.

Regulation and neurochemical identity of melanin-concentrating hormone neurones in the preoptic area of lactating mice

Neurones expressing the melanin-concentrating hormone (MCH) can be found in the medial preoptic area (mPOA) and ventral aspects of the periventricular preoptic nucleus of rats by mid-to-late lactation and this expression disappears after weaning. The transitory expression of MCH in the preoptic area suggests a role for these neurones in the control of the end of lactation. However, the neurochemical identity of mPOA MCH neurones and the regulatory factors that control the transient MCH expression remain largely unknown, especially in the mouse. In the present study, we showed that mice also present the transitory expression of MCH in the mPOA at late lactation. mPOA MCH cells did not colocalise significantly with markers of GABAergic (VGAT), glutamatergic (VGLUT2 and VGLUT3) or dopaminergic (tyrosine hydroxylase) neurones. mPOA MCH cells also did not express Kiss1 or oxytocin. By contrast, approximately 70% and 90% of mPOA MCH neurones colocalised with oestrogen receptor α and prolactin-induced phosphorylated signal transducer and activator of transcription 5 (STAT5), respectively. Finally, we demonstrated that the number of MCH neurones in the mPOA is significantly higher in females during the first lactation, compared to mice on the second lactation or pregnant mice during the first lactation or brain-specific STAT5 knockout mice during the first lactation. In summary, our findings indicate that MCH neurones in the mPOA of lactating mice are sensitive to oestrogens and prolactin. Thus, mPOA MCH expression is possibly influenced by hormonal variations. Furthermore, the STAT5 signalling pathway is likely involved in the regulation of MCH expression in the mPOA of lactating mice.

The Melanin-Concentrating Hormone (MCH) System: A Tale of Two Peptides

The melanin-concentrating hormone (MCH) system is a robust integrator of exogenous and endogenous information, modulating arousal and energy balance in mammals. Its predominant function in teleosts, however, is to concentrate melanin in the scales, contributing to the adaptive color change observed in several teleost species. These contrasting functions resulted from a gene duplication that occurred after the teleost divergence, which resulted in the generation of two MCH-coding genes in this clade, which acquired distinctive sequences, distribution, and functions, examined in detail here. We also describe the distribution of MCH immunoreactivity and gene expression in a large number of species, in an attempt to identify its core elements. While initially originated as a periventricular peptide, with an intimate relationship with the third ventricle, multiple events of lateral migration occurred during evolution, making the ventrolateral and dorsolateral hypothalamus the predominant sites of MCH in teleosts and mammals, respectively. Substantial differences between species can be identified, likely reflecting differences in habitat and behavior. This observation aligns well with the idea that MCH is a major integrator of internal and external information, ensuring an appropriate response to ensure the organism’s homeostasis. New studies on the MCH system in species that have not yet been investigated will help us understand more precisely how these habitat changes are connected to the hypothalamic neurochemical circuits, paving the way to new intervention strategies that may be used with pharmacological purposes.

Melanin Concentrating Hormone Signaling Deficits in Schizophrenia: Association With Memory and Social Impairments and Abnormal Sensorimotor Gating

BACKGROUND

Evidence from anatomical, pharmacological, and genetic studies supports a role for the neuropeptide melanin concentrating hormone system in modulating emotional and cognitive functions. Genome-wide association studies revealed a potential association between the melanin concentrating hormone receptor (MCHR1) gene locus and schizophrenia, and the largest genome-wide association study conducted to date shows a credible genome-wide association.

METHODS

We analyzed MCHR1 and pro-melanin concentrating hormone RNA-Seq expression in the prefrontal cortex in schizophrenia patients and healthy controls. Disruptions in the melanin concentrating hormone system were modeled in the mouse brain by germline deletion of MCHR1 and by conditional ablation of melanin concentrating hormone expressing neurons using a Cre-inducible diphtheria toxin system.

RESULTS

MCHR1 expression is decreased in the prefrontal cortex of schizophrenia samples (false discovery rate (FDR) P < .05, CommonMind and PsychEncode combined datasets, n = 901) while pro-melanin concentrating hormone is below the detection threshold. MCHR1 expression decreased with aging (P = 6.6E-57) in human dorsolateral prefrontal cortex. The deletion of MCHR1 was found to lead to behavioral abnormalities mimicking schizophrenia-like phenotypes: hyperactivity, increased stereotypic and repetitive behavior, social impairment, impaired sensorimotor gating, and disrupted cognitive functions. Conditional ablation of pro-melanin concentrating hormone neurons increased repetitive behavior and produced a deficit in sensorimotor gating.

CONCLUSIONS

Our study indicates that early disruption of the melanin concentrating hormone system interferes with neurodevelopmental processes, which may contribute to the pathogenesis of schizophrenia. Further neurobiological research on the developmental timing and circuits that are affected by melanin concentrating hormone may lead to a therapeutic target for early prevention of schizophrenia.