Mahesh M. Karnani

Multiple hypothalamic circuits sense and regulate glucose levels.

The hypothalamus monitors body energy status in part through specialized glucose sensing neurons that comprise both glucose-excited and glucose-inhibited cells. Here we discuss recent work on the elucidation of neurochemical identities and physiological significance of these hypothalamic cells, including caveats resulting from the currently imprecise functional and molecular definitions of glucose sensing and differences in glucose-sensing responses obtained with different experimental techniques. We discuss the recently observed adaptive glucose-sensing responses of orexin/hypocretin-containing neurons, which allow these cells to sense changes in glucose levels rather than its absolute concentration, as well as the glucose-sensing abilities of melanin-concentrating hormone, neuropeptide Y, and proopiomelanocortin-containing neurons and the recent data on the role of ventromedial hypothalamic steroidogenic factor-1 (SF-1)/glutamate-containing cells in glucose homeostasis. We propose a model where orexin/hypocretin and SF-1/glutamate neurons cooperate in stimulating the sympathetic outflow to the liver and pancreas to increase blood glucose, which in turn provides negative feedback inhibition to these cells. Orexin/hypocretin neurons also stimulate feeding and reward seeking and are activated by hunger and stress, thereby providing a potential link between glucose sensing and goal-oriented behavior. The cell-type-specific neuromodulatory actions of glucose in several neurochemically distinct hypothalamic circuits are thus likely to be involved in coordinating higher brain function and behavior with autonomic adjustments in blood glucose levels.

Dichotomous cellular properties of mouse orexin/hypocretin neurons

Non‐technical summary  Orexin/hypocretin neurons are widely projecting, ‘multi‐tasking’ brain cells that promote alertness, reward seeking and feeding. They are vital for stable consciousness in higher mammals. Loss of orexin/hypocretin cells produces narcolepsy. It was originally assumed that orexin/hypocretin neurons are one uniform population of cells, but recent studies hinted that they may be split into subsystems. To explore this, we performed unbiased statistical analysis of electrical properties of orexin/hypocretin cells in combination with 3‐D analysis of their shape. Our results pointed to an existence of two subgroups of orexin/hypocretin neurons, that have unique ‘electrical fingerprints’ and distinct ways of receiving information from other neurons.

Lateral hypothalamic GAD65 neurons are spontaneously firing and distinct from orexin- and melanin-concentrating hormone neurons

Lateral hypothalamus (LH) contains GABA neurons involved in controlling metabolism and sleep. LH glutamic acid decarboxylase 65 (GAD65) GABA neurons are intrinsically depolarized, unlike classical GAD65 neurons of the cortex. LH GAD65 GABA neurons are distinct from most studied LH neurons (orexin and melanin‐concentrating hormone cells). A subset of LH GAD65 neurons are glucose inhibited. Our study adds new populations of glucose sensing neurons to the list of hypothalamic sugar sensors and introduces inhibitory circuit elements of the LH.

Opening Holes in the Blanket of Inhibition: Localized Lateral Disinhibition by VIP Interneurons

Inhibitory interneurons in the neocortex often connect in a promiscuous and extensive fashion, extending a “blanket of inhibition” on the circuit. This raises the problem of how can excitatory activity propagate in the midst of this widespread inhibition. One solution to this problem could be the vasoactive intestinal peptide (VIP) interneurons, which disinhibit other interneurons. To explore how VIP interneurons affect the local circuits, we use two-photon optogenetics to activate them individually in mouse visual cortex in vivo while measuring their output with two-photon calcium imaging. We find that VIP interneurons have narrow axons and inhibit nearby somatostatin interneurons, which themselves inhibit pyramidal cells. Moreover, via this lateral disinhibition, VIP cells in vivo make local and transient “holes” in the inhibitory blanket extended by SOM cells. VIP interneurons, themselves regulated by neuromodulators, may therefore enable selective patterns of activity to propagate through the cortex, by generating a “spotlight of attention”. SIGNIFICANCE STATEMENT Most inhibitory interneurons have axons restricted to a nearby area and target excitatory neighbors indiscriminately, raising the issue of how neuronal activity can propagate through cortical circuits. Vasoactive intestinal peptide-expressing interneurons (VIPs) disinhibit cortical pyramidal cells through inhibition of other inhibitory interneurons, and they have very focused, “narrow” axons. By optogenetically activating single VIPs in live mice while recording the activity of nearby neurons, we find that VIPs break open a hole in blanket inhibition with an effective range of ∼120 μm in lateral cortical space where excitatory activity can propagate.

