How The Hypothalamus Regulates Hormone Release During Fasting

Post by Baldomero B. Ramirez Cantu

The takeaway

Fasting activates agouti-related peptide (AgRP)-expressing neurons in the hypothalamus which disinhibit neurons in the ​​paraventricular hypothalamus (PVH). This process leads to the release of corticosterone, a hormone that helps manage glucose levels which provides energy during fasting.

What's the science?

During fasting, the body undergoes various essential survival responses, including the activation of the hypothalamic-pituitary-adrenal (HPA) axis, which increases the levels of stress hormones (e.g. cortisol in humans or corticosterone in rodents). These hormones prevent drops in blood sugar caused by fasting and maintain glucose balance. This response is crucial for preventing low blood sugar during fasting, and although its importance is recognized, the exact mechanism behind this activation has remained a mystery. This week in Nature, Douglass, Resch, Madara et al. delve into the underlying neural mechanisms and specific neuron-type roles in the activation of the HPA axis during fasting.

How did they do it?

The authors used a variety of techniques to investigate the role of the hypothalamus in regulating corticosterone release during fasting in mice. They primarily relied on plasma corticosterone measurements to measure HPA-axis activation, optogenetic and chemogenetic manipulations to probe the function of different cell types and to map connectivity between different hypothalamic regions, and ex-vivo preparations to assess the role of different receptor types in this pathway.

First, the authors confirmed previously reported data that fasting activates the HPA axis and increases corticosterone levels - by fasting mice for 24-hours and measuring their plasma corticosterone levels. The authors then used chemogenetics to activate or inhibit AgRP neurons and measured the effects on corticosterone levels. To further confirm the role of AgRP neurons in activating the HPA axis, the authors conducted experiments where they monitored the activity of a PVH-Crh (a specific subclass of PVH neurons crucial for initiating the release of corticosterone) by measuring their activity levels using fiber photometry, while simultaneously performing chemogenetic activation of AgRP neurons as a function of chemogenetic manipulation of AgRP neurons.

Next, they wanted to understand how AgRP neurons synaptically influence the activity of PVH-Crh neurons. Since AgRP neurons release inhibitory neurotransmitters and do not directly excite PVH-Crh neurons, they hypothesized that AgRP neurons might inhibit other neurons that in turn inhibit PVH-Crh neurons - thereby activating PVH-Crh neurons via reduced inhibition. They conducted experiments using ex-vivo electrophysiology, recording inhibitory currents onto PVH-Crh neurons while using receptor-specific agonists or antagonists (NPY and GABA). They also created genetic mutants of the NPY and GABA receptors in order to probe their role for PVH-Crh neuron inhibition in-vivo.

Finally, the authors wanted to identify the source of inhibitory GABAergic input that influences PVH-Crh neurons. They used a technique called retrograde rabies mapping to identify brain regions sending GABAergic signals to PVH-Crh neurons. Next, they employed an optogenetic-based method called channelrhodopsin assisted circuit mapping (CRACM) to confirm that neurons from a specific brain region inhibit PVH-Crh neurons ex-vivo, and fiber photometry to confirm that projections from this brain area to PVH are inhibited by AgRP neurons in-vivo.

What did they find?

The authors found that the activation of AgRP neurons increased corticosterone levels even in well-fed mice, while inhibiting these neurons suppressed the usual increase in corticosterone seen during fasting. This indicates that AgRP neurons play a crucial role in releasing corticosterone and are essential for this response during fasting. Chemogenetic activation of AgRP neurons drove rapid and sustained activation of PVH-Crh neurons while inhibition appeared to have the opposite effect. These results further support the role of AgRP neurons in this pathway, given the crucial role of PVH-Crh neurons in the release of corticosteroids and the activation of the HPA axis.

They also found that NPY and GABA can reduce inhibitory tone onto PVH-Crh neurons through receptors located on GABAergic afferents in their ex-vivo preparation. Through in-vivo experiments in genetically modified mice, they discovered that both NPY and GABA are not individually necessary for AgRP neurons to activate the HPA axis, but their combined effect is crucial. The study suggests that GABA release from AgRP neurons acting on GABA-B receptors on GABA-ergic afferents to Crh neurons in the PVH is necessary for activating the HPA axis.

