Spinally projecting preproglucagon axons preferentially innervate sympathetic preganglionic neurons

I.J Llewellyn-Smith,N Marina,R.N Manton,F Reimann,F.M Gribble,S Trapp

doi:10.1016/j.neuroscience.2014.10.043

Abstract

Glucagon-like peptide-1 (GLP-1) affects central autonomic neurons, including those controlling the cardiovascular system, thermogenesis, and energy balance. Preproglucagon (PPG) neurons, located mainly in the nucleus tractus solitarius (NTS) and medullary reticular formation, produce GLP-1. In transgenic mice expressing glucagon promoter-driven yellow fluorescent protein (YFP), these brainstem PPG neurons project to many central autonomic regions where GLP-1 receptors are expressed. The spinal cord also contains GLP-1 receptor mRNA but the distribution of spinal PPG axons is unknown.Here, we used two-color immunoperoxidase labeling to examine PPG innervation of spinal segments T1–S4 in YFP-PPG mice. Immunoreactivity for YFP identified spinal PPG axons and perikarya. We classified spinal neurons receiving PPG input by immunoreactivity for choline acetyltransferase (ChAT), nitric oxide synthase (NOS) and/or Fluorogold (FG) retrogradely transported from the peritoneal cavity. FG microinjected at T9 defined cell bodies that supplied spinal PPG innervation.The deep dorsal horn of lower lumbar cord contained YFP-immunoreactive neurons. Non-varicose, YFP-immunoreactive axons were prominent in the lateral funiculus, ventral white commissure and around the ventral median fissure. In T1–L2, varicose, YFP-containing axons closely apposed many ChAT-immunoreactive sympathetic preganglionic neurons (SPN) in the intermediolateral cell column (IML) and dorsal lamina X. In the sacral parasympathetic nucleus, about 10% of ChAT-immunoreactive preganglionic neurons received YFP appositions, as did occasional ChAT-positive motor neurons throughout the rostrocaudal extent of the ventral horn. YFP appositions also occurred on NOS-immunoreactive spinal interneurons and on spinal YFP-immunoreactive neurons. Injecting FG at T9 retrogradely labeled many YFP-PPG cell bodies in the medulla but none of the spinal YFP-immunoreactive neurons.These results show that brainstem PPG neurons innervate spinal autonomic and somatic motor neurons. The distributions of spinal PPG axons and spinal GLP-1 receptors correlate well. SPN receive the densest PPG innervation. Brainstem PPG neurons could directly modulate sympathetic outflow through their spinal inputs to SPN or interneurons.

Highlights

Glucagon-like peptide 1 (GLP-1) is released postprandially from enteroendocrine cells in the gut and facilitates absorption of nutrients (Holst, 2007)
Native yellow fluorescent protein (YFP) fluorescence (Fig. 1) revealed that YFP-expressing neurons existed in the lumbosacral spinal cord, an anti-GFP antibody with peroxidase detection was better for defining the morphology of the spinal YFP-positive neurons (Fig. 2)
Similar to PPG projections to the hypothalamus (Vrang et al, 2007), we found that YFPPPG neurons located in the nucleus tractus solitarius (NTS), intermediate reticular nucleus (IRT) and midline all projected to the spinal cord, suggesting there is no spatial segregation within the medulla of PPG neurons that innervate different targets

Summary

Introduction

Glucagon-like peptide 1 (GLP-1) is released postprandially from enteroendocrine cells in the gut and facilitates absorption of nutrients (Holst, 2007). In the CNS preproglucagon (PPG) is processed posttranslationally to produce GLP-1, GLP-2, and oxyntomodulin (Holst, 2007). The highest levels of these PPG products occur in the dorsomedial hypothalamus (DMH) and hypothalamic paraventricular nucleus (PVN) and the lowest levels are found in the cortex and hindbrain (Jin et al, 1988; Vrang et al, 2007; Tauchi et al, 2008). The majority of GLP-1-producing somata in the brainstem occur in the caudal nucleus tractus solitarius (NTS). Some GLP-1 cell bodies are located in the dorsomedial region of the medullary reticular nucleus (Jin et al, 1988; Larsen et al, 1997).

Results

Discussion

Conclusion