Populations of Hindbrain Glucagon-Like Peptide 1 (GLP1) Neurons That Innervate the Hypothalamic PVH, Thalamic PVT, or Limbic Forebrain BST Have Axon Collaterals That Reach All Central Regions Innervated by GLP1 Neurons

May 29, 2024The Journal of neuroscience : the official journal of the Society for Neuroscience

Groups of Hindbrain GLP1 Neurons Targeting the Hypothalamus, Thalamus, or Limbic Forebrain Also Send Branches to All Brain Areas Connected to GLP1 Neurons

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Abstract

Glucagon-expressing neurons project to multiple brain regions and the spinal cord, revealing extensive connectivity.

Neurons in the caudal nucleus of the solitary tract (cNTS) and intermediate reticular nucleus (IRt) both contribute to glucagon-like peptide 1 (GLP1) axon projections.
Distinct subsets of GLP1-expressing neurons appear to have overlapping axonal projections across various brain regions.
Cre-dependent tracing indicated that cNTS and IRt neurons show reciprocal labeling, suggesting interconnectivity between these populations.
Microinjections into specific brain nuclei demonstrated that GLP1 neurons from both cNTS and IRt share axon collaterals in the spinal cord and other central regions.
The collective axon collaterals from GLP1 neuron populations innervate key areas such as the thalamic paraventricular nucleus and hypothalamic paraventricular nucleus.

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Neurons in the caudal nucleus of the solitary tract (cNTS) and intermediate reticular nucleus (IRt) that express the glucagon gene () give rise to glucagon-like peptide 1 (GLP1)-immunopositive axons in the spinal cord and many subcortical brain regions. Central GLP1 receptor signaling contributes to motivated behavior and stress responses in rats and mice, in which hindbrain GLP1 neurons are activated to express c-Fos in a metabolic state-dependent manner. The present study examined whether GLP1 inputs to distinct brain regions arise from distinct subsets of-expressing neurons, and mapped the distribution of axon collaterals arising from projection-defined GLP1 neural populations. Using our Gcg-Cre knock-in rat model, Cre-dependent adeno-associated virus (AAV) tracing was conducted in adult male and female rats to compare axonal projections of IRt versus cNTS GLP1 neurons. Overlapping projections were observed in all brain regions that receive GLP1 input, with the caveat that cNTS injections produced Cre-dependent labeling of some IRt neurons, and vice versa. In additional experiments, specific diencephalic or limbic forebrain nuclei were microinjected with Cre-dependent retrograde AAVs (AAVrg) that expressed reporters to fully label the axon collaterals of transduced GLP1 neurons. AAVrg injected into each forebrain site labeled-expressing neurons in both the cNTS and IRt. The collective axon collaterals of labeled neurons entered the spinal cord and every brain region previously reported to contain GLP1-positive axons. These results indicate that the axons of GLP1 neural populations that innervate the thalamic paraventricular nucleus, paraventricular nucleus of the hypothalamus, and/or bed nucleus of the stria terminalis collectively innervate all central regions that receive GLP1 axonal input. Gcg Gcg Gcg

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Populations of Hindbrain Glucagon-Like Peptide 1 (GLP1) Neurons That Innervate the Hypothalamic PVH, Thalamic PVT, or Limbic Forebrain BST Have Axon Collaterals That Reach All Central Regions Innervated by GLP1 Neurons

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