Fast neurotransmitter identity of MCH neurons: Do contents depend on context?

Hypothalamic melanin-concentrating hormone (MCH) neurons participate in many fundamental neuroendocrine processes. While some of their effects can be attributed to MCH itself, others appear to depend on co-released neurotransmitters. Historically, the subject of fast neurotransmitter co-release from MCH neurons has been contentious, with data to support MCH neurons releasing GABA, glutamate, both, and neither. Rather than assuming a position in that debate, this review considers the evidence for all sides and presents an alternative explanation: neurochemical identity, including classical neurotransmitter content, is subject to change. With an emphasis on the variability of experimental details, we posit that MCH neurons may release GABA and/or glutamate at different points according to environmental and contextual factors. Through the lens of the MCH system, we offer evidence that the field of neuroendocrinology would benefit from a more nuanced and dynamic interpretation of neurotransmitter identity.

Deletion of Androgen Receptor in LepRb Cells Improves Estrous Cycles in Prenatally Androgenized Mice

Androgens are steroid hormones crucial for sexual differentiation of the brain and reproductive function. In excess, however, androgens may decrease fertility as observed in polycystic ovary syndrome, a common endocrine disorder characterized by oligo/anovulation and/or polycystic ovaries. Hyperandrogenism may also disrupt energy homeostasis, inducing higher central adiposity, insulin resistance, and glucose intolerance, which may exacerbate reproductive dysfunction. Androgens bind to androgen receptors (ARs), which are expressed in many reproductive and metabolic tissues, including brain sites that regulate the hypothalamo-pituitary-gonadal axis and energy homeostasis. The neuronal populations affected by androgen excess, however, have not been defined. We and others have shown that, in mice, AR is highly expressed in leptin receptor (LepRb) neurons, particularly in the arcuate (ARH) and the ventral premammillary nuclei (PMv). Here, we assessed if LepRb neurons, which are critical in the central regulation of energy homeostasis and exert permissive actions on puberty and fertility, have a role in the pathogenesis of female hyperandrogenism. Prenatally androgenized (PNA) mice lacking AR in LepRb cells (LepRbΔAR) show no changes in body mass, body composition, glucose homeostasis, or sexual maturation. They do show, however, a remarkable improvement of estrous cycles combined with normalization of ovary morphology compared to PNA controls. Our findings indicate that the prenatal androgenization effects on adult reproductive physiology (ie, anestrus and anovulation) are mediated by a subpopulation of LepRb neurons directly sensitive to androgens. They also suggest that the effects of hyperandrogenism on sexual maturation and reproductive function in adult females are controlled by distinct neural circuits.

Distribution of androgen receptor mRNA in the prepubertal male and female mouse brain

Androgens are steroid hormones that play a critical role in brain development and sexual maturation by acting upon both androgen receptors (AR) and estrogen receptors (ERα/β) after aromatization. The contribution of estrogens from aromatized androgens in brain development and the central regulation of metabolism, reproduction, and behavior is well defined, but the role of androgens acting on AR has been unappreciated. Here, we map the sex specific expression of Ar in the adult and developing mouse brain. Postnatal days (PND) 12 and 21 were used to target a critical window of prepubertal development. Consistent with previous literature in adults, sex-specific differences in Ar expression were most profound in the bed nucleus of the stria terminalis (BST), medial amygdala (MEA) and medial preoptic area (MPO). Ar expression was also high in these areas at PND 12 and 21 in both sexes. In addition, we describe extra-hypothalamic and extra-limbic areas that show moderate, consistent and similar Ar expression in both sexes at both prepubertal time points. Briefly, Ar expression was observed in olfactory areas of the cerebral cortex, the hippocampus, several thalamic nuclei, and cranial nerve nuclei involved in autonomic sensory and motor function. To further characterize forebrain populations of Ar expressing neurons and determine whether they also coexpress estrogen receptors, we examined expression of Ar, Esr1 and Esr2 in prepubertal mice in selected nuclei. We found populations of neurons in the BST, MEA and MPO that coexpress Ar, but not Esr1 or Esr2, whereas others express a combination of the three receptors. Our findings indicate that various brain areas express Ar during prepubertal development and may play an important role in female neuronal development and physiology.

Dissociated Pmch and Cre Expression in Lactating Pmch-Cre BAC Transgenic Mice

The melanin-concentrating hormone (MCH) system plays a role in many physiological processes including reproduction and lactation. However, research regarding the function of MCH on different aspects of the reproductive function lags, due in part to a lack of validated genetic models with which to interrogate the system. This is particularly true in the case of female reproduction, as the anatomy and function of the MCH system is not well-characterized in the female mouse. We set out to determine whether the commercially available Pmch-Cre transgenic mouse line is a viable model to study the role of MCH neurons in distinct female reproductive states. We found that Pmch is transiently expressed in several nuclei of the rostral forebrain at the end of lactation. This includes the medial subdivision of the medial preoptic nucleus, the paraventricular nucleus of the hypothalamus, the ventral subdivision of the lateral septum, the anterodorsal preoptic nucleus and the anterodorsal nucleus of the thalamus. The Pmch expression in these sites, however, does not reliably induce Cre expression in the Pmch-Cre (BAC) transgenic mouse, making this line an inadequate model with which to study the role of MCH in behavioral and/or neuroendocrine adaptations of lactation. We also contribute to the general knowledge of the anatomy of the murine MCH system by showing that lactation-induced Pmch expression in the rostral forebrain is mostly observed in GABAergic (VGAT) neurons, in contrast to the typical MCH neurons of the tuberal and posterior hypothalamus which are glutamatergic (VGLUT2).

Neurotensin Receptor-1 Identifies a Subset of Ventral Tegmental Dopamine Neurons that Coordinates Energy Balance

Dopamine (DA) neurons in the ventral tegmental area (VTA) are heterogeneous and differentially regulate ingestive and locomotor behaviors that affect energy balance. Identification of which VTA DA neurons mediate behaviors that limit weight gain has been hindered, however, by the lack of molecular markers to distinguish VTA DA populations. Here, we identified a specific subset of VTA DA neurons that express neurotensin receptor-1 (NtsR1) and preferentially comprise mesolimbic, but not mesocortical, DA neurons. Genetically targeted ablation of VTA NtsR1 neurons uncouples motivated feeding and physical activity, biasing behavior toward energy expenditure and protecting mice from age-related and diet-induced weight gain. VTA NtsR1 neurons thus represent a molecularly defined subset of DA neurons that are essential for the coordination of energy balance. Modulation of VTA NtsR1 neurons may therefore be useful to promote behaviors that prevent the development of obesity.

Lateral Hypothalamic Neurotensin Neurons Orchestrate Dual Weight Loss Behaviors via Distinct Mechanisms

The central mechanism by which neurotensin (Nts) potentiates weight loss has remained elusive. We leveraged chemogenetics to reveal that Nts-expressing neurons of the lateral hypothalamic area (LHA) promote weight loss in mice by increasing volitional activity and restraining food intake. Intriguingly, these dual weight loss behaviors are mediated by distinct signaling pathways: Nts action via NtsR1 is essential for the anorectic effect of the LHA Nts circuit, but not for regulation of locomotor or drinking behavior. Furthermore, although LHA Nts neurons cannot reduce intake of freely available obesogenic foods, they effectively restrain motivated feeding in hungry, weight-restricted animals. LHA Nts neurons are thus vital mediators of central Nts action, particularly in the face of negative energy balance. Enhanced action via LHA Nts neurons may, therefore, be useful to suppress the increased appetitive drive that occurs after lifestyle-mediated weight loss and, hence, to prevent weight regain.