Finally, the authors identify the bed nucleus of the stria terminalis (BNST) as the source of tonic inhibition to PVH-Crh neurons. Chemogenetic inhibition of inhibitory BNST neurons increased plasma corticosterone levels, indicating that inhibiting these neurons stimulates the HPA axis. Specifically inhibiting the BNST → PVH pathway also stimulated the HPA axis. Additionally, their fiber photometry results showed that stimulation of AgRP neurons suppressed the synaptic activity of BNST axon terminals in the PVH. Overall, their findings suggest that inhibitory afferents from the BNST normally suppress PVH-Crh neuron activity and that during fasting, AgRP neurons inhibit these afferents, reducing GABAergic tone onto PVH-Crh neurons and stimulating the HPA axis. 

What's the impact?

Understanding how neurons in the hypothalamus influence the body's adaptive responses to energy deficit and stress is paramount to providing insights into potential therapeutic targets for managing conditions related to metabolic and hormonal imbalances. These results help us gain a better understanding of the neural mechanisms by which AgRP neurons play a pivotal role in activating the HPA axis during fasting.

The Impact of Deep Brain Stimulation on Brain Networks and Treatment Outcomes in Treatment-Resistant Depression

Post by Kulpreet Cheema

The takeaway

Deep brain stimulation of the subcallosal cingulate gyrus in patients with depression induces changes in brain network properties, which is correlated with improvements in depression symptoms.

What's the science?

Treatment-resistant depression (TRD) is a severe form of depression that does not respond to standard treatments. Deep brain stimulation (DBS) is an emerging therapy that involves implanting electrodes in specific brain regions to modulate neural activity and potentially alleviate depressive symptoms. The effectiveness of DBS varies among patients, and personal factors might be the key to understanding this variability. This week in Molecular Psychiatry, Ghaderi and colleagues aimed to understand how DBS affects brain networks in TRD patients using electroencephalography (EEG), a non-invasive technique that measures the brain’s electrical activity.

How did they do it?

The study involved twelve TRD patients who underwent subcallosal cingulate gyrus (SCG) DBS. The researchers collected resting-state EEG data and Hamilton Depression rating scale data from participants at three sessions: before surgery, one to three months after surgery, and six months after surgery. During the second and third sessions, EEG data was collected with DBS turned on and off sequentially.

To analyze brain network properties, the researchers used an analytical method known as graph theoretical analysis to decipher whether there were distinct differences in brain network properties between patients who responded positively to the DBS treatment (responders) and those who did not (non-responders). Using EEG to study oscillatory activity in brain networks, the authors analyzed four different frequency bands — the delta and alpha bands were of particular interest due to their relevance to depression.

What did they find?

The study revealed several important findings. First, the baseline brain network properties in patients — especially in the delta and alpha frequency bands — were associated with the outcomes of DBS treatment. Responders to DBS showed lower levels of network segregation, integration, and synchronization at baseline compared to non-responders. Second, DBS led to changes in brain network properties over time, characterized by increased integration and synchronization of brain regions. These changes were particularly evident in the delta frequency band.

Third, the researchers found that responders had higher centrality - a measure of the importance of a node in a network for information propagation - in the subgenual anterior cingulate cortex (ACC), a brain region associated with depression symptoms. DBS led to a reduction in centrality in this region, which correlated with treatment response. Other brain regions, like the primary somatosensory cortex and parahippocampal gyrus, also showed alterations in centrality, indicating their involvement in TRD and DBS response.

What's the impact?

This study sheds light on the complex effects of DBS on brain networks in treatment-resistant depression. By using EEG and advanced analytical techniques, researchers identified specific brain network features associated with treatment response and demonstrated how DBS induces changes in these networks over time. The findings provide valuable insights into the potential mechanisms underlying the therapeutic effects of DBS in TRD patients. This knowledge could contribute to the development of personalized treatments for individuals with treatment-resistant depression, ultimately offering new hope for improved mental health outcomes. 

Access the original scientific publication here. 

