Bradford B. Lowell

The paraventricular hypothalamus (PVH) controls many behavioral and physiologic processes, including appetite, social behavior, autonomic outflow, and pituitary hormone secretion. However, molecular markers for centrally-projecting PVH neuron populations remain largely undefined, and a complete census of PVH cell types has not been established. Therefore, we performed extensive single-cell/nucleus RNA sequencing to catalog PVH neuron subtypes and multiplexed error-robust fluorescence in situ hybridization (MERFISH) to map them spatially. Our spatial transcriptomic atlas resolves 26 Sim1+ and 29 GABAergic neuron populations from the PVH and surrounding areas, revealing multiple subtypes not described previously and distinct transcriptional programs between neuroendocrine and centrally-projecting neurons. Additionally, projection-based profiling determined neuronal subtypes that project to the parabrachial region (PB) and spinal cord, helping to identify PVH populations that regulate satiety and sympathetic nervous system activity, respectively. Notably, activation of PB-projecting PVH neurons expressing bombesin-like receptor 3 (Brs3) reduces food intake and silencing them causes obesity. Together, this atlas contributes high-resolution PVH spatial and circuit-based gene expression profiles, representing a valuable resource for the field of homeostasis.

Summary: AgRP neurons cause hunger, the drive to seek and consume food. Their activation by fasting is key for survival and is thought to be triggered by feedback when energy stores are low. However, we know that environmental cues can also regulate AgRP neurons, since cues that predict future food intake rapidly inhibit AgRP neurons. But is the converse true: can the prediction of future fasting rapidly activate AgRP neurons? Here we show that such rapid fasting activation of AgRP neurons does occur. This fasting response is driven by excitatory input from paraventricular hypothalamic neurons expressing Sim2 , which are bidirectionally sensitive to predictions of future energy state. In this way, cognitively-processed contextual information conveyed by PVH ^Sim2 neurons strongly activates AgRP neurons. Lastly, chronic silencing of PVH ^Sim2 neurons causes persistent hypophagia. This PVH ^Sim2 to AgRP neuron circuit, by anticipating and preventing negative energy balance, provides an important new dimension of hunger regulation.

Neuropeptides have important roles in neural plasticity, spiking and behaviour¹. Yet, many fundamental questions remain regarding their spatiotemporal transmission, integration and functions in the awake brain. Here we examined how MC4R-expressing neurons in the paraventricular nucleus of the hypothalamus (PVH^MC4R) integrate neuropeptide signals to modulate feeding-related fast synaptic transmission and titrate the transition to satiety^2,3,4,5,6. We show that hunger-promoting AgRP axons release the neuropeptide NPY to decrease the second messenger cAMP in PVH^MC4R neurons, while satiety-promoting POMC axons release the neuropeptide αMSH to increase cAMP. Each release event is all-or-none, stochastic and can impact multiple neurons within an approximately 100-µm-diameter region. After release, NPY and αMSH peptides compete to control cAMP—the amplitude and persistence of NPY signalling is blunted by high αMSH in the fed state, while αMSH signalling is blunted by high NPY in the fasted state. Feeding resolves this competition by simultaneously elevating αMSH release and suppressing NPY release^7,8, thereby sustaining elevated cAMP in PVH^MC4R neurons throughout a meal. In turn, elevated cAMP facilitates potentiation of feeding-related excitatory inputs with each bite to gradually promote satiation across many minutes. Our findings highlight biochemical modes of peptide signal integration and information accumulation to guide behavioural state transitions.

The “dorsal pons”, or “dorsal pontine tegmentum” (dPnTg), is part of the brainstem. It is a complex, densely packed region whose nuclei are involved in regulating many vital functions. Notable among them are the parabrachial nucleus, the Kölliker Fuse, the Barrington nucleus, the locus coeruleus, and the dorsal, laterodorsal, and ventral tegmental nuclei. In this study, we applied single-nucleus RNA-seq (snRNA-seq) to resolve neuronal subtypes based on their unique transcriptional profiles and then used multiplexed error robust fluorescence in situ hybridization (MERFISH) to map them spatially. We sampled ~1 million cells across the dPnTg and defined the spatial distribution of over 120 neuronal subtypes. Our analysis identified an unpredicted high transcriptional diversity in this region and pinpointed the unique marker genes of many neuronal subtypes. We also demonstrated that many neuronal subtypes are transcriptionally similar between humans and mice, enhancing this study’s translational value. Finally, we developed a freely accessible, GPU and CPU-powered dashboard (http://harvard.heavy.ai:6273/) that combines interactive visual analytics and hardware-accelerated SQL into a data science framework to allow the scientific community to query and gain insights into the data.

