Elsevier

Neuroscience

Volume 180, 28 April 2011, Pages 111-121
Neuroscience

Cognitive, Behavioral, and Systems Neuroscience
Research Paper
Preproglucagon neurons project widely to autonomic control areas in the mouse brain

https://doi.org/10.1016/j.neuroscience.2011.02.023Get rights and content

Abstract

Glucagon-like peptide 1 (GLP-1) and its analogue exendin-4 inhibit food intake, reduce blood glucose levels and increase blood pressure and heart rate by acting on GLP-1 receptors in many brain regions. Within the CNS, GLP-1 is produced only by preproglucagon (PPG) neurons. We suggest that PPG neurons mediate the central effects of GLP-1 by modulating sympathetic and vagal outflow. We therefore analysed the projections of PPG neurons to brain sites involved in autonomic control. In transgenic mice expressing yellow fluorescent protein (YFP) under the control of the PPG promoter, we assessed YFP-immunoreactive innervation using an anti-GFP antiserum and avidin-biotin-peroxidase. PPG neurons were intensely YFP-immunoreactive and axons could be easily discriminated from dendrites. YFP-immunoreactive cell bodies occurred primarily within the caudal nucleus tractus solitarius (NTS) with additional somata ventral to the hypoglossal nucleus, in raphé obscurus and in the intermediate reticular nucleus. The caudal NTS contained a dense network of dendrites, some of which extended into the area postrema. Immunoreactive axons were widespread throughout NTS, dorsal vagal nucleus and reticular nucleus with few in the hypoglossal nucleus and pyramids. The dorsomedial and paraventricular hypothalamic nuclei, ventrolateral periaqueductal grey and thalamic paraventricular nucleus exhibited heavy innervation. The area postrema, rostral ventrolateral medulla, pontine central grey, locus coeruleus/Barrington's nucleus, arcuate nucleus and the vascular organ of the lamina terminalis were moderately innervated. Only a few axons occurred in the amygdala and subfornical organ. Our results demonstrate that PPG neurons innervate primarily brain regions involved in autonomic control. Thus, central PPG neurons are ideally situated to modulate sympathetic and parasympathetic outflow through input at a variety of central sites. Our data also highlight that immunohistochemistry improves detection of neurons expressing YFP. Hence, animals in which specific populations of neurons have been genetically-modified to express fluorescent proteins are likely to prove ideal for anatomical studies.

Highlights

▶Central GLP-1 neurons project to feeding and non-feeding autonomic control regions. ▶Brainstem GLP-1 neurons have axons and dendrites in the area postrema. ▶Genetically altered animals expressing marker proteins are ideal to study anatomy.

Section snippets

Experimental procedures

These studies were performed on nine adult male and eight adult female mGLU-124 Venus YFP mice (Reimann et al., 2008). These will be referred to as YFP-PPG mice in this study. Mice weighed between 25 and 35 g and were 12–16 weeks of age. Males were consistently heavier than females of the same age. All animals were bred in-house, were kept on a 12 h light:dark cycle and had access to food and water ad libitum. All experiments were carried out in accordance with the UK Animals (Scientific

Results

YFP-immunoreactive innervation was assessed in nine adult male and eight adult female YFP-PPG mice. After both immunoperoxidase and immunofluorescent staining, YFP-immunoreactivity was intense throughout the entire cytoplasm of medullary YFP-PPG neurons and it was easy to distinguish between YFP-immunoreactive axons and YFP-immunoreactive dendrites. Varicose YFP-immunoreactive axons had clearly delineated, heavily immunoreactive varicosities and fine intervaricose segments that were constant in

Discussion

We have performed an immunohistochemical study to determine the distribution of brain GLP-1 neurons and their projections between the spinomedullary junction and the anterior wall of the third ventricle in YFP-PPG mice (Reimann et al., 2008). These mice express YFP specifically and exclusively in cells in which the PPG promoter is active (Reimann et al., 2008, Hisadome et al., 2010), that is cells that produce PPG, the precursor for glucagon, as well as for GLP-1, GLP-2 and oxyntomodulin (

Conclusion

In conclusion, our results demonstrate that medullary neurons capable of synthesising GLP-1 innervate primarily brain regions involved in autonomic control. Hence, central PPG neurons are in a prime position to modulate sympathetic and parasympathetic outflow through input at a variety of autonomic locations within the CNS.

Acknowledgments

This work was supported by grants from the Medical Research Council (G0600928), UK to ST and the National Health and Medical Research Council of Australia to ILS (Project Grant 480414, Principal Research Fellowship 229921). FMG and FR are supported by Wellcome Trust Senior Research Fellowships (WT088357 and WT084210). Lee Travis provided expert technical assistance.

References (36)

  • A. Flint et al.

    Glucagon-like peptide 1 promotes satiety and suppresses energy intake in humans

    J Clin Invest

    (1998)
  • K.B.J. Franklin et al.

    The mouse brain in stereotaxic coordinates

    (2007)
  • R. Goke et al.

    Distribution of GLP-1 binding sites in the rat brain: evidence that exendin-4 is a ligand of brain GLP-1 binding sites

    Eur J Neurosci

    (1995)
  • J.L. Guan et al.

    Synaptic relations of GABAergic neurons in the area postrema

    Acta Anat (Basel)

    (1994)
  • J.L. Guan et al.

    Observation of the ultrastructure and synaptic relationships of angiotensin II-like immunoreactive neurons in the rat area postrema

    Synapse

    (2000)
  • M.R. Hayes et al.

    Endogenous hindbrain glucagon-like peptide-1 receptor activation contributes to the control of food intake by mediating gastric satiation signaling

    Endocrinology

    (2009)
  • M.R. Hayes et al.

    Caudal brainstem processing is sufficient for behavioral, sympathetic, and parasympathetic responses driven by peripheral and hindbrain glucagon-like-peptide-1 receptor stimulation

    Endocrinology

    (2008)
  • K. Hisadome et al.

    Leptin directly depolarizes preproglucagon neurons in the nucleus tractus solitarius: electrical properties of glucagon-like Peptide 1 neurons

    Diabetes

    (2010)
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