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.