Orexin neurons as conditional glucosensors: paradoxical regulation of sugar sensing by intracellular fuels

Non‐technical summary  Brain orexin/hypocretin neurons stimulate wakefulness, feeding, reward‐seeking and healthy glucose balance. The activity of orexin neurons is tightly regulated by several hormones, neurotransmitters and nutrients. Intriguingly, elevated glucose concentration can block or silence the activity of orexin neurons. We identified an unexpected way to control these effects of glucose on orexin neurons. We found that supplying orexin neurons with other energy‐related molecules, such as pyruvate and lactate, can stop glucose from blocking orexin neurons. We hypothesize that orexin neurons only ‘see’ glucose changes when the levels of other energy molecules are low, whereas high energy levels can stop glucose from regulating orexin cells. This may shed new light on understanding how the brain is influenced by changes in glucose levels during different metabolic situations, such as fasting, eating different diets, or in disease states such as diabetes and obesity.

VIP+ interneurons control neocortical activity across brain states

GABAergic interneurons are positioned to powerfully influence the dynamics of neural activity, yet the interneuron-mediated circuit mechanisms that control spontaneous and evoked neocortical activity remains elusive. Vasoactive intestinal peptide (VIP+) interneurons are a specialized cell class which synapse specifically on other interneurons, potentially serving to facilitate increases in cortical activity. In this study, using in vivo Ca(2+) imaging, we describe the interaction between local network activity and VIP+ cells and determine their role in modulating neocortical activity in mouse visual cortex. VIP+ cells were active across brain states including locomotion, nonlocomotion, visual stimulation, and under anesthesia. VIP+ activity correlated most clearly with the mean level of population activity of nearby excitatory neurons during all brain states, suggesting VIP+ cells enable high-excitability states in the cortex. The pharmacogenetic blockade of VIP+ cell output reduced network activity during locomotion, nonlocomotion, anesthesia, and visual stimulation, suggesting VIP+ cells exert a state-independent facilitation of neural activity in the cortex. Collectively, our findings demonstrate that VIP+ neurons have a causal role in the generation of high-activity regimes during spontaneous and stimulus evoked neocortical activity.

Direct and indirect control of orexin/hypocretin neurons by glycine receptors

Normal wakefulness relies on brain cells called orexin/hypocretin neurons. Activity of these cells stimulates awakening while their loss produces the sleep disorder narcolepsy. By studying what makes orexin/hypocretin cells more or less active, we can thus gain insights into how the brain switches between different states of consciousness. We describe a new way to turn orexin/hypocretin cells off using a chemical called glycine. We show that glycine shuts down the electrical activity of orexin/hypocretin neurons from the adult brain, but has the opposite effect in the very young brain. Apart from these direct actions on orexin/hypocretin cells, glycine also enhances the ability of other nerve cells to communicate with orexin/hypocretin neurons. These data shed new light on the basic chemical and physical mechanisms regulating orexin/hypocretin neurons, which may also be useful in improving therapeutic strategies for disorders such as insomnia.

Cortical Control of Spatial Resolution by VIP+ Interneurons.

Neuronal tuning, defined by the degree of selectivity to a specific stimulus, is a hallmark of cortical computation. Understanding the role of GABAergic interneurons in shaping cortical tuning is now possible with the ability to manipulate interneuron classes selectively. Here, we show that interneurons expressing vasoactive intestinal polypeptide (VIP+) regulate the spatial frequency (SF) tuning of pyramidal neurons in mouse visual cortex. Using two-photon calcium imaging and optogenetic manipulations of VIP+ cell activity, we found that activating VIP+ cells elicited a stronger network response to stimuli of higher SFs, whereas suppressing VIP+ cells resulted in a network response shift toward lower SFs. These results establish that cortical inhibition modulates the spatial resolution of visual processing and add further evidence demonstrating that feature selectivity depends, not only on the feedforward excitatory projections into the cortex, but also on dynamic intracortical modulations by specific forms of inhibition. SIGNIFICANCE STATEMENT We demonstrate that interneurons expressing vasoactive intestinal polypeptide (VIP+) play a causal role in regulating the spatial frequency (SF) tuning of neurons in mouse visual cortex. We show that optogenetic activation of VIP+ cells results in a shift in network preference toward higher SFs, whereas suppressing them shifts the network toward lower SFs. Several studies have shown that VIP+ cells are sensitive to neuromodulation and increase their firing during locomotion, whisking, and pupil dilation and are involved in spatially specific top-down modulation, reminiscent of the effects of top-down attention, and also that attention enhances spatial resolution. Our findings provide a bridge between these studies by establishing the inhibitory circuitry that regulates these fundamental modulations of SF in the cortex.