Myelin Insulation and Risk of Neuron Degeneration in Autoimmune Environments

Post by Meredith McCarty

The takeaway

In the progression of autoimmune diseases such as multiple sclerosis, the degeneration of axons in the central nervous system leads to irreversible damage. Myelin sheaths encapsulating axons increase the risk of axonal degeneration in an autoimmune environment.

What's the science?

Multiple sclerosis (MS) is an inflammatory autoimmune disorder that affects the central nervous system and is characterized by axonal degeneration. Myelination, or the process by which oligodendrocytes encapsulate a neuron’s axons with an insulating sheath of myelin, is largely considered to serve an insulating and predominantly protective role for axons. Based on this assumption, demyelination, or the destruction of the myelin sheath, has been proposed as a likely cause of the axonal degeneration seen in autoimmune disorders. However, recent evidence that axonal damage actually occurs before the onset of demyelination suggests a more complex role for myelination in autoimmune disorders. This week in Nature Neuroscience, Schäffner, and colleagues utilize MS human biopsies and mouse models of demyelinating disease to investigate the role of demyelination and oligodendrocyte function in axonal degeneration.

How did they do it?

The authors first measured the relationship between axonal damage and demyelination in human MS. They performed electron microscopy on 4 MS biopsies in order to quantify the degree of demyelination of axons surrounding the lesions. They compared how the degree of demyelination correlated with the abundance of lesions and accumulation of organelles and condensed axoplasm, as measures of irreversible axonal damage.

In order to study the relationship between myelination, oligodendrocyte function, and acute inflammatory environments, the authors utilized several experimental mouse models and induced experimental autoimmune encephalomyelitis (EAE) to study axon survival.  First, to understand the temporal dynamics of axon degeneration, they induced EAE in mice via immunization. They quantified the proportion of myelinated and demyelinated damaged axons and characterized changes in myelin structure using electron microscopy and immunohistochemistry. To understand changes in gene regulation in EAE, the authors analyzed RNA sequencing datasets.

Next, in order to test whether demyelination could actually improve axon outcome, the authors used a cuprizone mouse model where axon demyelination occurs at an increased rate over time and measured changes in axon organelle accumulation. Lastly, in order to directly test whether the degree of myelination correlates with axonal damage, the authors used a mouse model with decreased expression of myelin basic protein (MBP). These hMbp mice exhibit an increased proportion of unmyelinated axons. They compared the disease progression and degree of axonal damage in hMbp mice relative to controls in an induced EAE environment.

What did they find?

The authors found that in both human and mouse models, irreversible axonal damage was restricted to myelinated axons. These damaged axons displayed increased organelle accumulation and highly condensed axoplasm, which are early biomarkers of axonal degeneration. The physical characteristics of myelin in damaged axons were found to be reflective of the dysfunctional movement of cellular materials to the axon by oligodendrocytes. They found evidence for the downregulation of RNA sequences essential for many cellular processes in EAE. In the cuprizone mouse model of axon demyelination, the authors found a decline in organelle accumulation in demyelinating axons, which suggests that demyelination can improve axonal survival.

When directly comparing axonal damage in hMbp mice in an EAE environment, the authors found significantly less axonal damage, fewer axons with organelle accumulation and condensed axoplasm, and overall better disease outcome relative to the control group. After ruling out differences in immunological response across these experimental groups, they found that axonal damage was limited to myelinated axons, even in a mouse model with an artificially increased proportion of demyelinated axons. Based on these data, the authors hypothesize that the progression to irreversible axonal degeneration is likely due to the accumulation of organelles and condensed cytoplasm, a process that would be prevented with efficient demyelination of the axon. They propose that oligodendrocyte dysfunction and the failure of efficient demyelination leads to this irreversible axonal damage.

What's the impact?

This study untangled the contradictory role of axonal myelination in the axonal damage characteristic of autoimmune diseases such as multiple sclerosis. Through untangling the molecular mechanisms involved in this axonal degeneration, this study puts forth novel theories as to the crucial role of oligodendrocytes and efficient demyelination of axons in an acute inflammatory environment. Future work building off of these findings has enormous implications for identifying new targets for therapeutic intervention to prevent axonal damage early in disease progression.