In addition to its motor functions, the cerebellum is involved in emotional regulation, anxiety and affect. We found that suppressing the firing of cerebellar Purkinje cells (PCs) rapidly excites forebrain areas that contribute to such functions (including the amygdala, basal forebrain and septum), but that the classic cerebellar outputs, the deep cerebellar nuclei, do not directly project there. We show that PCs directly inhibit parabrachial nuclei (PBN) neurons that project to numerous forebrain regions. Suppressing the PC–PBN pathway influences many regions in the forebrain and is aversive. Molecular profiling shows that PCs directly inhibit numerous types of PBN neurons that control diverse behaviors that are not involved in motor control. Therefore, the PC–PBN pathway allows the cerebellum to directly regulate activity in the forebrain, and may be an important substrate for cerebellar disorders arising from damage to the posterior vermis.

The “dorsal pons”, or “dorsal pontine tegmentum” (dPnTg), is part of the brainstem. It is a complex, densely packed region whose nuclei are involved in regulating many vital functions. Notable among them are the parabrachial nucleus, the Kölliker Fuse, the Barrington nucleus, the locus coeruleus, and the dorsal, laterodorsal, and ventral tegmental nuclei. In this study, we applied single-nucleus RNA-seq (snRNA-seq) to resolve neuronal subtypes based on their unique transcriptional profiles and then used multiplexed error robust fluorescence in situ hybridization (MERFISH) to map them spatially. We sampled ∼1 million cells across the dPnTg and defined the spatial distribution of over 120 neuronal subtypes. Our analysis identified an unpredicted high transcriptional diversity in this region and pinpointed many neuronal subtypes’ unique marker genes. We also demonstrated that many neuronal subtypes are transcriptionally similar between humans and mice, enhancing this study’s translational value. Finally, we developed a freely accessible, GPU and CPU-powered dashboard ( http://harvard.heavy.ai:6273/ ) that combines interactive visual analytics and hardware-accelerated SQL into a data science framework to allow the scientific community to query and gain insights into the data.

Brief summary: Grzelka et al. identified a hypothalamic circuit that regulates regain of body weight after weight loss. Body weight is one of the most regulated variables of our body, which is most likely due to the evolutionary history of times when access to food was scarce and volatile. Weight loss triggers a strong counterregulatory response. It increases the hunger drive and greatly increases the reward value of food, making it more difficult to adhere to a diet. Furthermore, energy metabolism is slowed down to reduce energy expenditure. Therefore, a weight loss diet is rarely successful in the long term (1). Caloric deprivation results in downregulation of nutrients and hormones, such as leptin, PYY and CCK, which usually suppress hunger, while the appetite-promoting hormone ghrelin becomes upregulated. By acting on orexigenic agouti-related peptide expressing (AgRP) neurons or anorexigenic pro-opio-melanocortin-expressing (POMC) neurons, these peripheral factors are thought to act on the brain to promote diet relapse and weight regain, but the precise mechanism of adaptations to dieting is not understood. The authors investigated mice after acute overnight fasting causing substantial (>10%) weight loss accompanied by an increased excitatory drive on AgRP neurons. They found that weight loss promotes a connection between AgRP neurons with another upstream neuron population, paraventricular hypothalamic thyrotropin releasing hormone neurons (PVH^TRH) and that activity of this PVH^TRH / AgRP circuit is necessary and sufficient to drive weight (re)gain. Most interesting, activation of this pathway induced persistent weight gain, while blocking the PVH^TRH / AgRP prevented weight gain after fasting. Although these results need validation in a clinical setting, Gzelka et al. provide evidence of a neuronal pathway consistent with the suggested set-point theory of body weight regulation. Since these experiments were performed in lean mice, it would be interesting to investigate if the identified pathway is also relevant in an obese setting. Furthermore, it would be of particular interest to investigate if this novel pathway interacts with leptin-melanocortin signalling. Reference: 1. Nordmo M, Danielsen YS, Nordmo M. The challenge of keeping it off, a descriptive systematic review of high-quality, follow-up studies of obesity treatments. Obes Rev 2020; 21. doi: 10.1111/obr.12949.